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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
SuWa 241
Geomorphology study (Study 6.5), mapping of geomorphic features and
turnover within the middle and lower Susitna River segments from 1950s,
1980s, and current aerials, technical memorandum
Author(s) – Personal:
Author(s) – Corporate:
Tetra Tech, Inc.
AEA-identified category, if specified:
September 26, 2014 technical memorandum filings
AEA-identified series, if specified:
Series (ARLIS-assigned report number): Existing numbers on document:
Susitna-Watana Hydroelectric Project document number 241
Published by: Date published:
[Anchorage, Alaska : Alaska Energy Authority, 2014] September 2014
Published for: Date or date range of report: Alaska Energy Authority
Volume and/or Part numbers:
Final or Draft status, as indicated:
Attachment B
Document type: Pagination:
Technical memorandum 625 p. in various pagings
Related work(s): Pages added/changed by ARLIS:
Cover letter to this report: Susitna-Watana Hydroelectric
Project, FERC Project no. 14241-000; Second set of 2014
technical memoranda for Initial Study Plan meetings. (SuWa
239)
Attachments A (SuWa 240) and C-G (SuWa 242-246)
Added cover letter
Notes:
All reports in the Susitna-Watana Hydroelectric Project Document series include an ARLIS-
produced cover page and an ARLIS-assigned number for uniformity and citability. All reports
are posted online at http://www.arlis.org/resources/susitna-watana/
September 26, 2014
Ms. Kimberly D. Bose
Secretary
Federal Energy Regulatory Commission
888 First Street, N.E.
Washington, D.C. 20426
Re: Susitna-Watana Hydroelectric Project, Project No. 14241-000
Second Set of 2014 Technical Memoranda for Initial Study Plan Meetings
Dear Secretary Bose:
As the Alaska Energy Authority (AEA) explained in its September 17, 2014 filing
with the Federal Energy Regulatory Commission (Commission or FERC) for the
proposed Susitna-Watana Hydroelectric Project, FERC Project No. 14241 (Project), the
June 3, 2014 Initial Study Report (ISR) provided for AEA to prepare certain technical
memoranda and other information based on 2014 work. In accordance with Commission
Staff direction, on September 17, 2014, AEA filed and distributed the first set of
technical memoranda and other information generated during the 2014 study season.
With this letter, AEA is filing and distributing the second set of technical
memoranda generated during the 2014 study season, as described below. As part of its
continued implementation of the study plan, AEA expects to file a third set of technical
memoranda prior to October 1, 2014.
This second set of technical memoranda includes:
• Attachment A: Geomorphology Study (Study 6.5) - Updated Mapping of
Aquatic Macrohabitat Types in the Middle Susitna River Segment from 1980s
and Current Aerials Technical Memorandum. This technical memorandum
updates the Middle Susitna River Segment portion of the aquatic macrohabitat
mapping results previously provided in the technical memorandum titled
Mapping of Aquatic Macrohabitat Types at Selected Sites in the Middle and
Lower Susitna River Segments from 1980s and 2012 Aerials (Tetra Tech
2013a).
• Attachment B: Geomorphology Study (Study 6.5) - Mapping of Geomorphic
Features and Turnover within the Middle and Lower Susitna River Segments
from 1950s, 1980s, and Current Aerials Technical Memorandum. This
technical memorandum updates the geomorphic mapping and assessment of
channel change that were initially provided in Mapping of Geomorphic
2
Features and Assessment of Channel Change in the Middle and Lower Susitna
River Segments from 1980s and 2012 Aerials (Tetra Tech 2013a). The initial
technical memorandum provided the results from tasks identified in Revised
Study Plan Study 6.5 Section 6.5.4.4. This update extends the previous 30
year analysis between the 1980s and 2012 by an additional 30 years with
aerial photography from the 1950s, and also provides a short term analysis of
geomorphic changes by comparing 2012 with 2013 aerial photography.
• Attachment C: Fluvial Geomorphology Modeling below Watana Dam Study
(Study 6.6) - Decision Point on Fluvial Geomorphology Modeling of the
Susitna River below PRM 29.9 Technical Memorandum. This technical
memorandum describes the decision of whether to extend the downstream
limit of the 1-D bed evolution model below Susitna Station at PRM 29.9.
• Attachment D: Fluvial Geomorphology Modeling Below Watana Dam (Study
6.6) - Winter Sampling of Main Channel Bed Material Technical
Memorandum. The overall purpose of this technical memorandum is to
quantify main channel bed material gradations at selected sites in the Upper,
Middle, and Lower Susitna River Segments. The data obtained from this
study serves as input for the 1-D and 2-D bed evolution modeling efforts
being conducted under the Fluvial Geomorphology Modeling Study (Study
6.6).
• Attachment E: Cook Inlet Beluga Whale Study (Study 9.17) - 2014 Cook Inlet
Beluga Whale Prey Study Implementation Technical Memorandum. This
technical memorandum summarizes activities implementing the Cook Inlet
Beluga Whale Study (Study 9.17) conducted in 2014 that tested methods to
document Cook Inlet Beluga Whale prey and prey habitat in the Susitna River
delta.
• Attachment F: River Productivity Study (Study 9.8) - 2013 Initial River
Productivity Results Technical Memorandum. This technical memorandum
provides a preliminary review and summary of 2013 river productivity sample
results based on laboratory data received after the ISR submittal in June 2014.
• Attachment G: River Productivity Study (Study 9.8) - 2014 Field Season River
Productivity Progress Report Technical Memorandum. This technical
memorandum presents an update on activities conducted during the Spring
field sampling event in June 2014, which was focused on data collection to
support the needs of the trophic modeling and stable isotope analysis
objectives of the River Productivity Study.
3
AEA appreciates the opportunity to provide this additional information to the
Commission and licensing participants, which it believes will be helpful in determining
the appropriate development of the 2015 study plan as set forth in the ISR. If you have
questions concerning this submission please contact me at wdyok@aidea.org or (907)
771-3955.
Sincerely,
Wayne Dyok
Project Manager
Alaska Energy Authority
Attachments
cc: Distribution List (w/o Attachments)
Attachment B
Geomorphology Study (Study 6.5) - Mapping of Geomorphic Features and Turnover within the
Middle and Lower Susitna River Segments from 1980s and Current Aerials
Technical Memorandum
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Geomorphology Study (6.5)
Mapping of Geomorphic Features and Turnover
within the Middle and Lower Susitna River Segments
from 1950s, 1980s, and Current Aerials
Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech, Inc.
September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
TABLE OF CONTENTS
Summary ...................................................................................................................................... vii
1. Introduction ........................................................................................................................1
2. Study Objectives.................................................................................................................2
3. Study Area ..........................................................................................................................2
3.1. General .....................................................................................................................2
3.2. Susitna River Segments ...........................................................................................3
4. Methods ...............................................................................................................................4
4.1. Variations from the Study Plan ................................................................................4
4.2. Aerial Photography ..................................................................................................4
4.3. Area of Geomorphic Delineation .............................................................................4
4.4. Delineation of Geomorphic Features .......................................................................5
4.4.1. Delineation Procedure ................................................................................. 5
4.4.2. Geomorphic Feature Classifications ........................................................... 5
4.5. Analysis....................................................................................................................8
4.5.1. Long-term Assessment of Middle River and Lower River Channel Change
from the 1950s, 1980s and 2012 Aerials .................................................... 8
4.5.2. Short-term Assessment of Channel Change from 2012 to 2013 Aerials .. 11
4.6. Quality Control ......................................................................................................12
4.6.1. Geomorphic Features ................................................................................ 12
4.6.2. Turnover .................................................................................................... 12
4.6.3. Registration Error in the 1950s Aerials .................................................... 13
5. Results ...............................................................................................................................13
5.1. Long-term Assessment of Middle River and Lower River Channel Change from
the 1950s, 1980s and 2012 Aerials ........................................................................13
5.1.1. Geomorphic Feature Area Tabulation ...................................................... 14
5.1.2. Geomorphic Feature Overlay Analysis ..................................................... 14
5.1.3. Turnover Results ....................................................................................... 14
5.1.4. Middle Susitna River Segment Long-term Channel Change Assessment 15
5.1.5. Lower Susitna River Segment Long-term Channel Change Assessment . 30
5.1.6. Chulitna River Reach ................................................................................ 37
5.1.7. Talkeetna River Reach .............................................................................. 38
5.1.8. Yentna River Reach .................................................................................. 39
5.2. Short-term Assessment of Channel Change from 2012 to 2013 Aerials ...............39
5.2.1. Qualitative Evaluation of 2012 Middle River Geomorphic Features and
the 2013 Aerial Photography .................................................................... 40
5.2.2. Quantitative Assessment of the 2012 and 2013 Geomorphic Features in
the Three Rivers Confluence .................................................................... 41
6. Discussion..........................................................................................................................43
7. References .........................................................................................................................46
8. Tables ................................................................................................................................48
9. Figures ...............................................................................................................................74
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
LIST OF TABLES
Table 4.2-1. 1950s Aerial Photo Summary. .................................................................................. 48
Table 4.2-2. Summary of 1980s aerial dates, discharges, and project river mile extents. ............ 49
Table 4.2-3. Summary of 2012 aerial dates, discharges, and project river mile extents. ............. 50
Table 5.1-1. Geomorphic feature areas for MR-1. ....................................................................... 51
Table 5.1-2. Geomorphic feature areas for MR-2. ....................................................................... 51
Table 5.1-3. Geomorphic feature areas for MR-3. ....................................................................... 52
Table 5.1-4. Geomorphic feature areas for MR-4. ....................................................................... 52
Table 5.1-5. Geomorphic feature areas for MR-5. ....................................................................... 53
Table 5.1-6. Geomorphic feature areas for MR-6. ....................................................................... 53
Table 5.1-7. Geomorphic feature areas for MR-7. ....................................................................... 54
Table 5.1-8. Geomorphic feature areas for MR-8. ....................................................................... 54
Table 5.1-9. Geomorphic feature areas for GAA-184. ................................................................ 55
Table 5.1-10. Geomorphic feature areas for GAA-173. .............................................................. 55
Table 5.1-11. Geomorphic feature areas for GAA-151. .............................................................. 56
Table 5.1-12. Geomorphic feature areas for GAA-144. .............................................................. 56
Table 5.1-13. Geomorphic feature areas for GAA-141. .............................................................. 57
Table 5.1-14. Geomorphic feature areas for GAA-138. .............................................................. 57
Table 5.1-15. Geomorphic feature areas for GAA-128. .............................................................. 58
Table 5.1-16. Geomorphic feature areas for GAA-115. .............................................................. 58
Table 5.1-17. Geomorphic feature areas for GAA-113. .............................................................. 59
Table 5.1-18. Geomorphic feature areas for GAA-104. .............................................................. 59
Table 5.1-19. Geomorphic feature areas for LR-1. ...................................................................... 60
Table 5.1-20. Geomorphic feature areas for LR-2. ...................................................................... 61
Table 5.1-21. Geomorphic feature areas for LR-3. ...................................................................... 62
Table 5.1-22. Geomorphic feature areas for LR-4. ...................................................................... 63
Table 5.1-23. Geomorphic feature areas for LR-5. ...................................................................... 64
Table 5.1-24. Geomorphic feature areas for LR-6. ...................................................................... 65
Table 5.1-25. Geomorphic feature areas for CL-1. ...................................................................... 66
Table 5.1-26. Geomorphic feature areas for TK-1. ..................................................................... 67
Table 5.1-27. Geomorphic feature areas for YN-1. ..................................................................... 68
Table 5.1-28. Turnover values for Middle River geomorphic reaches. ....................................... 69
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-29. Turnover values for Middle River Geomorphic Assessment Areas. ..................... 70
Table 5.1-30. Turnover values for Lower River geomorphic reaches. ........................................ 71
Table 5.1-31: Geomorphic reach delineations and classifications................................................ 72
LIST OF FIGURES
Figure 3.2-1. Susitna River geomorphology study area and large-scale river segments .............. 75
Figure 4.5-1. 1980s Geomorphic Feature Mapping of Channel or Land. ..................................... 76
Figure 4.5-2. 2012 Geomorphic Feature Mapping of Channel or Land ....................................... 76
Figure 4.5-3. Turnover Mapping of 1980 to 2012. ....................................................................... 77
Figure 4.5-4. Examples of vegetation encroachment and vegetation establishment. .................. 78
Figure 4.5-5. Example Focus Area and Geomorphic Assessment Areas. ................................... 79
Figure 4.5-6. Discharge measurements at USGS gaging station on or near the Susitna River
from June 2012 through October 2013. ................................................................................ 80
Figure 4.6-1. Example of Registration Error. ............................................................................... 81
Figure 5.1-1. Example of Turnover mapping in GAA-128 and GAA-138. ................................ 82
Figure 5.1-2. Map of the Middle Susitna River Segment showing the geomorphic reaches. ..... 83
Figure 5.1-3. Map of the Lower Susitna River Segment showing the geomorphic reaches. ....... 84
Figure 6.1-1: 1950s to 1980s turnover rates for Middle River geomorphic reaches. ................... 85
Figure 6.1-2: 1980s to 2012 turnover rates for Middle River geomorphic reaches. ..................... 86
Figure 6.1-3: 1950s to 2012 turnover rates for Middle River geomorphic reaches. ..................... 87
Figure 6.1-4. 1950s to 1980s turnover rates for Lower River geomorphic reaches. .................... 88
Figure 6.1-5. 1980s to 2012 turnover rates for Lower River geomorphic reaches. ...................... 89
Figure 6.1-6. 1950s to 2012 turnover rates for Lower River geomorphic reaches. ...................... 90
LIST OF APPENDICES
Appendix A. 1950s Middle Susitna River Segment Geomorphic Features
Appendix B. 1983 Middle Susitna River Segment Geomorphic Features
Appendix C. 2012 Middle Susitna River Segment Geomorphic Features
Appendix D. 1950s to 1980s Middle Susitna River Segment Channel Change Overlay
Appendix E. 1980s to 2012 Middle Susitna River Segment Channel Change Overlay
Appendix F. 1950s Lower Susitna River Segment Geomorphic Features
Appendix G. 1983 Lower Susitna River Segment Geomorphic Features
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iii September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Appendix H. 2012 Lower Susitna River Segment Geomorphic Features
Appendix I. 1950s to 1980s Lower Susitna River Segment Channel Change Overlay
Appendix J. 1980s to 2012 Lower Susitna River Segment Channel Change Overlay
Appendix K. 1950s to 1980s Middle Susitna River Segment Turnover
Appendix L. 1980s to 2012 Middle Susitna River Segment Turnover
Appendix M. 1950s to 2012 Middle Susitna River Segment Turnover
Appendix N. 1950s to 1980s Lower Susitna River Segment Turnover
Appendix O. 1980s to 2012 Lower Susitna River Segment Turnover
Appendix P. 1950s to 2012 Lower Susitna River Segment Turnover
Appendix Q. 2012 Geomorphic Features in the Three River Confluence
Appendix R. 2013 Geomorphic Features in the Three River Confluence
Appendix S. 2012 to 2013 Three River Confluence Channel Change Overlay
Appendix T. 2012 Middle Susitna River Segment Geomorphic Features on the 2013 Aerials
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
LIST OF ACRONYMS AND SCIENTIFIC LABELS
Abbreviation Definition
AEA Alaska Energy Authority
AOW Additional Open Water
BAB Bar/Attached Bar
BIC Bar Island Complex
Cfs Cubic feet per second
CL-1 Chulitna River Reach
ER Entrenchment ratio
EXP Exposed Substrate
ft² Square Feet
FERC Federal Energy Regulatory Commission
FIPS Federal Information Processing Standard
GIS Geographic Information System
GPS Global Positioning System
LiDAR Light Detection and Ranging
LR Lower River
LR-1 Lower River Reach 1 (PRM 102.4 to PRM 87.9)
LR-2 Lower River Reach 2 (PRM 87.9 to PRM 65.6)
LR-3 Lower River Reach 3 (PRM 65.6 to PRM 44.6)
LR-4 Lower River Reach 4 (PRM 44.6 to PRM 32.3)
LR-5 Lower River Reach 5 (PRM 32.3 to PRM 23.5)
LR-6 Lower River Reach 6 (PRM 23.5 to PRM 3.3)
MC Main Channel
MR Middle River
MR-1 Middle River Reach 1 (PRM 187.1 to PRM 184.6)
MR-2 Middle River Reach 2 (PRM 184.6 to PRM 169.6)
MR-3 Middle River Reach 3 (PRM 169.6 to PRM 166.1)
MR-4 Middle River Reach 4 (PRM 166.1 to PRM 153.9)
MR-5 Middle River Reach 5 (PRM 153.9 to PRM 148.4)
MR-6 Middle River Reach 6 (PRM 148.4 to PRM 122.7)
MR-7 Middle River Reach 7 (PRM 122.7 to PRM 107.8)
MR-8 Middle River Reach 8 (PRM 107.8 to PRM 102.4)
NAD North American Datum
NAVD North American Vertical Datum
PDF Portable document file
PRM Project River Mile
RM River Mile(s)
SC Side Channel
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page v September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Abbreviation Definition
SCC Side Channel Complex
SS Side Sloughs
TD Tributary Delta
TK-1 Talkeetna River Reach
TR Tributary
US Upland Slough
USGS U.S. Geological Survey
VI Vegetated Island
YN-1 Yentna River Reach
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page vi September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
SUMMARY
The purpose of this work was to update the geomorphic mapping and assessment of channel
change that were initially provided in Mapping of Geomorphic Features and Assessment of
Channel Change in the Middle and Lower Susitna River Segments from 1980s and 2012 Aerials
(Tetra Tech 2013a). The initial Technical Memorandum provided the results from tasks
identified in RSP Study 6.5 Section 6.5.4.4 Study Component: Assess Geomorphic Change
Middle and Lower Susitna River Segments. This update extends the previous 30 year analysis
between the 1980s and 2012 by an additional 30 years with aerial photography from the 1950s,
and also provides a short term analysis of geomorphic changes by comparing 2012 with 2013
aerial photography. The mapping presented in this technical memorandum supersedes the
mapping from the initial technical memorandum (Tetra Tech 2013a).
In addition to assessment of geomorphic feature changes, a turnover analysis was performed to
provide a graphical and quantitative determination of the area of floodplain that was converted to
river channel and the area of river channel that was converted to floodplain over the period from
the 1950s to 2012. Geomorphic features were simplified as either being a floodplain or channel
feature, and the turnover analysis assessed areas that changed from floodplain to channel or vice
versa over the given time periods. The turnover analysis showed a net channel to floodplain
turnover throughout the Middle and Lower Susitna rivers and supported the conclusion of the
initial geomorphic mapping technical memorandum, indicating that the primary factor in
geomorphic change is an increase in vegetation.
Also, additional aerial photography acquired in 2013 was used to assess short term geomorphic
changes observed between 2012 and 2013. The short-term analysis indicated that erosion was
the primary mode of channel change as opposed to vegetation encroachment or establishment.
This technical memorandum and Mapping of Geomorphic Features and Assessment of Channel
Change in the Middle and Lower Susitna River Segments from 1980s and 2012 Aerials (Tetra
Tech 2013a) represent the completion of the goals and objectives listed in RSP Study 6.5 Section
6.5.4.4 and ISR Study 6.5 Section 7.2.1.4 “Assess Geomorphic Change Middle and Lower
Susitna River Segments.” The turnover data and analyses presented in this 2014 technical
memorandum will be used to support the bed evolution modeling and bank energy index analysis
in ISR Study 6.6 Section 7.2.2.1, as well as the Riparian Instream Flow Study (Study 8.6), and
Ice Processes Study (Study 7.6).
The 1950s, 1980s, 2012, and 2013 aerials were used to assess the channel change and turnover in
the Middle and Lower River (ISR Study 6.5 Section 7.1.1.4). The Study Plan (RSP Sections
6.5.4.4.2.1 and 6.5.5.5.2.2.) indicates a decision will be made on whether to acquire additional
historical aerial photography for the Middle and Lower Susitna River Segments after completion
of the analysis of aerial photography from the 1950s, 1980s and present. In addition, the 2013
aerials that were flown to supplement the 2012 aerials document the river prior to, and after, the
peak flow of ~90,000 cfs at Gold Creek which is one of the highest flows recorded in the period
of record. As the objectives of this study were completed, no further analysis of aerial
photography is necessary.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
1. INTRODUCTION
The Alaska Energy Authority (AEA) is preparing a License Application that will be submitted to
the Federal Energy Regulatory Commission (FERC) for the Susitna-Watana Hydroelectric
Project using the Integrated Licensing Process. The Project is located on the Susitna River, an
approximately 320-mile-long river in the Southcentral region of Alaska. The Project’s dam site
will be located at Project River Mile (PRM) 187.1. The results of this study will provide
information needed to support the FERC’s National Environmental Policy Act analysis for the
Project license.
This technical memorandum updates the geomorphic feature mapping results presented in
technical memorandum Mapping of Geomorphic Features within the Middle and Lower Susitna
River Segments from 1980s and 2012 Aerials (Tetra Tech 2013a). The original Technical
Memorandum provided the results from efforts identified in RSP Study 6.5 Section 6.5.4.4 Study
Component: Assess Geomorphic Change Middle and Lower Susitna River Segments. The
mapping results presented in this technical memorandum supersede results from the
original technical memorandum (Tetra Tech 2013a).
The work presented in the original technical memorandum (Tetra Tech 2013a) involved two
primary efforts. The first was to use the digital aerial images collected as part of the aquatic
macrohabitat mapping task (Tetra Tech 2013b) and Geographic Information System (GIS)
software to delineate (map) geomorphic features identified in the 1980s and 2012 aerials for the
entire Middle and Lower Susitna River segments.
The second aspect of the original work consisted of identifying the channel change that had
occurred in the Middle and Lower Susitna River segments between the 1980s and present
condition (represented by 2012 aerial photography). As part of the original analysis, GIS was
used to create overlays of the 1980s and current geomorphic feature delineation. GIS was also
used to determine the area for each geomorphic feature for both periods. The geomorphic feature
overlays and tabulation of summed geomorphic feature areas was performed by geomorphic
reaches. An assessment of the channel change within each geomorphic reach between the 1980s
and 2012 was developed from this information.
The two primary updates conducted and presented in this technical memorandum include the
addition of the 1950s aerials and the results of the turnover analysis. The addition of the 1950s
aerials doubles the time frame over which channel change is evaluated. The turnover analysis
provides both a graphical and quantitative determination of the area of floodplain that was
converted to river channel and the area of river channel that was converted to floodplain over the
period from the 1950s to 2012. Both of these updates were added in response to licensing
participants’ comments.
Two secondary updates included in this technical memorandum involve the extension of the
geomorphic feature mapping on the lower reaches of the Chulitna and Talkeetna rivers and the
assessment of channel change between 2012 and 2013 as the result of large flow events.
In 2013, after the decision to model the Three Rivers Confluence with the 1-D Bed Evolution
Model was made, the upstream limits of the area of geomorphic delineation were extended 4.4
miles upstream on the Talkeetna River and 9.1 miles upstream on the Chulitna River (ISR Study
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
6.5 Section 4.4.2.2). Geomorphic mapping and turnover analysis was conducted for portions of
these two major tributaries where the 1-D Bed Evolution Model will be extended.
The large flows in September 2012 and June 2013 provided an opportunity to observe the
influence of large flow events on channel change. The 2012 and 2013 aerial photography were
used to qualitatively assess channel change in the Middle Susitna River Segment between 2012
and 2013. A more quantitative assessment of channel change between 2012 and 2013 in the area
of the Three Rivers Confluence was also conducted.
2. STUDY OBJECTIVES
The overall purpose of the work presented in this technical memorandum is to compare existing,
1980s and 1950s geomorphic feature data from aerial photograph analysis to characterize
channel stability and change and the spatial distribution of geomorphic features under
unregulated flow conditions. Specific objectives of the effort presented are listed below:
Map geomorphic features in the Middle and Lower Susitna River Segments, including the
Chulitna and Talkeetna rivers in the Three Rivers Confluence area, from the aerials
obtained representing the 1950s, 1980s and current conditions;
Determine and tabulate the area of the geomorphic features within each geomorphic reach
of the Middle and Lower Susitna River segments;
Evaluate channel change in the Middle and Lower Susitna River segments by:
— Preparing overlays of the 1950s, 1980s and current geomorphic features to assist in
qualitatively describing channel change;
— Quantitatively evaluate channel change with a turnover rate analysis that identifies
the area of channel converted to floodplain and floodplain converted to channel for
the periods of 1950s to 1980s and 1980s to present.
Qualitative assessment of channel change in the Middle River Segment between 2012 and
2013 resulting from the large flows (and/or ice jams and ice runs in the 2013 breakup)
through side by side comparison of the 2012 and 2013 aerial photography and the overlay
of the mapped 2012 geomorphic features on the 2013 aerial photography;
A quantitative assessment of channel change between 2012 and 2013 in the Three Rivers
Confluence area by delineation of the geomorphic features on the recently acquired 2013
aerial photographs in the Three Rivers Confluence area and comparison with the
geomorphic features delineated from the 2012 photography for the Talkeetna and Chulitna
rivers and geomorphic reaches MR-8 and LR-1 of the Susitna River.
3. STUDY AREA
3.1. General
The Susitna River, located in Southcentral Alaska, drains an area of approximately 20,010 square
miles and flows about 320 miles from its headwaters at the Susitna, West Fork Susitna, and East
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Fork Susitna glaciers to Cook Inlet (U.S. Geological Survey [USGS] 2012). The Susitna River
Basin is bounded on the west and north by the Alaska Range, on the east by the Talkeetna
Mountains and Copper River Lowlands, and on the south by Cook Inlet. The highest elevation in
the basin is Mt. McKinley at 20,320 feet while its lowest elevation is at sea level where the river
discharges into Cook Inlet. Major tributaries to the Susitna River between the headwaters and
Cook Inlet include the Chulitna, Talkeetna, and Yentna rivers, which are also glacially fed in
their respective headwaters. The basin receives, on average, 35 inches of precipitation annually
with average annual air temperatures of approximately 29°F.
