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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
SuWa 261
Glacier and runoff changes (Study 7.7) and fluvial geomorphology (Study
6.5), assessment of the potential for changes in sediment delivery to
Watana Reservoir due to glacial surges, technical memorandum
Author(s) – Personal:
Author(s) – Corporate:
Tetra Tech, Inc.
AEA-identified category, if specified:
November 14, 2014 technical memorandum filings
AEA-identified series, if specified:
Series (ARLIS-assigned report number): Existing numbers on document:
Susitna-Watana Hydroelectric Project document number 261
Published by: Date published:
[Anchorage, Alaska : Alaska Energy Authority, 2014] November 2014
Published for: Date or date range of report: Alaska Energy Authority
Volume and/or Part numbers:
Final or Draft status, as indicated:
Attachment G
Document type: Pagination:
Technical memorandum ii, 18 p.
Related work(s): Pages added/changed by ARLIS:
Cover letter to this report: Susitna-Watana Hydroelectric
Project, FERC Project no. 14241-000; Filing of Initial Study Plan
Meetings transcripts and additional information in response to
October 2014 Initial Study Plan Meetings. (SuWa 254)
Attachments A-F (SuWa 255-260) and H-N (SuWa 262-268)
Added cover letter (4 pages)
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/
November 14, 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
Filing of Initial Study Plan Meetings Transcripts and Additional Information in
Response to October 2014 Initial Study Plan Meetings
Dear Secretary Bose:
By letter dated January 28, 2014, the Federal Energy Regulatory Commission
(Commission or FERC) modified the procedural schedule for the preparation and review
of the Initial Study Report (ISR) for the proposed Susitna-Watana Hydroelectric Project,
FERC Project No. 14241 (Project).1 As required by the Commission’s January 28 letter,
the Alaska Energy Authority (AEA) filed the ISR with the Commission on June 3, 2014
and conducted ISR meetings on October 15, 16, 17, 21, 22, and 23, 2014. Attached as
Attachments A-1 through F-2 are the written transcripts (along with the agenda and
PowerPoint presentations) for these ISR meetings.
During the October ISR meetings, AEA and licensing participants identified
certain technical memoranda and other information that AEA would file with the
Commission by November 15, 2014. In accordance, AEA is filing and distributing the
following technical memoranda and other information:
• Attachment G: Glacier and Runoff Changes (Study 7.7) and Fluvial
Geomorphology (Study 6.5) - Assessment of the Potential for Changes in
Sediment Delivery to Watana Reservoir Due to Glacial Surges Technical
Memorandum. This technical memorandum documents AEA’s analysis of the
potential changes to sediment delivery from the upper Susitna watershed into
the Project’s reservoir from glacial surges.
• Attachment H: Riparian Instream Flow (Study 8.6) and Fluvial
Geomorphology (Study 6.6) - Dam Effects on Downstream Channel and
Floodplain Geomorphology and Riparian Plant Communities and Ecosystems
− Literature Review Technical Memorandum. This literature review technical
1 Letter from Jeff Wright, FERC Office of Energy Projects, to Wayne Dyok, Alaska Energy Authority,
Project No. 14241-000 (issued Jan. 28, 2014).
2
memorandum synthesizes historic physical and biologic data for the Susitna
River floodplain vegetation (including 1980s studies), studies of hydro project
impacts on downstream floodplain plant communities, and studies of un-
impacted floodplain plant community successional processes.
• Attachment I: Susitna River Fish Distribution and Abundance Implementation
Plan, Appendix 3. Protocol for Site-Specific Gear Type Selection, Version 5.
In accordance with the fish distribution and abundance studies, as described in
Revised Study Plan (RSP) Sections 9.5 and 9.6 and in the Fish Distribution
and Abundance Implementation Plan, this appendix establishes the protocol
for site-specific gear type selection for fish surveys. Throughout study plan
implementation, AEA has updated this appendix as needed to provide
consistent direction to all field teams. Version 1 of Appendix 3 was originally
filed with the Fish Distribution and Abundance Implementation Plan in March
2013. That version was updated twice (Versions 2 and 3) during the 2013
field season to accommodate protocol changes that related to FERC’s April 1,
2013 Study Plan Determination, field permits, and lessons learned during
study implementation. Version 4 was the protocol used for the 2014 field
season and was updated with respect to the prioritization of gear use and
based on 2013 data collected. This version herein, Version 5, will be followed
during the 2015 field season.
• Attachment J: Fish Distribution and Abundance in the Upper and
Middle/Lower Susitna River (Studies 9.5 and 9.6): Draft Chinook and Coho
Salmon Identification Protocol. This document established a Chinook and
coho salmon identification protocol to support accurate and consistent field
identification across field teams. It will allow for additional quality control
and assurance of field identification calls and for estimation and reporting of
any field identification error that may occur in future sampling efforts.
