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Susitna-Watana Hydroelectric Project Document
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Title:
Synthesis of the 1980s lower Susitna River segment, aquatic habitat
information : 2012 study technical memorandum
SuWa 56
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Prepared by Tetra Tech, Inc.
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2012 Environmental Studies
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Series (ARLIS-assigned report number):
Susitna-Watana Hydroelectric Project document number 56
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Published by:
[Anchorage, Alaska : Alaska Energy Authority, 2013]
Date published:
March 2013
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Prepared for Alaska Energy Authority
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Document type:
Study technical memorandum
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v, 34 p.
Related work(s):
Appendices (SuWa 57)
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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/
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Synthesis of the 1980s Lower Susitna River Segment
Aquatic Habitat Information
2012 Study Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech, Inc.
March 2013
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i March 2013
TABLE OF CONTENTS
1. Introduction ........................................................................................................................1
2. Study Objectives.................................................................................................................1
3. Study Area ..........................................................................................................................2
3.1. General ...............................................................................................................2
3.2. Specific Study Area ...........................................................................................2
4. Methods ...............................................................................................................................3
4.1. Deviations from Study Plan ...............................................................................3
4.2. Summary of 1980s Studies ................................................................................3
4.3. Hydrologic Information .....................................................................................3
4.3.1. Average Monthly Flows for the Susitna River at Sunshine ............ 3
4.3.2. Weekly Flows for the Susitna River at Sunshine............................ 4
4.3.3. Monthly Stage Change for the Susitna River at Sunshine .............. 4
4.4. Habitat Area versus Flow Relationship .............................................................4
4.5. Tributaries ..........................................................................................................5
4.5.1. Tributary Mouth Backwater Areas ................................................. 5
4.5.2. Tributary Access ............................................................................. 6
4.5.3. Tributary Mouth Stability ............................................................... 6
5. Results .................................................................................................................................6
5.1. Summary of 1980s Information .........................................................................6
5.1.1. Response of Aquatic Habitat Surface Area to Mainstem Discharge
Relationships in the Yentna to Talkeetna Reach of the Susitna
River (R&M Consultants and Trihey & Associates 1985a) ........... 7
5.1.2. Assessment of Access by Spawning Salmon into Tributaries of the
Lower Susitna River (R&M Consultants and Trihey & Associates
1985b) ............................................................................................. 8
5.2. Synthesis of 1980s Information with Current Project Flow Changes .............12
5.2.1. Habitat Area versus Flow Relationships ....................................... 12
5.2.2. Tributaries ..................................................................................... 13
6. Summary and Conclusions .............................................................................................15
6.1. Habitat Area versus Flow Relationships ..........................................................15
6.2. Tributaries ........................................................................................................16
6.3. 2013-2014 Lower River Studies ......................................................................16
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page ii March 2013
7. References .........................................................................................................................17
8. Tables ................................................................................................................................19
9. Figures ...............................................................................................................................27
LIST OF TABLES
Table 4.3-1. Comparisons of median monthly discharge (cfs) for pre-Project and Max LF OS-1
Conditions for the Susitna River at Sunshine. .......................................................20
Table 4.3-2. Comparison of monthly stage exceedences (ft) for the pre-Project and Max LF
OS-1 Conditions for the Susitna River at Sunshine.. .............................................21
Table 5.1-1. Summary of potential effects on the with-Project flows on tributaries of the Lower
Susitna River, reproduced from Table 4.1 of R&M Consultants and Trihey &
Associates (1985b). ................................................................................................22
Table 5.2-1. SC IV-4 accumulated wetted surface areas (ft2x103) for each habitat type for the
Open water period (May-September) and Ice Affected period (October-April) for
both the pre-Project and Maximum Load Following OS-1 conditions..................23
Table 5.2-2. Willow Creek accumulated wetted surface areas (ft2x103) for each habitat type for
the Open water period (May-September) and Ice Affected period (October-April)
for both the pre-Project and Maximum Load Following OS-1 conditions..... .......24
Table 5.2-3. Goose Creek accumulated wetted surface areas (ft2x103) for each habitat type for
the Open water period (May-September) and Ice Affected period (October-April)
for both the pre-Project and Maximum Load Following OS-1 conditions..... .......25
Table 5.2-4. Accumulated wetted surface areas for tributary mouth habitat for the salmonid
spawning period (June-September) for Willow Creek and Goose Creek. .............26
LIST OF FIGURES
Figure 3.2-1. Susitna River Geomorphology Study Area and Large-scale River Segments .......28
Figure 4.5-1. Comparison of median weekly discharge for the Susitna River at Sunshine
for the Natural and 2020 conditions presented in Figure 4.2 of R&M Consultants
and Trihey & Associates (1985b) and the pre-Project and Maximum Load
Following OS-1 conditions assessed by the 2012 Study. ......................................29
Figure 5.1-1. Wetted surface area (ft2 x 103) response to Susitna River discharge at
Sunshine by habitat type for Willow Creek. Reproduced from R&M
Consultants and Trihey & Associates (1985a).......................................................30
Figure 5.2-1. Accumulated wetted surface area (ft2x103) computed for SC IV-4 for the
Open Water period (May-September) and the Ice Affected period
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iii March 2013
(October-April) for the median monthly Susitna River discharge at Sunshine
under the pre-Project and Maximum Load Following OS-1 conditions for each
habitat type. ........................................................................................................ 31
Figure 5.2-2. Accumulated wetted surface area (ft2x103) computed for Willow
Creek for the Open Water period (May-September) and the Ice Affected
period (October-April) for the median monthly Susitna River discharge at
Sunshine under the pre-Project and Maximum Load Following OS-1
conditions for each habitat type. ...........................................................................32
Figure 5.2-3. Accumulated wetted surface area (ft2x103) computed for Goose Creek
for the Open Water period (May-September) and the Ice Affected period
(October-April) for the median monthly Susitna River discharge at Sunshine
under the pre-Project and Maximum Load Following OS-1 conditions for
each habitat type. .......................................................................................... 33
Figure 5.2-4. Accumulated wetted surface area (ft2x103) computed over the salmon
migration period (June-September) for the median monthly Sustina River
discharge at Sunshine presented for the Tributary Mouth habitat type for the
pre-Project and Maximum Load Following OS-1 conditions at Willow
Creek and Goose Creek.... .....................................................................................34
APPENDICES
Appendix 1. Lower River Habitat Type Area Tables
Appendix 2. Lower River Habitat Type Area Bar Charts
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iv March 2013
LIST OF ACRONYMS AND SCIENTIFIC LABELS
Abbreviation Definition
1-D One-Dimensional
AEA Alaska Energy Authority
ER Entrenchment Ratio
FERC Federal Energy Regulatory Commission
ft/mile Feet per Mile
IFS Instream Flow Study
ILP Integrated Licensing Process
LiDAR Light Detection and Ranging-based Topography
LR Lower Susitna River Segment
MC Multiple Channel Reach Classification
mi mile(s)
mm millimeter(s)
MR Middle Susitna River Segment
NEPA National Environmental Policy Act
PDF Portable Document Format
PRM Project River Mile (the current, Susitna-Watana Project river-mile system)
RM River Mile (the 1980s Project river-mile system)
SC Single Channel Reach Classification
UR Upper Susitna River Segment
USGS United States Geological Survey
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page v March 2013
Summary
This effort was designed to identify whether potential Project effects on aquatic habitat and
tributary access in the Lower River warranted additional study and, if necessary, help in planning
those studies. The analysis utilized information on aquatic habitat from the 1980s report
Response of Aquatic Habitat Surface Area to Mainstem Discharge Relationships in the Yentna to
Talkeetna Reach of the Susitna River (R&M Consultants, Inc. and Trihey & Associates. 1985a).
Information was also summarized from the report Assessment of Access by Spawning Salmon
into Tributaries of the Lower Susitna River (R&M Consultants, Inc. and Trihey & Associates.
1985b).
This effort also used results of two other 2012 technical memorandums. The first was the Stream
Flow Assessment (Tetra Tech 2013a) which was used to identify whether the pre- and post-
Project hydrology were sufficiently similar in the 1980s studies to the current Project to allow for
application of the results of the 1980s efforts to planning for the 2013-2014 studies. The Stream
Flow Assessment also provided the hydrology to evaluate habitat areas for current pre- and post-
Project conditions. The second technical memorandum utilized was Mapping of Aquatic
Macrohabitat Types at Selected Sites in the Middle and Lower Susitna River Segments from
1980s and 2012 Aerials (Tetra Tech 2013c). This information was used to assess geomorphic
change at tributary mouths over the past 30 years.