3.2. Susitna River Segments
The overall study area extends from Cook Inlet to the Maclaren River confluence at PRM 261.3.
Within the geomorphology study area, the Susitna River has been divided into three segments
whose general characteristics are governed by the basin geology as described by Wilson et al.
(2009). The segments are referred to as the Upper, Middle, and Lower Susitna River segments
and are identified in Figure 3.2-1 with the associated extents:
Upper Susitna River Segment: Maclaren River confluence (PRM 261.3 / RM 260)
downstream to the proposed Watana Dam site (PRM 187.1 / RM 184)1
Middle Susitna River Segment: Proposed Watana Dam site (PRM 187.1 / RM 184)
downstream to the Three Rivers Confluence (PRM 102.4 / RM 98.5)
Lower Susitna River Segment: Three Rivers Confluence (PRM 102.4 / RM 98.5)
downstream to Cook Inlet (PRM 3.3 / RM 0)
The work presented in this technical memorandum covers the Middle and Lower Susitna River
Segments. The Upper River is not part of the study effort documented in this or the initial
technical memorandum (Tetra Tech 2013a). The general characteristics of the Middle Susitna
River Segment are heavily influenced by bedrock outcrop as well as Quaternary-age glaciations.
The morphologic characteristics of the Lower Susitna River Segment are dominated by sediment
loading from the major tributaries and variable resistance to erosion of the Pleistocene-age,
glacially-derived materials including tills (moraines), glacio-fluvial sediments in various
elevation outwash-surfaces, and glacio-lacustrine sediments that control the width of the valley
(Tetra Tech 2014b).
1Note: Project River Miles (PRMs) are the river mile system used for the current Susitna-Watana Project. River Miles (RMs)
were the river mile system used in the 1980s project. The PRM delineation starts about 3 miles farther into Cook Inlet than the
RMs and has a slightly different thalweg than that of the 1980s. Thus, PRM values are generally 3 to 4 miles higher than the RM
values. Because this analysis is a temporal comparison, both systems are referenced.
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4. METHODS
4.1. Variations from the Study Plan
There are no variances associated with the Assess Geomorphic Change Middle and Lower
Susitna River Segments study component that is presented in this technical memorandum and
scope identified in RSP Study 6.5 Section 6.5.4.4 and ISR Study 6.5 Section 4.4.2.2.
4.2. Aerial Photography
The digital color aerial photographs were collected in 2012 at a scale of 1:12,000 and with a
pixel resolution of 1 foot or better. The processing methodology for the 2012 aerial photography
along with the acquisition procedures for the 2012 and 1980s aerials are explained in Mapping of
Geomorphic Features within the Middle and Lower Susitna River Segments from the 1980s and
2012 Aerials (Tetra Tech 2013a). The acquisition and processing of the 2013 aerial photography
followed the methods presented in ISR Study 6.5 Section 4.5.1.1.2. The acquisition of the 1950s
aerials was presented in ISR Study 6.5 Section 4.4.2.1.3. Acquisition dates for the 1950s, 1980s,
and 2012 aerial photography are shown in Table 4.2-1, Table 4.2-2, and Table 4.2-3,
respectively.
4.3. Area of Geomorphic Delineation
An area of geomorphic delineation was developed to define the outer limit of the geomorphic
features to be mapped. The limit is referred to as the “geomorphic boundary.” It encompasses the
active channel area and serves as an estimate of the outer limits of areas that may be affected by
the Project in terms of hydraulics and channel morphology. The boundary was defined in
coordination with the Riparian Instream Flow Study (Study 8.6) and the Riparian Vegetation
Study Downstream of the Proposed Watana Dam (Study 11.6). The upstream limit is the Watana
Dam site (PRM 187.1) and the downstream limit is Cook Inlet (PRM 3.3). The lateral limits of
the geomorphic boundary were set using the procedure described below.
As part of this updated memorandum, the extents of the geomorphic delineation were extended
farther upstream on the Chulitna and Talkeetna rivers at the Three Rivers Confluence. The area
of geomorphic delineation was extended an additional 5.1 miles to a total of 10.0 miles on the
Talkeetna River and was extended 9.1 miles to a total of 12.4 miles on the Chulitna River. This
was done to support the 1-D bed evolution modeling effort (Study 6.6) when the decision was
made to extend the model up both of these major Susitna River tributaries.
The geomorphic boundary was delineated at an approximate scale of 1:3,000 using the 2012
color aerial photographs and images from the 2011 Matanuska-Susitna Borough light detection
and ranging (LiDAR) survey. The outer limits were initially identified following the riverward
edge of a terrace that typically ranged from approximately 20 to 40 feet above the main channel
water-surface elevation at the time of this 2011 LiDAR survey. In areas where steep canyon or
hillsides existed rather than a terrace, these features were followed at an elevation of
approximately 20 to 40 feet above the adjacent channel water-surface elevation. In locations
where tributaries joined the main channel and cut through the terrace or canyon walls, the
boundary was extended upstream until the tributary water-surface elevation matched that of the
main channel terrace at the tributary mouth. The corridor was then narrowed in some locations to
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reflect man-made features that constrain the river, primarily the railroad embankment. It was
assumed that the Alaska Railroad will maintain this alignment, and the Susitna River will not
migrate through the embankment; if it does, the Railroad will repair and reestablish the
embankment in its original location. In a few locations, where hydrologic connections were
judged to continue through the railroad embankment to aquatic habitats on the landward side of
the embankment, the low-lying area landward of the railroad was included within the
geomorphic boundary. This was done in order to include the habitat within the geomorphic
features that may be affected by changes in water-surface elevations created by the Project.
Lastly, a minimum buffer of 150 feet was added along the main channel water’s edge, except in
locations where the railroad embankment was identified as the outer limits of the boundary. For
this exception, the boundary was not offset. The boundary for the 1950s and 2012 delineations
continued up the Chulitna River for 12.4 miles and the Talkeetna River for 9.4 miles.
4.4. Delineation of Geomorphic Features
4.4.1. Delineation Procedure
All geomorphic features within the defined geomorphic boundary were delineated. While the
delineations of geomorphic features reflected the aquatic habitat, they were not limited to the
wetted habitat, but rather encompassed the entire bank to bank extent of the feature. Therefore,
the geomorphic features followed defined bank lines and included the wetted habitat, exposed
substrate, and other low-lying areas within the banks of the feature, as discernable from the aerial
photography. Unlike aquatic habitat types, geomorphic features do not require a wetted
connection to the Susitna River (Note: The methods and results for the aquatic macrohabitat
mapping are presented in two separate technical memorandums [Tetra Tech 2013b and 2014a]).
It is only necessary that a geomorphic feature or series of features, wet or dry, connect to the
Susitna River. If the water body was isolated and there was not a connection, wet or dry, to the
Susitna River, then the wetted area was mapped as additional open water (AOW).
Geomorphic feature delineations were made using ArcGIS 10.0 at a scale of 1:3,000. At that
scale, it is estimated that the accuracy of delineation is within approximately 10-20 feet of the
actual feature. Geomorphic feature delineations from the 2012 aerials were assisted by use of the
2011 Matanuska-Susitna LiDAR (Mat-Su Borough 2011) to determine elevation differences to
better define the boundary between channel areas and floodplain or island areas. The LiDAR was
used to determine bank and water-surface elevations in areas of shadows and under vegetation
cover in the upper ends of sloughs. The LiDAR has a coordinate system of North American
Datum (NAD) 1983 State Plane Alaska 4 FIPS 5004 feet and North American Vertical Datum
(NAVD) 88. Mapping of geomorphic features was performed for all areas within the delineated
geomorphic boundaries of the Middle and Lower Susitna River Segments.
4.4.2. Geomorphic Feature Classifications
Two sets of geomorphic feature classifications were utilized: one for the Lower Susitna River
Segment and one for the Middle Susitna River Segment. Geomorphic features include the wetted
area along with the exposed substrate contained within the banks with perennial vegetation. The
geomorphic features are classified along a spectrum of connectivity between clear water sources
upland (precipitation and groundwater sources for those in the floodplain and riverine subsurface
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flow for those in the area of the main channel) and turbid water from the direct surface
connection to the main channel:
Additional Open Waters
Upland Slough
Side Slough
Side Channel
Main Channel
Side sloughs and side channels are a transition in the spectrum because their aquatic
macrohabitat changes when they become hydraulically disconnected at their upstream end from
the turbid main channel. In addition to turbidity, habitat value may also change based on the
water temperature or other water quality aspects of the source of flow.
4.4.2.1. Middle Susitna River Segment
The geomorphic classifications for the Middle Susitna River Segment were based on the aquatic
habitat types defined in Trihey & Associates (1985). The wetted perimeter of macrohabitat types
(main channel, side channel, side sloughs, upland sloughs) along with the exposed substrate and
other low-lying areas within the banks defined the extent of a geomorphic feature. With the
inclusion of tributaries, vegetated islands, and additional open water, the classification types
were defined in 2012 as follows:
Main Channels (MC) are those channels of the river that normally convey streamflow
throughout the entire year. They are visually recognizable by their turbid, glacial water and
high velocities. In general, they convey more than 10 percent (approximate) of the total flow
passing a given location. Main channels can contain tributary mouth habitat, which is clear
water plume that enters from an adjacent clear water tributary.
Side Channels (SC) are also characterized by turbid, glacial water. Velocities often appear
lower than in mainstem sites. In general, they convey less than 10 percent (approximate) of
the total flow passing a given location. Side channel habitat may exist in well-defined
channels or in areas possessing numerous islands and submerged gravel bars. When the
upstream berms of side channels are dewatered and the channels contain clear water, they are
classified as side sloughs. Side channels can contain tributary mouth habitat which is clear
water plume that enters from an adjacent clear water tributary.
Side Sloughs (SS) are off-channel features that contain clear water and typically occur
outside the main channel. They are single discrete side sloughs. Small tributaries, upwelling
groundwater, and local surface runoff are the primary sources of clear water for these areas.
Side sloughs have non-vegetated upper thalwegs that are overtopped during periods of
moderate to high mainstem discharge. When these areas are overtopped they convey turbid
water and are then classified as side channels.
Upland Sloughs (US) are off-channel features that typically occur outside the mainstem
channel area. They contain clear water and depend on small streams, upwelling, and local
surface runoff for their water supply. Upland sloughs possess vegetated upper thalwegs that
are rarely overtopped by mainstem discharge.
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Tributary (TR) features are the portion of a tributary channel flowing across the floodplain.
Tributaries are typically clear water channels.
Vegetated Island (VI) is a discrete, large vegetated island that exists within or along the
main channel. Vegetated islands have perimeters of perennial vegetation.
Additional Open Water (AOW) is defined as standing water areas that are not channels,
sloughs or rivers. These are isolated bodies of water without a discernible direct or indirect
surface connection to the Susitna River.
4.4.2.2. Lower Susitna River Segment
For the Lower Susitna River Segment, geomorphic feature mapping classifications were adapted
and modified from the habitat types in R&M Consultants, Inc. and Trihey & Associates (1985).
These included: vegetated areas, exposed substrate, and aquatic macrohabitat types (main
channel, side channels, side sloughs, tributaries, and upland sloughs). Features such as the side
channel complex (SCC), bar island complex (BIC), bar/attached bar (BAB), tributary delta, and
additional open water were added to the set of geomorphic features. Within this analysis
mainstem is defined as a total of the channel areas and vegetated islands associated with the main
channel, bar island complexes, and side channel complexes. Braid plain is defined as the total of
main channel and bar island complexes. The classification types were defined in 2012 as
follows:
Main Channel features consist of areas within the river that normally convey streamflow
throughout the entire year. They are visually recognizable by their turbid, glacial water and
high velocities. In general, they convey more than 10 percent (approximate) of the total flow
passing a given location. Vegetated islands, side channel complexes and bar island
complexes were not included within the area calculation of the main channel.
Side Channel features occur outside the main channel limits. They are single channels as
opposed to the multiple and often interlaced/braided channels of the side channel complexes
that occur within the mainstem. They are characterized by turbid, glacial water. Velocities
often appear lower than in main channel. In general, they convey less than 10 percent
(approximate) of the total flow passing a given location. When the upstream berms of side
channels are dewatered and the channels contain clear water, they are classified as side
sloughs.
Side Channel Complex is an area within the mainstem that contains multiple side channels
separated by vegetated islands. The islands are typically several to many channel widths
long. The side channels are typically not separated by gravel bars, though gravel bars may
occur within the side channels. Side channels within the side channel complexes convey
turbid water. Side channel complexes have vegetated islands which constitute more than 50
percent of the complex area, unlike bar island complexes whose non-wetted areas are
dominated by exposed substrate.
Bar Island Complex is an area where there are multiple channels in braided patterns
separated primarily by exposed substrate. Both gravel bars (exposed substrate) and vegetated
island may occur within these complexes. Vegetated islands form a relativity small
percentage of the total area of the complex (in contrast to side channel complexes). The
channel braids within the bar island complex convey turbid water. Bar island complexes have
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vegetated bars which constitute less than 50 percent of the complex, unlike side channel
complexes.
Bar / Attached Bar is an exposed substrate feature that is attached to the banks of the main
channel(s). They are typically single discrete point bars or alternate bars and are not dissected
by numerous channel threads. In some case, chute channels may dissect a bar / attached bars.
Side Slough features are single discrete channels that contain clear water. These are off-
channel features that typically occur outside the main channel. Small tributaries, upwelling
groundwater, and local surface runoff are the primary sources of clear water for these areas.
Side sloughs do not have mature trees in their upper thalwegs and are overtopped during
periods of moderate to high mainstem discharge. When these areas are overtopped, they
convey turbid water and are then classified as side channels.
Upland Slough features are off-channel features that typically occur outside the mainstem
channel area. They contain clear water and depend on small streams, upwelling, and local
surface runoff for their water supply. Upland sloughs possess mature trees in their upstream
thalwegs and are rarely overtopped by mainstem discharge.
Tributary features are the portion of a tributary channel flowing across the Susitna
floodplain. These are typically clear water channels.
Tributary Delta is an exposed substrate feature of sediment deposited where the tributary
meets the mainstem channel area. This would typically be a fan shaped area and the tributary
may branch out into several channels across the delta/fan. Tributary fans were delineated as
they enter areas from the apex (upstream end of the fan) downstream to its limits with the
mainstem channel area.
Vegetated Island is a discrete, large vegetated area surrounded by channels with a perimeter
of mature, perennial vegetation or trees. If a grouping of vegetated islands is broken by
numerous channels, it is defined as a side channel complex rather than several vegetated
islands. In this case, the vegetated islands are delineated within and are considered part of the
side channel complex.
Additional Open Water features represent standing water areas that are not channels, side
channels or sloughs.
For purpose of the GIS analysis, a background (BG) category was created to attribute the entire
area of geomorphic delineation and facilitate quality control. Background was defined as non-
wetted areas outside the main stem that are not relevant to the analysis. Riparian zone, islands
within tributaries, and areas not relevant to analysis are classified as Background.
4.5. Analysis
4.5.1. Long-term Assessment of Middle River and Lower River Channel Change
from the 1950s, 1980s and 2012 Aerials
The long-term assessment of channel change in the Middle River and Lower Susitna River
Segments covered two principle periods: the 1950s to 1980s and the 1980s to 2012. A third
period, from the 1950s to 2012, was added to the updated technical memorandum to assess the
geomorphic change observed over approximately six decades. The tools used to develop the long
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term channel change included the tabulation of geomorphic feature areas at the three points in
time, overlay of the geomorphic features mapped at the three points in time and the turnover
analysis for the three periods.
Like the geomorphic feature overlay analysis, the turnover analysis identifies changes in
geomorphic channel features and vegetation; however, it provides a more direct assessment of
the exchange between channel area and floodplain area. It provides a rate at which an area
occupied by channel is converted to floodplain and islands, and the rate at which area occupied
by floodplain and islands is converted to channel. The turnover analysis is an alternative to the
combination of the geomorphic feature area tabulation and overlay analysis for quantifying
channel change over time where channel change is assessed by changes in area of the various
geomorphic features. The turnover analysis simplifies the presentation of information in order to
view the system in terms of two components: floodplain (land) and channel; channel change is
then assessed by changes in these two components. Each has its purpose with the geomorphic
feature areas tabulation and overlay analysis geared to identify changes in the distribution of the
geomorphic features and the turnover analysis reflecting the dynamics of channel and floodplain
building and destruction processes.
4.5.1.1. Geomorphic Feature Area Tabulation
The surface areas of the geomorphic features derived from the GIS mapping polygons were
tabulated and compared between the 1950s, 1980s and 2012 for the Middle and Lower Susitna
River segments. Area measurements (square feet) were calculated to the sixth decimal point and
tabulated to a precision of 1,000 sq. ft. In some cases, the 1980s aerial coverage was a limiting
factor as to what areas could be incorporated into the analysis. Only portions of the area of
geomorphic delineation that were covered by both the 1980s and 2012 aerials could be used for a
comparison of geomorphic features. The 1950s aerials covered all the extents of the 1980s and
2012 aerials and was not a limiting factor.
In the Lower Susitna River Segment, several areas within the geomorphic delineation boundary
were not covered by the 1980s and 2012 aerial photography. These areas were identified and
tracked for quality control, but were not presented in the results tabulated in this document. The
areas of missing aerial coverage within the area of geomorphic delineation for 1980s or 2012
aerial photography sets were calculated and excluded from the geomorphic feature tabulation.
The various categories of geomorphic features were summed for comparison within each
geomorphic reach. Overbank is the portion of the floodplain that is along the valley wall and
does not include the vegetated islands. Overbanks were tabulated as the summation of tributary
(TR), tributary delta (TD), additional open water (AOW), and background (BG) areas. To verify
that all geomorphic features were accounted for, each feature was summed and the summation
compared to the area within the geomorphic reach boundary. Comparisons between summed
features area and the total area within the reach boundary were considered acceptable if the
difference was less than 0.2 percent.
4.5.1.2. Geomorphic Feature Overlay Analysis
A spatial comparison of the Middle Susitna River Segment and the Lower Susitna River
Segment was conducted qualitatively over the entire area of geomorphic feature mapping. This
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information was incorporated into the channel change discussion when it provided additional
understanding of important processes that occurred within a geomorphic reach.
This comparison included:
Changes in Geomorphic Form such as channel widths, lengths, and alignment
Identification of Geomorphic Processes such as bank erosion, bar formation, lateral
channel migration, meandering, and avulsion
Changes in Hydraulic Connections due to the breaching of side sloughs and side channels
Identification of Biogeomorphic Process such as beaver dam construction and failure
Identification of Vegetation Processes such as encroachment, establishment (primary
succession), and removal.
4.5.1.3. Turnover Analysis
To perform the turnover analysis, geomorphic features from the 1950s, 1980s, and 2012 were
merged into either land or channel categories. The channel category included geomorphic
features of upland sloughs (US), side sloughs (SS), side channels (SC), and main channels (MC).
In the Lower River, bar island complexes (BIC), side channel complexes (SCC), and
bar/attached bars (BAB) were also included as channels. The floodplain (land) category includes
the vegetated islands (VI), background (BG), tributaries (TR), tributary deltas (TD), and
additional open water (AOW) areas (TR and AOW were included as part of the floodplain (land)
category because they are not Susitna River channels). Example of the mapping of the 1980s and
2012 channel and floodplain (land) categories is provided in Figures 4.5-1 and 4.5-2.
The geomorphic feature shapefile of the 1950s, 1980s, and 2012 were unioned into a “turnover”
shapefile for both river segments. This preserved all the channel and land attributes of the parent
shapefiles. Each polygon had an attribute of channel or land for the 1950s, 1980s, and 2012. An
example of the turnover mapping is included as Figure 4.5-3. The spatial extents of the turnover
analysis coincided with the geomorphic feature delineation boundary (Section 4.3).
Turnover was mapped for periods of 1950s to 1980s and the 1980s to 2012. This was done by
categorizing the turnover category between the years. Categories were defined for the turnover of
channel to floodplain and floodplain to channel. A turnover rate (in terms of 1,000 sq. ft. per
year per mile) was calculated from the change in area divided by the duration of the period and
length of the geomorphic reach or Geomorphic Assessment Area. Two static categories were
defined for floodplain or channel areas that remained floodplain and channel areas, respectively,
over the turnover period. Net turnover was calculated as the difference of the channel to
floodplain area minus the floodplain to channel area. A total turnover rate was defined as the
sum of both the channel to floodplain rate and the floodplain to channel rate. To aid in the
discussion of vegetation processes, the terms for “vegetation encroachment” and “vegetation
establishment” were defined as follows:
Vegetation encroachment is an area of channel to floodplain turnover in a specific time
period (1950s to 1980s or 1980s to 2012) that is an extension or expansion of an area of
vegetation existing at the beginning of the specified time period.
Vegetation establishment is an area of channel to floodplain turnover in a specific time
period (1950s to 1980s or 1980s to 2012) that was completely surrounded by channel
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features that existed at the beginning of the specified time period. Areas of vegetation
establishment have perimeters that are not adjacent to vegetation existing at the beginning of
the specified time period.
Examples of vegetation encroachment and vegetation establishment are presented in Figure 4.5-
4.
The turnover analysis was conducted at four different spatial scales: segment, geomorphic reach,
focus area (FA) and geomorphic assessment area (GAA) as described in ISR Study 6.5 Section
4.1.2.3.2. All four scales apply to the Middle Susitna River Segment. The Lower Susitna River
Segment is divided into geomorphic reaches, but does not include Focus Areas or Geomorphic
Assessment Areas. The segment and reach breaks for the Middle and Lower Susitna River
segments were presented in the updated technical memorandum entitled Geomorphic Reach
Delineation and Characterization, Upper, Middle and Lower Susitna River Segments (Tetra
Tech 2014b). The Focus Areas are discussed in the technical memorandum entitled Adjustments
to Middle River Focus Areas (R2 Resource Consultants, Inc. 2013). The Geomorphic
Assessment Areas were presented in the Geomorphic Mapping of Focus Areas and Geomorphic
Assessment Areas in ISR Study 6.5 Section 5.1.3.5. The Geomorphic Assessment Areas are an
enlargement of the Focus Area boundaries to incorporate a complete set of geomorphic features
within the boundary. Examples of a Focus Area and a Geomorphic Assessment Area are
provided in Figure 4.5-5.
4.5.2. Short-term Assessment of Channel Change from 2012 to 2013 Aerials
The availability of aerial photography from 2012 and 2013 along with the large flow events that
occurred in 2012 and 2013 provided an opportunity to assess channel change over a short period.
To assess channel change between 2012 and 2013 resulting from the large flows on 6/2/2013
(90,700 cfs at Gold Creek and 146,000 cfs at Sunshine) and 9/21/2012 (72,900 cfs at Gold Creek
and 198,000 cfs at Sunshine) the 2012 geomorphic features were qualitatively compared to the
2013 Middle River aerial photography for the entire Middle Susitna River Segment. For the
Three Rivers Confluence area, a more detailed comparison of channel change was performed by
mapping geomorphic features from the 2013 aerial photography in the Three Rivers Confluence
area (MR-8, CL-1, TK-1, and LR-1) and comparing these with the geomorphic features mapped
from the 2012 aerial photography. To illustrate the hydrology over this period, the flows for the
Middle River from June 2012 to October 2013 is presented in Figure 4.5-6.
4.5.2.1. Qualitative Evaluation of 2012 Middle River Geomorphic Features and the
2013 Aerial Photography
The 2012 Middle River geomorphic features were compared to the 2013 aerial photography to
qualitatively assess channel change between 2012 and 2013. In ArcGIS, the 2012 Middle River
geomorphic features were overlaid on the 2013 aerial photography. Locations of erosion were
noted and documented in a polygon shapefile. The spatial distribution of erosion was evaluated
and summarized.
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4.5.2.2. Quantitative Assessment of the 2012 and 2013 Geomorphic Features in the
Three Rivers Confluence
Geomorphic features were delineated from the 2013 aerial photography for the reaches MR-8,
CL-1, TK-1, and LR-1. The 2012 and 2013 geomorphic features were compared to quantitatively
assess channel change between 2012 and 2013. The extent of the quantitative assessment was
focused on the two mile radius from PRM 102.4. Locations of erosion along vegetated islands
were noted and documented in a polygon shapefile. The maximum width of the erosion was
identified, measured, and related to the overall channel change observed in the Three Rivers
Confluence.
4.6. Quality Control
4.6.1. Geomorphic Features
The first step in quality control process involved the training of the staff developing the mapping.
The Geomorphology Program Lead and the Geomorphology Task Lead provided training to the
senior hydraulic engineers/geomorphologist and the GIS analysts to ensure appropriate
identification and application of the classification categories. Senior hydraulic engineers/
geomorphologists reviewed the feature delineations for completeness, adherence to the
classifications, and scale criteria. The senior hydraulic engineers/geomorphologists frequently
consulted with the Geomorphology Program Lead and the Geomorphology Task Lead on the
application of the definitions and for advice when differentiation between geomorphic
classifications and/or features was challenging.
The next step in the quality control process was for the senior hydraulic engineer/
geomorphologist to provide markups of the mapping to the staff performing the delineation in
comments on the GIS files, marked up portable document files (PDFs), and written instructions.