• Attachment K: Characterization and Mapping of Aquatic Habitats (9.9),
Errata to Initial Study Report Part A - Appendix A, Remote Line Mapping,
2012. This errata provides a corrected version of map book for Remote Line
Mapping, 2012. The version filed with the ISR (June 3, 2014) used a data
query to build the maps in geomorphic reaches MR-1 to UR-5 that mistakenly
did not include side slough habitat, so that no side sloughs were depicted on
the Appendix A maps 1 through 21. This version was corrected by including
side slough habitat in the data query for geomorphic reaches MR-1 to UR-5.
This version now includes side sloughs.
• Attachment L: Characterization and Mapping of Aquatic Habitats Study 9.9,
Revised Map Book for 2012 Remote Line Mapping. This map book represents
an update to the version published on June 3, 2014 with the Study 9.9 Initial
Study Report and the errata provided concurrently with this filing (see
Attachment K). The maps presented include all macrohabitat and mesohabitat
line identifications available in the 2012 Remote Line Mapping ArcGIS
3
shapefile. This map book should be considered a full replacement for
previous versions and represents the final product for the 2012 remote line
habitat mapping effort.
• Attachment M: Study of Fish Passage Barriers in the Middle and Upper
Susitna River and Susitna Tributaries (Study 9.12), Fish Passage Criteria
Technical Memorandum. This technical memorandum presents a proposed
final list of fish species that will be included in the fish barrier analysis as well
as depth, leaping and velocity passage criteria for selected fish species. AEA
previously consulted with the federal agencies and other licensing participants
regarding the information within the technical memorandum during a March
19, 2014 Fisheries Technical Meeting.
In addition to the technical memoranda and other information identified above,
AEA is filing a short errata (Attachment N) to the Mercury Assessment and Potential for
Bioaccumulation Study (Study 5.7), Evaluation of Continued Mercury Monitoring
Beyond 2014 Technical Memorandum. This technical memorandum, which was
originally filed on September 30, 2014, evaluates the need for continued monitoring of
mercury data beyond 2014 and whether the existing data collection efforts are sufficient
to satisfy objectives for characterizing baseline mercury conditions in the Susitna River
and tributaries (RSP Section 5.7.1). Since the filing of this TM and based upon the
ongoing QA/QC of the data reported in that TM, AEA discovered errors in the TM. The
attached TM corrects those errors. Additionally, the errata corrects corresponding errors
in the Mercury Assessment and Potential for Bioaccumulation presentation presented
during the October 16, 2014 ISR meeting.
Finally, AEA notes that data collected during the Study Plan implementation, to the
extent they have been verified through AEA’s quality assurance and quality control (QAQC)
procedures and are publicly available, can be accessed at http://gis.suhydro.org/isr_mtg. On
November 14, 2014, AEA posted the following data to this website:
• Baseline Water Quality Data (Study 5.5), 2013 QAQC water quality data
and DVRs per the Quality Assurance Project Plan.
• Breeding Survey Study of Landbirds and Shorebirds (Study 10.16),
cumulative 2013-2014 data.
• Characterization and Mapping of Aquatic Habitats (Study 9.9), ArcGIS
shapefile “ISR_9_9_AQHAB_RemoteLineMapping_2012.shp” used to
generate the maps in Attachment L.
4
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)
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Glacier and Runoff Changes (Study 7.7) and
Fluvial Geomorphology (Study 6.5)
Assessment of the Potential for Changes in Sediment
Delivery to Watana Reservoir Due to Glacial Surges
Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech, Inc.
November 2014
ASSESSMENT OF THE POTENTIAL FOR CHANGES IN SEDIMENT
TECHNICAL MEMORANDUM DELIVERY TO WATANA RESERVOIR DUE TO GLACIAL SURGES
TABLE OF CONTENTS
1. Introduction and Background ..............................................................................................1
2. Study Objectives ..................................................................................................................2
3. Study Area ...........................................................................................................................3
4. Review of Information on Glacier Retreat and Glacial Surges in Alaska ...........................3
5. Review of Information on Glacial Surges within the Susitna River Basin ..........................4
6. Potential for Increased Sedimentation in Watana Reservoir Due to Glacial Surge .............5
6.1. Glacial Surge Sediment Production .....................................................................................5
6.2. Sediment Transport in the Susitna River .............................................................................6
6.2.1. Sand Transport .....................................................................................................................6
6.2.2. Silt-clay Transport ...............................................................................................................7
7. Conclusions ..........................................................................................................................8
8. References ............................................................................................................................9
9. Figures................................................................................................................................13
LIST OF TABLES
Table 1 - Comparison of Gold Creek to Talkeetna Flow Volumes. .............................................. 7
LIST OF FIGURES
Figure 1. Map showing the upper Susitna River Basin including the locations of the West
Fork, Susitna, East Fork, Maclaren and Eureka Glaciers. Project River Miles (PRM) are
also shown from the upstream end of the Watana Reservoir pool (PRM 230) to the terminus
of the Susitna Glacier (PRM 320). ........................................................................................ 14
Figure 2. Photograph of the lower reach of the Susitna Glacier showing potholes that are
considered to be indicative of glacier stagnation. The photograph was taken on September
19, 2013. 15
Figure 3. Upstream view of the confluence of the West Fork Susitna River with the Susitna
River at PRM 301. The photograph was taken on September 19, 2013. ............................. 15
Figure 4. Total suspended sediment rating curves for the Denali (PRM 292), Cantwell
(PRM 225) and Gold Creek (PRM 140) USGS gages on the Susitna River. ....................... 16
Figure 5. Upstream view of the Denali Highway crossing the Susitna River at PRM 292.