Results of the comparison of the pre- and Post-Project hydrology used in the 1980s reports and
for the current Project indicated they are very similar for the adult salmon migration period of
May through September. Based on this finding, it was concluded that the potential impacts
identified for the 13 tributaries studied in the 1980s relative to salmon spawning access were
applicable to planning 2013-2014 studies. The earlier study identified the potential for a
moderate reduction in backwater (holding) area at nine out of the 13 tributaries studied. The
other four tributaries were identified as having the potential for a slight reduction in tributary
mouth backwater habitat area. Applications of the current pre- and post-Project hydrology and
habitat versus flow relationships from the 1980s indicated the potential for 19% to 26% average
reduction in the holding area at the mouths of the two tributaries evaluated. Of the 13 tributaries
studied, four were identified as potentially having problems associated with access by migrating
salmon due to reduction in flow depths for the post-Project condition.
In terms of aquatic macrohabitat, application of the habitat area versus flow relationships for the
main channel and lateral habitats indicated that the post-Project change in flows would alter
habitat area. Over the three sites and eight habitat types assessed, 65 percent of the results
showed reduction in wetted surface area under the post-Project condition. Three of the habitat
types, main channel, secondary side channel, and tributary mouth showed a decrease at all three
sites. The remaining habitat types indicated some instances of increases and some instances of
decreases in wetted surface area under post-Project conditions. Based on these results, the further
study of potential Project-related changes to aquatic habitat and tributary access conditions in the
Lower River is warranted.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 1 March 2013
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 (Project) using the Integrated Licensing Process (ILP). The Project is located on the
Susitna River, an approximately 300-mile-long river in the South-Central 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 that will serve as the basis for the 2013-14 formal study program,
Exhibit E of a license application, and FERC’s National Environmental Policy Act (NEPA)
analysis for the Project license.
This technical memorandum provides the results of Synthesis of the 1980s Aquatic Habitat
Information subtask of the Riverine Habitat-Flow Relationship Assessment task within the 2012
study Reconnaissance-Level Geomorphic and Aquatic Habitat Assessment of Project Effects on
Lower River Channel (G-S4). This effort summarizes information on aquatic habitat from the
1980s report Response of Aquatic Habitat Surface Area to Mainstem Discharge Relationships in
the Yentna to Talkeetna Reach of the Susitna River (R&M Consultants, Inc. and Trihey &
Associates 1985a). Information is also summarized from the report Assessment of Access by
Spawning Salmon into Tributaries of the Lower Susitna River (R&M Consultants, Inc. and
Trihey & Associates 1985b). The information relating aquatic macrohabitat and fish access to
flow and river stage in these reports are used along with the results of the 2012 technical
memorandum Stream Flow Assessment (Tetra Tech 2013a) to perform an initial identification of
potential Project effects in the Lower Susitna River Segment.
The pre-Project hydrology and post-Project hydrology under an operations scenario referred to as
Maximum Load Following Operation Scenario 1 (OS-1) are used to perform an initial
identification of potential Project effects. These two hydrologic scenarios were analyzed in detail
in Tetra Tech (2013a). The Maximum Load Following OS-1 hydrology is a simulated flow
record developed with the Project-conditions flow-routing model (MWH 2012) for the same 61-
year period as the pre-Project records. Maximum Load Following OS-1 is based on the
assumption that the load fluctuation of the entire Railbelt would be provided by the Susitna-
Watana Project, and all other sources of electrical power in the Railbelt would be running at base
load. This assumed condition is not realistic for an entire year, and the results of this condition
should be conservative with respect to assessing downstream impacts of load following.
2. STUDY OBJECTIVES
The effort described in this Technical Memorandum integrates the results of several earlier tasks
in the 2012 Reconnaissance-Level Geomorphic and Aquatic Habitat Assessment of Project
Effects on Lower River Channel Study (G-S4) to address several broader study objectives
including:
Assess potential changes to aquatic habitat pre- and post-Project;
Provide information to assist AEA and licensing participants to develop the 2013-2014 study
plans.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 2 March 2013
3. STUDY AREA
3.1. General
The Susitna River, located in south-central 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 Fork Susitna glaciers to the Cook Inlet (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 elevations in the basin are
at Mt. McKinley at 20,320 feet while its lowest elevations are 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 that 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 29oF.
3.2. Specific Study Area
For the Susitna-Watana Hydro Project licensing effort the Susitna River from Project River Mile
(PRM) 3.3 at Cook Inlet to the Maclaren River confluence at PRM 261.3, the river has been
subdivided into three segments (Tetra Tech 2013b) 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. The study effort presented in this Technical
Memorandum is concentrated on the Lower Susitna River Segment. The three segments are
identified on Figure 3.2-1 with the associated extents:
Upper Susitna River Segment: Maclaren River confluence (PRM 261.3) downstream to the
proposed Watana Dam site (PRM 187.1),
Middle Susitna River Segment: Proposed Watana Dam site (PRM 187.1) downstream to the
Three Rivers Confluence (PRM 102.4), and
Lower Susitna River Segment: Three Rivers Confluence (PRM 102.4) downstream to Cook
Inlet (PRM 3.3).
The upstream-most segment, referred to as the Upper River (UR), extends from PRM 261.3 to
PRM 187.1 at the Watana Dam site. The morphologic characteristics of this segment of the river
are dominated by the products of Quaternary-age glaciation. The Middle River (MR) segment
extends from the Watana Dam site to the Three Rivers Confluence at about PRM 102.4. The
general characteristics of the river in this segment are heavily influenced by bedrock outcrop as
well as Quaternary-age glaciations. The Lower River (LR) segment extends from the Three
Rivers Confluence (PRM 102.4) to the tidal flats at Cook Inlet (PRM 3.3). The morphologic
characteristics of the river in this segment are dominated by the 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.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 3 March 2013
4. METHODS
4.1. Deviations from Study Plan
There were no deviations from the 2012 study plan for Synthesis of the 1980s Lower Susitna
River Segment Aquatic Habitat Information presented as part of the 2012 study plan for the
Reconnaissance Level Assessment of Potential Channel Change in the Lower Susitna River
Segment G-S4.
4.2. Summary of 1980s Studies
In accordance with the 2012 study plan for this task, the two previous studies by R&M
Consultants and Trihey & Associates (1985a and 1985b) were obtained and summarized:
Response of Aquatic Habitat Surface Area to Mainstem Discharge Relationships in the
Yentna to Talkeetna Reach of the Susitna River.
Assessment of Access by Spawning Salmon into Tributaries of the Lower Susitna River.
The documents were downloaded as electronic documents in the Portable Document Format
(PDF) from the Alaska Resources Library & Information Service (ARLIS) online library. These
studies compared the surface area for aquatic habitat types for conditions prior to the
construction of the Susitna Hydroelectric Project and estimated for post-construction operation
conditions (R&M Consultants and Trihey & Associates 1985a) and evaluated Lower Susitna
River tributary access for spawning salmon (R&M Consultants and Trihey & Associates 1985b).
The tributary access PDF contains 66 pages, and includes 36 figures, 11 photographs, and five
tables; and, the habitat surface area PDF contains 201 pages, and includes 15 figures, 49
additional photographs included in two Exhibits, and five tables.
4.3. Hydrologic Information
As part of the larger 2012 study, hydrologic information was developed for completion of the
technical memorandum, Stream Flow Assessment (Tetra Tech 2013a). This information included
the probability of exceedence for weekly discharge for the Susitna River at Sunshine for both the
pre-Project and Maximum Load Following Operation Scenario 1 (OS-1). The hydrologic data
developed for the months of May through September are utilized in this technical memorandum
for comparison with R&M Consultants and Trihey & Associates (1985b) as part of the Tributary
Access analysis.