Comments were provided for both specific items such as changing the classification of a specific
feature or general concerns such as the quality of the digitization and proper interpretation of the
definitions. The GIS analysts performed the corrections per the instructions of the reviewers. The
reviewers conducted a back-check of the changes made to the classifications and provided
additional instructions on changes. The correction/ review cycle was repeated if necessary.
Throughout the process, the senior hydraulic engineers/geomorphologists consulted with the
Geomorphology Program Lead or the Geomorphology Task Lead to refine definitions and help
make decisions for unique situations encountered. The files were then reviewed for topology
errors such as gaps between delineations (slivers) and overlaps. A final check was run on the
tabulated areas for the reaches. Comparisons between summed features area, turnover, portions
missing aerial coverage, and the total reach boundary area were considered acceptable for the
geomorphic assessment areas, Middle, and Lower River reaches if the difference was less than
0.2 percent.
4.6.2. Turnover
The summed areas of the geomorphic features were compared to their outer boundary to ensure
complete and non-overlapping coverage. The percent change in floodplain and channel areas in
geomorphic features shapefiles were compared to the turnover analysis results.
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4.6.3. Registration Error in the 1950s Aerials
Mapping control for the 1950s photography was derived from Landsat imagery. While the 100-
foot pixel resolution of the Landsat imagery is rather low precision, the diagnostic statistics of
the aerotriangulation adjustments showed sub-pixel residuals for the control points that were
chosen. In an effort to improve the Landsat-based adjustment, three of the sets of photography
that cover the Middle River were subjected to a shift after an initial aerotriangulation solution
was reached. Holding rigid the relative locations and orientations of the exposures, the
preliminary solution was registered to a small collection of points identified in the 2012
orthophotos. In every case, the shifts that resulted from this procedure were smaller in magnitude
than the RMSE of the Landsat control. Improvement in the accuracy in some locations was likely
balanced by lower accuracy in other locations and the method was not extended to all blocks.
Additional details on the mapping control for the 1950s aerials were presented in ISR Study 6.5
Section 4.4.2.1.3.1.
Some systematic errors in the registration of the 1950s aerials were still identified after the
adjustments described in the previous paragraph were performed. In some locations, similar
features in the 1950s aerials are located about 20 to 50 feet north of their locations in the 1980s
and 2012 aerial photography within the east-west oriented portions of MR-1 through MR-5. An
example of the registration error in the 1950s geomorphic features as compared to the 2012
geomorphic features in the area of PRM 151 is presented in Figure 4.6-1. The shift here appears
to be primarily registration error as the bedrock along the right side of the main channel at PRM
151 has not shifted in location since the 1950s. The registration error is also present in the
turnover mapping and results as channel to floodplain turnover on the southeast side of the river
and floodplain to channel turnover on the northwest side of the river. Total turnover results were
influenced by the registration error for the 1950s to 1980s period. However, the shift in the 1950s
aerial photos is typically within the estimated 10-20 foot accuracy of the delineation as
previously mentioned in section 4.4.1. The net turnover values for the 1950s to 1980s period
should exclude the turnover error since the same shift in both the floodplain to channel turnover
and channel to floodplain turnover should cancel out.
5. RESULTS
5.1. Long-term Assessment of Middle River and Lower River Channel
Change from the 1950s, 1980s and 2012 Aerials
The long term assessment of channel change covered the Middle and Lower Susitna River
segments over two principle periods the 1950s to 1980s and the 1980s to 2012. A third overall
period, from the 1950s to 2012, was also analyzed to assess the geomorphic change observed
over approximately six decades.
The analysis of channel change in the Middle and Lower Susitna River segments is based on the
comparison and analysis of the geomorphic features mapped for the 1950s, 1980s and 2012. To
identify and quantify change tabulated areas were used for the various geomorphic features
within a reach. The analysis also identified processes that result in channel change including
vegetation encroachment, vegetation establishment (primary succession on gravel bars), bank
erosion, lateral migration, and biogeomorphic processes such as beaver dam construction.
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Comparative terms, such as increase and reduce, are a function of area differences (1980s vs.
1950s and 2012 vs. 1980s) determined from the tabulated geomorphic feature data. The overlay
mapping along with tabulation of areas associated with geomorphic features and turnover are
tools used in the discussion of the results.
5.1.1. Geomorphic Feature Area Tabulation
Aerial photographs with the geomorphic features mapped for the Middle Susitna River Segment
are provided in Appendix A for the 1950s, Appendix B for the 1980s, and Appendix C for 2012.
The Middle Susitna River Segment channel change overlay is provided in Appendix D for the
1950s and 1980s geomorphic features and in Appendix E for the 1980s to 2012 geomorphic
features. The geomorphic mapping for the Lower Susitna River Segment sites is provided in
Appendix F for 1950s, Appendix G for 1983, and Appendix H for 2012. The Lower Susitna
River Segment channel change overlay is provided in Appendix I for the 1950s and 1980s
geomorphic features and in Appendix J for the 1980s to 2012 geomorphic features. Tabulated
geomorphic feature areas are provided in Table 5.1-1 through Table 5.1-8 for the Middle River
geomorphic reaches, Table 5.1-9 through Table 5.1-18 for the Middle River Geomorphic
Assessment Areas, and Table 5.1-19 through Table 5.1-27 for the Lower River geomorphic
reaches.
5.1.2. Geomorphic Feature Overlay Analysis
Due to the qualitative comparison and comparatively subjective nature of geomorphic feature
classification, the limitations of the geomorphic feature area tabulation and overlay analysis were
addressed with a turnover analysis. For this reason, the analysis of geomorphic feature area
(tabular and qualitative summary) was limited to that which would be necessary to develop or
supplement the more robust turnover analysis. Changes in the classification of sloughs and
channels were included with the respective reaches and geomorphic assessment areas (GAAs) of
the turnover results.
5.1.3. Turnover Results
The turnover areas were tabulated by reach, as well as for Geomorphic Assessment Areas in the
Middle River. Areas that were gained or lost from the 1950s to 2012 between reaches at
confluences were calculated. The tabulated turnover areas represent areas that were located
within the reach and had aerial photography during both time periods. Figure 5.1-1 shows an
example of the turnover mapping from GAA-128 and GAA-138. The complete Middle River and
Lower River turnover mapping results are included as Appendix K through Appendix P.
Calculated turnover values are provided in Table 5.1-28 for the Middle River geomorphic
reaches, Table 5.1-29 for the Middle River Geomorphic Assessment Areas, and Table 5.1-30 for
the Lower River geomorphic reaches.
Relative stability rating criteria were established to categorize turnover rates. Categories
representing: stable (or static), moderately stable, moderately dynamic, and dynamic were
chosen for both the Middle and Lower Susitna River segments based on a total turnover rate
(defined as the sum of both channel to floodplain and floodplain to channel turnover rate).
Ranges for the relative stability criteria are listed below.
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Geomorphic Reach Stability Ratings Based on Total Turnover Rate
Relative Reach
Stability
MR reaches and
GAAs LR reaches
(ft²/yr/mile) (ft²/yr/mile)
Stable 0 to 10,000 0 to 100,000
Moderately Stable 10,000 to 20,000 100,000 to 200,000
Moderately Dynamic 20,000 to 30,000 200,000 to 300,000
Dynamic > 30,000 > 300,000
5.1.4. Middle Susitna River Segment Long-term Channel Change Assessment
The Middle Susitna River Segment can be generally divided into three geomorphic regions:
Watana Dam (PRM 187.1) to Devils Canyon (PRM 166.1), Devils Canyon (PRM 166.1 to PRM
153.9), and Devils Canyon to the Three Rivers Confluence (PRM 102.4). From the Watana Dam
site to the head of Devils Canyon, the slope is about 11 ft/mile and the channel is bounded by
meta-sedimentary and gneissic rocks. The channel slope in Devils Canyon is about 31 ft/mile
and the channel is bounded by granitic rocks. From Devils Canyon to the Three Rivers
Confluence, the channel slope decreases progressively from about 12 ft/mile to about 7 ft/mile
and the reduction in slope correlates to a reduction in the extent of bedrock outcrop along the
channel and in the erosion resistance of the bounding materials (Pleistocene- and Holocene-age
alluvial terraces) and the transition to an alluvial channel.
These general regions were further divided into eight geomorphically distinct reaches presented
in Figure 5.1-2 and listed in Table 5.1-31. Middle River geomorphic reaches 1 through 3
comprise the upper middle portion. Reach 4 is the Devils Canyon segment, and Reaches 5
through 8 comprise the lower middle portion. Each geomorphic reach was classified based on its
geomorphic characteristics including bed slope, bounding geology, average active channel width,
channel branching, sediment storage capacity, and sinuosity. Further characteristics for each of
the eight reaches are described in Geomorphic Reach Delineation and Characterization, Upper,
Middle and Lower Susitna River Segments Technical Memorandum (Tetra Tech 2014b).
5.1.4.1. MR-1 (PRM 187.1 to PRM 184.6)
The Susitna River (current 660 feet active channel width) flows in a narrow, approximately 780
feet wide, bedrock-bounded canyon downstream of the Watana Dam site. The canyon is formed
in Tertiary to Cretaceous-age gneiss. There is limited sediment storage potential in the reach
because of the narrow valley bottom an entrenchment ratio (ER) of 1.2 and relatively steep slope
(9.2 ft/mile). Alluvial sediments are stored within vegetated and non-vegetated mid-channel bars
that tend to be located in local hydraulic expansion zones. The sinuosity of the reach is 1.03, and
the average number of channels in the reach is 1.2.
Based on its turnover rate, MR-1 is classified as a stable reach. The total turnover rate for the
period of the 1950s to 2012 is 7,400 ft²/yr/mile. As there are few locations of sediment storage
and the main channel is formed within a bedrock-bounded canyon, turnover rates should be
similar between different time periods. The total turnover rate for the period from the 1950s to
1980s (15,000 ft²/yr/mile) appears to be more than twice the rate of the 1980s to 2012 period
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(6,100 ft²/yr/mile). Except for erosion along the vegetated islands at PRM 185.9 and PRM 185.3,
for the remainder of the turnover area in MR-1 during the 1950s to 1980s approximately half
appears to be registration error. The channel to floodplain turnover on the right bank is
approximately equal to the floodplain to channel turnover on the left bank. The channel to
floodplain turnover rate for MR-1 between the 1950s and 1980s is 7,900 ft²/yr/mile and
comparable to the floodplain to channel turnover rate of 7,300 ft²/yr/mile for the same period.
The channel to floodplain turnover rate in MR-1 for the 1980s to 2012 was 1,000 ft²/yr/mile, and
the floodplain to channel turnover rate was 5,100 ft²/yr/mile, for a net floodplain to channel
turnover rate of 4,000 ft²/yr/mile. The loss of floodplain area can be attributed to erosion on two
of the three vegetated mid-channel bars, as well as channel widening where the vegetation line
can be observed to have retreated 10-30 ft along the right bank at PRM 186.4, PRM 185.7, PRM
185, and PRM 184.7. Average channel widths remained relatively unchanged from 570 ft in the
1950s and 1980s, but increased to 660 ft in 2012.
5.1.4.1.1. GAA-184 Watana Dam (PRM 185.7 to PRM 184.7)
GAA-184 Watana Dam extends one mile from PRM 185.7 to PRM 184.7, in the downstream
portion of MR-1. GAA-184 is located in a bedrock-bounded canyon downstream of the Watana
Dam site and is comprised of Tertiary to Cretaceous-age gneiss (Tetra Tech 2014b). Alluvial
sediments in MR-1 are primarily stored within vegetated and non-vegetated mid-channel bars
that tend to be located in local hydraulic expansion zones. The channel gradient of 11.5 ft/mile in
GAA-184 is larger than the average channel gradient of 9.4 ft/mile in MR-1.
Similar to MR-1 as a whole, GAA-184 Watana Dam falls into the stable category based on
turnover rates. The total turnover rate for the period of the 1950s to 2012 was 8,200 ft²/yr/mile.
The active channel width of GAA-184 for all three time periods (1950s width = 740 ft, 1980s
width = 730 ft, 2012 width = 750 ft) is larger than the 2012 MR-1 current reach active channel
width of 660 ft because a portion of the right bank is composed of erodible glacial till.
Turnover from the 1950s to the 1980s in GAA-184 Watana Dam was limited to a few locations.
The floodplain island at PRM 185.2 decreased in size by about 80% from the 1950s to 1980s.
The floodplain island at PRM 185.3 right exhibited vegetation encroachment in the same period.
The main channel widened at PRM 185.6, PRM 185.1, and PRM 184.7 as the vegetation line
retreated 10 to 30 feet along the river right.
The net turnover trends were similar in GAA-184 and MR-1 for each time period, with a net
floodplain to channel turnover between the 1950s and 1980s, and net channel to floodplain
turnover for the 1980s to 2012 and 1950s to 2012 time periods. From the 1950s to the 1980s, the
net turnover rate in GAA-184 was 1,400 ft²/yr/mile, while the MR-1 reach average net turnover
rate was 600 ft²/yr/mile. For both the 1980s to 2012 and 1950s to 2012 time periods, net
turnover rates for GAA-184 (1980s to 2012 rate = 3,500 ft²/yr/mile, 1950s to 2012 rate = 1,100
ft²/yr/mile) were comparable to the MR-1 reach average net turnover rates (1980s to 2012 rate =
4,000 ft²/yr/mile, 1950s to 2012 rate = 1,800 ft²/yr/mile).
5.1.4.2. MR-2 (PRM 184.6 to PRM 169.6)
The Susitna River with a current 720 feet active channel width flows in a wider, approximately
1,500 feet, bedrock-bounded canyon between the Tsusena Creek confluence and about PRM
173, where the canyon narrows to about 1,000 feet. The wider, upper part of the reach is formed
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in more erodible Cretaceous-age Kahiltna Flysch meta-sedimentary rocks and the narrower,
lower part between PRM 173 and PRM 169.6 is formed in less-erodible Tertiary to Cretaceous-
age gneiss. The average slope of the reach is 10.8 ft/mile. There are considerably more,
compared with MR-1, alluvial sediments stored in vegetated islands, mid-channel bars and in
vegetated discontinuous floodplain segments in this reach with an entrenchment ratio of 2.1.
This is particularly true of the wider, upper portion of this reach. It is likely that Fog Creek, a
south bank tributary, is a local source of sediment in the upper portions of this reach. In the
lower, narrower part of the reach, alluvial sediments are stored within discontinuous vegetated
floodplain segments and in unvegetated mid-channel bars. The sinuosity of the reach is 1.06 and
the average number of channels in the reach is 1.4.
Based on the total turnover rate, MR-2 is classified as a stable reach. The total turnover rate for
the period of the 1950s to 2012 is 8,600 ft²/yr/mile. This value may be exaggerated due to
registration error along the main channel.
The channel to floodplain turnover in MR-2 can be described as vegetation encroachment
increasing the size of previously vegetated areas. This occurred primarily outside of the main
channel at PRM 183, PRM 173.3 left, PRM 176 left, within GAA-173 Stephan Lake Complex,
and PRM 172.3 right. MR-2 had a net turnover rate of 2,500 ft²/yr/mile of channel to floodplain
for the period from the 1950s to the 1980s.
Floodplain to channel turnover in MR-2 depicts different amounts in the mapping between the
two periods of the 1950s and 1980s and 1980s to 2012. The reason being that the 1950s to 1980s
floodplain to channel turnover rate results (5,700 ft²/yr/mile) were likely overestimated due to
shadows and registration error in the 1950s imagery. Floodplain to channel turnover appears to
have occurred between the 1980s and 2012 due to vegetation removal from the banks at
numerous locations such as PRM 183.6, PRM 179.8, PRM 175.8, PRM 175.2 and PRM 173.2.
While there were some small areas of vegetation encroachment (1980s to 2012 channel to
floodplain turnover rate = 3,400 ft²/yr/mile), the floodplain to channel turnover rate was larger
(turnover rate = 4,800 ft²/yr/mile). This led to a net floodplain to channel turnover rate of 1,400
ft²/yr/mile for the 1980s to 2012 time period. Channel widths over time also remained stable
with average widths of 720 ft in the 1950s, 700 feet in the 1980s, and 720 feet in 2012.
The tabulated geomorphic feature areas in MR-2 exhibited major differences between the two
periods of the 1950s to 1980s and 1980s to 2012. Side slough area decreased by 79 percent from
the 1950s to the 1980s, but increased by over 4,000 percent from the 1980s to 2012 (1950s area
= 139,000 sq. ft., 1980s area = 29,000 sq. ft., 2012 area = 1.4 million sq. ft.). The apparent
increase in side slough area is due to a lower discharge in the 2012 photos, leading to what had
been a side channel on river right at PRM 175 in the 1980s to be classified as a side slough in
2012 and likely does not represent a physical change in feature classification. Upland slough area
increased by 28 percent from the 1950s to the 1980s, and also increased by over 100 percent
from the 1980s to 2012 (1950s area = 90,000 sq. ft., 1980s area = 115,000 sq. ft., 2012 area =
268,000 sq. ft.). Channel conversion to uplands sloughs at PRM 178.4, PRM 176.3, and PRM
175.3 account for the majority of the upland slough area increase between the 1980s and 2012.
5.1.4.2.1. GAA-173 Stephan Lake Complex (PRM 175.7 to PRM 173.6)
GAA-173 Stephan Lake Complex extends 2.1 miles from PRM 175.7 to PRM 173.6, in the
lower half of MR-2. GAA-173 is located in a bedrock-bounded (Tetra Tech 2014b) canyon
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comprised of Cretaceous-Tertiary age gneiss. Alluvial sediments are stored in vegetated islands,
mid-channel bars, and vegetated discontinuous floodplain segments. The channel gradient of
GAA-173 is 10.8 ft/mile, the same as the MR-2 reach average channel gradient.
GAA-173 Stephan Lake Complex is moderately stable based on the turnover rates. The total
turnover rate for the GAA-173 Stephan Lake Complex was 15,000 ft²/yr/mile for the 1950s to
2012. The active channel widths of GAA-173 (1950s width = 990 ft, 1980s width = 940 ft, 2012
width = 890 ft) are considerably larger than the 2012 MR-2 reach active channel width of 720 ft.
Two main factors comprise the turnover values in GAA-173 Stephan Lake Complex: vegetation
encroachment and vegetation removal. Channel to floodplain turnover rates in GAA-173 Stephan
Lake Complex (1950s to 2012 rate = 12,000 ft²/yr/mile) were greater than MR-2 (1950s to 2012
rate = 4,600 ft²/yr/mile) due to the vegetation encroachment around the islands at PRM 175.2,
PRM 174.8, and PRM 174.2. Channel widening or vegetation retreat of 20 to 40 feet has
occurred at PRM 175.2 and PRM 173.6. Registration error is evident in the 1950s imagery in
GAA-173 Stephan Lake Complex and results in slight overestimation of turnover rates using this
imagery (1950s to 1980s and 1950s to 2012).
By comparing the net turnover values, a realistic comparison can be made between the overall
channel change in MR-2 and local changes at GAA-173 Stephan Lake Complex. Both had net
channel to floodplain turnover rates from the 1950s to 1980s (MR-2 net rate = 2,500 ft²/yr/mile)
and GAA-173 Stephan Lake Complex (7,900 ft²/yr/mile). However, for the 1980s to 2012 the net
turnover rate for MR-2 switched from floodplain to channel (1,400 ft²/yr/mile), while GAA-173
Stephan Lake Complex had a greater channel to floodplain turnover rate (8,900 ft²/yr/mile). The
primary difference between the two appears to be the channel widening through the majority of
MR-2 compared to the vegetation encroachment outside of the main channel in the GAA-173
Stephan Lake Complex.
Changes in the classification of geomorphic features within GAA-173 were affected both by
high discharges in the 1980s imagery and reduction in the hydraulic connection at the upper end
of a channel. The change in the side slough and side channel area follows the similar explanation
as MR-2. A physical reduction in the hydraulic connection at the head of the channel at PRM
175 was responsible for the classification change from SC in the 1950s to SS in the 1980s and is
not due to differences in flow rates in the imagery. The channel was completely encroached upon
by vegetation by 2012.
5.1.4.3. MR-3 (PRM 169.6 to PRM 166.1)
The Susitna River (590 feet active channel width) flows in a narrow (about 780 feet wide),
bedrock-bounded canyon from PRM 169.6 to PRM 166.1 (Tetra Tech 2014b). The canyon is
formed in Paleocene-age granitic rocks. Because of the relatively narrow canyon (ER=1.3) and
steep slope (12.3 ft/mile), the alluvial sediment storage potential in the reach is low. Alluvial
sediments are stored within a few vegetated mid-channel bars in the reach, and there is little
evidence of even discontinuous floodplain segments within the reach. The sinuosity of the reach
is 1.02 and the average number of channels in the reach is 1.1 indicating the reach is primarily
single channel.
MR-3 is classified as a stable reach based on its total turnover rate for the 1950s to 2012 of 3,000
ft²/yr/mile. Floodplain to channel turnover in the 1980s to 2012 period (rate of 4,200 ft²/yr/mile)
was the dominant turnover mechanism and occurred as 10 to 60 feet of main channel widening.
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The net rates for turnover in MR-3 depict opposite trends between the 1950s and 1980s and the
1980s to 2012. From the 1950s to 1980s, channel to floodplain turnover (turnover rate = 2,600
ft²/yr/mile) was larger than floodplain to channel turnover (turnover rate = 1,600 ft²/yr/mile) for
an overall net channel to floodplain turnover rate of 1,000 ft²/yr/mile. From the 1980s to 2012;
however, the channel to floodplain turnover rate of 600 ft²/yr/mile and the floodplain to channel
turnover rate of 4,200 ft²/yr/mile resulted in a net floodplain to channel turnover rate of 3,600
ft²/yr/mile. Channel widths reflect the stability of MR-3 with the reach average widths of 580 ft.
in the 1950s, 570 ft. in the 1980s, and 590 ft. in 2012.
5.1.4.4. MR-4 (PRM 166.1 to PRM 153.9)
The Susitna River (310 feet active channel width) flows in a very narrow (370 feet wide), very
steep (30.6 ft/mile), bedrock-bounded canyon, referred to as Devils Canyon (Tetra Tech 2014b).
The narrow canyon has formed in Paleocene-age granitic rocks that are probably not faulted
given the very narrow width of the canyon. Because of the narrow canyon (ER=1.2) and steep
slope, there is very little, if any, alluvial sediment stored within the reach. The sinuosity of the
reach is 1.03 and the reach is almost entirely single channel.
Devils Canyon is the most stable reach within the Middle Susitna River Segment. Turnover and
geomorphic change was minimal; most features did not change. As MR-4 is a bedrock-bounded
canyon with only two vegetated islands at PRM 163.5 and PRM 158.8, turnover rates were the
lowest in for all Middle and Lower River geomorphic reaches. The total turnover rate for the
reach from the 1950s to 2012 was 2,500 ft²/yr/mile with an appreciable portion of this likely
attributable to registration error in the 1950s aerials and shadows obscuring the banks in the 2012
aerials. Very little geomorphic change was documented in areas that were clearly visible in each
set of aerials. The stability of MR-4 is also evident in the minor changes in average channel
widths of 320 feet in both the 1950s and 1980s, and 310 feet in 2012. The 10 feet of difference is
within the resolution of the digitization process.
5.1.4.5. MR-5 (PRM 153.9 to PRM 148.4)
From PRM 153.9 to PRM 148.4, the Susitna River (510 feet active channel width) flows through
a bedrock-bounded canyon similar in width to MR-1 and MR-3 (about 850 feet), but nearly
double the width of Devils Canyon in MR-4. The relatively narrow canyon has formed in
Cretaceous-age Kahiltna Flysch meta-sedimentary rocks. The somewhat wider canyon and
lower slope (12.3 ft/mile) compared to MR-4 (Devils Canyon) allow some alluvial sediment
storage within the reach, primarily in a few vegetated mid-channel islands and discontinuous
floodplain segments in the slightly wider parts of the reach (ER=1.7). The sinuosity of the reach
is 1.03 and the average number of channels in the reach is 1.2.
In terms of its total turnover rate for the period of the 1950s to 2012 of 8,700 ft²/yr/mile MR-5 is
classified as a stable reach. Turnover primarily occurred as vegetation encroachment along the
vegetated bars of Slough 21A. There was an increase in floodplain area for all time periods
resulting in net channel to floodplain turnover for all periods (net channel to floodplain turnover
rates of 2,300 ft²/yr/mile for the 1950s to 1980s, 3,300 ft²/yr/mile for the 1980s to 2012, and
2,800 ft²/yr/mile for the 1950s to 2012).
Registration error appears to have been significant in the 1950s aerials. This error may represent
on the order of 30 to 70 feet of the main channel bank shift from PRM 151 to the downstream
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limit of MR-5 at PRM 148.3. The results for the total turnover rate for the period of the 1950s to
1980s was 15,000 ft²/yr/mile, although the 1980s to 2012 rate of 6,700 ft²/yr/mile is probably
more representative of the true stability of MR-5. Channel widths narrowed very slightly from
530 feet in the 1950s, 520 feet in the 1980s, and 510 feet in 2012.