The USGS Denali gage was located at the bridge between 1958 and 1986. The braid bars in
the river are composed of sand. The photograph was taken on September 19, 2013. .......... 17
Figure 6. Ranked mean annual discharge, average May-September discharge and maximum
daily discharge for 50 years of flow record at the Gold Creek gage (Tetra Tech ISR Study
6.6., Appendix E). ................................................................................................................. 18
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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LIST OF ACRONYMS AND SCIENTIFIC LABELS
Abbreviation Definition
AEA Alaska Energy Authority
cfs cubic feet per second
FERC Federal Energy Regulatory Commission
g/l grams per liter
ILP Integrated Licensing Process
M Million
mm Millimeter
OWFP open-water flow period
ppm parts per million
PRM Project River Mile
RM River Mile
RSP Revised Study Plan
sq mi square mile
SPD Study Plan Determination
USGS U.S. Geological Survey
WY Water Year
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1. INTRODUCTION AND BACKGROUND
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.
On December 14, 2012, AEA filed its Revised Study Plan (RSP) with the FERC for the Susitna-
Watana Hydroelectric Project (FERC Project No. 14241), which included 58 individual study
plans (AEA 2012). Included with the RSP are the Glacial Runoff Changes Study (RSP Study
7.7) and the Fluvial Geomorphology Study (RSP Study 6.5). Study 7.7 focuses on the potential
impacts of glacier wastage and retreat on the flow of water into the proposed reservoir. The goal
of Study 6.5 is to characterize the geomorphology of the Susitna River and includes determining
the sediment supply.
Harrison (2012), in his submission to FERC, suggested that glacial surges in the upper reaches of
the Susitna River Basin could significantly increase sediment loading to the Watana Reservoir.
This suggestion was based on observations of sediment production during surge periods of
glaciers both within and outside the Susitna basin.
On February 1 2013, FERC staff issued its study plan determination (February 1 SPD) for 44 of
the 58 studies, approving 31 studies as filed and 13 with modifications. RSP Section 7.7 was one
of the 13 approved with modifications. In the February 1 SPD, FERC recognized the following:
AEA proposes to analyze the potential effects of climate change on glacier wastage and retreat
and the corresponding effects on streamflow entering the proposed reservoir, and evaluate the
effects of glacial surges on sediment delivery to the reservoir. Specifically, AEA proposes to:
1. review existing literature relevant to glacier retreat in southcentral Alaska and the upper
Susitna watershed and summarize the current understanding of potential future changes in
runoff associated with glacier wastage and retreat;
2. develop a hydrologic modeling framework that utilizes a glacier melt and runoff model
(Hock 1999) and a Water Balance Simulation Model (WaSiM) to predict changes in glacier
wastage and retreat on runoff in the Susitna basin;
3. simulate the inflow of water to the proposed reservoir and predict changes to available
inflow using downscaled climate projections up to the year 2100; and
4. analyze the potential changes to sediment delivery from the upper Susitna watershed into the
reservoir from glacial surges.
FERC staff recommended the following in the February 1 SPD.
• We find that the analysis of the potential changes to sediment delivery from the upper Susitna
watershed into the reservoir from glacial surges as proposed by AEA is necessary, and therefore,
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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are recommending approval of this portion of AEA’s proposed study (item 4 as described above
in the applicant’s proposed study).
To address the FERC recommendation, the Geomorphology Study (Study 6.5) included the
following effort to provide for the potential analysis of the influence of glacial surge on reservoir
sediment accumulation rates in RSP Section 6.5.4.8.2.1 if sediment from glacial surge can
actually be delivered to the reservoir:
Potential additional sediment loading resulting from glacial surge will be investigated in the
Glacier and Runoff Changes Study ([RSP] Section 7.7.4.4, Analyze Potential Changes in
Sediment Delivery to Watana Reservoir). If this investigation indicates that the increased
sediment load can actually be delivered in substantial quantities to Watana Reservoir, more
detailed analyses of the increased loading will be performed and a sediment loading scenario
accounting for glacial surge will be added to the reservoir trap efficiency and sediment
accumulation analysis. This would include an estimate of the reduction in reservoir life that
could result from sediment loading associated with periodic glacial surges.
