HomeMy WebLinkAboutAPA2827SUSITNA HYDROELECTRIC PROJECT
Document No.2827
Susitna File No.4.3.1.2
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GEOMORPHIC CHANGE IN THE MIDDLE SUSITNA RIVER
SINCE 1949
Report by
Arctic Environmental
Information and Data Center
Joseph C.Labelle
Margaret S.Arend
Lynn D.Leslie
William J.Wilson
Under Contract to
Harza-Ebasco Susitna Joint Venture
Prepared for
Alaska Power Authority
.....t
Final Report
June 1985
ARLIS
Alaska Resources
Library &Infonnation SerVIces
Anchorage,Alaska
n
i \
I
I
NOTICE
ANY QUESTIONS OR COMMENTS CONCERNING
THIS REPORT SHOULD BE DIRECTED TO
THE ALASKA POWER AUTHORITY
SUSITNA PROJECT OFFICE
....................Sedimentation ••••••••••••••••••••
SUSITNA GLACIER SURGE •••••••••••••••
TABLE OF CONTENTS
LIST OF FIGURES ••••••••••••••••••••••••••••••••••••••••••••••••
L1ST OF TABLES .
I LIST OF APPENDIXES •••••••••••••••••••••••••••••••••••••••••••••
S~Y••••••••••••••••••••••••••••••••••••••••••••••••••••••••
INTRODUCTION •••••••••••••••••••••••••••••••••••••••••••••••••••
PURPOSE AND SCOPE ••••••••••••••••••••••••••••••••••••••••••••••
METHODS ••••••••••••••••••••••••••••••••••••••••••••••••••••••••
GEOMORPHIC CHANGES ••••0 ••••••••••••••••••••••••••••••••••••••••
PROBABLE CAUSES ••••••••••••••••••••••••••••••••••••••••••••••••
POSSIBLE INFLUENCES ••••••••••••••••••••••••••••••••••••••••••••
........................
Temporarily Increased Flows ••••••••••••••••••••••••••
Page No.
i
ii
iii
1
5
7
9
12
17
20
20
21
GREAT ALASKA EARTHqUAKE.....................................24
CONCLUS IONS.• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •28
EFFECTS OF THE PROJECT ••••••••••••••••.........................31
REFERENCES.• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •33
APPENDIXES ••
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...................................................35
Fig No.
1
LIST OF FIGURES
Page No.
Annual flood peak discharges,Susitna River
at Gold Creek •••••••••••••••••••••••••••.•.•••.•••.•••••.•.19
2 Areas of uplift and subsidence,1964 Alaska
Earthquake.•• •• . • . • . • •• . . . . . • . . . . • . . •. . . . . . • • . . •• • . . . • . • . . .26
i
LIST OF TABLES
Table No.Page No.
1 USGS discharge records for the Susitna River
at Gold Creek..............................................10
2 Slough changes since 1949 ••.•.••.•••••..••••••••.•••.••....
ii
16
Appendix A.
Appendix B.
Appendix C.
LIST OF APPENDIXES
Page No.
Comparative geomorphology,1949-present •••••••••••••••35
General descriptions of geomorphic change
since 1949............................................42
Fish habitat conditions •••••••••••••••••••••••••••••••51
iii
SUMMARY
In order to determine the natural geomorphic regime -the balance between
aggradation and degradation -of the middle Susitna River (between Devil Can-
yon and Talkeetna),analysis of geomorphic change since 1949 was carried out
by studying aerial photography for the periods 1949,1961-62,and 1977-82.
The purpose of the study was to understand how sloughs,which provide impor-
tant fish spawning and rearing habitats,have been evolving in recent decades.
This information could then be used to forecast the effects of the Susitna
Hydroelectric Project on natural slough evolution.
Photographic analysis showed the following:
1.Gravel bars and islands have apparently become more exposed,and
increasingly vegetated and,as a result,better stabilized.Several
gravel bars have become attached to the shoreline forming new ter-
races.
2.At eight places in the middle river,the mainstem channel has pro-
gressively shifted alignment since 1949,eroding out all or parts of
old islands and shoreline.
3.Many sloughs appear or relatively higher in elevation compared to
the water surface elevation of the mainstem at a given discharge.
Some sloughs have evolved from side channels to side sloughs,and
others from side sloughs to upland sloughs.In one case,a side
channel has evolved to an upland slough.
33RD4-011 - 1 -
The apparent emergence of gravel bars and islands,formation of new ter-
races,and perching of sloughs is probably due to a number of processes.Some
cases may be due to migration or deposition of gravel bars;deposition of new
material on old gravel bars,islands,and slough berms;and alteration by ice
processes.Analysis of the middle river as a whole,however,leads the au-
thors to believe that general river degradation since 1949 is responsible for
much of the noted change.Although some features have been eroded away during
that time,and some new gravel bars have appeared,most of the river's fea-
tures show evidence of emerging higher above the water surface at any given
discharge.The existence of several levels of terraces flanking both sides of
the middle river indicates that this process has gone on in the past,and
suggests that it may be continuing.Floodplain features are now apparently
less easily inundated by high mainstem flows,and vegetation has taken hold
and progressed through successional stages.
Changes in flood severity during the period since 1949 do not account for
the decreased overtopping of these features.Flooding increased from 1962-72,
and then decreased to about the same levels as 1949-61.However,the spread
of vegetation progressed steadily during these periods.
When a slough evolves through mainstem degradation,fish habitat condi-
tions within it change.As a side slough forms,conditions may become suit-
able for fish spawning or rearing.Continued evolution may change those con-
ditions over time and eventually make the slough habitat unsuitable for fish
spawning.However,the slough may remain suitable for rearing.Meanwhile.
however,new side sloughs are being converted from side channels,so that
there is probably always some suitable fish spawning habitat available.
The 1952-53 surge of Susitna Glacier does not appear to have had any
significant effect on the middle river's geomorphic regime.Bedload sediments
33RD4-011 - 2 -
j I
released by the surge were probably deposited initially in the upper river and
progress slowly through the middle river without accumulating there.
Temporarily increased river flows,caused by the glacier terminus moving
farther down into the ablation zone,do not appear to have significantly
altered river discharges during the few years after the surge.
The 1964 Alaska Earthquake caused a southward tilt of the Susitna River
basin of about 1.~·feet over a distance of 320 miles.This small amount would
have,at most,a very minor effect on the river's geomorphic regime.Addi-
tionally,the middle Susitna river flows almost perpendicular to the direction
of tilt,and the lower river is only slightly less so.The middle river
would,therefore,only begin downcutting after the lower river had deepened
its valley,and then only near the confluence.
The rate of general degradation throughout the middle river appears to
have been steady since 1949,and was probably not significantly affected by
the earthquake.However,there has been erosion in the confluence area,but
about 1/4 of it had occurred before the earthquake.It appears that the ero-
sion was caused by shifting of the main channel of the Chulitna River at the
confluence.Although the earthquake may have exacerbated the erosion,it does
not appear to have caused it.
After an initial period of riverbed scour,the operation of the Susitna
Hydroelectric project would probably stabilize the riverbed and cause the
apparent degradation regime in the middle river to slow substantially.
Evolution of sloughs from one type to another would stop.Slough types would
remain essentially the same as they were at project start~p,and slough fish
habitat would no longer be altered through perching of sloughs.
Slough habitat conditions,however,might still change due to project
operations.Controlled river flows would substantially reduce the frequency
33RD4-011 - 3 -
of overtopping of sloughs that normally occurs during high summer flows.This
would prevent the sloughs from being flushed of beaver dams anq accumulated
fine sediments contributed by small tributary streams.Unless remedial mea-
sures are taken,geomorphically stable sloughs are made unsuitable for fish
habitat by such processes as these,the total amount of suitable slough habi-
tat might be diminished over time since it would not be replenished through
slough evolution.