5.1.4.5.1. GAA-151 Portage Creek (PRM 152.3 to PRM 151.8)
GAA-151 Portage Creek extends 0.5 miles from PRM 152.3 to PRM 151.8 in the upper portion
of MR-5. GAA-151 is located in a bedrock-bounded (Tetra Tech 2014b) narrow canyon formed
in Cretaceous-age Kahiltna Flysch meta-sedimentary rocks and contains a significant tributary,
Portage Creek. GAA-151 is a single channel with no mid-channel bars or islands. As such,
sediment storage is limited. The channel gradient of GAA-151 of 12.2 ft/mile is nearly the same
as the MR-5 reach channel gradient of 12.1 ft/mile, but the active channel width of GAA-151 is
slightly smaller (1950s width = 460 feet, 1980s width = 440 feet, 2012 width = 440 feet) than the
2012 MR-5 reach active channel width of 510 feet.
The lowest amounts of turnover among the ten Geomorphic Assessment Areas occurred in
GAA-151 for all time periods, with a maximum turnover rate of 3,500 ft²/yr/mile from channel
to floodplain occurring between the 1950s and the 1980s. Channel to floodplain turnover rates
for GAA-151 (1950s to 1980s rate = 3,200 ft²/yr/mile, 1980s to 2012 rate = 700 ft²/yr/mile,
1950s to 2012 rate = 1,200 ft²/yr/mile) were less than the MR-5 reach average channel to
floodplain turnover rates (1950s to 1980s rate = 8,500 ft²/yr/mile, 1980s to 2012 rate = 5,000
ft²/yr/mile, 1950s to 2012 rate = 5,800 ft²/yr/mile) for all time periods.
The floodplain to channel turnover rates for GAA-151 were not appreciable for the 1950s to
1980s (300 ft²/yr/mile) and 1950s to 2012 (200 ft²/yr/mile), and were substantially less than the
MR-5 reach average floodplain to channel turnover rate for the same periods (1950s to 1980s
rate = 6,200 ft²/yr/mile, 1950s to 2012 rate = 3,000 ft²/yr/mile). From the period between the
1980s and 2012, however, the floodplain to channel turnover rate was essentially equal for GAA-
151 (2,000 ft²/yr/mile) to MR-5 reach average turnover rate (1,700 ft²/yr/mile). Turnover rates on
the order of 1,000 to 2,000 ft²/yr/mile are at the limit of the mapping ability considering map
resolution and accuracy of the line work.
Net channel to floodplain turnover occurred between the 1950s and 1980s (rate of 3,000
ft²/yr/mile) and 1950s to 2012 (rate of 1,000 ft²/yr/mile) time periods, whereas the turnover for
the 1980s to 2012 period were net floodplain to channel (rate of 1,200 ft²/yr/mile). The MR-5
reach turnover was net channel to floodplain for all three time periods (1950s rate = 2,300
ft²/yr/mile, 1980s rate = 3,300 ft²/yr/mile, 2012 rate = 2,800 ft²/yr/mile).
5.1.4.6. MR-6 (PRM 148.4 to PRM 122.7)
In Middle River geomorphic reach 6 (MR-6), the Susitna River flows through a bedrock-
bounded canyon composed of Cretaceous-age Kahiltna Flysch meta-sedimentary rocks
averaging 2,350 feet in width with an active channel width of 990 feet. In the wider parts of the
reach, alluvial sediments are stored in continuous vegetated floodplain segments and within
numerous vegetated islands and bars, as well as in unvegetated mid-channel bars (ER=2.4)
(Tetra Tech 2014b). Channel slope decreases nearly 15 percent from 12.3 ft/mile in MR-5 to
10.7 ft/mile. The reach is relatively straight at a sinuosity of only 1.09. The multiple channel
nature of this reach is evidenced by the average number of channels calculated at 2.4. Where the
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valley bottom is wider within the reach, the alluvial deposits tend to be more vegetated, and
where the valley bottom is narrower, the alluvial deposits tend to be less vegetated.
MR-6 is a dynamic reach and the turnover rates further support this conclusion. The total of
1950s to 2012 channel to floodplain (23,000 ft²/yr/mile) and floodplain to channel turnover rates
(14,000 ft²/yr/mile) is 37,000 ft²/yr/mile. The only Middle River geomorphic reach with a higher
total turnover rate is MR-8 (38,000 ft²/yr/mile). Both MR-6 and MR-8 had more than twice the
total turnover rate as the third most dynamic reach MR-7 (16,000 ft²/yr/mile). Compared to the
Geomorphic Assessment Areas in MR-6, the reach average for MR-6 (37,000 ft²/yr/mile) was
greater than GAA-144 Slough 21 (30,000 ft²/yr/mile) and GAA-141 Indian River (19,000
ft²/yr/mile) and less than GAA-138 Gold Creek (41,000 ft²/yr/mile) and GAA-128 Slough 8A,
(53,000 ft²/yr/mile).
In MR-6, the turnover analysis reveals that the even though the areas of channel to floodplain
turnover are similar for the reach, there were differences in the distribution of floodplain increase
attributable to vegetation encroachment versus vegetation establishment. In both periods, the
1950s to 1980s and 1980s to 2012, floodplain area increased by similar amounts of the total area
in the reach (1950s to 1980s turnover = 6.6 percent, 1980s to 2012 turnover = 5.5 percent). The
turnover rates for the channel to floodplain for the 1950s to 1980s was 28,000 ft²/yr/mile and for
the 1980s to 2012 it was 26,000 ft²/yr/mile, essentially equal. The period of the 1950s to 1980s
had two areas of vegetation encroachment that exceeded 1,000,000 sq. ft. They occurred at
PRMs 130 and PRM 127. In the period of the 1980s to 2012, there was only one area (PRM
124.7 area = approximately 575,000 sq. ft.) of vegetation encroachment that was larger than
500,000 sq. ft. The largest areas of the vegetation establishment ranged between 200,000 sq. ft.
and 500,000 sq. ft. These locations include PRMs 144.8 and PRM 138, PRM 137.2, and PRM
125 in the 1950s to 1980s and at PRM 137.9, PRM 133.4, PRM 128.4, and PRM 127.8 in the
1980s to 2012.
Unlike the channel to floodplain turnover, the amounts of floodplain to channel turnover differs
between the 1950s to 1980s and 1980s to 2012. Floodplain to channel turnover was twice the
amount during the 1950s to 1980s (turnover = 5.2 percent), than the 1980s to 2012 (turnover =
2.6 percent). The turnover rates for the floodplain to channel for the 1950s to 1980s was 22,000
ft²/yr/mile and for the 1980s to 2012 it was 12,000 ft²/yr/mile. The 1950s to 1980s floodplain to
channel turnover rate is higher than the 1980s to 2012 rate partially because of a few locations
with large turnover in the 1950s to 1980s period. Approximately 1,000,000 sq. ft. of floodplain
to channel turnover occurred at both PRMs 133.4 and PRM 146 in the period from the 1950s to
1980s as the main channel shifted. Channel widths decreased consistently from 1,090 feet in the
1950s, 1,060 feet in the 1980s and 990 feet in 2012.
5.1.4.6.1. GAA-144 Slough 21 (PRM 146.1 to PRM 143.6)
GAA-144 Slough 21 extends 2.5 miles from PRM 146.1 to PRM 143.6 and is the upper most
Geomorphic Assessment Area in MR-6, the longest geomorphic reach in the Middle River.
GAA-144 is located in a bedrock-bounded canyon comprised of Cretaceous-age Kahiltna Flysch
meta-sedimentary rocks with inset glacial and glacio-fluvial deposits. The channel gradient of
13.8 ft/mile in GAA-144 is larger than the MR-6 reach channel gradient of 10.8 ft/mile. The
active channel widths of GAA-144 for each time period (1950s width = 910 feet, 1980s width =
930 feet, 2012 width = 920 feet) are slightly smaller than the 2012 MR-6 reach active channel
width of 990 feet.
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Slough 21 GAA-144 is a moderately dynamic area. The total turnover rate for the 1950s to 2012
was 30,000 ft²/yr/mile. Registration error likely accounts for an appreciable portion of the higher
total turnover rate for the 1950s to 1980s (50,000 ft²/yr/mile) compared to the 1980s to 2012
(20,000 ft²/yr/mile).
From the 1950s to 1980s, channel to floodplain turnover primarily occurred as vegetation
establishment at PRM 144.8 and vegetation encroachment around the island in the mouth of
Slough 21. Vegetation encroachment was also prevalent along Slough 20, Side Channel 21, and
Slough 21. Vegetation establishment also occurred with the formation of the island at PRM 144.
From the 1980s to 2012, a similar trend of channel to floodplain turnover occurred. Vegetation
establishment between the vegetated islands along Side Channel 21 continued with the creation
of a new island at PRM 145.2 left. The inlet to Slough 21 continued to experience vegetation
encroachment. The train of islands along Side Channel 21 became more defined with the
establishment of vegetated islands at PRM 144.2 left and PRM 143.8 left and the merging of the
two islands at PRM 144.8.
The largest area of floodplain to channel turnover from the 1950s to 1980s occurs on river left
from PRM 146.3 to PRM 145.8, in the present-day location of the powerline crossing over the
Susitna River. Up to 480 feet of floodplain was eroded away between the 1950s and 1980s. A
major contributor to this behavior appears to be the loss of the vegetated island at PRM 146.3
between the 1950s and 1980s. In this area, the former island and split main channels of the 1950s
were replaced with a braided main channel by the 1980s. The erosion of the left bank decreased
between the 1980s and 2012, likely due to the formation of a main channel on the right with side
channels and vegetated islands on the left. The erosion at PRM 146 is the main reason the
turnover in Slough 21 GAA-151 had a net floodplain to channel turnover rate of 3,300 ft²/yr/mile
in the 1950s to 1980s as compared to the net channel to floodplain rate of 1,800 ft²/yr/mile in the
1980s to 2012.
Floodplain to channel turnover primarily occurred on the right side of the vegetated islands that
separate Side Channel 21 and Slough 21 from the main channel. Approximately, 100 feet of
erosion occurred along the left side of the main channel at PRM 145.5 and 145 between the
1950s and 1980s. Erosion continued along the left of the main channel at PRM 145 through the
period of the 1980s to 2012 for approximately 150 feet. Erosion of 200 feet along the left of the
main channel occurred at PRM 144.5 during the 1980s to 2012.
Changes in the classification of sloughs occurred in GAA-144 Slough 21 between the 1980s and
2012. The side slough at PRM 145.7 (the upstream branch of Slough 21) lost its hydraulic
connection to the main channel and was classified as an upland slough by 2012. Similarly, the
hydraulic connection between the side slough at PRM 144 and the main channel became
disconnected and was classified as an upland slough in 2012. The downstream end of this slough
is Waterfall Creek which outlets to the main channel at PRM 143.6.
5.1.4.6.2. GAA-141 Indian River (PRM 143.6 to PRM 140.1)
GAA-141 Indian River is 3.4 miles long within Geomorphic Reach MR-6. GAA-141 Indian
River is in a bedrock-bounded canyon. Additional geologic information for GAA-141 Indian
River is presented in the ISR Study 6.5 Section (5.1.3).
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GAA-141 Indian River is classified as a moderately stable area based on a total turnover rate of
18,900 ft²/yr/mile for the 1950s to 2012 period comprised of a channel to floodplain turnover
rate of (11,000 ft²/yr/mile) and floodplain to channel turnover rate (7,900 ft²/yr/mile). This is less
dynamic than the 1950s to 2012 MR-6 reach average total turnover rate (37,000 ft²/yr/mile).
In GAA-141 Indian River, channel to floodplain turnover appears to be due to approximately
equal parts of vegetation encroachment and vegetation establishment. Vegetation encroachment
is evident in Slough 19 and Slough 15 B in the 1950s to 1980s and the 1980s to 2012,
respectively. It was also present around the floodplain islands at PRM 143 and 140.6 in the
1950s to 2012 period. Three new vegetated floodplain islands were created in the period from the
1950s to 1980s and four new floodplain islands over the period of 1980s to 2012. The turnover
rates for the channel to floodplain for the 1950s to 1980s was 16,000 ft²/yr/mile and for the
1980s to 2012 it was 10,000 ft²/yr/mile. There was a net channel to floodplain turnover for both
the 1950s to 1980s (1,500 ft²/yr/mile) and 1980s to 2012 (4,600 ft²/yr/mile).
In GAA-141 Indian River, floodplain to channel turnover occurred through the loss of two
floodplain islands and a main channel shift. Floodplain islands at PRM 142.7 and PRM 142.5
were eroded away during the 1950s to 1980s period. The main channel shifted a maximum of
170 feet into the left bank at PRM 143.2.
Several channels in GAA-141 Indian River became less hydraulically connected between the
1950s and 2012. This conversion was accompanied by vegetation encroachment and manifested
as channel to floodplain turnover. Gold Creek Slough at PRM 140.2 appears to have converted
from an upland slough to floodplain between the 1980s and 2012. Also as a result of vegetation
encroachment, several side channels converted to side sloughs. Slough 15 (PRM 140.9 right) and
Slough 19 (PRM 143.2 left) became disconnected on the upstream end and were reclassified as
upland sloughs in the 1980s. In the 1950s, Slough 15 was a side slough and Slough 19 was a side
channel. Slough 15B (PRM 141.4) underwent a similar change from the 1950s to 1980s when it
changed from a side channel to a side slough. From the 1980s to 2012, Slough 16 (PRM 141.2
right) also changed from a side channel to a side slough.
Indian River itself experienced significant channel to floodplain turnover from the 1950s to
2012. In the 1950s, the lower mile was a braided channel approximately 400 feet wide. By 2012,
this portion of Indian River had become a single or split channel ranging from 40 to 100 feet
wide.
5.1.4.6.3. GAA-138 Gold Creek (PRM 140.1 to PRM 137.0)
The GAA-138 Gold Creek is 3.1 miles long within Geomorphic Reach MR-6 Gold Creek enters
within the boundary of the Geomorphic Assessment Area (Note: Gold Creek is not within the
Focus Area boundary). GAA-138 Gold Creek is classified as a dynamic area based on its total
turnover rate. The total of 1950s to 2012 channel to floodplain (27,000 ft²/yr/mile) and
floodplain to channel turnover rate (15,000 ft²/yr/mile) is 41,000 ft²/yr/mile. This is more
dynamic than the 1950s to 2012 MR-6 reach average total turnover rate (37,000 ft²/yr/mile).
In the period of the 1950s to 2012, floodplain area increased by an average of 5.9 percent of the
total area of GAA-138 (net channel to floodplain turnover of 5.4 percent for 1950s to 1980s and
6.4 percent for 1980s to 2012). There were notable differences between the vegetation behaviors
in these two periods that can be described as vegetation establishment and vegetation
encroachment. The majority of the channel to floodplain turnover during the 1980s to 2012 was
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encroachment around three groups of floodplain islands at PRM, 139.7, PRM 138.2, and PRM
137.2. Approximately 1 million square feet of vegetation established at the downstream end of
the floodplain islands at PRM 138 and PRM 137.2 between the 1950s and 1980s. The vegetation
establishment continued on the downstream end of the floodplain island at PRM 138 during the
period of the 1980s to 2012. The turnover rates for the channel to floodplain for the 1950s to
1980s was 27,000 ft²/yr/mile and for the 1980s to 2012 it was 34,000 ft²/yr/mile. There was a net
channel to floodplain turnover for both the 1950s to 1980s (3,100 ft²/yr/mile) and 1980s to 2012
(22,000 (ft²/yr/mile) though the former rate was nearly an order of magnitude smaller than the
latter rate.
Unlike the channel to floodplain turnover, the floodplain to channel turnover differs between the
1950s to 1980s and 1980s to 2012 which accounts for the large difference in the net turnover for
these periods. The turnover rates for the floodplain to channel for the 1950s to 1980s was 23,000
ft²/yr/mile and for the 1980s to 2012 it was 13,000 ft²/yr/mile. The largest areas of floodplain to
channel turnover occurred around PRM 137.7 right and PRM 137.4 left between the 1950s and
1980s.
There were several significant changes to geomorphic features in GAA-138 Gold Creek between
the 1950s and 2012 related to channel to floodplain turnover. Vegetation encroachment was
evident along Slough 10 (PRM 137.2 right), Slough 12 (PRM 138.8), Slough 13 (PRM 139 left)
and PRM 137.2 left during the 1950s to 1980s. Slough 13 was extended downstream a total of
1,100 feet by vegetation encroachment along the edge of a main channel gravel bar from the
1950s to 1980s. Vegetation encroachment filled in the former upstream connection of Slough 12
(PRM 139 left) between the 1950s and 1980s causing it to change classification from a side
slough to an upland slough. A slough was created between the 1980s and 2012 between the
floodplain islands at PRM 138 and 137.9. Vegetation encroachment is apparent in Slough 11
between the 1980s and 2012.
Floodplain to channel turnover is evident in the evolution of Slough 11 in the period between the
1950s and 1980s. This channel appears to have avulsed across the floodplain from PRM 139 to
PRM 140. In the 1950s Slough 11 was approximately 1,200 feet long and 40 feet wide but by the
1980s the slough had connected to the mainstem approximately 4,800 feet upstream and also
widened to over 100 feet in several places. As mentioned in the previous paragraph, some of the
widening was countered in the 1980s to 2012 period by encroachment of vegetation.
5.1.4.6.4. GAA-128 Slough 8A (PRM 130.4 to PRM 128.1)
GAA-128 Slough 8A (2.3 miles-long), part of MR-6, is inset between Kahiltna Flysch
metasediments. From 2012 and 2013 survey data, the gradient along this section of the Susitna
River is 8.6 feet/mile which is less than the reach-average gradient of 10.7 feet/mile. The
shallower slope within the Geomorphic Assessment Area may contribute to the slightly wider
active channel width compared to the reach average. Current (2012) active channel width of
GAA-128 (1,080 feet) is roughly 100 feet or 10 percent wider than the reach-average active
channel width (990 feet). The active channel width in GAA-128 has narrowed over time, as the
active channel width was 1,370 feet in the 1950s and 1,130 feet in the 1980s. The downstream
constrictions of Skull Creek (river left) and bedrock outcrop (river right) create backwater
conditions that under high flows are zones of preferred sediment deposition. This is evidenced by
the presence of younger geomorphic surfaces just upstream of the constriction (i.e. vegetated
bars, young floodplain, and mature floodplains) that become progressively older moving
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upstream (ISR Study 6.5 Section 5.1.3.5.1). These zones are similarly preferred locations for
ice-jam formation and are evidenced by documented historical and recent (2012) ice jams (HDR
2013a; HDR 2013b).
The channel to floodplain turnover rate in GAA-128 between the 1950s and 1980s was nearly
three times the turnover rate of floodplain to channel area (channel to floodplain= 62,000
ft²/yr/mile; floodplain to channel = 22,000 ft²/yr/mile). The resulting net turnover rate, 41,000
ft²/yr/mile channel to floodplain, is more than twice that of the other Geomorphic Assessment
Areas. It is roughly 6.5 times greater than the reach average net turnover rate (6,200 ft²/yr/mile)
and nearly three times greater than any of the average net turnover rates of the 8 geomorphic
reaches in the 1950s to 1980s or 1980s to 2012.
The net increase in channel to floodplain turnover (41,000 ft²/yr/mile) between the 1950s to
1980s resulted from vegetation establishment and encroachment on mid-channel gravel bars
(exposed at 19,000 cfs in 1950s aerial photography), mid-channel islands, and overbank
floodplain margins. About one-third of this turnover (1.5 million sq. ft.) occurred in the upstream
end of the Geomorphic Assessment Area through establishment of shrubs and a young poplar
stand on nearly 0.5 miles of a main channel gravel bar. Similarly, roughly 500,000 sq. ft. of
vegetation encroachment (likely in combination with sedimentation) caused the inlet of Half-
Moon Slough to become a side slough by the 1980s; by 2012, Half-Moon Slough had become
hydraulically disconnected and an upland slough. In Slough 8A vegetation encroachment has
occurred on most of the exposed sediment visible on the 1950s aerial photographs.
The channel to floodplain net turnover rates between the 1950s to 1980s reduced by nearly 85
percent during the 1980s to 2012 period, from 41,000 ft²/yr/mile to 6,400 ft²/yr/mile. Channel to
floodplain turnover in GAA-128 Slough 8A continued in the similar general locations in the
1980s to 2012 period but was significantly less around the island upstream of the Slough 8A inlet
in the more recent period. Locations of channel to floodplain turnover between the 1950s to the
1980s occur as vegetation establishment on previously barren mid-channel bars and
encroachment of vegetation around the side-channel margins as identified by the 2012 aerial
photography. Vegetation encroachment also continued in the west fork of Slough 8A. In the
1980s to 2012 period, a mid-channel island (PRM 128.4) became established upstream of a
1980s vegetated island. At Half-Moon Slough, a side slough in the 1980s, sand deposition and
vegetation colonization at the inlet and fine sediment deposition and beaver activity at the mouth
has reduced connectivity of the feature with lateral channels thereby causing a feature change to
an upland slough by 2012.
The floodplain to channel turnover rate of 20,000 ft²/yr/mile has remained approximately the
same over both periods in GAA-128, floodplain destruction and channel formation occurred
throughout the site between the 1980s to 2012 study period; however, the largest areas of erosion
occurred along the Skull Creek Side Channel and the mid-channel island upstream of Slough 8A
(up to 175 feet in width for nearly 2,000 feet in both places).
5.1.4.7. MR-7 (PRM 122.7 to PRM 107.8)
Within geomorphic reach 7 (MR-7), the Susitna River flows through a bedrock-bounded canyon
averaging 2,050 feet wide with an active channel width of 850 feet. Because of the relatively
wide valley and low slope (8.3 ft/mile), there is high sediment storage potential within the reach.
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Alluvial sediments are stored primarily within continuous vegetated floodplain segments and in
vegetated islands and mid-channel bars.
MR-7 is classified as a moderately stable reach with a total of the 1950s to 2012 channel to
floodplain (11,000 ft²/yr/mile) and floodplain to channel turnover rates (5,000 ft²/yr/mile) of
16,000 ft²/yr/mile. No other Middle River reach has a total turnover rate between 10,000
ft²/yr/mile and 30,000 ft²/yr/mile. The individual total turnover rates for MR-1 through MR-5 are
lower than 10,000 ft²/yr/mile and MR-6 and MR-8 are higher than 30,000 ft²/yr/mile. Compared
to the Geomorphic Assessment Areas in MR-7, the reach average for MR-7 (16,000 ft²/yr/mile)
was greater than GAA-115 Slough 6A (14,000 ft²/yr/mile) and less than Oxbow 1 GAA-113
(20,000 ft²/yr/mile). Ice shearing of vegetation (PRM 108.1) appears in the 1980s aerial imagery
and contribute to floodplain to channel turnover in MR-7.
In MR-7, a few primary locations are responsible for the vast majority of the channel to
floodplain turnover. At PRM 121, the main floodplain island experienced significant vegetation
encroachment through both the 1950s to 1980s and 1980s to 2012 periods. The largest area
(160,000 sq. ft.) of floodplain establishment in MR-7 between the 1950s and 1980s occurred just
upstream at PRM 121.3. By comparison the largest area of vegetation establishment in MR-7
was 580,000 sq. ft. at PRM 120.5. Downstream at PRM 117.3 the largest area (880,000 sq. ft.) of
floodplain encroachment in MR-7 occurred at the upstream extent of GAA-115 Slough 6A. The
two mile portion extending upstream experienced both vegetation encroachment and
establishment through the 1980s to 2012. The third main location of channel to floodplain
turnover in MR-7 was between PRM 115.3 and PRM 114.2, within Oxbow 1 GAA-113, and is
covered in more detail under the discussion of GAA-113.
The floodplain to channel turnover in MR-7 occurred in the same few main locations as those
listed for channel to floodplain turnover in the previous paragraph. At PRM 121 left, the main
channel experienced about 40 and 100 feet of erosion in the periods of the 1950s to 1980s and
1980s to 2012, respectively. Approximately 100 feet of width was lost from the island at PRM
117.8 in both the 1950s to 1980s and 1980s to 2012. The third main location of floodplain to
channel turnover in MR-7 was between PRM 115.3 and PRM 114.2, within Oxbow 1 GAA-113,
and is covered in more detail in the discussion of GAA-113. The turnover rates for the floodplain
to channel for the 1950s to 1980s period was 9,000 ft²/yr/mile and for the 1980s to 2012 period it
was 5,000 ft²/yr/mile.
Within MR-7 the channel to floodplain turnover and floodplain to channel turnover exhibited
opposite trends in the 1950s to 1980s and 1980s to 2012. Channel to floodplain turnover,
increased by more than twice the amount (percent of the total reach area) in the 1980s to 2012
compared to the 1950s to 1980s (1950s to 1980s turnover = 2.1 percent, 1980s to 2012 turnover
= 4.7 percent). Floodplain to channel turnover in the same periods had the opposite trend. It
decreased by 50 percent in 1980s to 2012 compared to the 1950s to 1980s. In effect, the net
channel to floodplain turnover for MR-7 is -0.3 percent in the 1950s to 1980s and +3.5 percent in
the 1980s to 2012. These percentages correspond to a net channel to floodplain rate of -1,200
ft²/yr/mile for in the 1950s to 1980s and 14,000 ft²/yr/mile in the 1980s to 2012. MR-7 and MR-4
were the only MR reaches to have a net floodplain loss in the 1950s to 1980s. Channel widths
also had alternating trends, with average width increasing slightly from 920 ft in the 1950s to
930 feet in the 1980s, and a decrease in average width to 850 feet in 2012.
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A few key geomorphic changes occurred in MR-7 between the 1950s and 2012. Most notable
were the conversion of side channels to side sloughs at PRM 121.5, PRM 119, PRM 117.2
during the 1950s to 1980s. Several upland sloughs between PRM 113 and PRM 109 and at PRM
117.3 left changed in extent due to biogeomorphic processes between the 1950s and 2012.
5.1.4.7.1. GAA-115 Slough 6A (PRM 117.3 to PRM 115.3)
GAA-115 Slough 6A (2.0 miles long) is laterally confined by riprap and a Pleistocene-age
moraine on river left and Holocene-age terrace inset from bedrock on river right. GAA-113 is
immediately downstream of GAA-115. GAA-115 is upstream of a constriction caused by riprap
and bedrock outcrop and just downstream of an alluvial fan at the mouth of Unnamed Tributary
PRM 117.4. The 8.6 ft/mile gradient and 910 feet active channel width are representative of MR-
7 reach average parameters (8.3 ft/mile and 850 feet active channel width).
