HomeMy WebLinkAboutAPA2781GLACIER MASS BALANCES AND RUNOFF
IN THE UPPER SUSITNA AND MACLAREN RIVER BASINS, 1981-1983
FINAL REPORT
Theodore S. Clarke, Douglas Johnson and William D. Harrison
Geophysical Institute
University of Alaska
Fairbanks, Alaska 99701
June 1985
ABSTRACT
Mass balance measurements have been made on the major glaciers at the
headwaters of the Susitna and Maclaren Rivers during 1981, 1982 and 1983. The
primary purpose of the work has been to estimate the acount of water
originating from this 790 km 2 glacierized area, in connection with the
development of water forecast models for the proposed Susitna hydroelectric
project. The study has been at the reconnaissance level, s ince only one
measurement stake per 50 km 2 has been monitored. Annual balances, when summed
over the three year measurement period, were estimated at +0 .1 T 0.6 m water
equivalent. Average runoff due to the melting of ice, firn and snow was about
1.3 m/yr, as estimated by monitoring melt on the glacier surfaces. Average
rain runoff was about 0.25 m/yr, as estimated from rain gauge data. This is
probably a lower limit on rainfall runoff. Overall, the glaciers produced
about 1.5 T 0.3 m/yr of water. This is compared to 0.95 m/yr for the
unglacierized portion of the basin above the Denali Highway and 0.59 m/yr from
the basin as a whole above the Susitna River gauge at Gold Creek for the same
period. This suggests that precipitation in the glacierized portion of the
basin is about 2.5 times greater than the basin as a whole. The 5.77. and 7.17.
glacie~ized areas above the proposed Devil Canyon and Watana dam sites
produced about 157. and 177. of the water at the respective sites. It is
estimated that nearly 757. of the melt watet originating from glaciers ran off
in July and August, while the remaining 25 7. was distr1buted between May, June,
September and October.
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I.
II.
TABLE OF CONTENTS
ABSTRACT
Table of Contents .••...••..............•.....•...............
List of Tables ...•...•..•...•................................
List of Figures ••.
ACKNOWLEDGEMENTS •••••••••••••••••••••••••••••••••••••••••••••
INTRODUCTION •••.••••••..••.•••••••••••••....•..•••...........
MASS BALANCE •••••••••••••••••••••••••••••••••••••••••••••••••
1.
2.
3.
4.
5.
6.
Introduction and Terminology •.
Point Balance Measurements •••••••••••••••••••••••••••••••
Balance-Time and Balance-Elevation Curves •...............
Average Balances .•.......................................
Eureka Glacier.
Error •••••••••••••••
Ill. GLACIER RUNOFF •.•..•••.•.•.