4.3.1. Average Monthly Flows for the Susitna River at Sunshine
Average monthly flows for the Susitna River at Sunshine for the pre-Project conditions were
developed from 61 years of extended record developed by the USGS (2012). Results from the
Maximum Load Following OS-1 simulations were utilized to develop average monthly flows for
this scenario and are presented in Table I-3 of the Stream Flow Assessment technical
memorandum (Tetra Tech 2013a). Changes in discharge for the Maximum Load Following OS-1
scenario are indicated by comparing the monthly 10-, 50-, and 90-percent probability of
exceedence discharge values to those of the pre-Project conditions. For the months of June, July,
and August the discharges decrease for all three exceedence probabilities for the Maximum Load
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 4 March 2013
Following OS-1 case. For the months of May and September, the 90-percent probability of
exceedence discharge values increase for the Maximum Load Following OS-1 scenario
compared to the pre-Project conditions, but decrease for the 10- and 50-percent probabilities of
exceedence. Increases for the Maximum Load Following OS-1 scenario are indicated for each of
these three probabilities of exceedence for the months of October, November, December,
January, February, March, and April. The median monthly values are used in this report to
determine wetted surface area associated with the aquatic macrohabitat types developed by R&M
Consultants and Trihey & Associates (1985a) for the sites Side Channel Complex IV (SC IV-4),
Goose Creek, and Willow Creek. The monthly median flow values for the Susitna River at
Sunshine for the pre-Project and Maximum Load Following OS-1 conditions are presented in
Table 4.3-1.
4.3.2. Weekly Flows for the Susitna River at Sunshine
The 61 years of daily flows for the pre-Project and Maximum Load Following OS-1 conditions
for the Susitna River at Sunshine were further processed to provide weekly 10-, 50-, and 90-
percent probability of exceedence discharge values for the months of May through September.
These months were picked since they represent the primary period of adult salmon migration in
the Lower Susitna River (R&M Consultants and Trihey & Associates 1985b). The weekly values
were used to extrapolate the Project effects for tributary access indicators from the 1980s study
to the current Project based on the similarity of pre- and post-Project hydrology in both studies.
4.3.3. Monthly Stage Change for the Susitna River at Sunshine
The Stream Flow Assessment technical memorandum (Tetra Tech 2013a) includes analysis of
probabilities of exceedance for monthly stage values at Sunshine for the pre-Project and
Maximum Load Following OS-1 conditions. These values are summarized for the 10-, 50-, and
90-percent probabilities of exceedence for the months of May through September. These values
were compared to the results of main channel discharge changes on tributary backwater area
presented by R&M Consultants and Trihey & Associates (1985b). The monthly stage
exceedance values at Sunshine are presented in Table 4.3-2.
4.4. Habitat Area versus Flow Relationship
The wetted surface area relationships developed by R&M Consultants and Trihey & Associates
(1985a) were used for the analysis of pre-Project and Maximum Load Following OS-1
conditions presented here. This was accomplished by log-linearly interpolating wetted surface
areas for three sites, SC IV-4, Goose Creek, and Willow Creek, using the median monthly
discharge values at Sunshine (Table 4.3-1). In order to expand the range of the discharge and
wetted surface area relationships beyond the minimum discharge of 13,900 cfs and maximum
discharge 75,200 cfs that were used by R&M Consultants and Trihey & Associates (1985a), the
following assumptions were applied:
Wetted surface areas were assumed to be remain at zero if they were zero for the bounding
discharge values of 13,900 and 75,200 cfs;
Wetted surface areas were assumed to remain constant at the bounding discharge value of
13,900 and 75,200 cfs, if the relationship indicated that wetted surface areas were increasing
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 5 March 2013
with decreasing discharge from 21,100 to 13,900 cfs or were increasing with increasing
discharge from 59,100 to 75,200 cfs; and
Wetted surface areas were assumed to decrease log-linearly for discharge values less than
13,900 cfs and more than 75,200 cfs, if the previous relationship indicated that wetted
surface areas were decreasing with decreasing discharge from 21,100 to 13,900 cfs or were
decreasing with increasing discharge from 59,100 to 75,200 cfs.
The monthly results were used to aggregate the wetted surface areas associated with each of the
habitat types for the pre-Project and Maximum Load Following OS-1 conditions for specific
periods. Two periods were used, with the first representing the general open-water period
accumulated from for May through September, and the second representing the general ice-
affected period accumulated from October through April. It is cautioned that the results
presented during the ice-affected months are relative indicators only since the habitat
relationships of R&M Consultants and Trihey & Associates (1985a) are based on open-water and
the associated hydraulics that help determine the areas are likely substantially different for
periods with ice cover. Therefore, the ice-affected period values can be used for relative
comparison between the pre-Project and Maximum Load Following OS-1 conditions but should
not be used to identify absolute differences in wetted surface areas between conditions.
4.5. Tributaries
Three aspects of the clearwater tributaries were investigated by R&M Consultants and Trihey &
Associates (1985b) including tributary backwater surface area, fish access into tributaries, and
morphologic stability of tributary mouth access for migrating salmon. These three aspects were
utilized in the current study to assess the pre-Project and Maximum Load Following OS-1
conditions. The methods used previously and for the current study for each of these three
indicators of access changes are described here.
4.5.1. Tributary Mouth Backwater Areas
The differences in median weekly discharges were used by R&M Consultants and Trihey &
Associates (1985b) to describe potential changes in water depth and potential subsequent
reductions to backwater areas for tributaries identified as having relatively low stream gradients.
The 1980 tributary information along with the 2012 Stream Flow Assessment were applied to
make an initial determination of the potential current Project effects on tributary backwater areas.
This was accomplished by evaluating the similarities between the 50% exceedence (median)
weekly discharge for the pre-Project conditions and the post-Project conditions for the 1980s
evaluation and the current effort. Figure 4.2 of R&M Consultants and Trihey & Associates
(1985b) presents the pre-Project (referred to as “Natural” on the figure) and the post-Project
(Case E-VI, 2020 load) conditions. Figure 4.5-1 presents these 1980s results and current median
weekly flows for the pre- and post-Project conditions. The values are very similar and it is
concluded that for this initial assessment, tributary access effects related to flow from the 1980s
study can serve as a good starting point to identify impacts of the current Project for the purpose
of planning studies.
Applying the habitat area versus flow relationships developed in the 1980s (Section 4.4) for
tributary mouths, the potential impacts to tributary backwater areas were further quantified in the
current study. This quantification was achieved by combining the habitat flow relationships
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 6 March 2013
developed by R&M Consultants and Trihey & Associates (1985a) for the period salmon
migration from May through September, which was identified by R&M Consultants and Trihey
& Associates (1985b), and evaluating those relationships using the median weekly discharge for
the current Project effects. The results of this analysis are presented in Section 5.2.
4.5.2. Tributary Access
Tributary access conditions in R&M Consultants and Trihey & Associates (1985b) were
evaluated by applying depth criteria. Chinook salmon were used as the limiting condition as they
required a minimum depth of 0.8 feet compared to 0.6 for the other four species of salmon
(Thompson 1972). Depths at tributary mouth were considered for the identified salmon
migration period of May 13 through September 30. It was also found that the depth at the
tributary mouth depends on whether the tributary mouth was located directly on the mainstem or
on a side channel. In the case of a side channel, the depth at the mouth was highly dependent on
whether the head of the side channel was breached or unbreached. In both cases, the depth at the
tributary mouth was related to the flow in the Susitna River.
In order to determine if the assessment of Project effects on tributary access could be
extrapolated to the current Project for study planning purposes, the comparison of flows between
the 1980s pre- and post-Project conditions and the flows for the current pre- and post-Project
conditions was made. Review of the weekly median flows for the pre- and post-Project
conditions during the period of adult salmon migration in the Lower River used in the 1985
(R&M Consultants and Trihey & Associates 1985) report were compared against the weekly
median flows for the pre-Project and Maximum Load Following OS-1 conditions. The
comparison of the median weekly flows is provided in Figure 4.5-1. The plots indicate that both
the pre- and post-Project condition hydrology from the 1980s study and for the current Project
are very similar in terms of median weekly flows for the period from May through September.
Based on this information, the results of the access analysis from 1985 were considered to be
transferrable to the current Project for study planning purposes.
4.5.3. Tributary Mouth Stability
The 1980s effort evaluated the potential for Project effects on the stability of tributary mouths
based on the observed stability of the tributary mouth. It was assumed in R&M Consultants and
Trihey & Associates (1985b) that tributaries that were observed to be stable under the existing
conditions would be less subject to instabilities from the Project than tributaries that had
exhibited dynamic behavior under per-Project conditions. Interpretation of tributary mouth
morphologic stability for current conditions was accomplished by summarizing the results of the
comparative analysis of 1980s and 2012 aerial photographs described by Tetra Tech (2013c).