Based on the total turnover rates of 20,000 ft²/yr/mile in both time periods of the 1950s to 1980s
and the 1980s to 2012, GAA-115 falls at the cutoff between classification as a moderately stable
and a moderately dynamic reach. The total turnover rate for the 1950s to 1980s is comprised of
9,800 ft²/yr/mile of channel to floodplain turnover and 10,000 ft²/yr/mile of floodplain to channel
turnover. The total turnover rate for the 1980s to 2012 is comprised of 16,000 ft²/yr/mile of
channel to floodplain turnover and 4,900 ft²/yr/mile of floodplain to channel turnover. The total
turnover rate from the 1950s to 2012 is 14,000 ft²/yr/mile based on a channel to floodplain
turnover rate of 9,800 and floodplain to channel turnover rate of 4,500 ft²/yr/mile over this
period. Active channel width has slightly decreased over time, from 960 feet in the 1950s and
1980s, to 910 feet by 2012. Historic ice jams were documented throughout the main channel
including a 2-mile long ice jam (i.e., Lane Creek Jam) at PRM 116 (HDR 2013a; HDR 2013b).
Over the two study periods of 1950s to 1980s and 1980s to 2012, the magnitude of the net
floodplain to channel turnover in GAA-115 increased. The floodplain to channel turnover from
the 1950s to 1980s was 300 ft²/yr/mile and 11,000 ft²/yr/mile from the 1980s to 2012. While
there is a net floodplain to channel turnover during the 1950s to 1980s study period in GAA-115,
it is minimal at 300 ft²/yr/mile. Floodplain to channel turnover, from the 1950s to the 1980s,
occurred along the right bank of the main channel, upstream of the constriction at PRM 116.5,
along the Terrace and Old Floodplain surface for nearly three-quarters of a mile. In the 1980s to
2012 floodplain to channel turnover, 4,900 ft²/yr/mile, was narrow (about 25 feet on the average
for the period) and limited to a few areas along the river right at PRM 116.5, PRM 115.8 right,
and along the left side of the dominant main channel island.
During the period of the 1950s to 1980s, channel to floodplain turnover occurred as vegetation
encroachment occurred on the mid-channel islands near PRM 116. This expanded the extent of
the vegetation around the small island approximately 100,000 sq. ft. and 50,000 sq. ft. along the
large island. On the upstream end of the sight, near PRM 117.3, a large gravel bar in the 1950s
was encroached upon by vegetation by the 1980s.
In the 1980s to 2012 study period GAA-115 had a channel to floodplain turnover rate of 16,000
ft²/yr/mile. Channel to floodplain turnover from the 1980s to 2012 include the channel margin of
the island at PRM 115.4. The mid-channel island at PRM 117.3 on the gravel bar also exhibited
vegetation encroachment. Lastly, over 150,000 sq. ft. of Slough 6A underwent channel to
floodplain turnover on the river right floodplain through biogeomorphic processes of beaver dam
construction verified during the 2013 field season.
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5.1.4.7.2. Oxbow 1 GAA-113 (PRM 115.3 to PRM 113.6)
GAA-113 Oxbow 1 (1.7 miles long) is laterally confined by riprap and a Pleistocene-age
moraine on river left and a Holocene-age outwash terrace inset below bedrock on river right.
Longitudinally, GAA-113 is upstream of a constriction caused by a moraine and outwash terrace
and downstream of GAA-115. GAA-113 contains three tributaries including Gash Creek (PRM
115.0), Slash Creek (114.9), and Unnamed Tributary PRM 113.7. The 7.7 ft/mile gradient of
GAA-113 and 850 feet active channel width are representative of MR-7 reach average
parameters (8.3 ft/mile and 850 feet active channel width). A paleo-channel network set on
Holocene terraces is present along the right bank of the site. Younger geomorphic surfaces (i.e.
vegetated bars, young floodplains and mature floodplains are present within the active channel
upstream of the constriction at PRM 114 (ISR Study 6.5, Appendix A.2).
Ice activity has been noted for both historical and current time periods in GAA-113 Oxbow 1.
Recent ice jams (2012) were observed within the main channel at the following locations:
upstream of Unnamed Tributary PRM 113.7, at the inlet of a side channel at PRM 114.3,
upstream of a side channel inlet at PRM 114.5, upstream of a side channel inlet at 114.8 and
along the mid-channel island at PRM 115 (HDR 2012; HDR 2013). Historical ice jams were all
observed within the main channel and were distributed throughout the site. Vegetation margins
on mid-channel islands appear to be sheared in the 1983 and 2012 aerial photography, likely due
to ice.
GAA-113 Oxbow 1 can be described as moderately dynamic based on the total turnover rate
between the 1950s and 2012 of 20,000 ft²/yr/mile. This is comprised of a channel to floodplain
turnover rate of 13,000 ft²/yr/mile and the floodplain to channel rate of 7,000 ft²/yr/mile. Active
channel width has fluctuated over time, where width increased between the 1950s to 1980s, from
920 feet to 950 feet and decreased by roughly 10 percent to 850 feet by 2012 (due to vegetation
encroachment on mid-channel islands).
GAA-113 Oxbow 1 had the greatest net floodplain to channel turnover rate (5,700 ft²/yr/mile)
compared to the other nine Geomorphic Assessment Areas in both periods of the 1950s to 1980s
and 1980s to 2012. The primary erosion of floodplain occurred on approximately 2,000 feet
length off the left bank with a width of over 200 feet. On the right, the bank was eroded roughly
75 feet for approximately 1,000 feet in length. Between the 1980s and 2012, the net turnover rate
was 18,000 ft²/yr/mile of channel to floodplain turnover.
Channel to floodplain turnover primarily occurred as vegetation encroachment in the side
channel at PRM 114 right and around the mid–channel islands, and vegetation establishment
associated with the creation of an island at PRM 114.4. The channel to floodplain turnover in
GAA-113 Oxbow 1 between the 1950s and 1980s (7,000 ft²/yr/mile) is comparable with the
reach average rate (7,800 ft²/yr/mile). Between the 1980s and 2012, there was an increase in
channel to floodplain turnover to 18,000 ft²/yr/mile which is 30 percent greater than the reach
average channel to floodplain turnover rate of 14,000 ft²/yr/mile.
Two locations of geomorphic feature change in GAA-115 occurred as side channels became side
sloughs. Side channels at PRM 113.7 and PRM 115.0 in 1983 changed classifications to side
sloughs by 2012.
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5.1.4.8. MR-8 (PRM 107.8 to PRM 102.4)
The Susitna River in geomorphic reach 8 (MR-8) averages 1,130 feet in active width with an
increase in width as it approaches the Three Rivers Confluence at PRM 102.4. The average
gradient of the river increases about 0.5 ft/mile to 8.8 ft/mile compared with MR-7. Sediment
storage in MR-8 is predominantly within limited areas of active, relatively low elevation
floodplain and vegetated islands that are inset below fairly extensive areas of relatively inactive,
higher elevation terrace and vegetated islands. Just upstream of the confluence, the bulk of the
alluvial sediments are stored in active unvegetated braid bars. The large entrenchment ratio of
7.9 is the result of the wide floodplain created by the transition into the confluence of the
Susitna, Chulitna, and Talkeetna rivers. This reach is a transition between the confined single-
channel dominated morphology of the Middle River to the multiple channel and braided
morphology of the Lower River.
MR-8 is rated as a dynamic reach with a total turnover rate from the 1950s to 2012 of 38,000
ft²/yr/mile comprised of a channel to floodplain turnover rate of 21,000 ft²/yr/mile and floodplain
to channel turnover rate of 17,000 ft²/yr/mile. Geomorphic reach MR-8 has the greatest reach
average total turnover rate in the Middle River. Both MR-6 (37,000 ft²/yr/mile) and MR-8 had
more than twice the total turnover rate as the third most dynamic reach MR-7 (16,000
ft²/yr/mile). Compared to the Geomorphic Assessment Areas in MR-8, the reach average total
turnover rate for MR-8 (38,000 ft²/yr/mile) was greater than GAA-104 Whiskers Slough (23,000
ft²/yr/mile). In the water years 2012 and 2013, there were ice jams in MR-8 (HDR 2013a; HDR
2013b). Ice features such as ice channels (PRM 139.8 and PRM 135.4) and vegetation shearing
appear in the aerial imagery and contribute to the dynamic nature of GAA-104 Whiskers Slough
and MR-8.
From the 1950s to 2012, floodplain area increased by 3.1 percent of the total reach area (1950s to
1980s channel to floodplain turnover = 1.8 percent, 1980s to 2012 channel to floodplain turnover
= 1.6 percent). Although, these percentages are similar, the floodplain to channel turnover differs
spatially in the periods of the 1950s to 1980s and 1980s to 2012. Approximately 750,000 sq. feet
of vegetation establishment occurred at both PRM 105.0 and PRM 102.7 in the 1950s to 1980s.
During 1980s to 2012, the largest area of vegetation establishment was approximately 250,000
sq. ft. (PRM 104.0) and the majority of the channel to floodplain turnover resulted from
vegetation encroachment.
Unlike the channel to floodplain turnover, the area of floodplain to channel turnover differs
appreciably between the 1950s to 1980s and 1980s to 2012. Floodplain to channel turnover was
50 percent greater during the 1950s to 1980s (turnover = 1.7 percent), than the 1980s to 2012
(turnover = 1.1 percent). The turnover rates for the floodplain to channel for the 1950s to 1980s
period was 23,000 ft²/yr/mile and for the 1980s to 2012 period it was 16,000 ft²/yr/mile. The
1950s to 1980s floodplain to channel turnover rate is higher than the 1980s to 2012 rate due to
relatively more turnover at PRMs 105.4 left, PRM 104.4 right, PRM 102.9 right, and PRM 102.5
left. Approximately 1,500,000 sq. ft. of floodplain to channel turnover occurred at PRM 104.4 in
the period from the 1950s to 2012 as the main channel shifted to the right. About 70 percent of
this turnover occurred between the 1950s to 1980s period.
Floodplain to channel turnover occurred between the Middle and Lower Susitna River segments
at the confluence of the Chulitna and Susitna Rivers where the confluence shifted 1,400 feet to
the northeast between the 1950s and 2012. This was due to the leftward migration of the
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Chulitna River into MR-8. During the period of the 1950s to 2012, this caused a 3.3 million sq.
ft. reduction in the floodplain area of MR-8. Channel widths remained relatively constant with
only a slight decrease in average widths of 1,290 feet in the 1950s, 1,210 feet in the 1980s, and
1,130 feet in 2012.
Several geomorphic feature classification changes occurred in MR-8 between the 1950s and
2012. Most of these were within GAA-104 Whiskers Slough and are described in the following
subsection. Downstream of GAA-104 Whiskers Slough, a side slough at PRM 103.3 left in the
1950s and 1980s lost its upstream hydraulic connection and was classified as an upland slough in
2012.
5.1.4.8.1. GAA-104 Whiskers Slough (PRM 107.4 to PRM 104.2)
The GAA-104 Whiskers Slough is 3.2 miles long and spans the majority of Geomorphic Reach
MR-8. GAA-104 Whiskers Slough is rated as a moderately dynamic portion of MR-8 with the
total of 1950s to 2012 channel to floodplain (16,000 ft²/yr/mile) and floodplain to channel
turnover (7,200 ft²/yr/mile) rates of 23,000 ft²/yr/mile. This is not as dynamic as the reach
average total turnover rate from 1950s to 2012 for MR-8 of 37,000 ft²/yr/mile. The total
turnover rate for MR-8 is higher than FA-104 Whiskers Slough Geomorphic Assessment Area
because of the very high turnover rates at the downstream end of MR-8 near the Three Rivers
Confluence. In water years 2012 and 2013, there were ice jams in GAA-104 Whiskers Slough
(HDR 2013a; HDR 2013b). Ice processes that result in the formation of chute channels (PRM
105.4 left 2012) and vegetation shearing (1980s and 2012) may be responsible for some of the
features observed in the aerial imagery and contribute to the dynamic nature of GAA-104
Whiskers Slough.
Changes to specific geomorphic features in GAA-104 Whiskers Slough were noted in the aerial
imagery. Between the 1950s and 1980s, vegetation establishment on the gravel bar at PRM 105.0
left split the main channel and created the Whiskers Slough East Side Channel. Slough 3A
decreased its connection to the Whiskers West Side Channel due to conversion to an upland
slough from a side slough during the 1950s to 1980s period. There was vegetation encroachment
evident along Whiskers Slough PRM 105.1 left, Slough 3B PRM 105.5, and Slough 3A. Slough
2 was breached by the turbid water and was classified as a side channel in 2012 (at 12,900 cfs at
Gold Creek) when it had been a side slough in 1980s (at 12,500 cfs at Gold Creek).
5.1.5. Lower Susitna River Segment Long-term Channel Change Assessment
This section presents the channel change developed for the Lower Susitna River Segment. This
includes channel change for major Susitna River tributaries, the Chulitna, Talkeetna, and Yentna
rivers. Tables 5.1-19 through 5.1-27 present the geomorphic feature areas mapped by
geomorphic reach for 1983 and 2012 in the Lower Susitna River and Chulitna, Talkeetna, and
Yentna tributaries. The tables also include the percent change from 1983 to 2012.
In the Lower Susitna River Segment, from the Three Rivers Confluence (PRM 102.4) to Cook
Inlet (PRM 3.3), the bed slope progressively decreases from 6 ft/mile to about 1.5 ft/mile. The
channel is bounded primarily by Pleistocene-age glacial, fluvio-glacial, and glacio-lacustrine
deposits. The Lower River is distinctly different from the Middle River. The character of the
river changes dramatically below the Three Rivers Confluence as the width of the river more
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than triples from the widest portions in the Middle Susitna River Segment, and it adopts a
braided channel form.
The Lower Susitna River Segment is divided into six distinct geomorphic reaches based on bed
slope, bounding geology, average active channel width, channel branching, sediment storage
capacity, and sinuosity. The six Lower River geomorphic reaches are presented in Figure 5.1-3
and Table 5.1-31. Further geomorphic characterization of the six reaches is provided in
Geomorphic Reach Delineation and Characterization, Upper, Middle and Lower Susitna River
Segments (Tetra Tech 2014b).
5.1.5.1. LR-1 (PRM 102.4 to PRM 87.9)
This reach of the Susitna River (3,340 feet active width) includes the Three Rivers Confluence
downstream of PRM 102.4 and extends downstream to a valley bottom constriction at PRM
87.9. The Susitna River triples its width in LR-1 compared with MR-8. This is the result of the
added flow and sediment loads from the Chulitna and Talkeetna rivers. However, the width of
the valley floor at approximately 9,200 feet wide is nearly identical to 8,960 feet width in the
MR-8 immediately upstream. LR-1 is confined on the east side primarily by Upper Pleistocene-
age moraines and glacial outwash surfaces and on the west side by Upper Pleistocene-age
lacustrine deposits intercalated with glacial outwash surfaces. The average channel gradient for
the reach is 6.2 ft/mile. In general, because of the combined sediment delivery from the Three
Rivers Confluence, the reach is net aggradational and the bulk of the alluvial sediment is stored
in active, unvegetated braid bars upstream of the valley floor constriction at PRM 87.9. Within
the reach, there are also locations where alluvial sediments are stored within vegetated islands
and mid-channel bars, and the reach is bounded on each side by a vegetated floodplain of
varying width. The sinuosity of the reach is 1.12 and the average number of channels is 4.0.
Based on 13 samples collected in 2013 as part of ISR Study 6.6, the average median bed material
size is 53 mm.
LR-1 is rated as a moderately stable Lower River reach based on its total turnover rate. The total
of 1950s to 2012 channel to floodplain (84,000 ft²/yr/mile) and floodplain to channel turnover
rate (58,000 ft²/yr/mile) is 142,000 ft²/yr/mile. In comparison to adjacent reaches, this is
approximately 4 times the total turnover rate for MR-8 (38,000 ft²/yr/mile), but only 30 percent
of the total turnover rate of Chulitna River above the confluence CL-1 (487,000 ft²/yr/mile). It is
similar to the next downstream Lower River reach at 80 percent of the total turnover rate of LR-2
(180,000 ft²/yr/mile). The total turnover rate in LR-1 of 142,000 ft²/yr/mile is 25 percent greater
than the total turnover rate of 107,000 ft²/yr/mile for the Talkeetna River (TK-1) at the
confluence.
From the 1950s to 2012, LR-1 channel to floodplain turnover occurred over 13 percent of the
total reach area. The channel to floodplain turnover area percentages were nearly double in the
1980s to 2012 (9.9 percent of the reach area) compared to the 1950s to 1980s (5.4 percent of the
reach area). The primary location of channel to floodplain turnover from the 1950s to 2012 in
LR-1 occurred from PRM 100 right to PRM 95 right over the period (36 million sq. ft.). The
largest area of floodplain establishment (3 million sq. ft.) occurred during the 1980s to 2012
period at PRM 101. This is the vegetated island in the dominant mid-channel bar of the Three
Rivers Confluence.
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The majority of the floodplain to channel turnover in LR-1 occurred in three locations (PRM
101, PRM 95 and PRM 89.5) and was due to main channel shifting. Over the period of the 1950s
to 2012, the main channel progressively shifted to the left approximately two miles at PRM 95
and one mile left at PRM 89.5. At PRM 101, there was approximately 2,000 feet of lateral
migration into the right overbank floodplain between the 1950s and 2012. This has occurred
concurrently with the shift of the Three Rivers Confluence intersection point to the northeast by
1,400 feet, the vegetation establishment on the dominant mid-channel bar, and the progressive
leftward (northward) shift of the Chulitna River (CL-1). The floodplain to channel turnover was
approximately equal in both the 1950s to 1980s (5.8 percent of the reach area) and 1980s to 2012
(5.7 percent of the area).
The net turnover rate for LR-1 was from 4,800 ft²/yr/mile floodplain to channel in the 1950s to
the 1980s and switched in the 1980s to 2012 period to 59,000 ft²/yr/mile of channel to
floodplain. Over the entire 1950s to 2012 period, the net turnover rate was 26,000 ft²/yr/mile
channel to floodplain. The 1950s to 2012 net turnover rate corresponds to the second highest net
channel to floodplain rate of turnover in the LR and equal to that of LR-4. It is more than 5 times
the net rate of MR-8 (4,600 ft²/yr/mile). The channel width history over the period shows a small
decrease in channel area as the reach average active channel widths were 3,500 feet, 3,490 feet
and 3,340 feet in the 1950s, 1980s and 2012, respectively.
5.1.5.2. LR-2 (PRM 87.9 to PRM 65.6)
This reach of the Lower Susitna River Segment is dominated by multiple channels with an
average of 5.6 channels totaling an average channel width of 3,120 feet. The average slope of the
channel is 4.9 ft/mile, the sinuosity is 1.16, with an entrenchment ratio of 2.5. This reach of the
Susitna River can be further subdivided into upper and lower subreaches. The upper reach
extends from the valley floor constriction formed by Upper Pleistocene-age glacial outwash on
the east bank and Upper Pleistocene-age moraines on the west bank at PRM 87.9, down to about
PRM 74.4. Within this subreach, the valley floor is confined on the east by Upper Pleistocene-
age glacial outwash and on the west side by similar aged moraines. The valley floor width varies
from about 4,000 to 5,200 feet and the alluvial sediments are primarily stored in vegetated
islands, bars and continuous floodplain segments (channel classified as MC2). Between PRM
74.4 and PRM 65.6, where there is a valley floor constriction most probably created by the
Kashwitna River fan on the east bank and Upper Pleistocene-age moraines on the west bank, the
planform of the river changes to anastomosed as a result of the imposed baselevel control. The
bulk of the alluvial sediments within the lower subreach are stored in longitudinally extensive,
relatively stable, vegetated floodplain segments (channel classified as MC3). The valley floor
width is on the order of 8,000 feet upstream of the constriction. The east side of the valley is
composed of Upper Pleistocene-age glacio-lacustrine deposits and the west side by similar-aged
moraines. The greater width of the valley in the lower subreach suggests that the Upper
Pleistocene-age glacial outwash that forms the east bank in the upper subreach is more erosion-
resistant than the glacio-lacustrine deposits in the lower subreach.
LR-2 is rated as a moderately dynamic reach in terms of its 1950s to 1980s and 1980s to 2012
turnover rates. The two 30-year periods had total turnover rates greater than 200,000 ft²/yr/mile
and the 1950s to 2012 period was similar at180,000 ft²/yr/mile. The total turnover rate for the
1950s to 1980s is 237,000 ft²/yr/mile and is comprised of 136,000 ft²/yr/mile of channel to
floodplain turnover and 100,000 ft²/yr/mile of floodplain to channel turnover. The total turnover
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rate for the 1980s to 2012 is 212,000 ft²/yr/mile and is comprised of 155,000 ft²/yr/mile of
channel to floodplain turnover and 57,000 ft²/yr/mile of floodplain to channel turnover. LR-2
experienced greater turnover than reach LR-1 which had a total turnover rate from 1950s to 2012
of 142,000 ft²/yr/mile and less dynamic than LR-3 with a total turnover rate from 1950s to 2012
of 237,000 ft²/yr/mile.
From the 1950s to 2012, LR-2 channel to floodplain turnover represented 20 percent of the reach
area. The channel to floodplain turnover area percentages were nearly equal in both the 1950s to
1980s (11.6 percent of the reach area) and 1980s to 2012 (11.9 percent of the reach area). The
primary location of channel to floodplain turnover in LR-2 occurred from PRM 79 left to PRM
67 left. Over the period of the 1950s to 2012, the channel to floodplain turnover had consolidated
two main side channel complexes from PRM 74 to PRM 69 and PRM 71 to PRM 67. This began
in the 1950s to 1980s as floodplain establishment between PRM 73 and 71 and floodplain
encroachment at PRM 75 left, PRM 70 left and PRM 68 left. Then in the 1980s to 2012, the
vegetated islands within side channel complexes coalesced with the vegetation encroachment at
PRM 71.8 left, PRM 71 left, and PRM 69.2 left.
Floodplain to channel turnover in LR-2 was the greatest around PRM 71. The high turnover in
this area is related to a westward shift of the main channel. In the 1950s the main channel was
along the left (east side) between PRM 73 and PRM 71. By 1980s, the main channel had shifted
to the right (west) between PRM 73 and PRM 71, while two side channels became more
prominent at PRM 71 left. The inlets to the side channels occur at PRM 74 left and PRM 73
right. The channel to floodplain turnover from the 1950s to 1980s was 8.5 percent of the total
reach area. Over the period from the 1980s to 2012, the floodplain to channel percent reduced by
nearly one half to 4.4 percent of the total reach area. Another large portion of the floodplain to
channel turnover occurred at PRM 78.5. The main channel eroded a maximum width of 1,500
feet between the 1950s and 1980s.
Accompanying the erosion discussed in the previous paragraph, there was also floodplain
creation on the opposite side of the river. This erosion was followed by vegetation encroachment
on river left at PRM 70 between the 1980s and 2012. The overall effect from the 1950s to 2012
was a region of new floodplain area PRM 79 left to PRM 67 left. This was the primary area of
channel to floodplain turnover in LR-2. Floodplain to channel turnover was significantly less
than channel to floodplain turnover in LR-2. For the period of 1950s to 2012, LR-2 exhibited the
greatest net gain in floodplain area (10 percent of the reach area) compared to the other LR
reaches. This corresponds to a net channel to floodplain turnover rate of (66,000 ft²/yr/mile).
The corresponding active channel widths reflect this net reduction in channel area with widths of
3,890 in the 1950s, 3,670 in the 1980s, and 3,120 in 2012.
5.1.5.3. LR-3 (PRM 65.6 to PRM 44.6)
This reach has the highest average number of channels of all the Lower River reaches at an
average value of 8.8. Consistent with the multiple channels, the river planform is anastomosed
for most of the reach (classified as MC3). The valley floor constriction at PRM 44.6 forms a
downstream baselevel control for the river in this reach and is likely partially responsible for the
planform (Smith and Smith 1980; Knighton and Nanson 1993; Makaske 2001). The Susitna
River (4,040 feet active channel width) within this reach is bounded by Upper Pleistocene-age
lacustrine deposits on the east and west sides of the valley, the apparent reason for a wider valley
floor (16,000 feet). The bulk of the alluvial sediments within the reach are stored in
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longitudinally extensive, relatively stable, vegetated floodplain segments that are referred to as
the Delta Islands. The average slope of the channel in the reach is 4.7 ft/mile and the sinuosity of
the primary anastomosed channel averages 1.23.
Based on the total turnover rates of 300,000 ft²/yr/mile in the 1950s to 1980s and 300,000
ft²/yr/mile in the 1980s to 2012 turnover rates, MR-3 falls at the cutoff between classification as
a moderately dynamic and a dynamic reach. The total turnover rate for the 1950s to 1980s is
comprised of 140,000 ft²/yr/mile of channel to floodplain turnover and 160,000 ft²/yr/mile of
floodplain to channel turnover. The total turnover rate for the 1980s to 2012 is comprised of
180,000 ft²/yr/mile of channel to floodplain turnover and 120,000 ft²/yr/mile of floodplain to
channel turnover. The total turnover rate from the 1950s to 2012 is 240,000 ft²/yr/mile based on
a channel to floodplain turnover rate of 130,000 and floodplain to channel turnover rate of
110,000 ft²/yr/mile over this period. In terms of turnover, it was more dynamic than the other
five Lower River reaches but the Chulitna River total turnover rate for the same period was over
twice as high at 490,000 ft²/yr/mile.