Glacial surges in the upper reaches of the Susitna River Basin, where 5 large glaciers and 50-60
small glaciers are located on the southern flank of the Alaska Range (Clarke 1991), could
significantly increase suspended sediment loading to the Watana Reservoir. The basis for this
concern (Harrison 2012) was the measured behavior of the intensively studied Variegated
Glacier located near Yakutat, AK that surged in 1982 and 1983 (Kamb et al. 1985; Harrison and
Raymond 1986; Raymond 1987; Raymond et al. 1987). Suspended sediment measurements at
the Variegated Glacier outlet streams indicated that there had been on the order of 0.3 m depth of
erosion of the underlying bedrock over the 20-year surge cycle ending in 1983. About two-thirds
of the erosion occurred during the 2 years of the surge peak and the bulk of this occurred during
a two-month period of very high peak discharges from the glacier that occurred as the glacier
surge ceased (Humphrey and Raymond 1994). If such extreme bedrock erosion were to occur
during a surge of one of the large Susitna Basin surging glaciers (West Fork, Susitna) that greatly
exceed the size of the Variegated Glacier (Harrison et al. 1994), Clarke et al. (1986) estimated it
would be equivalent to about 30 years of the annual total sediment load estimated at the Gold
Creek gage.
This estimate assumes that 100 percent of the sediment evacuated from the glacier over a short
time period would be delivered to the proposed Watana Reservoir. Given the extensive presence
of braid plains below the glaciers (Raymond and Benedict c. 1990) and the distance
(approximately 90 river miles) between the glaciers and the head of the reservoir (Guymon
1974), it may not be reasonable that an appreciable portion of the sediment load associated with
a glacial surge would be delivered to the proposed Watana Reservoir. If increased loads do
occur, then the reservoir sediment storage and longevity would be affected.
2. STUDY OBJECTIVES
The objectives of this Technical Memorandum are to:
1. Review the literature regarding glacier retreat and glacial surges and pulses in Alaska
2. Review the literature regarding glacial surges within the Susitna River Basin, and
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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3. Assess the likelihood of a glacial surge in the Susitna River Basin significantly increasing
sediment delivery to the proposed Watana Reservoir.
3. STUDY AREA
The study area extends from the headwaters of the Susitna River above PRM 320 and the
Maclaren River downstream to the upper extent of the proposed Watana Reservoir at PRM 232
(See Figure 1).
4. REVIEW OF INFORMATION ON GLACIER RETREAT AND
GLACIAL SURGES IN ALASKA
Glaciers in Alaska cover an area of about 28,500 square miles (Post and Mayo 1971). For the
last 100-200 years there has been a general retreat of the glaciers from their maximum
Neoglacial positions (Molnia 2008). In general, the response of Alaskan glaciers to the warming
climate has been to retreat (Hall et al. 2005), thin (Adalgeirsdóttir et al. 1998; Arendt et al. 2002;
Berthier et al. 2010; Luthcke et al. 2008), and decelerate (Heid & Kääb 2012a). A large number
of the retreating glaciers in Alaska have been categorized as surge- and pulse-type glaciers (Post
1960, 1969; Mayo 1978; Ommanney 1980; Clarke et al. 1986; Wilbur 1988; Copland et al. 2003;
Turrin 2014). Many have experienced substantial surges and pulses that have resulted in cyclical
flow instabilities (Meier and Post 1969; Lawson 1977; Clarke et al. 1986; Kamb et al. 1985;
Harrison et al. 1986a; Harrison et al. 1986b; Raymond and Harrison 1988; Kamb 1987;
Eschelmeyer et al. 1987; Molnia and Post 1995, 2010; Nolan 2003; Burgess et al. 2012;
Tangborn 2013; Turrin 2014). Within the Alaska Range surging glaciers tend to be found in
fault-shattered valleys (Post 1969) while the pulsing glaciers tend to be floored by deformable
glacial tills (Turrin 2014).
Surging glaciers are distinguished by their periods of brief but anomalously fast flow, known as
the active phase, and their often long periods of nearly stagnant flow, known as the quiescent
phase (Post 1969). The active phase, or surge, rapidly transports large volumes of ice down-
glacier from a reservoir area, often located in the accumulation zone, to the ablation zone and to
the terminus, often causing the terminus to thicken by tens of meters and to advance by
kilometers (Kamb et al. 1985). Mayo (1978) defined a glacier pulse as periodic, short duration,
unstable flow lesser in magnitude than surge-type behavior.