1. Snow Me 1 t ......•............••.••.•......................
2. Firn and Ice Melt ....................................... .
3. Rain
4. Evaporation •••.••.••.
5. Timing of Runoff .•.•...•.•...•..•..........•••....•......
IV. DISCUSSION AND CONCLUSIONS ••.••.•...•.••••....•••.•.••..••...
REFERENCES •••••••••••••••••••••••••••••••••••••••••••••••••••
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LIST OF TABLES
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Table I. Snow pack density................................... 24
Table II. Internal accumulation............................... 25
Table III. Debris cover on glaciers............................ 26
Table IV. Average winter season balance average summer
season balance and annual balances.................. 27
Table V. Snow melt runoff, lee melt runoff and 1:ainfall runoff
quanti ties.......................................... 28
Table VI. Comparison of glacier water to total water.......... 29
Table VIla. Rainfall data collected near Susitna Glacier by
R & M Consultants................................... 30
Table VIIb. Summer precipitation on the Susitna Glaciers •••••••• 30
Table VIII. Approximate equilibrium line elevations •....•....... 31
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LIST OF FIG URES
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Figure 1. Location map........................................... 32
Figure 2. Glacier names and locations, stake locations, and
drainage divides....................................... 33
Figure 3. Snowpack density variations with depth and elevation ... 34
Figure 4a. Balance-time relations on West Fork Glacier .••.•••••••• 35
Figure 4b. Balance-time relations on Susitna Glacier •.....•.•..... 36
Figure 4c. Balance-time relations on Northwest tributary o f
Susi tna Glacier .............................. , .......... 37
Figure 4d. Balance-time relations on Turkey Tributary of
Susitna Glacier .... ,................................... 38
Figure 4e. Balance-time relations on East Fork Glacier............ 39
Fitr,ure 4f. Balance-time relations on Maclaren Glacier............. 40
Figure Sa. Balance-elevation and area-elevation relations for
West Fork Glacier...................................... 41
Figure Sb. Balance-elevation and area-elevation relations for
Susi tna Glacier. . . . . . . . • . . • . . . . . . • • . . . . . . . . . . . . . . . . . . . . 42
Figure 5c. Balance-elevation and area-elevation relations for
Northwest tributary of Susitna Glacier................. 43
Figure 5d. Balance-elevation and area-elevation relations for
Turkey tributary of Susitna Glacier.................... 44
Figure Se. Balance-elevation and area-elevation relations for
East Fork Glacier...................................... 45
Figure Sf. B ~lance-elevation and a~ea-elevation relations for
Maclaren Glacier....................................... 46
Figure 6a. Winter accumulation for 1980-1981...................... 4 7
Figure 6b. Winter accumulation for 1981-1982...................... 4 8
Figure 6c . Winter accumulation for 1982-1983...................... 4 9
Figure 7. Runoff from glaciers compared to total runoff at
stream gauges on the Susitna and Maclaren Rivers....... 50
Figure 8. Ablation per year, in excess of that used f or internal
accumulation , versus elevation......................... 51
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ACKNOWLEDGEMENTS
It is a pleasure to acknowledge that this report has been the result of
the efforts of many people. For the field efforts, credit is due especially
to Carl Schoch of R & M Consultants and Clifton Moore of University of Alaska,
but we also acknowledge the contributions, at times almost herculean, of
Steven and Randy Bergt and Elizabeth Senear. Helicopter support was supplied
by Air Logistics. Cooperating companies or agencies have been R & M
Consultants, Acres American Inc., Harza-Ebasco Joint Venture, North Pacif i c
Aerial S~rveys, the Alaska Power Authority, and the State of Alaska Division
of Geological and Geophysical surveys. We are particularly grateful to Steven
Bredthauer of R & M for his advice and perspective, to colleagues at
University of Alaska, especially Carl Benson, and to Lawrence Mayo, Dennis
Trabant, and Rod March at the U.S. Geological Survey. The latter three
colleagues have helped with ideas concerning snow accumulation and
metamorphism, and mass balance in general. Finally, financial support has
been from subcontractors of the Alaska Power Authority, the Division of
Geological Surveys, and the University of Alaska.
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I. INTRODUCTION
This is a report summarizing the glacier balance data obtained for the
glaciers of the Susitna basin in 1981, 1982 and 1983, and the implications for
the amount and timing of runoff produced by those glaciers. The immediate
objective has been to obtain an assessment of the effect of glaciers on basin
water supply, to aid in the development of water forecast models for the
proposed Susitna hydroelectric project (Figure l).
The reason water from glacierized portions of the basin has been singled
out for special attention is that it has several unique properties. First and
most obvious, its influence is several times greater than might be suggested
by the 77. or so a real coverage above the proposed Watana dam site, because it
originates at high elevation where precipitation is high. Second, there is no
simple relationship between winter precipitation on glaciers and summer
runoff, which makes seasonal prediction of runoff from glaciet~?.ed areas a
difficult task, one for which there are few analogies fro~ unglacierized
basins. On the positive side, runoff from glacierized basins shows reduced
variability with meltwater from glaciers b~ing abundant in clear weather when
water from rain is not. Krimmel and Tangborn (1974} and Fountain and Tangborn
(1985), working in Washington state and southeastern Alaska, have found
minimua variability to occur at 367. glacierization ~Fountain and Tangborn
1985). Chacho (personal communication, 1985), working in south-central and
interior Alaska, finds that variability reaches a minimum from about 57. to 707.
glacier cover, but variability for any given percent glacierization can vary
considerably. And third, evaporative losses are very small and in fact
usually found to be negative (Patterson, 1969; Sharp, 1960).