From the 1950s to 2012, LR-3 channel to floodplain turnover represented 9.5 percent of the total
reach area. The increases in floodplain area were nearly equal in both the 1950s to 1980s (5.4
percent of the reach area) and 1980s to 2012 (6.3 percent of the reach area). The primary areas of
channel to floodplain turnover in LR -3 occurred around PRM 55, PRM 51, and PRM 45. Areas
of vegetation establishment at each of these locations exceeded 1 million sq. ft. in the period of
the 1950s to 1980s. Creation of floodplain continued at a high rate during the 1980s to 2012
period at all three locations with the area converted to floodplain ranging from 4 million sq. ft. to
9 million sq. ft.
Floodplain to channel turnover was also appreciable and covered 8.1 percent of the total area of
LR-3 from the 1950s to 2012. The majority of the floodplain to channel turnover appears to have
happened in stages in three portions of LR-3; right along PRM 62 to PRM 56, left of the main
channel from PRM 55 to PRM 53, and right from PRM 53 to PRM 47. During the 1950s to
1980s, the majority of the bank erosion was along the river right from PRM 62 to PRM 56.
Erosion resulted in a maximum width of 1,800 feet at PRM 59.3. Also during the period of the
1950s to 1980s, the inlets to the dominant side channel (PRM 56 to PRM 47) east of the Delta
Islands widened through floodplain to channel turnover between PRM 57 to PRM 55.
During the 1980s to 2012, floodplain to channel turnover switched over to the left side of the
main channel between PRM 55 to PRM 53. This area of turnover was created as the main
channel shifted leftward from the 1950s to a more central location in the braid plain. An avulsion
of a side channel, through the overbank floodplain at PRM 54 right happened at this location
between the 1980s and 2012. From PRM 53 to PRM 47 along the river right, floodplain to
channel turnover continued through both periods of the 1950s to 1980s and 1980s to 2012. This
includes erosion of more than 1,000 feet into the right overbank floodplain at PRM 51 and PRM
47.2. The main channel presently occupies these locations.
Over the course of the period from the 1950s to 2012, LR-3 has undergone a 1.4 percent net
conversion from channel to floodplain. This corresponds to a net rate of 19,000 ft²/yr/mile. The
average of the actual rates of channel to floodplain (128,000 ft²/yr/mile) and floodplain to
channel turnover (109,000 ft²/yr/mile) are an order of magnitude greater. Review of the average
channel widths of these periods show a reduction commensurate with the increase in floodplain
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with an average total channel width in 1950s of 4,290 feet, 4,400 feet in the 1980s and 4,040 feet
in 2012.
5.1.5.4. LR-4 (PRM 44.6 to PRM 32.3)
The Susitna River has a 2,750 feet active channel width in this reach as it flows between Upper
Pleistocene-age lacustrine deposits on both the east and west sides of the valley. Valley floor
width is about 12,300 feet and a valley floor constriction is created by the Yentna River alluvial
fan and the moraine-draped, Late Cretaceous-age, granodiorite outcrop on the west side and
volcanic tuff on the east side of the river at PRM 29.9 (Susitna Station downstream reach
boundary). The bulk of the alluvial sediments in the reach are stored in vegetated islands and
mid-channel bars and in continuous, vegetated floodplains on both sides of the river. The average
slope of the channel in the reach is very flat at 1.0 ft/mile, the sinuosity is 1.24, and on average
there are 5.1 channels in the reach. Based on 15 samples collected in 2013 as part of ISR Study
6.6 Section 6.1.9.1, the average median bed material size is 33 mm.
Based on the turnover results, LR-4 is rated as a moderately stable reach of the Lower River. The
total turnover rate from the 1950s to 2012 is 124,000 ft²/yr/mile of which channel to floodplain
turnover contributes 75,000 ft²/yr/mile and floodplain to channel turnover contributes 49,000
ft²/yr/mile. In terms of comparison its adjacent Lower River reaches it had a lower turnover rate
than LR-3 just upstream (total turnover from 1950s to 2012 = 240,000 ft²/yr/mile) and a higher
rate of turnover than LR-5 just downstream (total turnover from 1950s to 2012 = 95,000
ft²/yr/mile).
Floodplain to channel turnover in LR-4 has occurred including significant erosion of the
overbank floodplain. The majority of the erosion occurred during the 1980s to 2012 period and at
PRM 40 there was 800 feet of erosion of the right overbank floodplain. This resulted in the main
channel avulsing to the left side of the braid plain. From the 1950s to 2012, erosion of the main
channel into the overbank floodplain occurred at PRM 38.5 to the right and at PRM 37 to the
left. Main channel length and sinuosity of LR-4 increased by 4 percent from the 1950s to 1980s
and 5 percent from the 1980s to 2012 (1950s length = 59,700 ft., 1980s length = 61,700 ft., 2012
length = 64,400 ft.). From the 1950s to 1980s, the floodplain to channel turnover was 4.1 percent
of the total reach area and primarily occurred between PRM 41.5 and PRM 34 in large sections
ranging from 600 feet to 1000 feet wide and up to one-half mile long. This trend continued
during the 1980s to 2012 period erosion consuming 3.1 percent of the total reach area.
Channel to floodplain turnover followed a similar trend as the floodplain to channel turnover in
LR-4. Vegetation encroachment occurred on the inside bend of the main channel adjacent to
where it had eroded the outside banks at PRM 40 right, PRM 38.5 right, and PRM 37 left over
the period of the 1950s to 2012. Additional vegetation encroachment occurred on the inside of
the bend at PRM 43.5 during the 1950s to 1980s. From the 1950s to 1980s channel to floodplain
turnover was 6 percent of the total reach area and primarily occurred mid-channel as vegetation
encroachment of islands in bar island complexes and side channel complexes. From the 1980s to
2012, floodplain to channel turnover occurred over 4.2 percent of the reach and primarily
consisted of vegetation encroachment around the vegetated islands. From the 1950s to 2012,
there was a net channel to floodplain turnover in LR-4 of 26,000 ft²/year/mile. The
corresponding active channel widths reflect this net reduction in channel area with widths of
2,980 in the 1950s, 2,780 in the 1980s, and 2,750 in 2012.
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5.1.5.5. LR-5 (PRM 32.3 to PRM 23.5)
Within LR-5 which extends from the Yentna River confluence downstream to PRM 32.3 and
PRM 23.5, the Susitna River has an active channel width of 3,250 feet. It is confined by Upper
Pleistocene-age glacio-lacustrine deposits on the east bank as well as the Late Cretaceous-age
granodiorite outcrop on the west side of the river at Susitna Station (PRM 29.9). The valley
floor width is about 8,880 feet and the river slope is very low at 1.3 ft/mile. The relatively
constricted valley limits the sediment storage potential within the reach and the bulk of the
sediment is stored in mid-channel bars, vegetated islands and discontinuous floodplain segments.
The sinuosity is 1.13 and on average there are 1.9 channels in the reach, with the initial portion
of channel single threaded and the lower portion splitting into 2 channels.
Based on total turnover rates, LR-5 is the least dynamic reach in the Lower River. The total
turnover rate over the six decades was 95,000 ft²/yr/mile. The floodplain to channel turnover rate
at 62,000 ft²/yr/mile was almost double the channel to floodplain turnover rate of 34,000
ft²/yr/mile resulting in a net floodplain to channel turnover rate of 28,000 ft²/yr/mile.
Floodplain to channel turnover dominated the turnover in LR-5. This majority of this turnover
occurred in the left side channel (PRM 27 to PRM 25) during the 1950s to 1980s. The bank
erosion had a maximum width of 1,000 ft on the outside of the channel bends at PRM 25 and
PRM 24.5 and resulted from lateral meander migration. The erosion of the outer bends continued
through the 1980s to 2012 to a lesser degree, about 400 feet in width. From the 1950s to the
1980s there was 4.9 percent of floodplain to channel turnover. This was the largest amount of
turnover in LR-5 during either study period. During the 1980s to 2012, the floodplain to channel
turnover reduced by over 50 percent to 2 percent of the total reach area.
The majority of channel to floodplain turnover was isolated to a few locations in LR-5. There
was 2.5 million sq. ft. of vegetation establishment at PRM 25 in the 1950s to 1980s. From the
1950s to 1980s the channel to floodplain turnover was 2.7 percent of the reach total area. In the
1980s to 2012, vegetation establishment occurred between the two vegetated islands at PRM 28
and PRM 27 which were previously not vegetated. Vegetation encroachment proceeded during
the 1980s to 2012 around the vegetated island at PRM 25 that formed since the 1950s. During
the 1980s to 2012 the channel to floodplain turnover dropped to 1.4 percent of the total reach
area.
Between the 1950s and 2012, LR-5 experienced a net floodplain to channel turnover of 2.8
percent of the total reach area over the six decades studied corresponding to a rate of 27,600
ft²/yr/mile. LR-5 had the greatest amount of net floodplain to channel turnover in the Lower
River between the 1950s and 2012. The geomorphic reach of CL-1 was the only other reach to
have a net floodplain to channel turnover rate (27,000 ft²/yr/mile). The channel width history
over the period reflect the net increase in channel area as the reach average active channel widths
were 2,910 feet, 3,240 feet and 3,250 feet in the 1950s, 1980s and 2012, respectively.
5.1.5.6. LR-6 (PRM 23.5 to PRM 3.3)
This is the downstream most reach as the Susitna River flows into Cook Inlet and forms a delta-
distributary system with longitudinally continuous, vegetated and relatively stable inter-
distributary channel delta plain segments. The delta is bounded to the east by Upper Pleistocene-
age, glacioestuarine deposits and to the west by Holocene-age estuarine deposits. The active
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Castle Mountain Fault, with evidence of Holocene-age displacement (Labay and Haeussler
2001), crosses the river at the head of the reach. The width of the delta plain is about 31,000 feet
and the river slope in the reach is extremely flat (1.5 ft/mile). The sinuosity is 1.43, and on
average, there are 6.2 channels in the reach. The average active channel width of the reach is
5,280 feet.
The total turnover rate from the 1950s to 2012 of 213,000 ft²/yr/mile places LR-6 in the
moderately dynamic reach category relative to other Lower River reaches. The total turnover rate
is comprised of nearly equal contributions from channel to floodplain turnover at 114,000
ft²/yr/mile and floodplain to channel turnover 100,000 ft²/yr/mile. It was the second most
dynamic reach in the Lower River with the highest 1950s to 2012 total turnover rate being
attributed to LR-3 at 240,000 ft²/yr/mile.
Floodplain to channel turnover occurred in LR-6 as lateral channel migration and side channel
widening. Lateral channel migration of areas of larger than 5 million sq. ft. occurred at PRM
21.8, PRM 16, and PRM 13.5 during the 1950s to 1980s. Floodplain to channel turnover
continued in these areas to a lesser degree during the 1980s and 2012. Over the 1950s to 2012
period, each of the three areas either equaled or exceeded 10 million sq. ft. The channel between
PRM 11 and PRM 9 widened by 600 percent from the 1950s to 2012 (1950s width = 100 ft,
1983 width = 200 ft., 2012 width = 600 ft.). Floodplain to channel turnover was 4.0 percent of
the reach from the 1950s to 1980s and 2.8 percent of the reach in the 1980s to 2012.
From the 1950s to 2012, LR-6 channel to floodplain turnover represented 6.9 percent of the total
reach area. The channel to floodplain turnover area percentage doubled from the 1950s to 1980s
(2.5 percent of the reach area) and 1980s to 2012 (5.1 percent of the reach area). In both periods,
the channel to floodplain turnover areas (1950s to 1980s area = 51 million sq. ft., 1980s to 2012
area = 103 million sq. ft.) was predominantly vegetation encroachment; although there were a
few areas of vegetation establishment occurred that were approximately 1 to 2 million sq. ft. The
largest channel to floodplain turnover areas was the result of vegetation encroachment along
vegetated islands in side channels during the 1980s to 2012 at PRM 20 (8 million sq. ft.) and
PRM 7 left (21 million sq. ft.)
The net rate of turnover for LR-6 was 14 ft²/yr/mile of channel to floodplain turnover. As a
percent of the total reach area, this is only 0.9 percent. Although, 0.9 percent is a relatively low
percentage of net turnover compared to other reaches, it was a large area at 17 million sq. ft. The
channel width over the period corresponding decreased from 5,440 ft in the 1950s, to 5,720 ft in
the 1980s, to 5,280 ft in 2012.
5.1.6. Chulitna River Reach
The Chulitna River Reach (CL-1) joins the Susitna River at PRM 102 at the Three Rivers
Confluence. The reach extends approximately 3.2 miles upstream of the Susitna River. Aerial
coverage from the 1980s was limited to the portion downstream of Chulitna River Mile 1.4. The
tabulated values for turnover analysis were similarly limited to the lower 1.4 Chulitna River
miles for the 1950s to 1980s and 1980s to 2012. Turnover was mapped for the period of 1950s to
2012 for lower 12.4 miles of the Chulitna River, but the calculated values of turnover reported
below are all for the lower 1.4 miles to be consistent with the data available from the 1980s.
The average annual sediment load of the Chulitna River was estimated to be 8.6 million
tons/year (Tetra Tech 2014c). This is 62 percent of the total sediment load estimated for the
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Three Rivers Confluence. The remainder of the estimated average annual load is composed of
3.4 million tons/year from the Susitna River or 25 percent and the 1.9 million tons from the
Talkeetna River or 13 percent. In contrast, the percentage of average annual flow contributed at
Three Rivers Confluence is 36, 46, and 17 for the Chulitna, Susitna, and Talkeetna rivers,
respectively (Tetra Tech 2013c). The dominance of the Chulitna River in terms of sediment
supply is reflected in the channel from downstream of the Three Rivers Confluence, which
resembles the Chulitna River much more so than the Susitna River even though their flow
contributions are similar.
The river planform changes from a single channel to multiple channels at Chulitna River Mile
3.2 as the river exits a more confined area. Evidence indicates that the Chulitna River Reach is
net aggradational and the bulk of the alluvial sediment is stored in active unvegetated braid bars
(Tetra Tech 2014b). Within the reach, there are also locations where alluvial sediments are stored
within vegetated islands and mid-channel bars, and the reach is bounded on each side by a
vegetated floodplain of varying width.
The lower 1.4 miles of the CL-1 is a very dynamic reach; between the 1950s and 2012, it had
more than twice the total turnover rate at 490,000 ft²/yr/mile compared to the Lower River reach,
LR-3, with the highest total turnover rate of 240,000 ft²/yr/mile. The CL-1 total turnover rate
from the 1950s to 2012 was based on 230,000 ft²/yr/mile of channel to floodplain turnover and
260,000 ft²/yr/mile of floodplain to channel turnover. This is due to the northward shift of the
Chulitna in CL-1. There was a maximum of 3,200 feet of erosion at Chulitna River Mile 0.9
which accounted for 26 million sq. ft. of floodplain to channel turnover over the 1950s to 2012
time period. Erosion of this area contributed to the northeast migration of the confluence of the
Chulitna River and Susitna Rivers. This topic is also discussed for MR-8 in Section 5.1.1.8
above. The net turnover rate for CL-1 was 27,000 ft²/yr/mile of floodplain to channel for the
period of the 1950s to 2012. The channel change in channel widths over the period
corresponding varied over this period from 4,970 feet in the 1950s, to 6,980 feet in the 1980s, to
5,510 feet in 2012.
5.1.7. Talkeetna River Reach
The Talkeetna River Reach (TK-1) joins the Susitna River at PRM 101, which is part of the
Three Rivers Confluence area. Aerial coverage from the 1980s was limited to the portion
downstream of Talkeetna River Mile 2.0. The tabulated values for turnover analysis were
similarly limited to the lower 2 miles of the Talkeetna River. However, turnover was mapped for
the period of 1950s to 2012 for the lower 9.4 miles of the Talkeetna River, though all turnover
rates provided are only for the lower 2.0 miles.
The Talkeetna River is the smallest of the three rivers at the confluence and on an average annual
basis, contributes 13 percent of the sediment load and 17 percent of the flow at the Three Rivers
Confluence (Tetra Tech 2013c, 2014c). The planform changes from a single channel to
anastomosed about a quarter of the way downstream through the reach. The bulk of the alluvial
sediments within the reach are stored in longitudinal vegetated floodplain islands and attached
bars.
Turnover in the lower 4 miles of the Talkeetna River is dominated by shifting of the multiple
channels and vegetation encroachment around islands. The lower 2 miles of the TK-1 is rated as
moderately dynamic relative to turnover in the Lower Susitna River Segment based on a total
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turnover rate from the 1950s to 2012 of 107,000 ft²/yr/mile. The total turnover rate is comprised
of a channel to floodplain contribution of 66,000 ft²/yr/mile and floodplain to channel
contribution of 40,000 ft²/yr/mile. For the lower 2 miles of TK-1 reach, there was a net turnover
channel to floodplain at a rate of 26,000 ft²/yr/mile for the period of the 1950s to 2012. The
channel widths over the three periods decreased from 1,210 feet in the 1950s, to 980 feet in the
1980s, to 890 feet in 2012.
5.1.8. Yentna River Reach
The Yentna River Reach (YN-1) joins with the Susitna River at PRM 32.3 and defines the reach
break between geomorphic reaches LR-4 and LR-5. Aerial coverage from the 1980s was limited
to the portion downstream of Yentna River Mile 1.9. The tabulated values for turnover analysis
were similarly limited to the lower 1.9 Yentna River miles. Turnover was mapped for the period
of 1950s to 2012 for the portion downstream of Yentna River Mile 3.1.
Review of the average annual flow and estimated sediment load for Susitna Station indicates that
the Yentna River contributes 40 percent of the total Susitna River mean annual flow of 48,600
cfs at the Susitna Station and 43 percent of the total sediment load of 33.7 million tons/year
(Tetra Tech 2013c, 2014c). The bulk of the alluvial sediments in the reach are stored in vegetated
islands, mid-channel bars, and continuous vegetated floodplains on both sides of the river.
The lower 2.1 miles of the Yenta River reach YN-1 is rated moderately dynamic relative to
turnover in the Lower Susitna River Segment. The total turnover rate for the period of 1950s to
2012 was 168,000 ft²/yr/mile with a channel to floodplain rate of 98,000 ft²/yr/mile and a
floodplain to channel turnover rate of 70,000 ft²/yr/mile. A large area of floodplain to channel
turnover over the period from the 1950s to 2012 occurred about 1.5 miles upstream of the
Yentna River confluence with the Lower River. At this location, the river eroded 700 feet into
the left bank between the 1950s and 1980s. During the period of the 1980s to 2012, floodplain to
channel turnover in YN-1 resulted primarily from the widening of a side channel on river right
between 0.5 miles to 1.7 miles upstream of the confluence. In this area, the width of the side
channel increased from about 100 feet in the 1980s to 700 feet in 2012. Adjacent to the channel
widening, there was substantial vegetation encroachment around the islands in the main channel
between the 1980s and 2012. The encroachment accounted for the majority of the channel to
floodplain turnover in YN-1 over the period of the 1950s to 2012.
Over the reach as a whole, there was a net turnover of channel to floodplain at an average rate of
28,000 ft²/yr/mile for the period of the 1950s to 2012. The channel widths over the three periods
decreased from 3,100 ft in the 1950s, to 3,030 ft in the 1980s, to 2,460 ft in 2012. The reductions
in channel widths reflect the overall increase in floodplain relative to channel area over the past
six decades.
5.2. Short-term Assessment of Channel Change from 2012 to 2013
Aerials
There were two types of analyses conducted to assess short term channel change. They differed
in spatial extent and methods. A qualitative analysis was conducted for the entire Middle River
and a quantitative analysis was done for the Three Rivers Confluence. Both were used to assess
short-term channel change between 2012 and 2013 resulting from the large flows on 6/2/2013
(90,700 cfs at Gold Creek) and 9/21/2012 (72,900 cfs at Gold Creek) and/or ice jams and ice
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runs in the 2013 breakup. The qualitative comparison identified the locations of channel change
in the entire Middle River between the 2012 geomorphic features and the 2013 aerial
photographs. For the quantitative comparison, the 2013 geomorphic features were first
delineated from the 2013 aerial photographs within the Three Rivers Confluence for the reaches
of MR-8, CL-1, TK-1, and LR-1. The 2013 geomorphic features were then overlain with the
2012 geomorphic features so the dimensions of channel change could be identified and
measured. The quantitative comparison focused on the channel change within a 2-mile radius
from PRM 102.4.
5.2.1. Qualitative Evaluation of 2012 Middle River Geomorphic Features and
the 2013 Aerial Photography
Based on the comparison of the 2012 and 2013 aerial photography, erosion was the dominant
geomorphic process in the Middle River during the period of comparison. Incidences of erosion
were identified in the mapping presented in Appendix T. Likely potential causes of the erosion
observed include high flows and ice processes. On 9/21/2012 the peak flow at Gold Creek
reached 72,900 cfs (198,000 cfs at Sunshine); corresponding to approximately a 20-year flood.
On 6/2/2013 there was 90,700 cfs flood at the Gold Creek gage (146,000 cfs at Sunshine). This
was greater than the 50-year return period discharge of 87,500 cfs at the Gold Creek gage and
approaching the 100-year peak of 98,000 cfs. The discharges during the open-water period for
the following gages were plotted for water years 2012 and 2013 in Figure 4.5-6: Susitna River at
Gold Creek, Chulitna River near Talkeetna, Talkeetna River near Talkeetna, Susitna River at
Sunshine, and Susitna River at Susitna Station. In water year 2013, ice jams were noted at the
following locations (HDR 2013a; HDR 2013b) and are potential locations where ice processes
may have contributed to the observed erosion in MR-6 through MR-8.
PRM 146 (main channel)
PRM 137 (main channel)
PRM 134 (main channel)
PRM 130
PRM 128
PRM 125.2 (main channel)
PRM 115.5 (main channel)
PRM 104 (main channel)
In the 2013 aerial photography there is evidence of the ice jam locations listed above. There was
vegetation shearing at PRM 145.6 left and PRM 135.5 to PRM 133.0. The erosion around PRM
135 is mentioned in the results for MR-6.
The channel change observed in the Middle River between 2012 and 2013 was limited to
erosion. Changes in the hydraulic connectivity of channels or in the vegetation and
biogeomorphic processes were not observed. The magnitude of the erosion is discussed per reach
in the following sections.
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5.2.1.1. MR-1, MR-2, MR-3, MR-4, and MR-5 (PRM 187.1 to PRM 148.4)
No appreciable channel change was detected from review of the 2012 and 2013 aerial
photography in the upper five reaches of the Middle River between 2012 and 2013.
5.2.1.2. MR-6 (PRM 148.4 to PRM 122.7)
In MR-6, there were 47 instances of channel change between 2012 and 2013 in seven primary
locations. These locations had multiple instances of erosion centered around PRM 143.9, PRM
143.1, PRM 139.7, PRM 138.7, PRM 137.6, PRM 135.5, PRM 131.7, PRM 130.0, PRM 129.2,
and PRM 125.2. Between PRM 146 and PRM 143 there was a reduction in the length of the
small floodplain islands. At PRM 139, vegetation was removed from within Slough 11.
Approximately 100 feet was removed from the head of the island in the side channel at PRM
138.6 left. At PRM 137.8, the main channel widened by about 100 feet on both the right and left
sides. The area near PRM 135 saw several instances of erosion and vegetation shearing by ice in
2013. Over 300 feet of erosion occurred at the head of the floodplain island at PRM 132.1 and in
the side channel downstream at PRM 131.6. There were several instances of main and side
channel erosion at PRM 130 (the upstream end of GAA-128 Slough 8A) and around the side
channels at PRM 129. There were three instances of erosion in the side channels near PRM 125.1
right.
5.2.1.3. MR-7 (PRM 122.7 to PRM 107.8)
There were only seven instances of erosion between 2012 and 2013 in MR-7. This occurred near
four primary areas of PRM 122, PRM 121, PRM 118, and PRM 115. The width of erosion
ranged between 20 to 50 feet, with a maximum of 75 feet at PRM 121.9. The main channel
experienced widening of about 10 to 25 feet at PRM 122.2 and PRM 117.9. Greater main
channel widening of about 70 feet occurred at PRM 121.9 and PRM 117.1. The head of the
vegetated islands were eroded at PRM 121.1 and PRM 115.2. The side channel at PRM 118.2
widened by 25 feet.
5.2.1.4. MR-8 (PRM 107.8 to PRM 102.4)
There were six instances of erosion between 2012 and 2013 in MR-8. This occurred in the
primary locations of PRM 104.3 and PRM 103.1. Three islands around PRM 104.3 experienced
erosion of 10 to 30 feet on their left sides. The head of the island at PRM 103.5 was eroded by
about 110 feet. Entire islands and portions of adjacent islands were eroded at PRM 103.1 and
PRM 102.9.
5.2.2. Quantitative Assessment of the 2012 and 2013 Geomorphic Features in
the Three Rivers Confluence
The channel change observed in the Three Rivers Confluence between 2012 and 2013 was
limited to erosion. Changes in the hydraulic connectivity of channels or in the vegetation and
biogeomorphic processes were not observed. Vegetation establishment and encroachment were
not observed within the comparison of geomorphic features. This would be expected for a short
term view as these are processes that progress at a slower but likely more consistent rate than
erosion. The locations of erosion were identified in the Three River Confluence between 2012
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and 2013 within an approximate 2-mile radius of PRM 102.4. The locations of erosion were
numbered to facilitate identification and added as labels in Appendix Q through Appendix S.
The mapping of geomorphic features in the Three Rivers Confluence is presented in Appendix Q
for 2012 and Appendix R for 2013. The overlay mapping of the 2012 and 2013 geomorphic
features is presented in Appendix S. The magnitude of the erosion is discussed per geomorphic
reach in the following sections.