33RD4-011 - 4 -
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INTRODUCTION
All river systems undergo a natural process of geomorphic evolution over
time.Although their geomorphic regimes -the balance between erosion and
deposition over a period of years -may remain relatively stable at times,
river channels often aggrade or degrade to adjust to changes in local climate,
runoff,sediment supply,or tectonic effects on land slope.A river usually
remains under a given regime until one or more of these factors changes,or
until conditions remain stable long enough for the river to aggrade or degrade
its channel to an equilibrium condition.A certain regime may last from a few
years to thousands of years,depending on the frequency and severity of causa-
tive factors.
Since the end of the Pleistocene ice age about 10,000 years ago,the
Susitna River in the Devil Canyon-to-Talkeetna reach,(hereafter called the
middle river)has undergone general valley deepening and canyon cutting.Most
of the middle river is incised into canyons a few hundred feet deep.The
modern channel sits below the level of a series of old sedimentary terraces on
either side of the river.However,this general,long-term degradation might
have been interrupted at times by change or even reversal of its regime,and
it is of interest to know what regime has been active in recent decades in
order to assess its effects on fish habitat.
Fish habitat investigations in the middle Susitna River,related to the
proposed Susitna Hydroelectric Project development,have concentrated on the
river's sloughs,because they constitute the bulk of salmon spawning habitats
that would be affected by the project.Slough types and their suitability as
fish habitat are dependent,in part,on river channel evolution and stability.
Aggradation or degradation of the river could alter slough types and cause
33RD4-011 - 5 -
natural fish habitat transformation.Some sloughs might evolve into more
favorable habitat conditions (suitable water depths,temperatures,velocities,
and substrate conditions;see appendix D),while others might evolve into
]
I poorer habitats.Investigators do not know how long presently-favorable
f I
slough habitats have existed,since systematic slough habitat investigations
in the middle river by the Alaska Dept.of Fish and Game (ADF&G)have only
been carried out since the mid-1970s.
In order to determine geomorphic change in the middle Susitna River since
1949,the date of the earliest aerial photography,a photo interpretive analy-
sis of aerial photography from the beginning,middle,and end of the period
was undertaken.This study was conducted as part of the ongoing effort to
characterize fish habitats in the Susitna River and to determine the effects
of the proposed Susitna Hydroelectric Project on them.
33RD4-011 - 6 -
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PURPOSE AND SCOPE
In order to assess natural processes leading to slough habitat changes in
the middle Susitna River,it is necessary to know whether the river is now
aggrading,degrading,or stable,and whether sloughs have evolved from one
type to another in recent years.This information could give an index of
natural habitat stability that might provide clues to the effects of hydro-
electric development on future habitat stability.Proj ect operations would
alter the river's natural geomorphic regime by controlling seasonal flows,
trapping bedload sediments in the reservoirs,causing riverbed scour down-
stream of the dams,and changing river ice processes.This altered regime
might disturb the natural evolution of the river system and fish habitat suit-
ability.
Accordingly,we undertook an analysis of aerial photography of the"middle
Susitna river taken in three periods during the past 36 years:1949 (the first
year for which aerial photography was available),1961-62 (just before the
1964 Alaska Earthquake),and 1977-80.Also used for comparison was the 1982
aerial photography presented in EWT&A (1984).The purposes of the study were
to determine:
1.whether aggradation,degradation,or stability has been the dominant
geomorphic regime during the period from 1949 to present;
I III
2.
33RD4-011
whether significant geomorphic changes,such as alteration of slough
types,have occurred during that period;
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3.whether there is any apparent evidence of alteration of regime
caused by the two natural catastrophic events that occurred in the
Susitna Basin during the period:the 1952 Susitna Glacier surge
(Post 1960),and the 1964 Alaska Earthquake (Plafker 1969).
I III, I
[I
II
4.
33RD4-011
whether the proposed Susitna Hydroelectric Proj ect would have any
effect on the geomorphic regime of the middle river,and therefore
on fish habitat in sloughs.
- 8 -
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II
[I
METHODS
For comparison purposes,sets of photographic prints were assembled for
the earliest time for which aerial photography was available (1949 USGS
black-and-white photography),for very recent years (1977-80 Bureau of Land
Management infrared photography,and 1982 black-and-white photography),and
for a period about midway (1961-62 USGS black-and-white photography).The
middle period was selected for two reasons:(1)it would show whether any
identified geomorphic regime was in effect continuously from 1949 to the
present,and (2)it would allow examination of the river just before the 1964
Alaska Earthquake.
Photography of the entire middle river below Devil Canyon was examined
for evidence of general geomorphic changes since 1949,and imagery of several
individual sloughs that have been investigated by ADF&G (Friese 1975)was
analyzed more closely to discern possible changes in slough types.Three
additional sloughs,herein designated 8E,8F,and 8G,were also examined.
These are shown in EWT&A (1984)between sloughs 8A and 8B (appendix A).
The year 1949 was also the first year the U.S.Geological Survey (USGS)
kept continuous records of r1ver discharge for the middle river.Using these
records,we gathered photography for all periods with similar discharges,
which gave us a rough water surface elevation base level for comparisons be-
tween years.It was possible to obtain photography for most periods with a
discharge near the range 23,000-30,000 cfs.The only significantly different
year was 1977 with 41,000 cfs;this photography was used only for a short
reach of the river near the Susitna/Chulitna confluence.Daily discharges
from USGS records for each photo set are shown in table 1.
33RD4-011 - 9 -
Table 1.USGS discharge records from Gold Creek station,corresponding to
dates of aerial photography.(Sources:USGS 1957, 1961, 1962, 1978,
1979,1980,1981)
Photography date
August 10,1949
August 14,1949
July 14,·1961
July 5,1962
August 10,1962
August 11,1962
June 19,1977
August 1,1980
August 11,1980
June 1,1982
Discharge (cfs)
29,900
28,600
25,000 (est.)*
25,900
23,000 (est.)
23,000 (est.)
41,000
31,100
22,600
23,000
[I
(I
I
i I
*(est.)indicates no record for that date;discharge estimated on basis of
weather records,and records from other stations.
33RD4-011 -10 -
II
I i
II
Since the photography of the river was taken at somewhat different
scales,the map sets in Appendix A were adjusted to bring all illustrations to
approximately the same scale of 1:60,000 (about one inch to one mile).
Earlier analysis of 1949-51 aerial photography from old USGS glass
slides,discussed in a draft preliminary report of this study (AEIDC 1984),
appeared to show that there were fewer gravel bars in the middle river at that
time than there are now.Further investigation showed that the photography had
either been poorly processed or had deteriorated with age,underexposing the
river and its features in a way that washed out the presence of unvegetated
gravel bars in the river,making those areas appear as open water.New and
better photography,all from 1949,was obtained from the USGS for the final
phase of the study,and show that the number of gravel bars in the river has
not significantly changed.This fact does not change the overall conclusions
of the draft report in r"egard to riverbed degradation and slough perching
since 1949,which are substantiated here.
33RD4-011 -11 -
GEOMORPHIC CHANGE
Analysis of aerial photography of the middle Susitna River from 1949 to
1
\present shows that geomorphic changes have occurred throughout the reach.The
overall evolutionary trend has been as follows:
1.Gravel bars and islands have become more exposed,better stabilized,
and increasingly vegetated.That is,they now apparently stand
higher above the water surface at a given discharge,are not
overtopped and eroded as frequently by floods,and vegetation has
taken hold on them and passed through successional stages.
Vegetation becomes established on gravel bars when the bars are not
flooded frequently or severely enough to erode away the topsoil in which
plants grow.If the gravel bars remain unflooded long enough,vegetation
succession takes place,in which plants grow to maturity and then are slowly
replaced by other species which dominate the site.In the Susitna Basin,
vegetation succession on gravel bars usually starts with the establishment of
stands of horsetail,horsetail-willow,horsetail-balsam poplar,or dryas.
Within 10 to 25 years after a site is stabilized,thinleaf alder succeeds the
earlier vegetation,followed within 25 to 55 years after stabilization by
immature balsam poplar stands of tall shrubs or trees.Later,75 to more than
100 years after stabilization,mature balsam poplar dominate the site,fol-
lowed finally by birch-spruce forest (McKendrick et al.1982).