The complex relatfonship between precipitation and runoff for glacierized
basins is due to the unique storage processes which oc:ur in these basins.
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Although storage of winter snow well into the succeeding summer is familiar
enough for unglacierized mountain basins, glaciers store some of the winter
snow for hundreds of years, releasing each year not only sooe of the previous
winters' accumulated snow, but ice melt that originated as snow many years
earlier. In the long run this ice melt tends to be replenished by glacier
flow, but in a given year there is generally no positive correlation between
winter snow accumulation and summer runoff; in fact the correlation, if any,
is generally negative. Heavy snows lead to the late appearance of underlying
low-albedo glacier ice and therefore lower runoff. These phenomena are
aspects of glacier storage of solid precipitation. Storage of liquid water
w~thin glaciers also occurs, both in early summer, when a sizeable fraction of
the surface melt, or rain, is stored and released later in the year (Paterson,
1981; Tangborn and others, 1975; Stenborg, 1970), and from year to year which
is usually released as jokulhlaups (Bjornsson, 1977; Hodge, 1974).
Storage by glaciers is the source of errors that have sometimes been made
in estimates of long-term water availability from stream flow records
(Bezinge, 1979). The errors have been due to failure to take into account the
component of runoff from secular decrease in glacier volume, which is usually
due to warm temperatures over the peri~d of stream flow record. Glaciers are
extremely sensitive to temperature; a one degree change in summer temperature
may lead to significant volume change (Tangborn, 1980; Meier, 1965; Ahlmann,
1953).
In the work reported here the so-called glaciological method was used to
assess the effe~t of the Susitna basin glaciers on water supply. This method
is indirect in that glacier balar.~~. or the accumulation and ablaticn of mass
over the glaciers' surfaces, is monitored at several points, and the water
pro~uction at the termini of the glaciers is estimated from the results,
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rather than measured directly. With the exception of EurekJ Gla ci er, which
straddles the eastern boundary of the basin, and the small glaciers of the
Talkeetna Mountains to the south, all major glaciers of the basin were studied
(Figure 2 ). The total area of glacierization is about 790 km 2 , or 7.1 /. of the
total basin area above the proposed Watana dam site. Limitations on the
interpretation of the data are imposed by the very sparse coverage (3
measurement points per glacier), and perhaps more important, by the short time
span (1981 to 1983) of the data.
Three previous reports describe earlier phases of the work, including
thermal and flow regimes of the glaciers, the effect of surges on sediment and
water supplies, and the effect of long term glacier volume change (R & M and
Harrison, 1981; R & M and Ha rrison, 1982; Harrison and others 1983). In this
report all the balance data are presented, and reduced in a consistent
fashion. As stated above, this report is primarily concerned with the
determination and timing of glacier runoff, and the mass balance of the
glaciers for the 1981-1983 period over which data were collected. The results
are summarized in Section IV, which is self-contained and can be read directly
by the reader not concerned with the intervening details.
II. MASS BALANCE
(l) Introduction and Terminology
The glaciological m~thod, as already noted, was used to assess the water
supply from the Susitna basin glaciers. This method employs a network of
stakes drilled into the glacier surface. Mel t and accumula tion are monitored
at each stake. These point changes in water equivilant thickness are then
extrapolated over the glacier surface. Melting of snow and ice, when
integrated over the hydrologic year, is called the annual ablation, and the
discussion of its determination would fulfill the major objectives of this
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report. However, the discussion is broadened to include data relating to
accumulation of snow as well.
Having embarked upon this slightly more ambitious program, a certain
amount of terminology is required. Mass balance, measured by the
glaciological method, concerns the gain or loss of mass of a glacier, and its
distribution over the glacier and over time. The four balance definitions
most important for this report are as follows:
1. Annual balance, the balance at a specified point on a glacier during the
hydrologic year (1 October to 30 September). Units of water equivalent
thickness are used throughout.