5.2.2.1. Middle River: MR-8 (PRM 104.5 to PRM 102.4)
The erosion in the lower 2.1 miles of MR-8 was predominantly along river right. The erosion
ranged from about 40 to 70 feet. The only instance of erosion on river left happened on the
downstream left side of the reach. The main channel migrated into the overbank floodplain for a
maximum of 130 feet. This is labeled in the mapping as erosion location number 5.
5.2.2.2. Lower River: LR-1 (PRM 102.4 to PRM 99.0)
Erosion in the Lower River reach of the Three Rivers Confluence is most prominent in the
vicinity of the bar island complex at PRM 101.5. At this location flow is split around this bar
island complex or depositional bar and floodplain islands. The erosion on the floodplain islands
that define the outer channels’ banks has served to increase the width of the braid plain by about
130 feet across the section with end points of numbered 13 and 14. Erosion represents an
eastward shift of the boundary between the Lower River and the Talkeetna River at number 13.
The main channel shifted to the right about 90 feet into the floodplain island in the main channel
at number 12.
A trend of width increases in the braid plain continued in the downstream direction at PRM 101.
The Lower River braid plain increased by about 100 feet just downstream of the confluence with
the Talkeetna River. The main channel shifted to the right about 60 feet into the floodplain island
in the main channel at number 15. Further downstream the main channel eroded about 50 feet
into a floodplain island at PRM 100.
5.2.2.3. Chulitna River: CL-1
Erosion in the lower 1.6 miles of the Chulitna River mainly occurred along the right side of the
braid plain. They ranged between 50 to 150 feet, but had a maximum of 460 feet at number 7. A
secondary main channel eroded 50 feet into the downstream side of the floodplain island
(number 9) that represented the boundary between the Chulitna River and the Middle River.
Erosion at numbers 8, 9 and 10 indicate a widening of the Chulitna River braid plain by about
100 feet near its confluence with the Susitna River.
5.2.2.4. Talkeetna River: TK-1
Erosion in the lower 0.8 miles of the Talkeetna River occurred mostly on the outside of bends
with attached bars. The erosion ranged between 50 and 130 feet on the outside bends at numbers
19, 20, and 21. The island that comprises the majority of the boundary between the Talkeetna
River and Lower River was eroded on several sides. The other noted erosion location, number
22, was 30 feet wide on the east side of the island that forms the boundary between the Lower
River and Talkeetna River.
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6. DISCUSSION
This 2014 study effort updates the previous geomorphic feature mapping technical memorandum
(Tetra Tech 2013a) by incorporating geomorphic feature mapping from 1950s aerial
photography, a turnover analysis, and a short term analysis of geomorphic feature changes using
2013 aerial photography. The analysis of channel change and the turnover analysis were
conducted on the three sets of aerial photography and provided an excellent history over the past
60 years. The set of 1950s aerial photography is the earliest high quality coverage available; no
additional high quality aerial photography covering large portions of the study area was located.
The turnover analysis provided a simplified method of describing geomorphic changes in the
Susitna River. Rather than assessing changes in each geomorphic feature, the features were
combined as being either a channel feature or a floodplain feature. When comparing channel or
floodplain features over time, increases in vegetation and narrowing of channels is associated
with net channel to floodplain turnover. A net floodplain to channel turnover indicates an
increase in channel width and a removal of vegetation by erosion. As each geomorphic reach
varies in length, turnover was presented as normalized rates so that turnover in different
geomorphic reaches could be compared to each other. Turnover rates were calculated by
dividing the calculated turnover areas by the length of each reach and the span of time between
each set of aerials: 30 years for the 1950s to 1980s and 1980s to 2012 analyses, and 60 years for
the 1950s to 2012 analysis.
The previous technical memorandum identified increased vegetation as the primary factor in
geomorphic change, the turnover analysis described in this technical memorandum further
validates that conclusion, identifying greater channel to floodplain turnover throughout the
Middle and Lower Susitna River segments. The Middle River becomes less stable approaching
the Three Rivers Confluence. However, the Lower River had less consistent trends in stability by
geomorphic reach as shown in the following tables:
Relative Reach
Stability
MR Total Turnover
Rate Middle River Reaches
(ft²/yr/mile)
Dynamic > 30,000 MR-6, MR-8
Moderately Dynamic 20,000 to 30,000 N/A
Moderately Stable 10,000 to 20,000 MR-7
Stable 0 to 10,000 MR-1, MR-2, MR-3, MR-4, MR-5
Relative Reach
Stability
LR Total Turnover
Rate Lower River Reaches
(ft²/yr/mile)
Dynamic > 300,000 CL-1, LR-3
Moderately Dynamic 200,000 to 300,000 LR-2, LR-6
Moderately Stable 100,000 to 200,000 LR-1, TK-1, LR-4, YN-1
Stable 0 to 100,000 LR-5
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In the Middle River, net channel to floodplain turnover rates increased between the periods of the
1950s to 1980s and the 1980s to 2012 for MR-4 through MR-8. Net channel to floodplain
turnover rates decreased in MR-1 through MR-3 over the same two periods. From the 1950s to
the 1980s, there was net channel to floodplain turnover in all the Middle River reaches, except
for MR-4 and MR-7. During the 1980s to 2012, all the Middle River reaches, expect for MR-1,
MR-2, and MR-3, had net channel to floodplain turnover.
Among the Middle River geomorphic reaches, MR-6, MR-7, and MR-8 exhibited the greatest
amounts of turnover during the period of the 1980s to 2012. The net channel to floodplain
turnover rate exceeded 7,000 ft²/yr/mile between 1980s to 2012 for MR-6, MR-7, and MR-8. All
the other Middle River geomorphic reaches had net turnover rates less than 4,000 ft²/yr/mile. The
relative sizes of the net turnover rates are compared to the channel to floodplain and floodplain to
channel rates in the Middle River in Figure 6.1-1, Figure 6.1-2, and Figure 6.1-3 for the periods
of the 1950s to 1980s, 1980s to 2012, and 1950s to 2012, respectively.
Compared to the Middle River, the Lower River had a stronger trend of increasing channel to
floodplain turnover between the two periods of the 1950s to 1980s and 1980s to 2012. Five out
of six of the Lower River geomorphic reaches had an increase in net channel to floodplain rate
between those two periods. LR-4 was the exception. During the 1950s to 1980s, LR-2 and LR-4
were the only Lower River reaches to have a net channel to floodplain turnover rate. For
comparison, during the 1980s to 2012 period, LR-1, LR-2, LR-3, LR-4, and LR-6 all had net
channel to floodplain turnover rates. The relative sizes of the net turnover rates are compared to
the channel to floodplain and floodplain to channel rates in Figure 6.1-4, Figure 6.1-5, and
Figure 6.1-6 for the periods of the 1950s to 1980s, 1980s to 2012, and 1950s to 2012,
respectively. The portions of Chulitna River and Yentna River studied followed the channel to
floodplain trend in the Middle and Lower River. Net channel to floodplain turnover rates
increased between the 1950s to 1980s and 1980s to 2012 for the portions of Chulitna River and
Yentna River studied, but not for the Talkeetna River. The portion of the Talkeetna River studied
had net channel to floodplain turnover for both time periods of the 1950s to 1980s and the 1980s
to 2012.
The mapping of channel change that occurred between 2012 and 2013 indicates that erosion was
the primary process contributing to channel change in LR-1, the Middle River and the Three
Rivers Confluence, as opposed to vegetation establishment or encroachment. The incidences of
erosion were the greatest in MR-6 followed by MR-7, MR-8, and MR-3, while the degree of
erosion was the greatest in CL-1, followed by LR-1, TK-1 and MR-8. These two sets of recent
aerial photographs provide an understanding of short-term channel change in relation to a large
flow event that was the intent of acquiring additional historical aerial photography.
This technical memorandum and Mapping of Geomorphic Features and Assessment of Channel
Change in the Middle and Lower Susitna River Segments from 1980s and 2012 Aerials (Tetra
Tech 2013a) represent the completion of the goals and objectives listed in RSP Study 6.5 Section
6.5.4.4 and ISR Study 6.5 Section 7.2.1.4 “Assess Geomorphic Change Middle and Lower
Susitna River Segments.” The turnover results data presented in this 2014 technical
memorandum will be used to support the bed evolution modeling and bank energy index in ISR
Study 6.6 Section 7.2.2.1, as well as the Riparian Instream Flow Study (Study 8.6), and Ice
Processes Study (Study 7.6).
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The 1950s, 1980s, 2012, and 2013 aerials were used to assess the channel change and turnover in
the Middle and Lower River (ISR Study 6.5 Section 7.1.1.4). The Study Plan (RSP Sections
6.5.4.4.2.1 and 6.5.5.5.2.2.) indicates a decision will be made on whether to acquire additional
historical aerial photography for the Middle and Lower Susitna River Segments after completion
of the analysis of aerial photography from the 1950s, 1980s and present. In addition, the 2013
aerials that were flown to supplement the 2012 aerials document the river prior to, and after, the
peak flow of ~90,000 cfs at Gold Creek which is one of the highest flows recorded in the period
of record. As the objectives of this study were completed, no further analysis of aerial
photography is necessary.
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7. REFERENCES
AEA. 2012. Revised Study Plan: Susitna-Watana Hydroelectric Project FERC Project No.
14241. December 2012. Prepared for the Federal Energy Regulatory Commission by the
Alaska Energy Authority, Anchorage, Alaska. http://www.susitna-
watanahydro.org/study-plan.
HDR Alaska, Inc. (HDR). 2013a. Susitna River Ice Processes Study Report. Prepared for Alaska
Energy Authority, March 2013.
HDR. 2013b. Susitna River Ice Processes Study Draft Report. Prepared for Alaska Energy
Authority, August 2013.
Knighton, A.D., and G.C. Nanson. 1993. Anastomosis and the continuum of channel pattern.
Earth Surface Processes and Landforms, v.18: 613-625.
Labay, K.A. and Haeussler, P.J. 2001. GIS coverages of the Castle Mountain Fault, south central
Alaska: U.S. Geological Survey Open-File Report 2001-504.
Labelle, J.C., M. Arend, L. Leslie, and W. Wilson. 1985. Geomorphic Change in the Middle
Susitna River since 1949. Report by Arctic Environmental Information and Data Center.
Prepared for the Alaska Power Authority.
Makaske, B. 2001. Anastomosing rivers: review of their classification, origin and sedimentary
products. Earth-Science Reviews, v.53: 149-196.
Matanuska-Susitna Borough. 2011. Matanuska Susitna Borough LiDAR/Imagery Project.
http://matsu.gina.alaska.edu.
R&M Consultants, Inc. and Trihey & Associates. 1985a. Response of Aquatic Habitat Surface
Areas to Mainstem Discharge in the Yentna to Talkeetna Reach of the Susitna River.
Prepared under contract to Harza-Ebasco, for Alaska Power Authority, document No.
2774, June.
R2 Resource Consultants, Inc. 2013. Adjustments to Middle River Focus Areas. Susitna-Watana
Hydroelectric Project. 2012 Study Technical Memorandum. Prepared for the Alaska
Energy Authority. Anchorage, Alaska.
Smith, D.G., and N.D. Smith. 1980. Sedimentation in anastomosed river system: Examples from
alluvial valleys near Banff, Alberta. J. Sedimentary Petrology. v. 50 (1): 0157-0164.
Tetra Tech. 2013a. Mapping of Geomorphic Features and Assessment of Channel Change in the
Middle and Lower Susitna River Segments from 1980s and 2012 Aerials. Susitna-Watana
Hydroelectric Project. 2012 Study Technical Memorandum. Prepared for the Alaska
Energy Authority. Anchorage, Alaska.
Tetra Tech. 2013b. Mapping of Aquatic Macrohabitat Types at Selected Sites in the Middle and
Lower Susitna River Segments from 1980s and 2012 Aerials. Susitna-Watana
Hydroelectric Project. 2012 Study Technical Memorandum. Prepared for the Alaska
Energy Authority. Anchorage, Alaska.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 46 September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Tetra Tech, Inc., 2013c. Stream Flow Assessment. Susitna-Watana Hydroelectric Project. 2012
Study Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage,
Alaska.
Tetra Tech. 2014a. Updated Mapping of Aquatic Macrohabitat Types in the Middle Susitna
River Segment from 1980s and Current Aerials. Susitna-Watana Hydroelectric Project.
Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage, Alaska.
Tetra Tech. 2014b. Geomorphic Reach Delineation and Characterization, Upper, Middle, and
Lower Susitna River Segments. Susitna-Watana Hydroelectric Project. Updated
Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage, Alaska.
Tetra Tech. 2014c. 2014 Update of Sediment-Transport Relationships and a Revised Sediment
Balance for the Middle and Lower Susitna River Segments. Susitna-Watana
Hydroelectric Project. Technical Memorandum. Prepared for the Alaska Energy
Authority. Anchorage, Alaska.
Trihey & Associates. 1985. Response of Aquatic Habitat Surface Areas to Mainstem Discharge
in the Talkeetna-To-Devil Canyon Segment of the Susitna River, Alaska. Prepared under
contract to Harza-Ebasco, for Alaska Power Authority, document No. 2945.
U.S. Geological Survey (USGS). 2012. Streamflow Record Extension for Selected Streams in
the Susitna River Basin, Alaska, Scientific Investigations Report 2012–5210. 46 p.
Wilson, F. H., C. P. Hults, H. R. Schmoll, P. J. Haeussler, J. M. Schmidt, L. A. Yehle and K. A.
Labay. 2009. Preliminary Mapping of the Cook Inlet Region Alaska Including Parts of
the Talkeetna, Talkeetna Mountains, Tyonek, Anchorage, Lake Clark, Seward, Iliamna,
Seldovia, Mount Katmai, and Afognak 1:250,000 Scale Quadrangles. USGS Open-File
Report 2009-1108. 54p plus maps.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
8. TABLES
Table 4.2-1. 1950s Aerial Photo Summary.
Aerial Coverage (PRM) Date Used for Mapping
Discharge (cfs)
From To (MM/DD/YYYY) Gold Creek Sunshine¹
Middle Susitna River Segment
191.5 187 8/15/1949 25,800 --
187 158.5 8/15/1949 X 25,800 --
158.9 158.5 8/10/1949 29,900 --
158.5 151.8 8/10/1949 X 29,900 --
151.8 140.9 8/10/1949 29,900 --
151.8 102 7/3/1951 X 19,000 --
Lower Susitna River Segment
102 33.4 7/3/1951 X (19,000)² 45,100¹
45.2 40 7/11/1954 (19,000) 47,200
40 38.5 7/11/1954 X³ (19,000) 47,200
38.3 33.4 7/23/1953 (19,300) 48,000
33.4 28.5 7/23/1953 X (19,300) 48,000
31.3 28.5 7/25/1953 (20,000) 49,800
28.5 27.4 7/25/1953 X (20,000) 49,800
27.4 26 7/25/1953 (20,000) 49,800
27.4 21.5 8/12/1952 X (24,400) 61,400
21.5 20.6 8/12/1952 (24,400) 61,400
21.5 0 9/2/1952 X (28,700) 70,600
Notes:
1 Discharges shown in italics are synthesized flows from the extended flow record developed by the USGS (2012) and may
not reflect actual flows
2 Discharges in parentheses are measured flows at Gold Creek and were used to develop the USGS extended flow record
(USGS 2012)
3 07/11/1954 Aerial photos only used on the river right floodplain
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 4.2-2. Summary of 1980s aerial dates, discharges, and project river mile extents.
Aerial Coverage (PRM) Date Discharge (cfs)
From To (MM/DD/YYYY) Gold Creek Sunshine Station
Upper Susitna River Segment
251 187 7/19 and 7/20/1980 35,800 & 31,600 ---
Middle Susitna River Segment
187 152 7/19 and 7/20/1980 35,800 & 31,600 ---
158 102 9/11/1983 12,500 (12,200
published) ---
Lower Susitna River Segment
102 0 9/6/1983 --- 36,600
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 4.2-3. Summary of 2012 aerial dates, discharges, and project river mile extents.
Aerial Coverage (PRM) Date Discharge (cfs)
From To (MM/DD/YYYY) Gold Creek Sunshine Station
Upper Susitna River Segment
266.5 231.5 10/20/2012 7,410 ---
231.5 187 9/30/2012 17,000 ---
Middle Susitna River Segment
187 143.6 9/30/2012 17,000 ---
143.6 102 9/10/2012 12,900 ---
119 102 7/27/2012 22,200 ---
Lower Susitna River Segment
102 63 7/27/2012 --- 53,000
102 78 9/10/2012 38,200
78 69 9/30/2012 --- 48,000
69 33.5 10/10/2012 --- 55,000
33.5 22.5 9/30/2012 48,000
22.5 0 10/10/2012 --- 55,000
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-1. Geomorphic feature areas for MR-1.
MR-1
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 7,516,000 0 0 424,000 4,775,000 5,199,000
1983 7,471,000 0 0 376,000 4,868,000 5,244,000
2012 7,794,000 0 0 279,000 4,643,000 4,922,000
Percent Change 1950s to 1983 -1% 0% 0% -11% 2% 1%
Percent Change 1983 to 2012 4% 0% 0% -26% -5% -6%
Percent Change 1950s to 2012 4% 0% 0% -34% -3% -6%
Table 5.1-2. Geomorphic feature areas for MR-2.
MR-2
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 56,644,000 139,000 90,000 10,997,000 98,655,000 109,652,000
1983 55,551,000 29,000 115,000 8,448,000 102,383,000 110,831,000
2012 54,702,000 1,416,000 268,000 8,998,000 101,140,000 110,138,000
Percent Change 1950s to 1983 -2% -79% 28% -23% 4% 1%
Percent Change 1983 to 2012 -2% 4783% 133% 7% -1% -1%
Percent Change
1950s to 2012 -3% 919% 198% -18% 2% 0%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-3. Geomorphic feature areas for MR-3.
MR-3
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 10,711,000 0 0 414,000 9,230,000 9,644,000
1983 10,601,000 0 0 436,000 9,318,000 9,754,000
2012 11,008,000 0 1,000 368,000 8,979,000 9,347,000
Percent Change 1950s to 1983 -1% 0% 0% 5% 1% 1%
Percent Change 1983 to 2012 4% 0% N/A -16% -4% -4%
Percent Change 1950s to 2012 3% 0% N/A -11% -3% -3%
Table 5.1-4. Geomorphic feature areas for MR-4.
MR-4
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 20,487,000 0 0 38,000 22,390,000 22,428,000
1983 20,767,000 0 0 45,000 22,104,000 22,149,000
2012 20,093,000 0 0 46,000 22,778,000 22,824,000
Percent Change 1950s to 1983 1% 0% 0% 18% -1% -1%
Percent Change
1983 to 2012 -3% 0% 0% 2% 3% 3%
Percent Change
1950s to 2012 -2% 0% 0% 21% 2% 2%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-5. Geomorphic feature areas for MR-5.
MR-5
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 15,445,000 0 0 1,507,000 18,364,000 19,871,000
1983 15,023,000 0 0 1,636,000 18,657,000 20,293,000
2012 14,494,000 0 0 2,073,000 18,748,000 20,821,000
Percent Change 1950s to 1983 -3% 0% 0% 9% 2% 2%
Percent Change 1983 to 2012 -4% 0% 0% 27% 0% 3%
Percent Change 1950s to 2012 -6% 0% 0% 38% 2% 5%
Table 5.1-6. Geomorphic feature areas for MR-6.
MR-6
Year
Channel Floodplain
Main and Side
Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 144,165,000 3,927,000 457,000 60,836,000 137,214,000 198,050,000
1983 134,189,000 8,572,000 681,000 67,832,000 135,326,000 203,158,000
2012 127,708,000 3,934,000 1,431,000 73,700,000 139,827,000 213,527,000
Percent Change 1950s to 1983 -7% 118% 49% 11% -1% 3%
Percent Change
1983 to 2012 -5% -54% 110% 9% 3% 5%
Percent Change
1950s to 2012 -11% 0% 213% 21% 2% 7%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-7. Geomorphic feature areas for MR-7.
MR-7
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 72,068,000 29,000 196,000 15,231,000 86,663,000 101,894,000
1983 71,295,000 1,000,000 572,000 15,233,000 86,089,000 101,322,000
2012 65,180,000 921,000 737,000 20,863,000 86,488,000 107,351,000
Percent Change 1950s to 1983 -1% 3348% 192% 0% -1% -1%
Percent Change 1983 to 2012 -9% -8% 29% 37% 0% 6%
Percent Change 1950s to 2012 -10% 3076% 276% 37% 0% 5%
Table 5.1-8. Geomorphic feature areas for MR-8.
MR-8
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 35,289,000 747,000 771,000 9,337,000 187,784,000 197,121,000
1983 32,851,000 875,000 850,000 9,813,000 185,469,000 195,282,000
2012 31,887,000 319,000 1,103,000 9,435,000 185,887,000 195,322,000
Percent Change 1950s to 1983 -7% 17% 10% 5% -1% -1%
Percent Change
1983 to 2012 -3% -64% 30% -4% 0% 0%
Percent Change
1950s to 2012 -10% -57% 43% 1% -1% -1%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-9. Geomorphic feature areas for GAA-184.
GAA-184
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 3,908,000 0 0 258,000 2,088,000 2,346,000
1983 3,865,000 0 0 249,000 2,141,000 2,390,000
2012 3,976,000 0 0 235,000 2,044,000 2,279,000
Percent Change 1950s to 1983 -1% 0% 0% -3% 3% 2%
Percent Change 1983 to 2012 3% 0% 0% -6% -5% -5%
Percent Change 1950s to 2012 2% 0% 0% -9% -2% -3%
Table 5.1-10. Geomorphic feature areas for GAA-173.
GAA-173
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 10,632,000 39,000 72,000 4,210,000 8,080,000 12,290,000
1983 10,159,000 11,000 73,000 3,028,000 9,762,000 12,790,000
2012 8,734,000 864,000 60,000 3,653,000 9,722,000 13,375,000
Percent Change 1950s to 1983 -4% -72% 1% -28% 21% 4%
Percent Change 1983 to 2012 -14% 7755% -18% 21% 0% 5%
Percent Change
1950s to 2012 -18% 2115% -17% -13% 17% 8%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-11. Geomorphic feature areas for GAA-151.
GAA-151
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 1,299,000 0 0 0 1,820,000 1,820,000
1983 1,246,000 0 0 0 1,872,000 1,872,000
2012 1,265,000 0 0 0 1,853,000 1,853,000
Percent Change 1950s to 1983 -4% 0% 0% 0% 3% 3%
Percent Change 1983 to 2012 2% 0% 0% 0% -1% -1%
Percent Change 1950s to 2012 -3% 0% 0% 0% 2% 2%
Table 5.1-12. Geomorphic feature areas for GAA-144.
GAA-144
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 11,251,000 670,000 0 2,299,000 11,444,000 13,743,000
1983 11,829,000 352,000 0 2,396,000 11,088,000 13,484,000
2012 11,806,000 69,000 172,000 2,288,000 11,330,000 13,618,000
Percent Change 1950s to 1983 5% -47% 0% 4% -3% -2%
Percent Change
1983 to 2012 0% -80% N/A -5% 2% 1%
Percent Change
1950s to 2012 5% -90% N/A 0% -1% -1%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-13. Geomorphic feature areas for GAA-141.
GAA-141
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 15,690,000 132,000 105,000 3,179,000 26,280,000 29,459,000
1983 15,403,000 138,000 222,000 3,101,000 26,523,000 29,624,000
2012 14,705,000 354,000 239,000 4,600,000 25,490,000 30,090,000
Percent Change 1950s to 1983 -2% 5% 111% -2% 1% 1%
Percent Change 1983 to 2012 -5% 157% 8% 48% -4% 2%
Percent Change 1950s to 2012 -6% 168% 128% 45% -3% 2%
Table 5.1-14. Geomorphic feature areas for GAA-138.
GAA-138
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 16,555,000 364,000 215,000 2,993,000 28,895,000 31,888,000
1983 15,545,000 1,136,000 138,000 7,809,000 24,394,000 32,203,000
2012 14,081,000 109,000 673,000 5,292,000 28,867,000 34,159,000
Percent Change 1950s to 1983 -6% 212% -36% 161% -16% 1%
Percent Change
1983 to 2012 -9% -90% 388% -32% 18% 6%
Percent Change
1950s to 2012 -15% -70% 213% 77% 0% 7%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-15. Geomorphic feature areas for GAA-128.
GAA-128
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 14,750,000 2,176,000 0 13,740,000 5,856,000 19,596,000
1983 11,127,000 2,766,000 2,000 16,119,000 6,507,000 22,626,000
2012 12,401,000 968,000 97,000 16,662,000 6,395,000 23,057,000
Percent Change 1950s to 1983 -25% 27% N/A 17% 11% 15%
Percent Change 1983 to 2012 11% -65% 4750% 3% -2% 2%
Percent Change 1950s to 2012 -16% -56% N/A 21% 8% 15%
Table 5.1-16. Geomorphic feature areas for GAA-115.
GAA-115
Year
Channel Floodplain
Main and
Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 10,102,000 0 63,000 2,302,000 16,682,000 18,984,000
1983 9,862,000 35,000 284,000 2,379,000 16,589,000 18,968,000
2012 9,380,000 0 185,000 2,730,000 16,855,000 19,585,000
Percent Change 1950s to 1983 -2% N/A 351% 3% -1% 0%
Percent Change
1983 to 2012 -5% -100% -35% 15% 2% 3%
Percent Change
1950s to 2012 -7% 0% 194% 19% 1% 3%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-17. Geomorphic feature areas for GAA-113.