In all parts of the middle river,many old,barren gravel bars have now
become more exposed and vegetated.Some are now stabilized,vegetated is-
lands.The vegetation has steadily progressed from initial grassy cover to
33RD4-011 -12 -
low brush and finally to high brush and small trees.(At the scale of the
photography,species identification is not possible without ground-truth in-
formation,and was not attempted).Many barren gravel bars that were attached
to shore have now become vegetated terraces,and some gravel bars originally
near shore have now become attached to shore and are now vegetated.Some
nearshore gravel bars also have become terraces,while others are trending
that way but have not yet become fully isolated from the river.Examples of
each type of change can be found in appendix C.
Overflow channels in some islands and terraces,that flooded during high
flows in 1949,are now emerged so high relative to the water surface of the
river that they are seldom flooded and have become increasingly vegetated.
Also,older forested terraces and islands that have long-abandoned overflow
channels exist throughout the length of the middle river.These are now fully
vegetated,sometimes to mature vegetational stages.
On many islands that were already stable and vegetated in 1949,new grav-
el beaches have become increasingly exposed,especially on their gentlysloped
upstream and downstream ends.Vegetation has encroached onto these new-
ly-exposed beaches in a manner similar to that on barren gravel bars.(See
appendix C).
All of these changes are most noticeable in split-channel reaches of the
river and are not as apparent in single-channel reaches.This is interpreted
to be an artifact of the method of analysis.Increased exposure,or emergence,
of features shows up in aerial photography as lateral increases in area,which
only occurs on gentle slopes.Features with vertical or very steep side
slopes show little or no increase in area with emergence.Split-channel
reaches of the river have more gravel bars,islands,and terraces with gentle
side slopes,while single-channel reaches usually lack these features and have
33RD4-0l1 -13 -
steep banks.In braided reaches,which occur only from RM 102 to the
Susitna/Chulitna confluence,rapid and continuous shifting of channels makes
this kind of analysis more difficult and less meaningful.
2.At eight places in the middle river,the mainstem channel has pro-
gressively shifted since 1949,eroding out all or parts of some old
islands or shoreline,while depositing or exposing new gravel bars
in the old position of the channel.This happened at river miles
(RM)113.8,119.5,125.0-125.5,130.0,132.2,133.6,and 139.0 and
142.4 (appendix A).
Even in these locations,however,the main evolutionary trend has been
toward increasing land exposure,stability,and vegetation.This is apparent
on nearby gravel bars and islands on all sides of these eroded areas.
3.Some sloughs have come into existence since 1949,some have changed
character and/or type,and others have not yet changed enough to be
noticeable in the photography.Details of slough changes are shown
in table 2.
Many sloughs have evolved from side channels to side sloughs,and others
from side sloughs to upland sloughs.In one case,slough evolution has pro-
gressed from side channel to upland slough.Of the sloughs studied,only
slough 11,which was observed to be altered by ice jam flooding and erosion in
1976,changed in the opposite direction,from an upland slough to a side
slough.
33RD4-011 -14 -
With the exception of slough 11,all of the studied sloughs that have
changed type appear to have emerged;that is,they,or at least their berms,
are now higher in elevation relative to the mainstem water surface at a given
discharge.Of the 34 sloughs studied,15 have changed type.Fourteen of
those have emerged,while one (slough 11)has been eroded out and lowered in
elevation by ice processes (table 2).
Of the 19 sloughs that did not appear to have changed type,9 were al-
ready upland sloughs and would have remained so if they had become further.
perched.Many side sloughs changed in morphologic character even when they
did not evolve enough to change type.At present,some have less wetted sur-
face area at a given discharge,and some appear to have larger berm areas
separating them from the mainstem than in 1949,perhaps suggesting higher berm
heights.At some sloughs,enclosing gravel bars,which separate the sloughs
from the mainstem,have grown larger by emergence and/or deposition,and are
often further vegetated.
Only one of 10 sloughs below Lane Creek changed type,although 3 of these
were already upland sloughs..Even in this area,however,some degree of
perching at present is still suggested by decreased wetted surface areas in
some sloughs and vegetation excroachment on gravel bars since 1949.
33RD4-011 -15 -
Table 2.Slough-changes,middle Susitna River,since 1949
(Sloughs listed in order ascending upstream)
Slough 1949 1961-2 1977-83 Emerged?Reach type
SC
US
SC
SC
SS
SS
SC
SS
US
SC
US
US
SS
SC
SC
SC
SC
US
SS
SC
SC/SS
sc
sc
r
I i
1 SC
2 SS
Whiskers SC
3A US
3B SS
4 US
5 US
6 US
6A US
7 ?
8
8D
8C
8B
8E
8F
8G
8A
9B
9A
10
11
12
13
14
15
16
17
19
20
21
22
21A
SS
SS
SC
US
SS
US
US
US
US
?
SS
US
SC
SC
SS
SS
SC
SS
US
SC
US
US
SS
SS
SC
SS
SC
US
SS
sc
SC/SS
SC
SC
SS
SS
SC
US
SS
US
US
US
US
SS
SS
US
SS
SC
US
US
SS
SS
US
SC
US
SS
US
SS
SC
US
SC
US
US
SS
SC/SS
SC
SS
Yes
No
No
No
No
No
No
No
No
?
Yes
No
Yes
Yes
Yes
Yes
Yes
No
No
Yes
No
No
Yes
Yes
No
Yes
No
No
Yes
Yes
Yes
No
Yes
Split
Split
Split
Split
Split
Single
Single
Single
Split
Single
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split
Split -
Split
Split
Split
Split
Split
DEFINITIONS
(Source:EWT&A 1984)
SC -Side Channel.These contain turbid,glacial waters,the same as found in
the mainstem.These channels convey less than ten percent of the total flow.
SS -Side Slough.These contain clear water.Local surface runoff and
upwelling are the primary water sources that supply clear water to the side
sloughs.Side sloughs have non-vegetated upper thalwegs that are overtopped
during periods of moderate to high mainstem discharge.Once overtopped,side
sloughs are considered side channels.
US -Upland Sloughs.These contain clear water and depend upon upwelling
and/or local runoff as their clear water sources.Upland sloughs possess vege-
tated upper thalwegs that are seldom overtopped by mainstem discharge.
Emerged -indicates a floodplain feature has risen higher relative to the
water surface elevation,at any given discharge,since 1949.
33RD4-011 -16 -
PROBABLE CAUSES
The apparent emergence or increased exposure of gravel bars and islands,
formation of new terraces,and the perching of sloughs could be caused by a
number of processes.Some individual cases can probably be explained by the
migration of gravel bars;deposition of new gravel bars and new material on
old gravel bars,islands,and slough berms;and alteration of features by ice
processes.These processes can cause apparent emergence of features with
I 'I
I
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associated reduction in frequency and severity of flood overtopping events.
Migration of the mainstem channel across its floodplain can result in de-
creased flooding of certain floodplain features and consequently an increase
of vegetation coverage of those features.Localized aggradation or degrada-
tion of the river bed could also account for many of these changes.
Many of these processes were active during the period studied and proba-
bly account for some of the cases of apparent emergence of features reported
herein.Analysis of the middle river as a whole,however,leads the authors
to believe that general river degradation since 1949 probably is responsible
for much of the noted change.Although some features have been eroded away
during that time and some new gravel bars have appeared,especially in old
positions of the migrating mainstem,most of the reach's features show evi-
dence of emergence.We believe that,as the river slowly eroded its bed,the
mainstem water surface elevation at any discharge lowered in relation to
I '
floodplain features.The existence of several levels of terraces flanking
both sides of the middle river at several places (as many as five levels can
be seen at RM 110)indicates that this process has gone on in the past,and
suggests that it may be continuing.