2. Annual ablation, the total ablation (which consists of snow, firn and ice
melt) at a point during the hydrologic year.
3. Winter season balance, the balance at a point from 1 October to 14 May.
4. Summer season balance, the balance at a point from 15 May to 30
September.
These four quantities, which are defined at a measurement point, can be
converted to total volume quantities by integration over the glacier surface,
in which case they are usually divided by the total area of the glacier and
termed "average annual balance", "average annual ablation", "average winter
season balance" and "average summer season balance". The average annual
balance is a measure of the "health" of a glacier, as it represents the mass
gain or loss in a given hydrologic year. The algebraic sum of average annual
ablation and rainfall runoff is approximately equal to total annual runoff.
Annual ablation is not the same as a\•erage summer season balance because the
upper reaches of a glacier might accumulate summer snow, so the summer season
balance can be less than, or even the opposite sign from, annual ablation
there. This terminology applies in the "fixed date" system of balance
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description (Anon ymo us , 1969). In the "stratigraphic" s yste r.t of balance
description, t h e maximum a nd minicum gtacier volume in a given year are
defined as the winter and summer balances, respectively (Anonymous, 1969 ).
This report uses winter season balance and summer season balance as
approximations to winter and summer balances. These are only approximations
because maximum and minimum glacier ma3s does not necessarily occur on 14 May
and 30 September, r espectively . The complex terminology reflects the
supri3ingly complex processes involved in accumulation and ablation of glacier
mass. Mayo and others (1972) presented a good discussion of the complexities
involved in mass balance determinations and terminology.
Some further backgroun~ is needed. Snow that has survived a summer of
ablation is "firn". In the fixed date system, the line on the glacier where
annual balance is zero is the "equilibrium line''. l t is the boundary be tween
the "accumulation zone" (annual balance > 0 ) and the "ablation zone" (annual
balance < 0). The stratigraphic boundary between firn and new snow in the
upper glacier, or the ice-snow boundar;· in the lower, is the "summer
surface". Sometimes summer surfaces f r om one year or several years can be
identified from snow stratigraphy studies in the accumulation zone. This was
attempted on the first visit to the glaciers in 1981; subsequent measurements
have been made on stakes drilled into the surfaces of the glaciers.
Great care is necessary in the interpretation of stake or strati g raphic
measurements, because snow that melts at the curface is not necessarily
available for runoff, even after some delay. This is because refree z i ng ma y
take place, even to depths exceeding that of the most recent summer surface
(Benson, 1962). The process depends critically upon the thermal regime of the
glacier, which is one reason that glacier temperature, measured in 19 8 1 at one
site, is relevant to glacier hydrolo g y. Helt or rain water frozen on an ice
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surface is called superimposed ice. Water also free z es in snow and firn
(Bazhev, 1973), a process that is sometimes called "internal accumulation ". A
discussion by Trabant and Mayo (1984 ) points out that t h ere are actuall y tw c
processes involved in its formation; the freezing in early summer of do~nwar d
per~olating water and the freezing in winter of the irreducible capill ~ry
water remaining in the firn.
When the term "mass" balance is used by glaciologists, "mass" usually
means the mass of solid phase H2 o. This is sometimes confusing, because it is
known that a great deal of liquid H2 o is also stored in glaciers, particularly
in the first part of the melt season (Tangborn and others, 1975; Stenborg,
1970) and is released later either gradually or catastrophically. As with ice
storage, liquid storage may be unequal to zero in a given year (Hodge,
1974).
(2) Point Balance Measurements
Balance at a point on a glacier was measured primarily by monitoring the
position of the surface with respect to a stake set into the glacier an~
maintained throughout the year. Snow deneity was measured as a function of
depth and used to convert the stake measurements to water equivalent
balances. Three stakes were placed on each of the major glaciers, one in the
ablation zone at about 1000 m elevation, one near the equilibrium line at
about 1500 m, and one in the accumulation zone at about 2000 m (Figure 2 ).