GAA-113
Year
Channel Floodplain
Main and Side Channel Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 8,023,000 0 0 3,124,000 5,994,000 9,118,000
1983 8,328,000 0 0 3,164,000 5,651,000 8,815,000
2012 7,124,000 322,000 0 3,947,000 5,751,000 9,698,000
Percent Change 1950s to 1983 4% 0% 0% 1% -6% -3%
Percent Change 1983 to 2012 -14% N/A 0% 25% 2% 10%
Percent Change 1950s to 2012 -11% N/A 0% 26% -4% 6%
Table 5.1-18. Geomorphic feature areas for GAA-104.
GAA-104
Year
Channel Floodplain
Main and Side
Channel
Side Slough Upland
Slough Islands Overbank Islands and
Overbank
ft²
1950s 16,163,000 628,000 408,000 3,193,000 59,444,000 62,637,000
1983 15,690,000 535,000 320,000 3,687,000 59,601,000 63,288,000
2012 14,955,000 271,000 333,000 4,465,000 59,811,000 64,276,000
Percent Change 1950s to 1983 -3% -15% -22% 15% 0% 1%
Percent Change 1983 to 2012 -5% -49% 4% 21% 0% 2%
Percent Change
1950s to 2012 -8% -132% -23% 28% 1% 3%
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-19. Geomorphic feature areas for LR-1.
LR-1
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total
Islands1 Overbank2
ft²
1950s 82,434,000 29,725,000 152,041,000 0 2,316,000 859,000 235,000 118,448,000 202,920,000
1983 73,434,000 22,858,000 163,389,000 0 5,579,000 615,000 1,190,000 101,520,000 216,269,000
2012 71,135,000 18,218,000 148,951,000 0 3,207,000 730,000 1,911,000 111,545,000 233,636,000
Percent Change 1950s to 1983 -11% -23% 7% 0% 141% -28% 406% -14% 7%
Percent Change 1983 to 2012 -3% -20% -9% 0% -43% 19% 61% 10% 8%
Percent Change 1950s to 2012 -14% -39% -2% 0% 38% -15% 713% -6% 15%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-20. Geomorphic feature areas for LR-2.
LR-2
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total Islands Overbank
ft²
1950s 152,886,000 120,573,000 181,902,000 0 594,000 1,361,000 126,000 213,984,000 168,977,000
1983 132,878,000 79,412,000 216,331,000 0 1,288,000 752,000 1,159,000 225,927,000 182,655,000
2012 142,407,000 84,228,000 130,472,000 0 7,846,000 1,061,000 2,057,000 289,022,000 183,309,000
Percent Change 1950s to 1983 -13% -34% 19% 0% 117% -45% 820% 6% 8%
Percent Change 1983 to 2012 7% 6% -40% 0% 509% 41% 77% 28% 0%
Percent Change 1950s to 2012 -7% -30% -28% 0% 1221% -22% 1533% 35% 8%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-21. Geomorphic feature areas for LR-3.
LR-3
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total Islands Overbank
ft²
1950s 141,780,000 193,170,000 126,048,000 0 9,344,000 1,820,000 3,169,000 656,515,000 599,675,000
1983 112,019,000 120,577,000 247,828,000 0 3,253,000 2,631,000 680,000 591,190,000 653,343,000
2012 126,610,000 160,038,000 155,122,000 0 5,343,000 2,451,000 1,225,000 635,057,000 645,675,000
Percent Change 1950s to 1983 -21% -38% 97% 0% -65% 45% -79% -10% 9%
Percent Change 1983 to 2012 13% 33% -37% 0% 64% -7% 80% 7% -1%
Percent Change 1950s to 2012 -11% -17% 23% 0% -43% 35% -61% -3% 8%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-22. Geomorphic feature areas for LR-4.
LR-4
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total Islands Overbank
ft²
1950s 81,006,000 66,999,000 34,303,000 0 10,138,000 1,024,000 0 341,866,000 101,521,000
1983 60,670,000 65,136,000 45,564,000 0 8,557,000 481,000 0 345,500,000 106,478,000
2012 94,451,000 55,676,000 12,423,000 0 9,294,000 1,248,000 0 348,877,000 110,039,000
Percent Change 1950s to 1983 -25% -3% 33% 0% -16% -53% 0% 1% 5%
Percent Change 1983 to 2012 56% -15% -73% 0% 9% 159% 0% 1% 3%
Percent Change 1950s to 2012 17% -17% -64% 0% -8% 22% 0% 2% 8%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-23. Geomorphic feature areas for LR-5.
LR-5
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total
Islands Overbank
ft²
1950s 104,784,000 7,110,000 20,590,000 0 1,262,000 1,254,000 180,000 46,846,000 335,741,000
1983 86,976,000 0 61,382,000 0 853,000 1,508,000 29,000 45,390,000 325,762,000
2012 129,912,000 926,000 16,816,000 0 4,811,000 1,664,000 0 49,103,000 319,266,000
Percent Change 1950s to 1983 -17% -100% 198% 0% -32% 20% -84% -3% -3%
Percent Change 1983 to 2012 49% U -73% 0% 464% 10% -100% 8% -2%
Percent Change 1950s to 2012 24% -87% -18% 0% 281% 33% -100% 5% -5%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-24. Geomorphic feature areas for LR-6.
LR-6
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total Islands Overbank
ft²
1950s 212,750,000 220,216,000 138,925,000 0 643,000 3,651,000 4,360,000 620,599,000 811,782,000
1983 189,444,000 210,871,000 199,858,000 0 0 4,027,000 6,043,000 614,063,000 788,619,000
2012 298,210,000 106,821,000 142,664,000 0 7,410,000 3,700,000 4,524,000 681,302,000 768,294,000
Percent Change 1950s to 1983 -11% -4% 44% 0% -100% 10% 39% -1% -3%
Percent Change 1983 to 2012 57% -49% -29% 0% U -8% -25% 11% -3%
Percent Change 1950s to 2012 40% -51% 3% 0% 1052% 1% 4% 10% -5%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-25. Geomorphic feature areas for CL-1.
Chulitna
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total
Islands Overbank
ft²
1950s 9,599,000 10,875,000 15,100,000 0 195,000 938,000 0 8,961,000 34,777,000
1983 7,696,000 3,956,000 36,703,000 0 1,971,000 1,234,000 0 13,783,000 23,621,000
2012 6,155,000 10,508,000 26,348,000 0 695,000 288,000 137,000 22,297,000 20,207,000
Percent Change 1950s to 1983 -20% -64% 143% 0% 911% 32% 0% 54% -33%
Percent Change 1983 to 2012 -20% 166% -28% 0% -65% -77% U 62% -113%
Percent Change 1950s to 2012 -36% -3% 74% 0% 256% -69% U 149% -42%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-26. Geomorphic feature areas for TK-1.
Talkeetna
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total Islands Overbank
ft²
1950s 5,238,000 1,451,000 3,018,000 2,811,000 280,000 0 0 18,794,000 40,264,000
1983 3,933,000 1,615,000 934,000 2,463,000 674,000 68,000 636,000 20,682,000 40,519,000
2012 5,021,000 1,407,000 354,000 1,618,000 809,000 186,000 0 20,717,000 40,416,000
Percent Change 1950s to 1983 -25% 11% -69% -12% 141% U U 10% -100%
Percent Change 1983 to 2012 28% -13% -62% -34% 20% 174% -100% 0% -29%
Percent Change 1950s to 2012 -4% -3% -88% -42% 189% U 0% 10% 14%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-27. Geomorphic feature areas for YN-1.
Yentna
Year
Channel Floodplain
Main
Channel
(MC)
Side Channel
Complex (SCC)
Bar Island
Complex
(BIC)
Bar Attached
Bar (BAB)
Side
Channel
(SC)
Upland
Slough
(US)
Side
Slough
(SS)
Total
Islands Overbank
ft²
1950s 20,611,000 0 13,238,000 0 517,000 13,000 0 8,352,000 16,336,000
1983 9,363,000 2,000 19,665,000 0 4,538,000 0 0 9,021,000 16,797,000
2012 9,144,000 0 0 0 16,426,000 0 1,691,000 16,890,000 15,017,000
Percent Change 1950s to 1983 -55% U 49% 0% 778% -100% 0% 8% 3%
Percent Change 1983 to 2012 -2% -100% -100% 0% 262% 0% U 87% -11%
Percent Change 1950s to 2012 -56% 0% -100% 0% 3077% -100% U 102% -8%
Notes:
1 Total Islands is the summation of all vegetated islands (VI MC, VI SCC, VI BIC, VI SC, and VI SS).
2 Overbank is summation of Tributary (TR), Tributary Delta (TD), Additional Open Water (AOW), and Background (BG).
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-28. Turnover values for Middle River geomorphic reaches.
GAA Period (years) Turnover (sq. ft. x 10³) Turnover as % of GAA Turnover in ft²x103/yr/mile
Channel to Floodplain Floodplain to Channel Net¹ Channel to Floodplain Floodplain to Channel Net¹ Channel to Floodplain Floodplain to Channel Net¹ Total Turnover
184
∆t1 292 248 44 4.7 4.0 0.7 9.4 8.0 1.4 17
∆t2 50 161 -110 0.8 2.6 -1.8 1.6 5.0 -3.5 6.6
∆t3 225 293 -68 3.6 4.7 -1.1 3.6 4.6 -1.1 8.2
173
∆t1 922 421 501 4.0 1.8 2.2 14 6.6 7.9 21
∆t2 882 298 584 3.8 1.3 2.5 13 4.5 8.9 18
∆t3 1,520 439 1,090 6.6 1.9 4.7 12 3.4 8.4 15
151
∆t1 58 5 53 1.9 0.2 1.7 3.2 0.3 3.0 3.5
∆t2 12 31 -19 0.4 1.0 -0.6 0.7 2.0 -1.2 2.7
∆t3 41 7 34 1.3 0.2 1.1 1.2 0.2 1.0 1.4
144
∆t1 1,860 2,120 -260 7.2 8.2 -1.0 23 27 -3.3 50
∆t2 798 665 133 3.1 2.6 0.5 11 9.2 1.8 20
∆t3 2,180 2,300 -130 8.5 9.0 -0.5 14 15 -0.8 30
141
∆t1 1,770 1,600 165 3.9 3.5 0.4 16 15 1.5 31
∆t2 1,020 560 461 2.2 1.2 1.0 10 5.6 4.6 16
∆t3 2,280 1,650 626 5.0 3.6 1.4 11 7.9 3.0 19
138
∆t1 2,660 2,340 316 5.4 4.8 0.6 27 23 3.1 50
∆t2 3,130 1,170 1,960 6.4 2.4 4.0 34 13 22 47
∆t3 5,080 2,800 2,270 10 5.7 4.6 27 15 12 41
128
∆t1 4,660 1,630 3,030 13 4.5 8.3 62 22 41 84
∆t2 1,760 1,330 431 4.8 3.6 1.2 26 20 6.4 46
∆t3 5,520 2,060 3,460 15 5.6 9.5 39 14 24 53
115
∆t1 629 646 -16 2.2 2.2 -0.1 9.8 10 -0.3 20
∆t2 902 286 616 3.1 1.0 2.1 16 4.9 11 20
∆t3 1,150 549 600 3.9 1.9 2.1 9.4 4.5 4.9 14
113
∆t1 384 688 -300 2.2 4.0 -1.8 7.2 13 -5.7 20
∆t2 1,060 180 882 6.2 1.0 5.1 22 3.7 18 26
∆t3 1,290 712 578 7.5 4.2 3.4 13 7.0 5.7 20
104
∆t1 1,780 1,130 649 2.2 1.4 0.8 18 11 6.4 29
∆t2 1,570 577 996 2.0 0.7 1.2 17 6.3 11 23
∆t3 3,040 1,400 1,640 3.8 1.8 2.1 16 7.2 8.5 23
Notes: ∆t1 = 1950s to 1980s. ∆t2 = 1980s to 2012 ∆t3 = 1950s to 2012 ¹A positive net value indicates greater channel to floodplain turnover, a negative net value indicates greater floodplain to channel
turnover
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-29. Turnover values for Middle River Geomorphic Assessment Areas.
Reach Period (years)
Turnover (sq. ft. x 10³) Turnover as % of Reach Turnover in ft²x103/yr/mile
Channel to
Floodplain
Floodplain to
Channel Net Channel to
Floodplain
Floodplain to
Channel Net Channel to
Floodplain
Floodplain to
Channel Net Total
Turnover
MR-1
∆t1 614 568 46 4.8 4.5 0.4 7.9 7.3 0.6 15
∆t2 82 406 -320 0.6 3.2 -2.5 1.0 5.1 -4.0 6.1
∆t3 441 719 -280 3.5 5.7 -2.2 2.8 4.6 -1.8 7.4
MR-2
∆t1 3,820 2,640 1,180 2.3 1.6 0.7 8.2 5.7 2.5 14
∆t2 1,650 2,320 -680 1.0 1.4 -0.4 3.4 4.8 -1.4 8.3
∆t3 4,320 3,820 508 2.6 2.3 0.3 4.6 4.0 0.5 8.6
MR-3
∆t1 280 170 110 1.4 0.8 0.5 2.6 1.6 1.0 4.2
∆t2 66 474 -410 0.3 2.3 -2.0 0.6 4.2 -3.6 4.8
∆t3 160 458 -300 0.8 2.2 -1.5 0.7 2.1 -1.4 2.8
MR-4
∆t1 605 883 -280 1.4 2.1 -0.6 1.6 2.3 -0.7 3.9
∆t2 1,090 415 673 2.5 1.0 1.6 2.8 1.1 1.7 3.8
∆t3 1,150 755 396 2.7 1.8 0.9 1.5 1.0 0.5 2.5
MR-5
∆t1 1,540 1,120 422 4.4 3.2 1.2 8.5 6.2 2.3 15
∆t2 801 272 528 2.3 0.8 1.5 5.0 1.7 3.3 6.7
∆t3 1,960 1,010 951 5.6 2.9 2.7 5.8 3.0 2.8 8.7
MR-6
∆t1 23,000 17,900 5,110 6.6 5.2 1.5 28 22 6.2 50
∆t2 19,200 8,870 10,400 5.5 2.6 3.0 26 12 14 38
∆t3 36,700 21,200 15,500 11 6.1 4.5 23 14 9.9 37
MR-7
∆t1 3,700 4,280 -570 2.1 2.5 -0.3 7.8 9.0 -1.2 17
∆t2 8,190 2,160 6,030 4.7 1.2 3.5 19 5.0 14 24
∆t3 9,980 4,520 5,460 5.7 2.6 3.1 11 5.0 6.0 16
MR-8
∆t1 4,190 3,940 248 1.8 1.7 0.1 24 23 1.4 47
∆t2 3,700 2,460 1,250 1.6 1.1 0.5 24 16 8.0 39
∆t3 6,980 5,450 1,530 3.1 2.4 0.7 21 17 4.6 38
Notes:
∆t1 = 1950s to 1980s.
∆t2 = 1980s to 2012
∆t3 = 1950s to 2012
¹A positive net value indicates greater channel to floodplain turnover, a negative net value indicates greater
floodplain to channel turnover
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Table 5.1-30. Turnover values for Lower River geomorphic reaches.
Reach Period (years)
Turnover (sq. ft. x 10³) Turnover as % of Reach Turnover in ft²x103/yr/mile
Channel to
Floodplain
Floodplain to
Channel Net Channel to
Floodplain
Floodplain to
Channel Net Channel to
Floodplain
Floodplain to
Channel Net Total
Turnover
LR-1
∆t1 31,600 33,800 -2,200 5 6 -0.4 68 73 -4.8 141
∆t2 58,100 33,400 24,800 10 5.7 4.2 138 79 59 218
∆t3 74,100 51,200 22,900 13 9 3.9 84 58 26 142
LR-2
∆t1 97,300 71,500 25,800 12 9 3.1 136 100 36 237
∆t2 100,000 36,600 63,800 12 4.4 7.6 155 57 99 212
∆t3 167,000 77,600 89,600 20 9 11 123 57 66 180
LR-3
∆t1 93,900 106,000 -12,000 5 6.1 -0.7 140 157 -17 297
∆t2 109,000 72,400 36,400 6 4.2 2.1 179 119 60 298
∆t3 164,000 140,000 24,700 9 8 1.4 73 82 -9.1 237
LR-4
∆t1 38,100 26,000 12,200 6 4.1 1.9 97 66 31 163
∆t2 26,800 19,500 7,300 4 3.1 1.2 75 55 20 130
∆t3 56,200 36,800 19,400 9 5.8 3.1 145 96 48 124
LR-5
∆t1 14,000 25,500 -11,000 3 4.9 -2.2 52 95 -43 147
∆t2 7,500 10,300 -2,800 1.4 2.0 -0.5 29 40 -11 70
∆t3 17,800 32,200 -14,000 3 6.2 -2.8 314 266 47 95
LR-6
∆t1 51,300 81,000 -30,000 2.5 4.0 -1.5 82 129 -47 211
∆t2 103,000 56,200 46,900 5 2.8 2.3 176 96 80 272
∆t3 138,000 121,000 17,200 7 6.0 0.9 114 100 14 213
CL-1
∆t1 4,200 14,000 -9,700 5.2 17.3 -12.1 93.8 311.4 -217.6 405
∆t2 16,900 9,290 7,570 20.3 11.2 9.1 415.3 228.9 186.4 644
∆t3 19,700 21,900 -2,300 24.7 27.5 -2.9 230.2 256.9 -26.8 487
TK-1
∆t1 6,250 3,710 2,540 8.8 5.2 5.2 97.7 58.0 39.8 156
∆t2 4,460 3,800 660 6.3 5.4 5.4 76.9 65.5 11.4 142
∆t3 8,090 4,910 3,180 11.5 7.0 7.0 66.3 40.3 26.0 107
YN-1
∆t1 3,010 5,400 -2,400 5.1 9.1 9.1 44.8 80.4 -35.6 125
∆t2 10,500 4,750 5,760 17.8 8.0 8.0 172.6 78.0 94.5 251
∆t3 12,500 8,960 3,550 21.1 15.1 15.1 97.7 69.9 27.7 168
Notes:
∆t1 = 1950s to 1980s.
∆t2 = 1980s to 2012
∆t3 = 1950s to 2012
¹A positive net value indicates greater channel to floodplain turnover, a negative net value indicates greater
floodplain to channel turnover
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Table 5.1-31: Geomorphic reach delineations and classifications
Reach Designation
Reach Breaks
( PRM / RM )1 Reach Classification Slope (ft/mi) Lateral Constraints
Upstream Downstream
Upper Susitna River Segment (UR)
UR-1 261.3 / 260.0 248.6 / 247.7 SC2 NA Quaternary Basin Fill
UR-2 248.6 / 247.7 234.5 / 233.0 SC1 NA Quaternary Basin Fill
UR-3 234.5 / 233.0 224.9 / 223.1 SC1 NA Quaternary Basin Fill
UR-4 224.9 / 223.1 208.1 / 205.7 SC2 NA Granodiorite
UR-5 208.1 / 205.7 203.4 / 200.8 SC1 NA Quaternary Basin Fill
UR-6 203.4 / 200.8 187.1 / 184.3 SC2 NA Quaternary Basin Fill
Middle Susitna River Segment (MR)
MR-1 187.1 / 184.3 184.6 / 181.9 SC2 9 Tertiary-Cretaceous Gneiss
MR-2 184.6 / 181.9 169.6 / 166.4 SC2 10 Cretaceous Kahiltna Flysch Tertiary-Cretaceous
Gneiss
MR-3 169.6 / 166.4 166.1 / 163.0 SC2 17 Paleocene Granites
MR-4 166.1 / 163.0 153.9 / 150.3 SC1 30 Paleocene Granites
MR-5 153.9 / 150.3 148.4 / 144.9 SC2 12 Cretaceous Kahiltna Flysch
MR-6 148.4 / 144.9 122.7 / 118.9 SC3 10
Cretaceous Kahiltna Flysch with undifferentiated
Upper Pleistocene moraines, kames, lacustrine
deposits
MR-7 122.7 / 118.9 107.8 / 104.1 SC2 8
Cretaceous Kahiltna Flysch with undifferentiated
Upper Pleistocene moraines, kames, lacustrine
deposits
MR-8 107.8 / 104.1
102.4 / 98.6
MC1/SC3
(Reach is a
transition from
SC3 to MC1 as
the Three
Rivers
Confluence is
approached)
8 Upper Pleistocene moraines, outwash and
Holocene Alluvial Terrace deposits
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Reach Designation
Reach Breaks
( PRM / RM )1 Reach Classification Slope (ft/mi) Lateral Constraints
Upstream Downstream
Lower Susitna River Segment (LR)
LR-1 102.4 / 98.6 87.9 / 83.8 MC1 5 Upper Pleistocene Outwash, Moraine and
Lacustrine deposits
LR-2 87.9 / 83.8 65.6 / 61.4 MC2/MC3 5 Upper Pleistocene Outwash, Moraine and
Lacustrine deposits
LR-3 65.6 / 61.4 44.6 / 40.3 MC3 4 Upper Pleistocene Glaciolacustrine deposits
LR-4 44.6 / 40.3 32.3 / 28.3 MC2 2 Upper Pleistocene Glaciolacustrine deposits
LR-5 32.3 / 28.3 23.5 / 19.4 SC2 2 Upper Pleistocene Glaciolacustrine and Moraine
deposits and Late Cretaceous granodiorite
LR-6 23.5 / 19.4 3.3 / 0.0 MC4 1.4 Upper Pleistocene Glaciolacustrine and Holocene
Estuarine deposits
Notes:
1 First Value is in current Project River Miles (PRM). Second value, in italics, is in the 1980s River Mile
(RM) System.
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9. FIGURES
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Figure 3.2-1. Susitna River geomorphology study area and large-scale river segments.
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Figure 4.5-1. 1980s Geomorphic Feature Mapping of Channel or Land.
Figure 4.5-2. 2012 Geomorphic Feature Mapping of Channel or Land.
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TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Figure 4.5-3. Turnover Mapping of 1980 to 2012.
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Figure 4.5-4. Examples of vegetation encroachment and vegetation establishment.
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Figure 4.5-5. Example Focus Area and Geomorphic Assessment Areas.
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Figure 4.5-6. Discharge measurements at USGS gaging station on or near the Susitna River from June 2012 through October 2013.
0
50,000
100,000
150,000
200,000
250,000
4/1/2012 7/10/2012 10/18/2012 1/26/2013 5/6/2013 8/14/2013 11/22/2013 3/2/2014Discharge (cfs) Date
Susitna R. at Gold Creek (USGS 15292000)
Chulitna R. near Talkeetna (USGS 15292400)
Talkeetna R. near Talkeetna (USGS 15292700)
Susitna R. at Sunshine (USGS 15292780)
Susitna R. at Susitna Station (USGS 15294350) {Data not available for 2012}
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Figure 4.6-1. Example of Registration Error.
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Figure 5.1-1. Example of Turnover mapping in GAA-128 and GAA-138.
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Figure 5.1-2. Map of the Middle Susitna River Segment showing the geomorphic reaches.
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Figure 5.1-3. Map of the Lower Susitna River Segment showing the geomorphic reaches.
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Figure 6.1-1: 1950s to 1980s turnover rates for Middle River geomorphic reaches.
-30
-20
-10
0
10
20
30
MR-1 MR-2 MR-3 MR-4 MR-5 MR-6 MR-7 MR-8Turnover in ft²x10³/yr/mile Geomorphic Reach
1950s to 1980s Turnover
Net Turnover Channel to Floodplain Floodplain to Channel
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Figure 6.1-2: 1980s to 2012 turnover rates for Middle River geomorphic reaches.
-30
-20
-10
0
10
20
30
MR-1 MR-2 MR-3 MR-4 MR-5 MR-6 MR-7 MR-8Turnover in ft²x10³/yr/mile Geomorphic Reach
1980s to 2012 Turnover
Net Turnover Channel to Floodplain Floodplain to Channel
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Figure 6.1-3: 1950s to 2012 turnover rates for Middle River geomorphic reaches.
-30
-20
-10
0
10
20
30
MR-1 MR-2 MR-3 MR-4 MR-5 MR-6 MR-7 MR-8Turnover in ft²x10³/yr/mile Geomorphic Reach
1950s to 2012 Turnover
Net Turnover Channel to Floodplain Floodplain to Channel
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Figure 6.1-4. 1950s to 1980s turnover rates for Lower River geomorphic reaches.
-200.0
-150.0
-100.0
-50.0
0.0
50.0
100.0
150.0
200.0
LR-1 LR-2 LR-3 LR-4 LR-5 LR-6Turnover in ft²x10³/yr/mile Geomorphic Reach
1950s to 1980s Turnover
Channel to Floodplain Floodplain to Channel Net Turnover Rate
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 88 September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Figure 6.1-5. 1980s to 2012 turnover rates for Lower River geomorphic reaches.
-200.0
-150.0
-100.0
-50.0
0.0
50.0
100.0
150.0
200.0
LR-1 LR-2 LR-3 LR-4 LR-5 LR-6Turnover in ft²x10³/yr/mile Geomorphic Reach
1980s to 2012 Turnover
Channel to Floodplain Floodplain to Channel Net Turnover Rate
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 89 September 2014
TECHNICAL MEMORANDUM MAPPING OF GEOMORPHIC FEATURES AND TURNOVER
Figure 6.1-6. 1950s to 2012 turnover rates for Lower River geomorphic reaches.
-200.0
-150.0
-100.0
-50.0
0.0
50.0
100.0
150.0
200.0
LR-1 LR-2 LR-3 LR-4 LR-5 LR-6Turnover in ft²x10³/yr/mile Geomorphic Reach
1950s to 2012 Turnover
Channel to Floodplain Floodplain to Channel Net Turnover Rate
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 90 September 2014