Surges of the glaciers influence their mass balance by transporting large masses of ice to a lower
elevation where higher temperatures increase ablation rates and runoff (Eisen et al. 2001;
Tangborn 2013). The surges also increase sediment discharge from the glacier (Hance 1937;
Humphrey et al. 1986; Eschelmeyer et al. 1987; Humphrey and Raymond 1994; Merrand and
Hallet 1996; Harrison et al. 1994; Raymond and Benedict, c.1990). Under non-surging
conditions, suspended sediment concentrations from glaciers are generally less than 10g/l
(10,000 ppm) (Gaddis 1974; Raymond and Benedict, c.1990) but measured concentrations at the
end of surges of both the Variegated and West Fork Glaciers were as high as 30g/l (30,000 ppm)
for relatively short periods of time as a result of ice erosion of the underlying bedrock
(Humphrey and Raymond 1994: Harrison et al. 1994). Bed load, which can constitute up to 30
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percent of the total load directly downstream of the glacier (Ostrem et al. 1973), is generally
deposited within a short distance of its source (Guymon 1974; Raymond and Benedict, c.1990).
Termination of the surges can also result in one or more large magnitude outburst floods
(jokulhaups) as water is released from englacial storage (Kamb et al. 1985; Humphrey and
Raymond 1994; Harrison et al. 1994; Lingle and Fatland 2003).
Surge frequencies are glacier specific, but reportedly range from about 7 years for the Ruth
Glacier (Turrin 2014), to 17-20 years for the Bering Glacier (Molnia and Post 2010; Tangborn
2013) and Variegated Glacier (Humphrey and Raymond 1994), to 50-60 years for the West Fork
and Susitna Glaciers (Clarke et al. 1986; Clarke 1991; Harrison et al. 1994). Exactly how surging
glaciers will respond to warming climatic trends, either by increasing or decreasing surge
frequency and magnitude, is unknown (Turrin 2014). However, within Alaska, general glacier
retreat and loss of ice mass is expected to continue in the future (Radic and Hock 2011).
5. REVIEW OF INFORMATION ON GLACIAL SURGES WITHIN THE
SUSITNA RIVER BASIN
Glaciers cover about 305 mi2 (790 km2) or 5.9 percent of the Susitna Basin area above the
proposed Watana Dam site. Because of the significance of the glacierized portion of the basin
for both runoff and sediment supply to the Susitna River, a number of investigations were
conducted in the 1980s for the Alaska Power Authority (Harrison 1981; Harrison and R&M
Consultants, Inc. 1982; R&M Consultants, Inc. and Harrison 1981; Harrison et al. 1983; Clarke
et al. 1985, 1986; Clarke 1986). The glacierized portion of the Susitna River Basin produces
about 13 percent of the flow at the Gold Creek gage (Clarke et al. 1986), which is approximately
16 percent of the flow to the proposed Watana Dam site. Glacial retreat in the 1949-1980 period
was estimated to have produced about 3-4 percent of the flow at the Gold Creek gage (Clarke et
al. 1986). Approximately 34 percent of the runoff measured north of the Denali Highway using
the Susitna River at Denali and Maclaren River at Denali gages is attributed to the glacierized
area (ISR, Study 7.7., Section 4.2.4.2). In addition, it has been suggested that glacial surges
could significantly increase the suspended sediment load delivery to the proposed reservoir
(Clarke et al. 1986; Harrison 2012).
Five large glaciers, the West Fork, Susitna, East Fork Glaciers on the mainstem Susitna River
and the Maclaren and Eureka Glaciers in the Maclaren River Basin, and 50-60 small glaciers
form the headwaters of the Susitna River (Clarke 1991) (Figure 1). The West Fork (about 99
mi2) and Susitna (about 88 mi2) Glaciers are underlain by faults associated with the Denali Fault
System (Post 1969; Clarke et al. 1986; Harrison et al. 1994) and are classified as surge-type
glaciers (Post 1960, 1969; Clarke et al. 1986; Wilbur 1988; Eschelmeyer and Harrison 1987;
Clarke 1991; Harrison et al. 1994). The East Fork Glacier (about 16 mi2) is classified as a non-
surging-type glacier (Post 1969; Wilbur 1988). Eureka Glacier (about 15 mi2) has been
classified as a surge-type glacier (Post 1969). The Maclaren Glacier (about 23 mi2) was
classified as inconclusive by Post (1969), but Mayo (1978) suggested that it had undergone a
strong pulse in 1971. Indicators of surge activity include: a) the presence of looped moraines, b)
deformed ice structures, c) heavily crevassed glacier surface in the active phase, d) potholes on
the glacier surface during the quiescent phase, e) rapid advance of the glacier terminus when
surrounding glaciers are relatively stable in ice-margin position, f) shear margins on the glacier
surface, g) surface velocities that are typically an order of magnitude higher than during the
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quiescent phase, and h) strandlines of ice on surrounding bedrock (Meier and Post 1969;
Copland et al. 2003). Wilbur (1988) using the 0-5 scale Canadian Glacier Inventory Code
(Ommanney 1980) where Code 0 represents no evidence of surge activity and Code 5 represents
the presence of extensive surge features, classified the Eureka Glacier as a Code 2 (few surge
features present) and the Maclaren Glacier as Code 3 (moderate presence of surge features). In
contrast, the West Fork and Susitna Glaciers were assigned Codes of 4 and 5, respectively which
reflect the extensive presence of surge features.