33RD4-011 -17 -
.1
These floodplain features are now less easily inundated by high mainstem
flows.Emergence has caused some side channels to be converted to side
sloughs.Some berms separating sloughs from the mainstem have become too high
to be overtopped at all,changing the associated sloughs from side sloughs to
upland sloughs (see table 2).Old overflow channels on some islands and ter-
races now seldom carry flows and are becoming vegetated.Reduced effects of
overtopping flood events,which allowed vegetation to take hold,could result
either from the emergence of these features to a higher elevation relative to
a given discharge,or from a reduction in the severity of flood events.An
analysis of flood events (H-E 1985A)shows that the maximum annual flood in-
creased during the period 1962-72,with six of those years having floods be-
tween the lO-and 50-year recurrence interval.Maximum floods then declined
again during.the period 1973-82 to about the same levels as during 1949-61
(figure 1).
The greater severity of flooding from 1962-72 would have caused increased
overtopping of gravel bars and islands if they had remained at the same eleva-
tions relative to a given discharge.Photo analysis shows that vegetation of
most features appeared to progress at a steady rate during the period 1949-82
and these features were not affected by increased flooding during 1962-72.
Therefore,the increased vegetation coverage of gravel bars and islands is
probably due to their emergence.
Erosion of gravel bars and islands at eight sites in the middle river was
caused by progressive shifting of the mainstem channel alignment in those
parts of the river.In many of these cases the mainstem in 1949 has been
converted to a side channel at present,and a side channel in 1949 has changed
to the present mainstem.
33RD4-011 -18 -
Figure 1.
ANNUAL FLOOD PEAK DISCHARGES
Susitna River at Gold Creek
10
--C)
C)
C)80
~
x
(J)
4-
U 60--
w
UJ
0:::<:;:r:40uen---0
20
o
1950 1955 1960 1965 1970
1975 1980
50 --(J)
25 '-(0
Q)
>---10 --.J<:>5 0:::
W
I-
Z---2 wuz
W
0:::
0:::
::J
U
W
0:::
WATER YEAR
Source::modified from H-E 1985A.
~-~_.'
POSSIBLE INFLUENCES
During the period from 1949 to present,two natural catastrophic events
occurred in the Susitna Basin that could have affected the river's geomorphic
regime..These were the 1952 surge of the Susitna Glacier and the 1964 Alaska
Earthquake.
SUSITNA GLACIER SURGE
In 1952 or 1953 The Susitna Glacier underwent a surge,a period of rapid
forward motion of the glacier,that extended the glacier terminus down valley
about 2.5 miles (Post 1960,Meier and Post 1969).When a glacier surges it
may have two effects on river regime below it.There may be a large sediment
discharge and a temporary (a few years)increase in melt water production
caused by an increased ablation area (area within in the seasonal melting
zone)(Harrison et al.1983).
SEDIMENTATION
Most of the sediment released from beneath a surging glacier is probably
suspended sediment.It occurs as water rushes from beneath the surging ice
and lasts for a short period.The amount of bedload sediment that accompanies
the suspended load is unknown,but most of it would probably be deposited a
short distance below the glacier terminus due to the abrupt decrease in water
velocities upon exiting from beneath the glacier.It would then take a period
of time for river flows 'to move the bedload further downstream.The bedload
sediments would eventually be distributed over a certain distance downstream,
and the amount deposited would decrease with distance.
33RD4-011 -20 -
r!
\'
The reach of the river investigated in this study begins at the mouth of
Devil Canyon,nearly 170 miles downstream from the glacier.Analysis of the
aerial photography from 1949 to present indicates that the number of exposed
gravel bars has not significantly changed.It seems that little,if any,of
the bedload sediment released by the Susitna Glacier surge has reached this
part of the river and does not account for the morphologic changes seen since
1949.
TEMPORARILY INCREASED FLOWS
When the Susitna Glacier surged,it moved the glacier terminus forward
about 2.5 miles.This put the terminal area at a substantially lower eleva-
tion and increased the ablation area of the glacier.Normally,an increase in
melting of the glacier would occur,resulting in an increase in its contribu-
tion of meltwater to the Susitna River.The increased discharge would proba-
bly peak the first summer and then proceed at a decreasing rate for several
years until the terminus retreated to approximately its original position.
For several years after 1952-53,then,discharges in the middle Susitna
Riv~r may have been somewhat higher than they would have been under normal
conditions.A cursory look at Susitna river discharge data from Gold Creek
did not reveal any notable increase,but climatic variability could have
masked it..For this reason a statistical analysis of the data was conducted
to look for any important variations in discharge that were not linked to
climatic factors.
Several multiple regression analyses were performed for time blocks of
weekly,monthly,and interannual discharge characteristics.In each of the
three test cases (weekly,monthly,and interannual)the dependent variable was
i I
33RD4-011 -21 -
I
I
streamflow rates in ft 3 /sec from the Gold Creek station during open water
season.
The thrust of the analysis for the two former cases (weekly and monthly)
was to observe whether or not a correlation exists between streamflow rates,
air temperature,and precipitation.At the onset of the open water season,
the fluctuation of the air temperature above and below normal should influence
the rate at which the snowpack melts and,thereby,inputs water into the
streamflow system.Similarly,the amount of measured precipitation for the
respective water weeks and months during open water season would influence the
water levels in the system.The analysis was performed for the years 1950 to
1958 and 1970 to 1975.
Our hypotheses stated that these climatic factors should account for the
explained variance in the streamflow rates.Any residual unexplained variance
in the years following the glacial surge might be attributed to the surge.
In both the weekly and monthly open water cases there was a fairly high
correlation between temperature and streamflow (R 2 from .54 to .87),and lower
correlation (R 2 from .19 to .56)between precipitation and streamflow.The
variation from year to year for the different weekly and monthly cases did not
point to any pattern that might be attributable to the glacial surge during
the 1952-53 season.Therefore,the results from these analyses were inconclu-
sive and will not be presented in detail.
The first two test cases did provide some physical insight as to the
climatic mechanisms controlling streamflow rates in the basin.The relatively
high correlation of temperature,and low correlation of precipitation,to
streamflow indicated that the depth and density of the snowpack was probably
important as a water source for the system.Thus,a third test case was de-
signed to examine the correlation between total seasonal snowfall,in inches,
33RD4-011 -22 -
from Talkeetna,and precipitation measurements,in inches,during open water
season (April to October)from Talkeetna,with the average streamflow rates in
ft 3 /sec from the Gold Creek station for the open water season for the years
1952 to 1979.
Talkeetna is the only station within the basin which has enough detail
and period of record to indicate long-term climatic trends.It would be more
desirable to have measurements of snow depth or water equivalent inches on the
ground rather than snowfall data,bu~none is available within the Susitna
drainage basin p~ior to 1964.It would also be preferable to use snow,pre-
cipitation,and flow data from stations further upstream,nearer to the gla-
cier and nearer to each other,but these do not exist.The distances and
geographic variability between Talkeetna,Gold Creek,and the Susitna Glacier
introduced potential error to the analysis.Nevertheless,it was the most
comprehensive data available,and if a substantial increase in streamflow due
to the glacial surge occurred,this was the most reasonable method available
by which it might be detected.
Temperature measurements in the interannual case were neglected.We
assumed that regardless of short-term variability in local air temperature,
that by the end of the open water season,all snow present on the ground would
have melted.
In the interannual case,analysis was performed on four-year time blocks
beginning in 1952 at the time of the glacial surge and extending through 1979.
The adjusted correlation coefficient (R 2 )ranged from a high of .95 to a low
of .26,but the lowest R2 value did not occur in the time period following the
1952-53 season.The second lowest R2 ,.39,did occur in the years immediately
following the glacial surge.
33RD4-011 -23 -
Thus,while the glacial surge might partially account for the variation
in streamflow rates in the weeks,months,and years following its occurrence,
we were unable to indicate its influence with certainty using a statistical
analysis.The lack of climate data from the upper basin makes the results
indicative only,and not conclusive.
GREAT ALASKA EARTHQUAKE
On March 27,1964,the largest earthquake ever recorded in North America
rocked southcentral Alaska.With a Richter magnitude of about 8.4,the earth-
quake was centered in northwestern Prince William Sound,but had effects over
a wide area.The 'land within a radius of hundreds of miles tilted,with sub-
sidence taking place throughout a broad area northwest of the epicenter.Sub-
sidence decreased with distance from the epicenter until reaching the line of
zero displacement,which lies near the middle Susitna River (figure 2).North
of that,slight uplifts were recorded (Plafker 1969).