5. RESULTS
5.1. Summary of 1980s Information
Information from the two 1980s reports is summarized in this section. The reports summarized
are Response of Aquatic Habitat Surface Area to Mainstem Discharge Relationships in the
Yentna to Talkeetna Reach of the Susitna River (R&M Consultants and Trihey & Associates
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
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1985a) and Assessment of Access by Spawning Salmon into Tributaries of the Lower Susitna
River (R&M Consultants and Trihey & Associates 1985b).
5.1.1. Response of Aquatic Habitat Surface Area to Mainstem Discharge
Relationships in the Yentna to Talkeetna Reach of the Susitna River
(R&M Consultants and Trihey & Associates 1985a)
R&M Consultants and Trihey & Associates (1985a) identified eight sites in the Lower Susitna
River that were determined to be representative of the varied channel morphologies and the
habitat types mapped on aerial photography. Wetted surface area for each habitat type was
determined for five specific flows from Aerials acquired at flows of 75,200 cfs, 59,100 cfs,
36,600 cfs, 21,100 cfs and 13,900 cfs as measured at Sunshine. These sites were:
Side Channel Complex Segment IV (SC IV-4),
Willow Creek,
Caswell Creek,
Sheep Creek,
Goose Creek,
Montana Creek,
Sunshine Slough, and
Birch Creek Slough.
The wetted surface areas in each of the sites was classified by eight general aquatic habitat
categories that could be identified by physical characteristics, rather than specific biological
characteristics or geographical locations, from either aerial reconnaissance or examination of
aerial photography. The eight aquatic habitat categories were:
Mainstem1,
Primary Side Channel,
Secondary Side Channel,
Turbid Backwater,
Clearwater,
Side Slough,
Tributary Mouth, and
Tributary.
The descriptions for each of the habitat categories from R&M Consultants and Trihey &
Associates (1985a) are included here, with the change of Mainstem to Main Channel for
consistency with the Tetra Tech (2013c) terminology.
1 It should be noted that as part of the work conducted by Tetra Tech for the mapping of aquatic macrohabitat types (Tetra Tech
2013c) the habitat type associated with Mainstem was further defined and rephrased as Main Channel. The use of the term Main
Channel is continued within this report and corresponds to the R&M Consultants and Trihey & Associates (1985a) Mainstem
habitat type.
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Main Channel habitats represent the main river channel, consisting of the thalweg channel,
major sub-channels and alluvial island complexes. This habitat type was, in most cases,
outside the boundaries of the control areas used to define representative areas.
Primary Side Channel habitats are those channels which normally convey streamflow
throughout the entire year. They exhibit characteristics similar to middle Susitna River
main channel habitat types, as described by Klinger and Trihey (1984). They are
characterized by turbid glacial water, high velocities, and few mid channel gravel bars.
Secondary Side Channel habitats also have turbid water, but exhibit characteristics of the
middle river side channels. For example, there are middle channel gravel bars and riffles
or water surface features that indicate slower-moving, shallower water.
Turbid Backwater habitats are nonbreached channels containing turbid water. They have non-
vegetated upper thalwegs that are overtopped during periods of moderate to high main
channel discharge. They represent transitional habitat type between breached secondary
side channel habitats and nonbreached clearwater or side slough habitats.
Clearwater habitats are nonbreached channels containing clear water that dewater completely at
a main channel discharge of 13,900 cubic feet per second (cfs) or higher. These channels
have non-vegetated upper thalwegs that are overtopped during periods of moderate to
high main channel discharge. Groundwater and local surface runoff appear to supply
water to these areas at main channel flows above 13,900 cfs.
Side Slough habitats contain clear water. Upwelling and local surface runoff appear to supply
sufficient clear water to these areas to maintain wetted areas at a main channel discharge
of 13,900 cfs. Side Sloughs also have non-vegetated upper thalwegs that are overtopped
at moderate to high main channel discharges.
Tributary Mouth habitats are clear water habitats that exist between the downstream extent of a
clear water plume and upstream into the tributary, to the upper extent of the backwater
influence. The surface area depends on the discharge of both the tributary and main
channel.
Tributary habitat exists upstream of the tributary mouth habitat. In this analysis, tributary
habitat was measured only to the boundary of the digitized photo enlargement. Tributary
habitat may increase dramatically when the tributary flows into a nonbreached side
channel and the clear tributary flows through the side channel to join the Susitna River.
The wetted surface area in units of thousand square feet (ft2x103) were measured at each site and
for each habitat type for five Susitna River discharge measurements at Sunshine, including
13,900; 21,100; 36,600; 59,100; and 75,200 cfs. These values are presented in Table 3.3 and the
surface area response to discharge relationships are plotted in Figures 3.1 through 3.4 of R&M
Consultants and Trihey & Associates (1985a). The Willow Creek site information from Figure
3.1 is recreated in this report as Figure 5.1-1.
5.1.2. Assessment of Access by Spawning Salmon into Tributaries of the
Lower Susitna River (R&M Consultants and Trihey & Associates 1985b)
R&M Consultants and Trihey & Associates (1985b) identified clearwater tributaries to the
Lower Susitna River Segment as the primary salmon spawning areas within the Lower Susitna
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
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Basin and as such carried out analysis to evaluate potential Project impacts on fish access to
these clearwater tributaries. Their primary interest was the backwater areas at the tributary
mouths. The backwater areas near the mouths of the clearwater tributaries provides lower
velocity areas for migrating salmon to hold and rest. The mouths were also the areas that Project-
related changes in water surface elevations in the mainstem could affect depths in the tributary
mouth and consequently fish access to the tributaries. The 1980s analysis focused on the
potential impacts during the months of June through September, as these months comprise the
typical period during which adult salmon migrate within the Lower River. Their data indicated
peak migration occurs in July and August for sockeye, chum, pink and coho salmon and June to
early July for Chinook. Three clearwater tributar y evaluation categories were investigated by
R&M Consultants and Trihey & Associates (1985b):
Tributary backwater surface area,
Fish access into tributaries, and
Morphologic stability of tributary mouth access for migrating salmon.
R&M Consultants and Trihey & Associates (1985b) assessed possible impacts of with-project
conditions on the access of spawning salmon to the tributaries of the Lower Susitna River. The
five Pacific salmon species identified as important to the commercial and sport fisheries of Cook
Inlet identified were Chinook (Ck), chum (Ch), sockeye (S), pink (P), and coho (Co), and the
associated combined migration period for all five of these salmon species was generally defined
as June through September. The three evaluation categories were applied to 13 tributaries,
including:
Alexander Creek,
Deshka River,
Willow Creek,
Lower Willow Creek,
Kashwitna River,
Caswell Creek,
Sheep Creek,
Goose Creek,
Montana Creek,
Rabideux Creek,
Sunshine Creek,
Birch Creek, and
Trapper Creek.
R&M Consultants and Trihey & Associates (1985b) generalized post-Project effects compared to
pre-Project seasonal discharge patterns for the Lower Susitna River as potentially increasing
discharge in late fall and winter seasons, decreasing discharge in the summer season, and
resulting in similar discharge in the spring and early fall seasons. These changes in discharge
patterns were summarized as potentially causing:
Decreased size of backwater areas for migrating fish resting and holding at tributary mouths,
Decreased water depth in tributary mouths that may prevent adult salmon access, and
Decreased morphologic stability for tributary mouths or adjoining side channels that may
inhibit tributary access.
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The results of R&M Consultants and Trihey & Associates (1985b) were summarized in Table
4.1 of that report, and this table is recreated in as Table 5.1-1. Additional discussions for each of
the three tributary access categories evaluated are presented below.
5.1.2.1. Tributary Mouth Backwater Area (Holding Area)
The tributary backwater areas serve as holding areas for adult salmon. The low gradient
tributaries were identified as having the most extensive backwater areas. Caswell Creek and
Sheep Creek were selected in the 1985 study as representative of low gradient tributary mouth
backwater areas and response curves of water depth to mainstem discharge were developed.
Time series plots of water depth versus 50-percent exceedence weekly pre-Project and post-
Project (Case E-VI , 2020 load) were developed to identify the degree of change in water depth
caused by Project flows. This category was assessed on a semi-qualitative basis with ranked
effects for tributary mouth backwater areas rated as moderate change and slight change.