The West Fork Glacier surged between 1935 and 1937 (Clarke 1991) and again about 50 years
later in 1987/1988 (Eschelmeyer and Harrison 1989; Harrison 1994; Harrison et al. 1994;
Raymond and Benedict, c.1990). The Susitna Glacier last surged either in 1951/1952 (Clarke
1991) or 1952/1953 (Post 1960; Meier and Post 1969; Labelle et al. 1985). Clarke (1991)
suggested that the next surge of the Susitna Glacier would occur in the first decade of the 21st
century. This estimate was based on the ratio of actual ice flux to balance flux developed from
data collected between 1981 and 1983. However, there has been no recent evidence of imminent
surge activity, and in fact the surface of the glacier is potholed (Figure 2) which suggests that it
is still in a stagnation or quiescent, non-surging phase (Harrison et al. 1994; Copland et al. 2003).
Assuming that the historical West Fork Glacier surge cycle is maintained, the next surge is likely
to occur in about 2,038. However, the effects of climate warming on the frequency and
magnitude of glacial surge cycles are unknown (Turrin 2014) and Harrison (personal
communication, September 10, 2012) hypothesized that surging may no longer occur at many
glaciers, including the West Fork Glacier, because non-surge glacial processes would be able to
maintain equilibrium. It is unknown whether the failure of the Susitna Glacier to surge in the
first decade of the 21st century supports Harrison’s hypothesis.
6. POTENTIAL FOR INCREASED SEDIMENTATION IN WATANA
RESERVOIR DUE TO GLACIAL SURGE
6.1. Glacial Surge Sediment Production
The 1982-1983 surge of the Variegated Glacier produced sediment transported in suspension
approximately equivalent to 0.3 m of bed lowering during the 20-year surge cycle (0.015 m/yr).
Two-thirds of this lowering occurred over the 2-year surge period (0.1 m/yr), and the bulk of that
during a two-month period (Humphrey and Raymond 1994). Clarke et al. (1986) suggested that
a similar surge of one of the very large Susitna Basin glaciers (West Fork or Susitna) could
produce about 20 x 107 tons of suspended sediment, which they indicated was equivalent to
about 30 years of the annual total sediment load estimated at the Gold Creek gage (about 6.8 x
106 tons/yr). However, downstream storage in extensive braid plains (Figure 1) makes it highly
unlikely that the entire estimated sediment production would be transported to the proposed
Watana Reservoir (Guymon 1974).
The West Fork Glacier surged in 1987 and 1988 and suspended sediment data were collected at
the outlet streams and in the Susitna River upstream of the confluence with the West Fork
Susitna River (Figure 3) (Harrison et al. 1994; Raymond and Benedict, c.1990). Suspended
sediment concentrations as high as 30 g/l (30,000 ppm) were measured for short periods of time
in 1988 (concentrations exceeded 10 g/l for about 16 days) towards the cessation of the surge.
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These values greatly exceed the measured concentrations at non-surging glaciers (< 10 g/l;
10,000 ppm) (Gaddis 1974; Raymond and Benedict, c.1990). Measured concentrations in the
Susitna River upstream of the confluence with West Fork Susitna River (PRM 301) in 1988 were
one to two orders of magnitude less than at the West Fork Glacier outlet (Harrison et al. 1994).
For a point of reference, measured suspended sediment concentrations at the USGS Denali
gaging station (USGS 15291000) during non-surge conditions in the basin (1958-1986) range
from 100 mg/l (100 ppm) to 6,000 mg/l (6,000 ppm) (Figure 4) (Tetra Tech 2014a). Based on
the suspended sediment rating curve for the Denali gage (Figure 4), the average annual
suspended sediment load is about 3 x 106 tons, of which 57 percent (about 1.7 x 106 tons) is
composed of silts and clays and 43 percent (1.3 x 106 tons) is composed of sand (Tetra Tech
2014a). The average annual suspended sediment concentration at the gage is about 1,000 ppm
(Guymon 1974; Tetra Tech 2014a).