Interpolation of measured displacements along the Alaska Railroad and the
Richardson Highway where they cross the Alaska Range indicates that the upper
Susitna River basin near the glaciers was uplifted about 0.5 feet.Interpola-
tion from measurements along the Alaska Railroad near Wasilla and Anchorage
indicate that the river mouth at Cook Inlet subsided approximately 1 foot.
Total southward tilt of the river basin,therefore,was approximately 1.5 feet
over a distance of 320 miles,increasing the river's gradient very slightly.
The zero displacement contour,or hinge line,crossed the upper Susitna River
near the Tyone River confluence.
Measured subsidence along the Susitna River,where the Alaska Railroad
follows it,showed a range of subsidence in the middle river varying from 0.2
to 0.6 feet from Gold Creek to the Susitna/Chulitna confluence,to a maximum
33RD4-011 -24 -
in the lower river of 0.8 feet between Kashwitna and Willow (Plafker 1969).
The upper river,above the Oshetna River confluence,lies parallel to the
direction of tilt,while the middle and lower river reaches are aligned per-
pendicular to the direction of tilt.Normally,the effects of seaward tilting
of a river basin cause it to downcut and to deepen its valley.However,the
effect is felt immediately only by streams that lie parallel to the direction
of tilt.Streams flowing perpendicular to the direction of tilt begin down-
cutting only after the stream it flows into has deepened its valley,lowering.
the base level of the first stream and leaving it out of adjustment.Even
then the effect is felt only at the mouth of the first stream rather than
along its entire length (Thornbury 1954).
Only the upper river flows parallel to the direction of tilt.The total
amount of tilt within that reach is about 0.5 feet.Consequently,the upper
river may have degraded slightly since 1964 as a result of the earthquake-
caused tilt,but the upper river has not been studied to confirm this.
The middle river lies almost exactly perpendicular to the direction of
tilt,while the lower river is'only slightly less so (figure 2).Therefore,
only a small rejuvenation effect would be expected in the lower river,a less-
er effect might have occurred near the Susitria/Chulitna confluence,and almost
no effect should have occurred in the middle river reach.
A large amount of erosion has occurred at the Susitna/Chulitna conflu-
ence,especially along the north bank of the Susitna and east bank of the
Chulitna,where they join.It appears that this erosion was caused by the
main channel of the Chulitna River swinging slowly to the east since 1949.
Most of this erosion did occur after the earthquake,but it had begun before
that,with perhaps 1/4 of the erosion occurring before 1962.Although the
effects of the earthquake may have exacerbated this effect,it does not appear
that the earthquake caused it.
•Falrbankl
ALASKA
600
Chlrlhf I.
(J
0 50 100 150 Kllom.'."I','1 ','i i IiI I I
0 50 100 150 Mil ..
Submarine contour.in f ••t
.......o'''~''''-_·····················_
.."b
....~
Conlour Ihowlno uplill (+1 or lublld.nc.(-I In f ••,
(Dtleh.d wher.oppro~lmo',;doll.d whore Inf.rr.d)
Approxfmol.oxfl 0'maximum uplill
Approxlmal.axil of moxlmum lubllden••
Arterlhockl
*'Acli••or dormonl volcono ..
1540 152.0 1500 1480 1460 1440
Figure 2.Areas of uplift and subsidence,
1964 Alaska Earthquake.
Source:Selkregg 1974
r I
\ I.)
Analysis of the aerial photography in the middle river shows that the
rate of apparent degradation between 1949 and the present has progressed con-
tinually throughout the period.There was no notable change in rate between
1949-62 and 1962-83.Therefore,it seems that there was little effect on the
middle river's geomorphic regime by the 1964 Alaska Earthquake.
33RD4-011 -27 -
CONCLUSIONS
Although some of the geomorphic changes seen in the middle Susitna River
since 1949 can be explained by locally important processes,such as localized
deposition,mainstem channel migration,or ice processes,the results of this
study appear to indicate that,during the past 36 years at least,the middle
river has been slowly degrading its bed as it did during the larger part of
its history since the ice age.The amount of degradation since 1949 is not
known but probably has not exceeded a few feet.
The river is probably in dynamic equilibrium over the short term (a few
years).That is,although most bedload movement takes place during high-flow
events,about the same amount of bedload sediment is carried into the middle
river as leaves it,and any excess local deposition or scour during a given
event is adjusted for later by sediment redistribution.However,superimposed
upon this regime there appears to be a long-term regime of slow riverbed
degradation as the river continually adjusts to glacier downcutting at the
head of the basin.
As the main channel degrades and lowers its water surface elevation,
gravel bars and other features protruding from the riverbed slowly emerge,
eventually standing high enough above the water surface that they are seldom
overtopped by floods.Those gravel bars near shore often become attached to
shore and eventually emerge high enough to become new terraces.Vegetation
then takes hold on these exposed surfaces and goes through natural succession,
eventually becoming mature forests.
As the mainstem migrates across its floodplain,old side channels often
become mainstem and old mainstem positions become side channels.As the river
cuts down,intervening berms between the mainstem and side channels become
effectively higher and some side channels evolve into side sloughs.As the
33RD4-011 -28 -
I II I
berms emerge even higher,some sloughs are less frequently flooded,until the
berms eventually become high enough to prevent any overtopping flows.These
sloughs have then evolved into upland sloughs.
When a slough evolves,fish habitat conditions (appendix C)within it
change.As a side sloughs forms,it may eventually reach a stage of develop-
ment in which water depth and temperature,velocity,substrate,and fish pas-
sage conditions become,for a time,suitable for fish spawning or rearing.As
the slough continues to evolve,those conditions change too,and all or part
of a slough may eventually become unsuited for fish habitat.This probably
takes several decades to occur.For example,as a slough mouth rises,water
depths over the entrance may become so shallow that -adult fish passage into
the slough is effectively prevented.When a side slough evolves into an up-
land slough,it is no longer flooded under any circumstances except at its
mouth,and changing conditions within the slough may diminish fish spawning
habitat,while it continues as important rearing habitat.
It is probable that some sloughs never become suitable fish habitat at
any time in their evolution.The requisite conditions may never be met.In
those sloughs that do become suitable,the length of time they remain suitable
is unknown,and probably varies considerably from slough to slough depending
on local conditions.
At any given time in the degradational process,there are probably some
sloughs entering the low end of the evolutionary cycle as others leave the top
of the cycle.In other words,some side channels are being converted to side
sloughs as other side sloughs are being altered to upland sloughs,_and some
upland sloughs are emerging high enough to be effectively isolated from the
river.If this is so,then some amount of suitable fish habitat might always
be available in the natural system.
33RD4 ....011 -29 -
f i
I I
I)
!IIi
The variation in quantity and quality of this habitat over time,however.
is unknown because of the short period of record in the middle Susitna River.
Also,some suitable slough habitat probably forms and is later altered through
unrelated,short-term sedimentation events,such as deposition or migration of
a gravel bar into a suitable position near the shore.
33RD4-011 -30 -
EFFECTS OF THE PROJECT
With the Susitna Hydroelectric Project in operation,the river's geomor-
phic regime would be altered.River flows would be regulated,with discharges
being lower than normal in the summer as water is stored in the reservoirs,
and higher than normal in the winter when power demands would be the greatest.
Flows would also be more stable than normal,with flood effects reduced in
severity except in extreme events.Bedload sediments normally carried
downriver would be trapped in the reservoirs above the mouth of Devil Canyon.
Therefore,little bedload would be carried into or through the middle river.
Studies show that the river would try to adjust itself to a new equilib-
rium condition.The main channel would tend to become narrower and more con-
fined,and may recede from the heads of some sloughs and side channels.Since
the river would be unburdened with bedload and have a greater capacity for
carrying sediments,the middle river would initially accelerate scour of its
bed somewhat.This would last for only a few years.The total amount of
scour in the mainstem would average about 1-1.5 feet,with the effect most
pronounced near the dam face and decreasing with distance downstream.In
sloughs and side channels the amount of degradation would range from none to
0.3 foot (H-E 1985b).