From the analysis several observations were made in R&M Consultants and Trihey & Associates
(1985b) on the potential Project effects on the tributary mouth holding areas. The report
indicated Project effects vary depending on the season, precipitation, the channel gradient and
the location of the channel relative to the mainstem. The reduction in surface area of tributary
mouth backwaters was identified as primarily due to a reduction in the length that the backwater
extends upstream. As the mainstem stage drops, the location where the tributary flow meets the
backwater moves further downstream in the tributary.
Of the 13 tributaries, four were rated as slight change and nine were rated as moderate change as
to the potential for reduction in backwater area at the tributary mouth for post-Project conditions.
The four tributaries assigned a slight change rating were Montana Creek, Goose Creek, Willow
Creek and the Kashwitna River. The nine tributaries assigned a moderate change rating were
Trapper Creek, Birch Creek, Sunshine Creek, Rabideux Creek, Sheep Creek, Caswell Creek,
Little Willow Creek, Alexander Creek and the Deshka River.
Based on the time series plots of water depth in R&M Consultants and Trihey & Associates
(1985b), for June post-Project flows the reduction in the depth for the holding area at Caswell
Creek was estimated to be 4.5 to 5.7 feet in the last week in June and 3.5 to 4.1 feet in the last
week of August. At Sheep Creek the decreases in post-Project holding area depth was predicted
to be 4.0 to 5.4 feet in the last week of June and 3.2 to 3.8 feet in the last week of August.
5.1.2.2. Tributary Access
R&M Consultants and Trihey & Associates (1985b) determined by field observations that
tributary access would not be impeded by velocity but could be impeded by depth and channel
stability. Depth criteria developed for each species of salmon was used to compare with field
observed water depths in tributary mouths, which were noted to depend on the discharge of both
the tributary and main channel Susitna River. The report further caveated the measurements for
tributary water depths with respect to main channel discharge as being collected during a period
of below average tributary flow. Thus, the water depth analysis provided accounts mostly for the
impact of main channel discharge on tributary mouth water depths and in turn may have
overestimated decreases in depth associated with the post-Project conditions. The study indicated
that during the adult salmon migration period (May 13 through September 30) water depths in
tributary mouths would equal or exceed 0.8 feet, the minimum depth for adult Chinook passage
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
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in Thompson (1972). The depth criterion for the other four species of salmon was identified as
0.6 feet and less restrictive.
For tributary mouths that connected to side channels of the main channel of the Susitna River
rather than directly to the main channel of the Susitna River, discharges measured at Sunshine
were compared with the elevation needed to provide breached connection between the main
channel and the tributary mouth. Changes in backwater depth or side channel breaching flows
were used to determine potential Project effects on tributary access by adult salmon in the 1985
study. The discharge of 21,100 cfs at Sunshine was selected as the baseline since it corresponded
to the discharge on the date that the aerial photographs, used in the assessment, were acquired.
This category was assessed on a semi-qualitative basis with ranked effects for fish access into
tributaries at 21,100 cfs rated as “possible problem” and “no problem” based on whether a
minimum depth of 0.8 feet was met for a flow of 21,100 cfs.
Goose Creek and Trapper Creek were the only tributaries with minimum water depth at or below
the passage criteria in Thompson (1972) at 21,100 cfs based on the text of R&M Consultants and
Trihey & Associates (1985b). Each of the tributaries flowed into a side channel that had a berm
at its head that was unbreached at 21,100 cfs. The Goose Creek side channels breached at 22,000
cfs and the Goose Creek at 44,000 cfs. For the flows less than the breaching flows, the depths in
the backwater holding areas were determined by the tributary flow. The study indicated that
during the salmon migration period (May 13 through September 30) water depths in these
tributary mouths would equal or exceed 0.8 feet, the minimum depth for adult Chinook passage
in Thompson (1972). Table 4.1 in R&M Consultants and Trihey & Associates (1985b) also
indicates potential problems for Caswell and Montana Creek under the post-Project condition
and 21,100 cfs.
5.1.2.3. Tributary Mouth Morphologic Stability
Tributary mouth morphologic stability from the years 1951 to 1983 was determined by R&M
Consultants and Trihey & Associates (1985b) by comparing aerial photographs of the tributary
mouths for relative change at the tributaries evaluated. This category was assessed on a semi-
qualitative basis which rated morphologic stability of tributary mouths for pre-Project conditions
from “good” meaning no change in tributary mouth morphology since 1951, to “fair” meaning
some change in tributary mouth morphology since 1951, and to “poor” meaning change in
tributary mouth morphology from 1951 to present. Post-Project stability received the same or
slightly higher morphologic stability rating based on the reasoning that the Project would not
change tributary hydrology or sediment supply and the amount of sediment deposition would
remain similar to the pre-Project condition; it would just occur further downstream.
None of the results presented for morphologic stability of tributary mouths by R&M Consultants
and Trihey & Associates (1985b) indicated that post-Project conditions would cause a decrease
in stability. Of the 13 tributaries, only one, Montana Creek, was rated as poor for tributary
mouth morphologic stability under pre-Project conditions. Five were rated as good: Alexander
Creek, Willow Creek, Little Willow Creek, Sheep Creek and Sunshine Creek. The remaining
seven tributary mouths were rated as having fair morphologic stability for the pre-Project
condition. The results provided for five of the tributary mouths suggested that the post-Project
conditions would result in improved stability. These tributaries were Caswell Creek, Goose
Creek, Montana Creek, Rabideux Creek, and Birch Creek. However, these results were based on
comparison of aerial photographs only and not on detailed geomorphic assessment of each
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AQUATIC HABITAT INFORMATION
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tributary and the impacts Project-related changes in main channel Susitna River discharge and
associated stages.
5.2. Synthesis of 1980s Information with Current Project Flow
Changes
The information from the 1980s habitat flow relationships in the Lower Susitna River (R&M
Consultants and Trihey & Associates 1985a) and the assessment of tributary access (R&M
Consultants and Trihey & Associates 1985b) was applied along with the changes in flows and
river stages that were developed for the current Project in the stream flow assessment (Tetra
Tech 2013a) to make initial assessments of potential effects of the current Project.
5.2.1. Habitat Area versus Flow Relationships
The analysis presented uses the previously developed habitat flow relationships (R&M
Consultants and Trihey & Associates 1985a) for the purpose of comparing the pre-Project and
Maximum Load Following OS-1 conditions. This was accomplished for SC IV-4, Goose Creek,
and Willow Creek by following the methodology described in Section 4.4. These three sites
were selected from the eight sites with habitat versus flow relationships for several reasons. SC
IV-4 was selected since it was the only site without a tributary. Willow Creek was selected since
it had the largest tributary mouth backwater area (784,000 sq. ft.), was indicated to not have a
possible access problem and was rated as having good morphologic stability in the 1980s study.
Goose Creek had a much smaller tributary mouth backwater area (maximum of 141,000 sq. ft.),
but it was indicated to possibly have an access problem in the 1980s report and to have only fair
morphologic stability. Between the two tributaries selected for further review under the current
post-Project conditions, a range of tributary mouth conditions were represented.
The results of the analysis for each of the 12 months of the year for each habitat type are
presented in Appendix 1 (Tables 1-1 to 1-3). The results of the analysis for the open water (May
through September) and ice affected (October through April) periods for each habitat type are
presented in Tables 5.2-1 to 5.2-3 (No primary side channel habitat was present at the three sites
for the range of flows investigated so it does not appear on any of the figures, but is identified as
have zero wetted surface area in the tables.). Results for each of the 12 months of the year are
also presented in Appendix 2 of this report as bar charts for each habitat type and the three sites
(Figures 2-1 to 2-7). Bar charts for the open water and ice affected results are presented in
Figures 5.2-1 to 5.2-3. Based on these results, a summary of the estimated change from the pre-
Project to the Maximum Load Following OS-1 conditions in terms of wetted surface area for
each habitat type during the open water and ice affected periods are as follows (The wetted
surface area for tributary mouth habitat is discussed in Section 5.2.2.):
Main Channel – Reductions in wetted surface area during the open water period were estimated
at each of the three sites, including a 3-percent reduction at SC IV-4, a 3-percent
reduction at Willow Creek, and a 26-percent reduction at Goose Creek. Conversely,
during the ice affected period an increase in wetted surface area is estimated for each of
the three sites, with an increase of 1 percent is indicated for SC IV-4 and 10 percent for
Willow Creek. During the ice affected period, there was no estimated wetted surface area
for Main Channel habitat for Goose Creek.