6.2. Sediment Transport in the Susitna River
6.2.1. Sand Transport
Transport of the sand fraction of the suspended load provided by the upstream glaciers at the
Denali gage (PRM 292) is transport capacity dependent and is based on the hydrology and
hydraulic conditions at the gage (Figure 4) where there is an almost unlimited supply of sand
sized material in the bed (Figure 5). Analysis of a 50-year hydrologic record at the Gold Creek
gage (PRM 140) (USGS 15292000), which included two glacier surge periods (Susitna Glacier
in 1952/1953 and West Fork Glacier in1987/1988), did not indicate that the surges appreciably
affected the mean annual discharge, the average May-September discharge or the maximum
daily discharge (Figure 6) which confirms the analysis of Labelle et al. (1985) for the Susitna
Glacier surge. The Gold Creek gage was selected because it is the longest operated gage on the
Susitna River and neither of the upstream gages (Denali and Cantwell) were in operation during
either of the surge periods (1952/1953 and 1987/1988). Ranking of the 50-years of mean annual
discharge records indicated that 1952, 1953, 1987 and 1988 years represented the 52nd, 46th, 54th
and 76th percentiles, respectively. Ranking of the 50-years of average May-September discharge
records indicated that 1952, 1953, 1987 and 1988 years represented the 52nd, 62nd, 58th and 76th
percentiles, respectively. Ranking of the 50-years of maximum daily discharge records indicated
that 1952, 1953, 1987 and 1988 years represented the 62nd, 40th, 66th and 46th percentiles,
respectively (Tetra Tech 2014b). Based on the hydrologic analysis, which indicates these were
all fairly typical years hydrologically, it is unlikely that the surges could significantly affect the
volume of capacity-limited sand transported at the Denali gage.
Though the above analysis shows the surge years were all very typical in terms of the recorded
flows on the Susitna at Gold Creek, it is possible that these may have been low flow years in
general. This would have then masked the effect of the glacier surge at Gold Creek. To
investigate this, the Gold Creek and Talkeetna USGS gage records were also compared to assess
whether the surge years for the Susitna basin produced higher than expected flows. The long-
term average runoff volume measured at Gold Creek is 2.41 times the runoff volume at
Talkeetna. For just the open-water flow season the average ratio is 2.43. These values are based
on 44 years when the gage observations are concurrent. This period includes the 1987-1988
surge for the West Fork Glacier but not the 1952-1953 surge of the Susitna Glacier. Table 1
shows the variability of this ratio for open-water periods and the values for 1987 and 1988 (flows
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for 1952 and 1953 are not available for the Talkeetna gage). The first year of the surge is nearly
equal to the long-term average and the second year is at the midpoint of the upper quartile.
Because there are four non-surge years that have ratios greater than 1987, there is no evidence
that Gold Creek flows in 1987 were unusual within the period of record. The year following the
cessation of the surge (1989) also shows no unusual difference when compared to the Talkeetna
gage and is slightly below average. Therefore, this is further indication that the occurrence of
surges in the Susitna glaciers does not appear to have an appreciable hydrologic effect on flow
volumes further downstream in the basin and thus sand delivery to the proposed Watana
Reservoir.
Table 1. Comparison of Gold Creek to Talkeetna Flow Volumes.
Quartile or Year observed Ratio of Open-Water Flow volumes
(Gold Creek/Talkeetna)
Minimum 1.92
1st Quartile 2.21
Median 2.42
3rd Quartile 2.58
Maximum 2.98
1987 2.46 (Rank is 24 of 44)
1988 2.78 (Rank is 40 of 44)
1989 2.34 (Rank is 16 of 44)
6.2.2. Silt-clay Transport
The volume of silt and clay transported is dependent on the supply from upstream. The total
glaciated area upstream of the Denali gage is on the order of 250 mi2 (Clarke et al. 1986) and the
contributing drainage area at the gage is 950 mi2, which provides a ratio of glaciated area to total
area of 0.26. The glaciated area produces approximately 34 percent of the flow for this drainage
area (ISR, Study 7.7, Section 4.2.4.2). If the average annual suspended sediment concentration
(silt/clay and sand) at the Denali gage (1,000 ppm) is divided by these ratios to reflect the
downstream dilution (Guymon 1974), the average annual sediment concentration at the glaciers
under non-surge conditions should be on the order of 3,800 ppm based on area to 2,900 ppm
based on flow, which is within the range of measured values (Gaddis 1974; Harrison et al. 1994;
Raymond and Benedict c1990). Therefore, the maximum measured suspended sediment
concentrations (30,000 ppm) at the West Fork Glacier during the surge are close to a factor of 10
higher than under non-surge conditions.
Based on this information, if there is a glacial surge every 50 years within the basin based on the
historical record, and it is assumed that the higher concentration would be representative of the
entire 6-month open-water flow period (OWFP) of that year rather than for a short period of time
(weeks), this would very conservatively provide about 55 years of total load within a 50-year
period since the silt-clay fraction represents about 50 percent of the total suspended load. Based
on an estimated reservoir life of 850 years (Tetra Tech 2014c) this could represent a reduction in
reservoir longevity by about 10 percent to 770 years. If, on the other hand, the elevated
suspended concentration (30,000 ppm) was applied for only the measured duration
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(approximately 2 weeks above 10,000 ppm of a single 6-month OWFP), this would provide
approximately 50.5 years of total load in a 50-year period. This would represent about a one
percent reduction in reservoir longevity (from 850 to 840 years).