After the initial period of scouring downstream of the dams,an armor
layer would develop and the riverbed would become more stable (H-E 1985b).
The main channel would tend to become reduced in width by encroachment of
vegetation and .sediment near the banks (R&M Consultants,Inc.1982).
Long-term natural degradation of the riverbed would be substantially reduced.
As degradation of the middle river ceased,the natural evolution of the
sloughs would stop.Sloughs would no longer change by this process from one
33RD4-011 -31 -
II
II
type to another.The type of each slough would remain essentially the same as
it was at project startup.Slough fish habitat would,therefore,no longer be
altered by degradational evolution.
However,it cannot be assumed that presently-suitable fish habitat would
necessarily remain so.Although slough types would remain unchanged,habitat
conditions within the sloughs might still undergo change due to processes
other than river degradation.Small streams that are tributary to sloughs
would continue to produce sediment and might alter substrate conditions in
sloughs,perhaps causing substrate siltation.Reduced flooding in the
[I
f I, I
sloughs,caused by stabilized project flows,might prevent the periodic
flushing of silts from the slough substrates and the flushing of beaver dams
that trap sediments and can hinder fish passage into sloughs.If so,unless
remedial measures are taken,sloughs might eventually become unsuitable for
fish spawning.
33RD4-011 -32 -
-1
l
Ll
i :
I 'U
Ii
LJ
REFERENCES
REFERENCES
Friese,N.V.1975.Preauthorization assessment of anadromous fish popula-
tions of the upper Susitna River watershed in the vicinity of the pro-
posed Devil Canyon Hydroelectric Project.Alaska Dept.of Fish &Game,
Div.of Commercial Fisheries,Anchorage,AK.108 pp.
AEIDC,See University of Alaska-Fairbanks,Arctic Environmental Information &
Data Center.
l~1 EST&A.See E.Woody Trihey and Assoc.
E.Woody Trihey and Assoc.1984.Response of aquatic habitat surface areas
to mainstem discharge in the Talkeetna to Devil Canyon reach of the
Susitna River,Alaska.Final report,June.E.W.Trihey &Associates.
Report for Alaska Power Authority.Document 1693.1 vol.
II
I
Harrison,W.D.,et al.1983.Reconnaissance of the glaciers of the Susitna
River basin in connection with proposed hydroelectric development.An-
nals of Glaciology.4:99-104.
II
II
II
I i
Harza-Ebasco Susitna Joint Venture.1985a.Flood frequency analyses for
natural and with-project conditions.Draft report,April.Prepared for
Alaska Power Authority.12 pp.plus appendices.
1985b.Middle Susitna River sedimentation study:stream channel
stability analysis of selected sloughs,side channels and main channel
locations.Draft report.Alaska Power Authority.Susitna Hydroelectric
Project.1 vol.
H-E.See Harza-Ebasco Susitna Joint Venture.
McKendrick,J.,et al.1982.Plant ecology studies,phase I.Final 'Report.
Agricultural Experiment Station,University of Alaska,Palmer,AK.Alas-
ka Power Authority.Susitna Hydroelectric Project.124 pp.
Meier,M.,and A.S.Post.1969.What are glacier surges?Canadian Journal
of Earth Sciences.6(4):807-817.
I I
l
Plafker,G.1969.Tectonics of the March 27,1964 Alaska Earthquake.U.S.
Geological Survey Professional Paper 543-1.174 pp.plus appendices.
Post,A.S.1960.The exceptional advances of the Muldrow,Black Rapids,and
Susitna glaciers.Journal of Geophysical Research.65:3703-3712.
II
R&M Consultants,Inc.1982.River morphology.R&M Consultants,Inc.,An-
chorage,AK.Alaska Power Authority.Susitna Hydroelectric Proj ect.
Report for Acres American,Inc.1 vol.
Selkregg,L.L.,ed.1974.Alaska regional profiles.Vol 1.Southcentral
region.Arctic Environmental Information &Data Center,University of
Alaska,Anchorage,AKa Prepared for Alaska Office of the Governor.
253 pp.
33RD4-011 -33 -
Thornbury,W.D.1954.
Inc.,New York,NY.
Principles of geomorphology.
618 pp.
John Wiley and Sons,
I T
I
II
[ I
II
II
i I
!!
LI
[ I
u.S.Geological Survey.1957.Compilation of records of quantity and quality
of surface waters of Alaska through September 1950.USGS Water-Supply
Paper 1372.1 vol.
1961.Surface water records for Alaska,1961.USGS Water Data
report.1 vol.
1962.Surface water records for Alaska,1962.USGS Water Data
report.1 vol.
1978.Water resources data for Alaska,water year 1977.USGS
Water data report AK-77-1.1 vol.
1979.Water resources data for Alaska,water year 1978.USGS
Water data report AK-78-1.1 vol.
1980.Water resources data for Alaska,water year 1979.USGS
Water data report AK-79-1.1 vol.
1981.Water resources data for Alaska,water year 1980.USGS
Water data report AK-80-1.1 vol.
USGS.See U.S.Geological Survey.
University of Alaska-Fairbanks,Arctic Environmental Information &Data Cen-
ter.1984.Geomorphic changes in the Devil Canyon to Talkeetna reach of
the Susitna River since 1949.Preliminary report.6 pp.plus appendi-
ces.
II
i I 33RD4-011 -34 -
!
I
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U
[I
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u
APPENDICES
I i
II
APPENDIX A.COMPARATIVE GEOMORPHOLOGY
The drawing in appendix A are included in this report only to give the
reader a very general idea of how the river has evolved since 1949.The draw-
ings were done by a technician independent of the geomorphic analysis of the
photography,and each year's set of drawings was done at a different time over
the span of almost two years..The scale of the photography is very small and
varies considerably between photographic sets,and lighting varies from flat
with no shadows to bright with deep shadows.Some of the photography is in
black-and-white while others are in false-color infrared,and water discharges
varied slightly between years.
Due to these limitations it was difficult and,at times,impossible to
portray each river feature accurately.Artistic license was taken at times in
order to distinguish important features of the river that would be indistin-
guishable at true scale,since line widths often exceeded the actual size of
features portrayed.The method of portrayal to achieve this end varied some-
what from one part of the river to another,and from year to year.All vege-
tation stands,regardless of successional stage,are shown on the drawings as
black areas.
The drawings were not used by the authors in their analyses,and,in
fact,were not finished until after the analyses were completed.The analyses
were performed using a magnifying stereoscope utilizing overlapping stereo
pairs of photographs.Without the three-dimensional images and magnification
of features,many of the reported geomorphic changes could not have been dis-
tinguished.Accordingly,although the drawings do illustrate general change
in the river,such as increased vegetation coverage and general changes in
shape and area of certain features,the drawings should not be used by the
reader to attempt to verify each geomorphic change reported in the text.
II
33RD4-011 -35 -
I-
I
II
II
II
II
I I
( i
II
I !
I iu
II
II
I,.
1949
SC
Whiskers Slough
1961-1962
u
1977-80,82
II Vegetation SS Side Slough
COMPARATIVE GEOMORPHOLOGY
G)Slough US Upland Slough 1949;1961-62;1977-80,82 ~N Approximate Scale:1:60,000
Middle Susitna River
sc Side Channel 116 River Mile River Mile 98-109 1 inch =.9 miles.
1 of 6
i-
1977-80,82
1949
u s
Lane Creek
Lane Creek
7
SS
[ i
!I
I !
II
II
I I
II
II
II
I
I
II III!Vegetation SS Side Slough
i I CD Slough US Upland SloughII
SC Side Channel 116 River Mile.
...---------Appendix A-------.....,
COMPARATIVE GEOMORPHOLOGY
1949;1961-62;1977-80,82
Middle Susitna River
River Mile 109-119
--"::::"N Approximate Scale:1:60,000
1 inch =.9 miles
2 of 6
·-
1949
1961-1962
Skull Creek
BC
SC
Deadhorse Creek
II
I!