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Primary Side Channel – There was no estimated wetted surface area computed for primary side
channel habitat for either the pre-Project or Maximum Load Following OS-1 conditions
for the three sites during the open water period or the ice affected period.
Secondary Side Channel – Reductions were estimated for the open water period for SC IV-4 of
7.9 percent, for Willow Creek of 6.8 percent, and for Goose Creek of 12 percent.
Increases were estimated for the ice affected period of 44 percent for SC IV-4, 42 percent
for Willow Creek, and 47 percent for Goose Creek.
Turbid Backwater – Reductions in wetted surface area during the open water period were
estimated for SC IV-4 as 27 percent, however increases were estimated for both Willow
Creek, at 8%, and Goose Creek, at 5 percent. Increases were estimated during the ice
affected Period for Willow Creek as 148 percent and for Goose Creek as 93 percent.
During the ice affected period, there was no estimated wetted surface area for turbid
backwater habitat for SC IV-4.
Clearwater – Increases in wetted surface area during the open water period were estimated for
SC IV-4 as 43 percent, for Willow Creek as 10 percent, and for Goose Creek of 36
percent. During the ice affected period, no wetted surface area was predicted for the three
sites under the pre-Project conditions or the Maximum Load Following OS-1 conditions.
Side Slough – During the open water period, no wetted surface area was predicted for SC IV-4
or Willow Creek, but for Goose Creek a decrease of 6 percent was predicted. During the
ice affected period reductions in wetted surface area were predicted for each of the three
sites, as 14 percent for SC IV-4, 14 percent for Willow Creek, and 2 percent for Goose
Creek.
Tributary - No wetted surface area was predicted for SC IV-4 for the pre-Project or Maximum
Load Following OS-1 conditions for either the open water or ice affected periods. For the
open water period, an increase in wetted surface area was predicted for Willow Creek of
14 percent, but a decrease was predicted for Goose Creek of 5 percent. For the ice
affected period, reductions were predicted for both Willow Creek and Goose Creek of 1
and 5 percent, respectively.
5.2.2. Tributaries
The results of R&M Consultants and Trihey & Associates (1985b) assessment of access by
spawning salmon in the Lower River tributaries were summarized in Table 5.1-1. Comparison
of the median weekly flows during the adult salmon migration period, May through September,
was made for both the pre- and post-Project conditions for the 1980s study and current Project
(Figure 4.5-1). It was determined that both the pre-Project and post-Project hydrology were
sufficiently similar that the evaluations for the 1980s report were applicable to the current effort
for use in study planning. The results from the 1980s evaluation of the adult salmon access issue
were previously presented in section 5.1.2 including summary Table 5.1-1 (reproduced from
Table 4.1 of the 1980s report) and will not be repeated in this section. Two additional analyses
were conducted to further address the potential effects of the current Project on access related
conditions at the tributary mouths and are presented under their evaluation categories.
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5.2.2.1. Tributary Mouth Backwater Area (Holding Area)
Holding areas used by migrating adult salmon and associated with the Tributary Backwater area
were evaluated by R&M Consultants and Trihey & Associates (1985b) at Caswell Creek and
Sheep Creek by comparing anticipated decreases in water depths associated with the with-project
discharge. For Caswell Creek, a decrease of 1.2 feet during the last week of June and of 0.6 feet
during the last week of August was estimated. For Sheep Creek, a decrease of 1.4 feet during the
last week of June and of 0.6 feet during the last week of August was estimated. The stage
analysis performed by Tetra Tech (2013a) for the Susitna River at Sunshine comparing the pre-
Project and Maximum Load Following OS-1 conditions estimated similar decreases in stage
(Table 4.3-2), including for the median monthly discharge a decrease of 1.43 feet in June and of
0.67 feet in August. R&M Consultants and Trihey & Associates (1985b) concluded that these
differences would not negatively impact the holding areas of these two tributary mouths.
In order to further estimate potential Project changes to the backwater holding area at the
tributary mouths, the wetted surface areas computed for Willow Creek and Goose Creek in
Section 5.2.1 of this report were summed for the salmon migration period of May through
September for the Tributary Mouth habitat type. These accumulated results are presented in
Table 5.2-2 and Figure 5.2-2. Reductions in the tributary mouth wetted surface area were
estimated for both Willow Creek and Goose Creek at 26 percent (from 2,286 ft2x103 to 1,697
ft2x103) and 19 percent (from 405 ft2x103 to 329 ft2x103) respectively (Table 5.2-2).
5.2.2.2. Tributary Mouth Morphologic Stability
Tributary mouth morphologic stability from the years 1951 to 1983 was determined by R&M
Consultants and Trihey & Associates (1985b) b y comparing aerial photographs of the tributary
mouths for relative change at the tributaries evaluated. None of results presented for morphologic
stability of tributary mouths by R&M Consultants and Trihey & Associates (1985b) indicated
that with-project conditions would cause a decrease in stability. The results provided for five of
the tributary mouths suggested that the with-project conditions would result in improved
stability. However, these results are based on comparison of aerial photographs only and not on
detailed geomorphic assessment of each tributary and the impacts alternative main channel
Susitna River discharge.
R&M Consultants and Trihey & Associates (1985b) indicated that the tributary mouth of Willow
Creek had no change in morphology between 1951 and 1983, and subsequently classified
Willow Creek as good for both pre- and -Project conditions. Additionally, it was noted that the
mouth of Willow Creek discharges to a side channel, as opposed to directly discharging to the
main channel, of the Lower Susitna River. The bulk of the flow in the Susitna River at the
Willow Creek site had run along the western edge of this site in 1983. In 2012, the majority of
the flow passed through a branch west of the site, so the area classified as main channel in 1983
was classified as secondary side channel in 2012. Though the site has changed since 1983, the
tributary mouth has remained stable. However, as is the case at nearly all the tributary mouths,
migration of the main channel has the potential to significantly change the characteristics of the
tributary mouth area.
The morphologic stability of the tributary mouth for Goose Creek was identified by R&M
Consultants and Trihey & Associates (1985b) as being fair under pre-Project conditions and
Fair/Good under post-Project conditions, indicating that some changes were observed between
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
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FERC Project No. 14241 Page 15 March 2013
1951 and 1983. Comparison of aerial photographs from 1983 and 2012 indicate that the tributary
mouth area for Goose Creek has increased by 29% for a mainstem discharge of 36,600 cfs (1983
area = 52,000 ft2, 2012 area = 68,000 ft2) indicating some changes may have occurred at the
tributary mouth. Inspection of the aerials reviews more significant change. In the 1983 aerials,
the mouth was considered to be at the upstream end of the site where the main channel flowed
along the tributary mouth. In the intervening 30 years vegetated islands now separate the mouth
of Goose Creek from the main channel. As a result, the mouth of Goose Creek is now in a
secondary side channel. In contrast, 3,000 feet downstream, the main channel migrated 400 feet
into the site since 1983. In 1983 Goose Creek split with the majority of the going south and
discharging into a secondary side channel with a smaller portion taking a much shorter path and
discharging into the Susitna River in the main channel at the north end of the site. However,
migration of either the main channel or the subsequent side channel at the north end of the site
resulting in the capture of the majority of the flow by side channel at the north end of the site.
Currently, the tributary mouth is on a completely different side channel than in 1983, over 3,000
feet to the north. The side channel that Goose Creek flowed into in 1983 has evolved into a side
slough. This illustrates the potential for the migration of the main channel to affect the conditions
at a tributary mouth.
6. SUMMARY AND CONCLUSIONS
This technical memorandum summarizes the analysis performed to assess potential changes to
aquatic habitat for pre-Project and Maximum Load Following OS-1 conditions, and to provide
information to AEA and licensing participants to assist with the development of the 2013-2014
study plans. This analysis has been accomplished by reviewing assessment work performed
previously for the effects of hydropower operations on the Susitna River by R&M Consultants
and Trihey & Associates (1985a and 1985b). These previous studies quantified the wetted
surface area versus flow relationships for eight habitat types and developed criteria to compare
tributary access by migrating salmon for potential post-Project changes to Susitna River
discharge.