Although it is highly unlikely that sediment concentrations at the glacier would be at the upper
limit for an entire OWFP, the period could also be more than 2 weeks given the greater size of
the West Fork and Susitna Glaciers compared to the Variegated Glacier. The assumption that
silt/clay and sand concentrations are approximately equal is based on downstream measurements.
Because sand is deposited in the extensive braid plains below the glaciers, the proportion of sand
is probably much higher at the glacier. This assumption makes either calculation conservative
resulting in greater reductions in estimated reservoir longevity. Therefore, the results are
probably conservative, though likely closer to the lower one percent estimate of increased long-
term sediment deliver to the reservoir.
7. CONCLUSIONS
1. Two large surging-type glaciers with a surge cycle of about 50 years are located within the
Upper Susitna River Basin. The West Fork Glacier surged in 1935/1937 and again in
1987/1988. The Susitna Glacier surged in 1952/1953 and was expected to surge again in the
first decade of the 21st century. The fact that it has not surged again could be taken as
evidence that surging is not required to maintain glacial equilibrium under the warming
climatic regime (W.D. Harrison, personal communication, 2012). However, the effects of
climate warming on the frequency and magnitude of glacial surge cycles are as yet unknown
(Turrin 2014).
2. Glacial surges result in increased ablation losses and increased runoff from the glacier.
Analysis of the hydrologic record at the USGS Gold Creek gage indicated that the 1952/1953
and 1987/1988 surges had no apparent effect on either the mean annual discharge, the
average May-September discharge or the maximum daily discharge and thus would have had
no significant influence on the volume of sand transported to the Watana Reservoir.
3. Glacial surges also increase the suspended sediment discharge from the glacier for relatively
short periods of time by an order of magnitude. Measurements at the outlet to the West Fork
Glacier during the 1987/1988 surge indicated suspended sediment concentrations of up to
30,000 ppm which is an order of magnitude higher than under non-surge conditions (3-4,000
ppm) and 30 times higher than the average measured annual suspended sediment
concentration (1,000 ppm) at the USGS Denali gage under non-surge conditions.
4. If a glacial surge was to occur within the Upper Susitna Basin that is very conservatively
estimated as producing 30,000 ppm concentrations for an entire OWFP during the year, then
the elevated silt-clay fraction of the annual suspended sediment load could result in 55 years
of sediment delivery to the proposed Watana Reservoir within a 50 year period. This could
reduce the longevity of the reservoir by approximately 10 percent from 850 to 770 years. If
the elevated concentrations are applied to the measured duration of approximately 2 weeks,
the surge could reduce reservoir longevity by about 1 percent (850 to 840 years).
5. Based on this review and evaluation, no further geomorphic investigations are warranted for
flow or sediment production from glacial surges. This includes a recommendation to not
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include a glacial surge sediment loading scenario in the reservoir sediment trap efficiency
and sediment accumulation modeling.
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Fullerton, September 10, 2012.
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40 (135), 241-254.
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surge-type glacier. Eidg. Tech. Hochschule,<:firich. Versuchanst. WasStTbau, Hydro!.
Gladol. Milt. 90, pp. 55-56.
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hydroelectric development. Annals of Glaciology, 4, pp. 99-104.
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decrease in land-terminating glacier speeds. The Cryosphere, 6(2), pp. 467–478.
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Kamb, B., C.F. Raymond, W.D. Harrsion, H. Engelhardt, K.A. Echelmeyer, N. Humphrey, M.M.
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Labelle, J.C., M.S. Arend, L.D. Leslie and W.J. Wilson. 1985. Susitna Hydroelectric Project:
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surges? Annals of Glaciology, 36(1), pp. 14–20.
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9. FIGURES
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Figure 1. Map showing the upper Susitna River Basin including the locations of the West Fork, Susitna, East Fork,
Maclaren and Eureka Glaciers. Project River Miles (PRM) are also shown from the upstream end of the Watana
Reservoir pool (PRM 230) to the terminus of the Susitna Glacier (PRM 320).
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Figure 2. Photograph of the lower reach of the Susitna Glacier showing potholes that are considered to be indicative of
glacier stagnation. The photograph was taken on September 19, 2013.
Figure 3. Upstream view of the confluence of the West Fork Susitna River with the Susitna River at PRM 301. The
photograph was taken on September 19, 2013.
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Figure 4. Total suspended sediment rating curves for the Denali (PRM 292), Cantwell (PRM 225) and Gold Creek (PRM 140) USGS gages on the Susitna River.
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Figure 5. Upstream view of the Denali Highway crossing the Susitna River at PRM 292. The USGS Denali gage was located at the bridge between 1958 and 1986. The braid bars in the river are composed of sand. The photograph was
taken on September 19, 2013.
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Figure 6. Ranked mean annual discharge, average May-September discharge and maximum daily discharge for 50 years of flow record at the Gold Creek gage (Tetra Tech ISR Study 6.6., Appendix E).
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