II
II
[I
II 1977-80,83
II
Deadhorse Creek
II Appendix A
Vegetation SS Side Slough
COMPARATIVE GEOMORPHOLOGY
I CD Slough US Upland Slough 1949;1961-62;1977-80,82 Approximate Scale:1:60,000IIMiddleSusitnaRiver.,>N
SC Side Channel 116 River Mile River Mile 119-128 1 inch =.9 miles.
3 of 6
.......,....:iAJ
"-
SC
16
1949
1977-80,82
Fourth of July Creek
III I
II
II
I i
I i
I I
Vegetation
CD Slough
SC Side Channel
ss
us
116.
Side Slough
Upland Slough
River Mile
...---------Appendix A ---,.----------.
COMPARATIVE GEOMORPHOLOGY
1949;1961-62;1977-80,82
Middle Susitna River
River Mile 128-138
Approximate Scale:1:60,000
1 inch =.9 miles
4 of 6
I
I
!I
I
I.I
\ j
II
Indian River
Indian River
@
SS
1949
SC
22
Jack Long Creek
1961-1962
1971-80,82
sc
22
Vegetation SS Side Slough
CD Slough US Upland Slough
SC Side Channel 116 River Mile.
I
,--------Appendix A-------.....
COMPARATIVE GEOMORPHOLOGY
1949;1961-62;1977-80,82
Middle Susitna River
River Mile 138-147
5016
Approximate Scale:1:60,000
1 inch =.9 miles
Portage Creek
Devil Canyon
151
1949
R I V
Portage Creek
1961-1962
151
Devil Canyon
Portage Creek
1977-80,82
151
R I
s U~S~~~/ T
I I II Vegetation SS Side SloughiI
CD Slough US Upland Slough
SC Side Channel 115 River Mile.
,---------Appendix A~--------.
COMPARATIVE GEOMORPHOLOGY
1949;1961-62;1977-80,82
Middle Susitna River
River Mile 147-153
6 of 6
Approximate Scale:1:60,000
1 inch =.9 miles
APPENDIX B -GENERAL DESCRIPTIONS OF GEOMORPHIC CHANGE
SINCE 1949.
RM 98.6.Confluence of Susitna/Chulitna rivers.Chulitna main channel has
eroded away about 1000 feet of the land north of the Susitna River and east of
the Chulitna River.The Susitna River mouth was widened in the process,as
the islands on the south side of it were also removed.About 1/4 of this
erosion took place from 1949-61,while about 3/4 took place from 1961-present.
RM 99.5-100.0.Gravel islands in this reach became progressively more vege-
tated.Most of this occurred 1949-61;only slightly more occurred from
1961-present.
RM 99.5,Slough 1.Changed from side channel to side slough from 1949-61.
Enclosing gravel bars,particularly near slough mouth,became somewhat vege-
tated with low brush from 1961-present.
RM 101.0-101.5.Many gravel bars in this reach became progressively more
vegetated from 1949-present.They now have low vegetation covers.
RM 111.Gravel bars in this area have shown progressively more brushy vegeta-
tion from 1949-present.
RM 111.5.Forested island has new area of vegetation at southeast end.It
progressed from no vegetation in 1949 to thic~brush at present.
33RD4-011 -42 -
RM 113.7,Slough 8.Changed from side channel to side slough from
1962-present.Enclosing gravel bar has grown in area and become vegetated
with low brush from 1962-present.
RM 113.7.Gravel bar on south shore has become progressively vegetated from
1949-present.There was no vegetation in 1949,and there is at present a
large stand of tall brush.
RM 113.8.Two forested islands were eroded from 1961-present.The upstream
island is totally gone,while the downstream island is diminished in size.
Also,a small,vegetated island downstream from there disappeared.It appears
that the mainstem has shifted its course northward.
RM 117.2.Islands show progressively encroaching vegetation from 1949-
present.The southwestern and central islands have doubled their vegetated
areas.In 1949 they were about half vegetated;in 1962 sparse brush had grown
on the unvegetated areas;and at present there is a dense cover of tall brush.
Northwest of there,a gravel bar has grown considerably in area from 1949 to
present and has become sparsely vegetated.
RM 117.8.Gravel bar has grown considerably in area and has become sparsely
vegetated.
RM 118.7.Margins of forested island have new brushy vegetation that has
increased progressively from 1949-present.
33RD4-011 -43 -
RM 119.1.Forested island has approximately doubled its area of vegetation
coverage.The new vegetation is tall brush at present.
RM 119.5.Mainstem channel has swung southward from 1949-present,eroding
away part of vegetated feature on the south side of the river.This feature,
an island in 1949 and 1962,is now connected to the mainland at its upstream
end,enclosing a new,unnamed,side slough.
RM 121.Gravel bar has become partly vegetated with brush from 1949-present.
RM 121.3.
1949-present.
Gravel bar has become partly vegetated with brush from
RM 121.8,Slough 8C.Changed from side channel to side slough from
r I
I I
1962-present.Enclosing gravel bar has increased in size and become sparsely
vegetated from 1949-present.
RM 122.Curved,forested island has progressively grown brushy vegetation at
west and southwest ends from 1949-present.
RM 123.4.Gravel bar attached to south shore of river in 1949 is at present a
terrace and has become progressively more vegetated with brush from
1949-present.
RM 122.3,Slough 8B.Although slough has not changed type,the enclosing
gravel bar has become increasingly vegetated from 1949-present.In 1949 there
33RD4-01l -44 -
was almost no vegetation;in 1962 it was about half covered with vegetation;
and at present it is fully vegetated with high brush and low trees.
RM 123.0,Slough 8E.Changed from side slough to upland slough from
II
I I
1962-present.Upper part of slough is now fully vegetated.
RM 123.3,Slough 8F.Changed from side slough to upland slough from
1962-present.Enclosing gravel bar has shown increasing vegetation from
1949-present.In 1949 there was only a slight coverage of low grasses;in
1962 the bar was about half covered with brushy vegetation;and at present the
bar is fully covered with high brush and low trees.
RM 123.8,Slough 8G.Changed from side channel to side slough from
1962-present.In 1949,the side channel was behind a small,unvegetated grav-
el bar in mid-river.By 1962 the gravel bar had grown in area,and the side
channel was reduced in size.At present,the gravel bar is larger,connected
to the south shore,and is sparsely vegetated.
RM 123.4.Gravel bar near the south shore in 1949 has become attached to
shore as a terrace,and has become progressively more vegetated from
1949-present.
RM 123.8.There is a new gravel bar attached to the south shore at present.
This bar did not exist in 1949;it was a small in-channel bar in 1962;and it
is a large bar attached to shore and beginning to vegetate at present.
33RD4-011 -45 -
RM 124.1.Gravel bar has become progressively more vegetated from
1949-present,and now has dense brush.Also,new gravel bars to the south of
it,formed or emerged since 1949,have progressively increased in area to the
present.
RM 124.7.Gravel bar has progressively increased vegetation coverage from
1949-present.
I RM 125-125.5.Mainstem channel has shifted position since 1949,eroding away.I I
the tips of several old,forested islands.There is a new gravel bar in the
old position of the mainstem channel.One old gravel bar,barren in 1949,has
become progressively vegetated from 1949-present.
RM 126.6-126.7.Large gravel bar near south shore in 1949 "has become attached
to the south shore as a terrace.It has also become progressively vegetated
with high brush from 1949-present.
RM 127.Gravel beaches on north side of channel have become vegetated since
1949.
RM 128.1-128.4.
1949-present.
Gravel bars have become increasingly vegetated from
RM 128.3,Slough 9.Wetted area of slough narrowed from 1949-62.A large
gravel bar in mid-slough became larger in area and increasingly vegetated from
1949-present.Another gravel at the lower end of the slough,partly vegetated
33RD4-0 11 -46 -
I i
I
Ii
in 1949,has progressively increased in vegetation cover,and is at present
completely covered with high brush and low trees.
RM 129.2-129.5.Gravel bars increased in area from 1949-62,and became in-
creasingly vegetated from 1962-present.