6.1. Habitat Area versus Flow Relationships
The analysis presented for the three selected sites, SC IV-4, Willow Creek, and Goose Creek in
this technical memorandum indicates that there is a potential for impacts for the Maximum Load
Following OS-1 conditions to the wetted surface areas of the eight aquatic macrohabitat types
assessed, main channel, primary side channel, secondary side channel, turbid backwater,
clearwater, side slough, tributary mouth, and tributary. Results are presented for the open water
period and the ice affected period, which has less certainty since the efforts conducted do not
account for the changes in hydraulics and wetted area that would result from ice cover.
For the three selected sites and eight habitat types, a total of 17 results were possible for the open
water period, and there were seven combinations of site and habitat type that had no aquatic
habitat for the open water period discharge evaluated using the habitat area versus flow
relationships. Of the 17 possible results, 11 indicated potential decreases in wetted surface area,
which constitutes 64% of the possible results. Habitat types that indicated potential decreases in
wetted surface area for all sites that had habitat present of the t ype being considered for the
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median discharge values included main channel, secondary side channel, and tributary mouth.
The remaining habitat types indicated both increases and decreases in wetted surface area.
6.2. Tributaries
Comparison of the pre- and post-Project hydrology used in the 1980s with the current hydrology
show that both pre- and post-Project flows from the 1980s effort and the current Project are very
similar and the results of the earlier study can be used in developing study plans for the 2013-
2014 study. The R&M Consultants and Trihey & Associates (1985b) indicated that of the 13
tributaries, four were rated as slight change and nine were rated as moderate change as to the
potential for reduction in backwater area at the tributary mouth for post-Project conditions. The
four tributaries assigned a slight change rating were Montana Creek, Goose Creek, Willow Creek
and the Kashwitna River. The nine tributaries assigned a moderate change rating were Trapper
Creek, Birch Creek, Sunshine Creek, Rabideux Creek, Sheep Creek, Caswell Creek, Little
Willow Creek, Alexander Creek and the Deshka River. Using current pre- and post-Project
hydrology and habitat versus flow relationships from the 1980s (R&M Consultants and Trihey &
Associates 1985a), reductions in the tributary mouth wetted surface area were estimated for both
Willow Creek and Goose Creek at 26% and 19%, respectively.
Goose Creek and Trapper Creek were the only tributaries with minimum water depth at or below
the passage criteria in Thompson (1972) at 21,100 cfs based on the text of R&M Consultants and
Trihey & Associates (1985b). The study indicated that during the salmon migration period (May
13 through September 30) water depth would equal or exceed 0.8 feet, the minimum depth for
adult Chinook passage in Thompson (1972). R&M Consultants and Trihey & Associates (1985b)
also indicated potential problems for Caswell and Montana Creek under the post-Project
condition and 21,100 cfs.
None of the results presented for morphologic stability of tributary mouths by R&M Consultants
and Trihey & Associates (1985b) indicated that post-Project conditions would cause a decrease
in stability. The results provided for five of the tributary mouths suggested that the post-Project
conditions would result in improved stability. These tributaries were Caswell Creek, Goose
Creek, Montana Creek, Rabideux Creek, and Birch Creek. However, these results were based on
comparison of aerial photographs only and not on detailed geomorphic assessment of each
tributary and the impacts from Project-related changes in main channel Susitna River discharge
and associated stages. The comparison of the 1983 and 2012 mapping of habitat showed that
significant changes had occurred at the Goose Creek site (Tetra Tech 2013c) due to changes in
the adjacent main channel.
6.3. 2013-2014 Lower River Studies
The results of the assessment presented in this technical memorandum indicate the potential for
Project-related effects on the wetted surface area of various habitat types and related to tributary
access by spawning salmon. To further quantify the potential Project effects on these aspects of
aquatic habitat in the Lower River Segment additional studies have been planned (R2 Resource
Consultants 2013). These efforts include instream flow studies in the main channel and lateral
habitats at two sites and assessment of access issues at five tributaries to be conducted as part of
the Fish and Aquatics Instream Flow Study (IFS). The two instream flow study sites are in the
vicinity of Trappers Creek between PRM 97 and PRM 95 in Geomorphic Reach LR-1 and
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 17 March 2013
between PRM 65 and PRM 68 in Geomorphic Reach LR-2. The Lower River tributaries to be
studied are Trappers Creek (PRM 94.5), Birch Creek (PRM 92.5), Sheep Creek (PRM 69.5),
Caswell Creek (PRM 67) and the Deshka River (PRM 45). The open water flow routing model
and the one-dimensional sediment transport model will be extended downstream to PRM 29.9.
The Riparian IFS has also added a study site in each of the six Lower River geomorphic reaches.
Results of the 2013 Lower River studies will be reviewed to determine if additional sampling
and analysis should be performed in 2014.
7. REFERENCES
Klinger, S. and W. Trihey. 1984. Response of Aquatic Habitat Surface Areas to Mainstem
Discharge in the Talkeetna to Devil Canyon Reach of the Susitna River, Alaska. Prepared
by Trihey & Associates under contract to Harza-Ebasco for Alaska Power Authority,
Document No. 1693.
MWH. 2012. Susitna-Watana Hydroelectric Project, Preliminary Susitna River Pre-Project and
Post-Project Flow Stages, presented at Technical Work Group Meetings, October 23-25.
R2 Resource Consultants, Inc. 2013. Selection of Focus Areas and Study Sites in the Middle and
Lower Susitna River for Instream Flow and Joint Resource Studies – 2013 and 2014.
Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage, Alaska.
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.
R&M Consultants, Inc. and Trihey & Associates. 1985b. Assessment of Access by Spawning
Salmon into Tributaries of the Lower Susitna River. Prepared under contract to Harza-
Ebasco, for Alaska Power Authority, document No. 2775, June.
Tetra Tech, Inc., 2013a.Stream Flow Assessment. Susitna-Watana Hydroelectric Project. 2012
Study Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage,
Alaska.
Tetra Tech, Inc. 2013b. Initial Geomorphic Reach Delineation and Characterization, Middle and
Lower Susitna River Segments. Susitna-Watana Hydroelectric Project. 2012 Study
Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage, Alaska.
Tetra Tech, Inc. 2013c. 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.
Thompson, K. 1972. Determining streamflows for the fish life. In Proceedings, Instream Flow
Requirement Workshop Pacific Northwest River Basin Commission. Vancouver,
Washington. p31-50.
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.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 18 March 2013
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.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 19 March 2013
8. TABLES
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 20 March 2013
Table 4.3-1. Comparisons of median monthly discharge (cfs) for pre-Project and Max LF OS-1 Conditions for the Susitna River at Sunshine.
Average Monthly Discharge (cfs) for the Susitna River at Sunshine
Oct Nov Dec Jan Feb Mar
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
13,600 15,800 6,100 11,600 4,520 11,300 3,700 11,400 3,280 11,700 2,930 9,510
Apr May Jun Jul Aug Sep
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
Pre-
Project
Max LF
OS-1
3,520 9,320 31,100 25,900 63,500 48,000 58,600 45,800 52,900 46,000 32,000 31,300
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 21 March 2013
Table 4.3-2. Comparison of monthly stage exceedences (ft) for the pre-Project and Max LF OS-1 Conditions for the Susitna River at Sunshine.
Percentile
May June July
Pre-
Project Max LF OS-1 Difference Pre-
Project Max LF OS-1 Difference Pre-
Project Max LF OS-1 Difference
90% 11.93 12.62 0.69 16.42 15.62 -0.8 16.98 16.07 -0.91
50% 15.37 14.76 -0.61 18.56 17.13 -1.43 18.13 16.92 -1.21
10% 18.76 17.18 -1.58 20.34 18.29 -2.05 19.68 18.08 -1.6
Percentile
August September
Pre-
Project Max LF OS-1 Difference Pre-
Project Max LF OS-1 Difference
90% 16.13 15.79 -0.34 13.73 13.78 0.05
50% 17.61 16.94 -0.67 15.48 15.39 -0.09
10% 19.23 18.67 -0.56 17.91 17.73 -0.18
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 22 March 2013
Table 5.1-1. Summary of potential effects on the with-Project flows on tributaries of the Lower Susitna River, reproduced from Table 4.1 of R&M Consultants and
Trihey & Associates (1985b).
General Tributary Information Effects of With-Project Flows On
Tributary River
Mile
Tributary Mouth in Breaching
Discharge
at
Sunshine
(cfs)
Summary of Adult Salmon
Usage of Stream Interface
Reach 1984(1)
Passage Conditions
Near Tributary Mouth
Fish Access Into
Tributaries at 21,100
cfs (2)
On Backwater Areas
(3)
Morphologic
Stability of Tributary
Mouth (4)
Side
Channel
Main
Channel Passage Spawning
Water
Depth
(ft)
Discharge
at
Sunshine
(cfs)
Possible
Problem
No
Problem
Moderate
Change
Slight
Change Present With-
Project
Alexander
Cr
9.1 X --- Not Surveyed -- -- X X Good Good
Deshka R 40.6 X --- Ch, S, P, Co -- -- -- X X Fair Fair
Willow Cr 49.1 X <13,900 S, P, Ch, Co P, Ch 2.8 18,300 X X Good Good
L Willow
Cr
50.5 X 58,000 Ck, S, P, Ch,
Co
P 1.5 18,300 X X Good Good
Kashwitna
R
61.0 X --- P, Ch -- -- -- X X Fair Fair
Caswell
Cr
64.0 X 35,000 Ck, S, P, Ch P, Ch 0.8 21,100 X X Fair Fair/
Good
Sheep Cr 66.1 X <13,900 S, P, Ch, Co P 3.0 18,300 X X Good Good
Goose Cr 72.0 X 21,000 Ck, S, P, Ch,
Co
P 0.4 -- X X Fair Fair/
Good
Montana
Cr
77.0 X 35,000 Ck, P, Ch,
Co
P, Ch 1.1 18,300 X X Poor Poor/
Fair
Rabideux
Cr
83.1 X --- Ck, S, P, Ch -- -- -- X X Fair Good
Sunshine
Cr
85.1 X <13,900 Ck, S, P, Ch P, Ch 1.5 28,400 X X Good Good
Birch Cr 89.2 X 54,100 Ck, S, P, Ch,
Co
P -- -- X X Fair Fair/
Good
Trapper
Cr
91.5 X 44,000 Ck, S, P, Ch,
Co
P, Ch 0.6 20,900 X X Fair Fair
Definitions:
1 The interface reach is first third mile from mouth up the tributary. Source Barrett et al. 1985. Ck = chinook, S= sockeye, P = pink, Ch = chum, and Co = coho.
2 Possible Problem - There is the potential for access problems depending on low tributary flows, debris jams, or channel changes.
No Problem - No problem with access currently exists.
3 Moderate Change - The extent of backwater area could be moderately reduced by with-project flows during June and July.
Slight Change - The extent of backwater area could be slightly reduced by with-project flows during June and July
4 Good - No change in tributary mouth morphology since 1951.
Fair - Some change in tributary mouth morphology since 1951.
Poor - Change in tributary mouth morphology from 1951 to present.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 23 March 2013
Table 5.2-1. SC IV-4 accumulated wetted surface areas (ft2x103) for each habitat type for the Open water period (May-September) and Ice Affected period (October-
April) for both the pre-Project and Maximum Load Following OS-1 conditions.
Accumulated wetted surface area (ft2x103) for each habitat type
Main Channel Primary Side Channel Secondary Side Channel Turbid Backwater
Period Accumulated Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1
Open Water (May-Sept) 30,512 29,524 NA1 NA 32,724 30,135 914 669
Ice Affected (Oct-Apr) 37,659 38,012 NA NA 10,882 15,683 0 0
Clearwater Side Slough Tributary Mouth Tributary
Period Accumulated Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1
Open Water (May-Sept) 848 1,212 0 0 NA NA NA NA
Ice Affected (Oct-Apr) 0 0 598 514 NA NA NA NA
1 NA = No aquatic habitat of this type present at this site
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 24 March 2013
Table 5.2-2. Willow Creek accumulated wetted surface areas (ft2x103) for each habitat type for the Open water period (May-September) and Ice Affected period
(October-April) for both the pre-Project and Maximum Load Following OS-1 conditions.
Accumulated wetted surface area (ft2x103) for each habitat type
Main Channel Primary Side Channel Secondary Side Channel Turbid Backwater
Period Accumulated Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1
Open Water (May-Sept) 14,518 14,113 NA1 NA 55,586 51,825 1,317 1,419
Ice Affected (Oct-Apr) 12,469 13,701 NA NA 21,699 30,724 134 332
Clearwater Side Slough Tributary Mouth Tributary
Period Accumulated Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1
Open Water (May-Sept) 754 832 0 0 2,647 2,150 6,228 7,107
Ice Affected (Oct-Apr) 0 0 5,334 4,578 233 622 14,280 14,214
1 NA = No aquatic habitat of this type present at this site
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 25 March 2013
Table 1.2-3. Goose Creek accumulated wetted surface areas (ft2x103) for each habitat type for the Open water period (May-September) and Ice Affected period
(October-April) for both the pre-Project and Maximum Load Following OS-1 conditions.
Accumulated wetted surface area (ft2x103) for each habitat type
Period Accumulated Main Channel Primary Side Channel Secondary Side Channel Turbid Backwater
Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1
Open Water (May-Sept) 24,751 18,234 NA1 NA 27,416 24,108 943 991
Ice Affected (Oct-Apr) 0 0 NA NA 11,793 17,362 42 81
Clearwater Side Slough Tributary Mouth Tributary
Period Accumulated Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1 Pre-Project Max LF OS-1
Open Water (May-Sept) 677 920 1,172 1,104 455 367 2,539 2,417
Ice Affected (Oct-Apr) 0 0 5,257 5,170 0 0 15,820 15,079
1 NA = No aquatic habitat of this type present at this site
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 26 March 2013
Table 5.2-4. Accumulated wetted surface areas for tributary mouth habitat for the salmonid spawning period (June-
September) for Willow Creek and Goose Creek.
Site
Accumulated wetted surface area (ft2x103)
for the salmonid spawning period (June-Sept)
Tributary Mouth
Pre-Project Max LF OS-1
Willow Creek 2,286 1,697
Goose Creek 405 329
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 27 March 2013
9. FIGURES
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 28 March 2013
Figure 3.2-1. Susitna River Geomorphology Study Area and Large-scale River Segments.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 29 March 2013
Figure 4.5-1. Comparison of median weekly discharge for the Susitna River at Sunshine for the Natural and 2020
conditions presented in Figure 4.2 of R&M Consultants and Trihey & Associates (1985b) and the pre-Project and
Maximum Load Following OS-1 conditions assessed by the 2012 Study.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 30 March 2013
Figure 5.1-1. Wetted surface area (ft2 x 103) response to Susitna River discharge at Sunshine by habitat type for Willow
Creek. Reproduced from R&M Consultants and Trihey & Associates (1985a).
1
10
100
1,000
10,000
100,000
0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000Wetted Surface Area (ft²x10³) Discharge (cfs)
Mainstem Primary Side Channel Secondary Side Channel
Turbid Backwater Clearwater Side Slough
Tributary Mouth Tributary Gravel Bar
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 31 March 2013
Figure 5.2-1. Accumulated wetted surface area (ft2x103) computed for SC IV-4 for the Open Water period (May-September) and the Ice Affected period (October-April)
for the median monthly Sustina River discharge at Sunshine under the pre-Project and Maximum Load Following OS-1 conditions for each habitat type.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 32 March 2013
Figure 5.2-2. Accumulated wetted surface area (ft2x103) computed for Willow Creek for the Open Water period (May-September) and the Ice Affected period (October-
April) for the median monthly Sustina River discharge at Sunshine under the pre-Project and Maximum Load Following OS-1 conditions for each habitat type.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 33 March 2013
Figure 5.2-3. Accumulated wetted surface area (ft2x103) computed for Goose Creek for the Open Water period (May-September) and the Ice Affected period (October-
April) for the median monthly Sustina River discharge at Sunshine under the pre-Project and Maximum Load Following OS-1 conditions for each habitat type.
SYNTHESIS OF THE 1980S LOWER SUSITNA RIVER SEGMENT
AQUATIC HABITAT INFORMATION
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 34 March 2013
Figure 5.2-4. Accumulated wetted surface area (ft2x103) computed over the salmon migration period (June-September)
for the median monthly Sustina River discharge at Sunshine presented for the tributary mouth habitat type for the pre-
Project and Maximum Load Following OS-1 conditions at Willow Creek and Goose Creek.
0
500
1,000
1,500
2,000
2,500
Willow Creek Goose CreekAccumulated wetted surface area (ft2x103) Pre-Project Max LF OS-1