RM 133.3,Slough 9A.Slough did not change type,but gravel bar at lower end
of slough enlarged in area from 1949-62,and became increasingly vegetated
from 1962-present.
RM 130.Mainstem channel shifted southward and widened from 1949-present.
New gravel bars have formed in the old position of the mainstem channel and
have become increasingly vegetated.
RM 131.2-131.7.Several gravel islands have become increasingly vegetated
from 1949-present.
RM 132.2.The mainstem channel shifted position slightly to the northwest,
eroding,away the southeastern sides of several forested islands.The northern
sides of the islands,however,have increased their area of vegetation cover-
age.Most of the island erosion took place before 1962.There are at present
new gravel bars in the old position of the mainstem channel.
RM 133.0-133.3.Forested islands have increased in area by apparent emergence
of new gravel beaches at the northeast and southwest ends,and these beaches
have become increasingly vegetated,from 1949-present.
33RD4-011 -47 -
I]
RM 133.6.The mainstem channel shifted to the south and eroded out a small,
forested island near the south shore.A lengthy gravel bar,which had just
started to form or emerge in 1949,now occupies the old position of the main-
stem channel,has become attached to the shore as the beginning of a new ter-
race,and has become increasingly vegetated from 1949-present.Two small,
forested islands near the south shore in 1949 have now become attached to the
south shore as a terrace,and have become increasingly vegetated with brush,
from 1949-present.
RM 134.5.Large gravel bar in mid-river has become increasingly vegetated
with brush from 1949-present.
RM 135.3.Two gravel bars have become increasingly covered with low vegeta-
tion from 1949-present.
RM 135.4,Slough 11.Large ice jam event,observed in 1976,overtopped the
terrace on the south side of the river and eroded out a large side slough,
known now as slough 11.There was only a small upland slough at the down-
stream end of the terrace previous to that event.
RM 135.3,Slough 12.Changed from side slough to upland slough from
1962-present.Upper end of channel bed is now fully vegetated.
RM 135.6,Slough 13.Changed from side channel to side slough from 1949~62.
In 1949 the channel was behind a small gravel bar offshore.By 1962 the bar
was attached the large terrace remnant enclosing slough 11,and at present the
bar is slightly covered with low vegetation.
33RD4-011 -48 -
I
I ,
RM 137.2.Gravel bar has become much more vegetated from 1962-present.
RM 137.3,Slough 15.Changed from side channel to side slough from 1949-62,
and then to an upland slough from 1962-present.
RM 138.6.Main channel of Indian River has swung progressively eastward in
its delta from 1949-present.
RM 139.2.Mainstem channel has shifted northward from 1962-present,eroding
out two small,forested islands.Gravel bar to the southeast of that has
become slightly more vegetated from 1949-present.
II
RM 139.8,Slough 19.
1962-present.
Changed from side slough to upland slough from
RM 140.Vegetated island near south shore in 1949 has become attached to the
shore as a terrace,and has become increasingly vegetated,from 1949-present.
RM 140.2,Slough 20.
1962-present.
Changed from side channel to side slough from
I !
i I
t I
RM 140.6.Gravel bar has become increasingly vegetated from 1949-present.
RM 141.5,Slough 21.Slough is enclosed by three forested islands,and is
composed of both a lower side channel area and an upper side slough area.
Overflow berm at upper end of side slough area has become narrowed and con-
fined by encroachment of vegetation from 1949-present.Channel between first
33RD4-011 -49 -
[I
and second island has formed or emerged a gravel bar,almost closing the chan-
nel,from 1962-present.Berm between second and third island has become
greatly vegetated with low brush from 1949-present.It appears that flow in
the lower side channel area has become severely restricted at comparable flows
due to emergence of intervening berms.
RM 141.9.Gravel island has become larger in area,and has become increasing-
ly vegetated in its center,from 1949=present.
RM 142.4.A large portion of the south shore terraced mainland,and a large
portion of a large,forested island in mid-river,have been eroded away.
There is a barren gravel bar now in the position of the old south shore ter-
race.The mainstem channel is now much wider through this reach.This ero-
sion all occurred since 1962;the old forested island had been increasing in
vegetation coverage from 1949-62.
RM 143.0-143.3.Gravel"bar on south shore has become larger in area and in-
creasingly vegetated from 1949-present.
RM 145.2.Gravel bar on south shore has become slightly more vegetated from
"1949-present.
RM 145.5,Slough 21A.Channel appears to be somewhat less flooded,at.similar
discharges,from 1949-present.
33RD4-011 -50 -
APPENDIX C.FISH HABITAT CONDITIONS
Riverine fish habitat in the middleSusitna River occurs in various loca-
tions--lakes,tributaries,the mainstem,side sloughs and channels,and upland
sloughs.Tributary confluence areas also have unique characteristics which
define their importance as fish habitat.Of these areas,side sloughs and
tributaries are used most heavily by fish,principally by salmon.From inves-
tigations over the past five years,habitat quality in these area is consid-
ered highest when certain physical characteristics are present.
The substrate most suitable for salmon spawning and embryo incubation is
a medium sized gravel with few fine sands or clay particles.Some coarse
cobble or boulder particles will not limit suitability for spawning/incubation
unless these large particles are too numerous for fish to successfully exca-
vate redds.Fine particulates are almost always present in suitable streambed
gravels,but excessive amounts accumulated over long periods of time can in-
hibit waterflow through the substrate which is necessary for embryo respira-
tion and evacuation of waste metabolites.Periodic flushing of excessive fine
materials by physical disturbance of the gravels,often accomplished by the
spawning act of adult fish,or high flows created when high mainstem discharg-
es overtop slough berms,can maintain the quality of spawning and incubation
gravel substrates.
Other characteristics of suitable fish habitat include a stable and con-
tinuous flow of high quality water which provides depths of about 0.3 to
0.5 ft or more for spawning and sufficient depth to ensure the surface is
wetted throughout the incubation period.Velocities of nearly zero to as high
as 3 to 5 feet per second are suitable,but such a generalization masks the
differences between preference by young fry for lower velocities and adult
fish which find higher velocities more suitable because of the role water flow
33RD4-011 -51 -
plays during excavation of streambed gravels by spawners.Seasonal flow fluc-
tuations can be beneficial by seasonally washing riparian matter into fish
habitats,providing organic food sources to invertebrates.These seasonal
high flows also scour streambeds of beaver dams and accumulated fine materials
and transport them out,thereby assuring substrates are available for use by
fish,and waterflow through gravels can continue to maintain incubating fish
embryos.Rapid daily or weekly fluctuations,however,do not permit success-
ful colonization of shorelines by algae or invertebrates.
Groundwater.upwelling through streambed gravels ensures a stable incuba-
tion environment for fish embryos,particularly during winter when surface
flow is 'significantly reduced and ice formation is extensive.This appears to
be most important in side sloughs and channels.
A~cess to habitat is necessary for adult movement to spawning areas and
for juvenile movement to rearing sites.Reaches of low velocity provided by
riffle/pool variability,large boulders or instream debris,or sills at en-
trances to habitat ensure unimpeded movement by fish into or out of preferred
habitats.
Juvenile fish rearing occurs year around since chinook,coho,and sockeye
salmon spend at least one year in fresh water prior to outmigrating.
Invertebrate production is important as these organisms provide food for rear-
ing fish.Rapid daily flow fluctuations,flooding,or other disruptions tend
to reduce habitat suitability;physical habitat stability increases its value
for rearing.Some form of cover (depth,riparian vegetation,turbidity,in-
stream debris,etc.)is necessary to provide resting places or predator escape
for rearing fish.Because of small size,rearing fish rely on low to moderate
flow velocities.
33RD4-011 -52 -
Other features contribute to good habitat such as cover for juvenile fish
predator escape or resting,high levels of dissolved oxygen,adequate water
quality,and suitable water temperatures.Many other biologic characteristics
affect the nature of good quality fish habitat,such as the presence of dis-
ease,competitors,and predators.However,in the context of this report,the
previously described features are the most relevant.
33RD4-011 -53 -