The stake data were supplemented by probing to the summer surface, where it
could be identified this way. Measurements were made in both April or May,
and late August or early September, and sometimes in mid summer.
Snow pits were dug to the most recent summer surface at representative
stakes in spring. Stratigraphy, snow temperature, and snow density were
measured in these pits. In spring 1981, the first year of the work, snow
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stratigraphy was used to estimate the 1980-81 winter snowpack and the 1980-8 1
winter balance. Snow density was estimated from samples taken from pit walls,
and from samples taken from the surface with thin-walled core tubes. Density
was measured in the 1981 and 1982 snow pits from 500 ml snow samples taken
from the pit walls every 0.50 m and assumed to be representative of the 0.50 m
interval. The cumulative thickne,s of ice lenses was also measured and
density was corrected accordingly. 1~ 1983 the same procedure was used exc~pt
samples were taken every 0.10 m. Cores, taken from the surface, were 54 mm in
diameter and 1.5 m long. Often several cores had to be taken in each hole to
complete a snowpack sample.
Table 1 'ummari?.es density data gathered on the Susltna glaciers between
1981 and 1983. Mean snc ~pack density, which is of primary concern when
calculating the water conLent of a given snowpack, is tabulated for late
spring, mid summer and early fall and for each field season. The density
shown for early fall is for the late summer snow only; the previous winter's
snow is not included in early fall snow density determinations. The data
listed in Table 1 a re averaged both over depth and elevation. While mean
snowpack density did not change significantly with elevation, density-depth
relations did. Almost invariably sno~packs above the equili~rium line showed
increasing density with depth and snowpacks below the equilibrium line ~howed
decreasing density with depth. The change in density with depth was greatest
in the surficial third of the high elevation snow packs and greatest in the
basal third of the lower elevation snowpacks. These relations are more
clearly illustrated in Figure 3. The density relations at high elevation can
be explained by compaction; the low altitude relations are caused, at least in
part, by the formation of a depth hoar.
The data in Table 1 show the spring snowpack mean density to lie
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consistently around 400 kg/m3 . The mid-summer data are limited but the
measured density of roughly 500 kg/m3 seems reasonable considering the late
spring 400 kg/m3 snowpack is wet then. The late summer-early fall densities
can vary considerably depending upon when the new snow accumulates. !n late
August of 1981 a very wet snowfall had a density of about 380 kg/m3 • Density
of the September 1982 and September 1983 snowfalls was not measured. These
relatively minor snowfalls had been on the ground for several weeks before
their depth was measured and were therefore assumed to have a density of
250 kg/m3 •
For the mass balance calculations a density of 400 kg/m 3 was used for
spring snowpacks, 500 kg/m3 was used for snow that was still present after
mid-summer, 400 kg/m3 was used for the late summer snowfall of 1981, and
250 kg/m3 was used for the fall snow of 1982 and 1983. !ce density was
assumed to ~e 900 kg/m3 as is standard practice in glaciological
investigations.
Superimposed ice, formed on the lower part of the glacier from downward
pe rcolating water as discussed earlier, was not studied in the field.
However, rough estimates of the amount of ice were made from earlier
temperature measurements in Black Rapids Glacier (Harrison and others, 19 7 5 )
located j u st east of the Susitna basin. Spring temperatures, measured in the
ice just before infiltrating water reaches it through the overlying snow,
characterize the strength of a "cold reservoir" which is available for
freezing of downward percolating water. These temperatures were used to
estimate the amount of superimposed ice that could form. The result is 0.3 to
0.4 m of water equivalent, which, although large, is probably an upper
limit. Mayo (pers. comm.), in 20 years of mass balance work on Gulkana
Glacier, has never observed this much superimposed ice.
The formation of superimposed ice would not significantly affect the
estimates of total ablation, and therefore of runoff. If some snow had melted
and refrozen as superimposed ice, this quantity would have been tabulated as
glacier ice melt rather than snow melt, but the total melt quantity would be
the same. Winter season balance estimates could be affected by superimposed
ice, but the glaciers were visited early enough in 1982 and 1983 that little
melt is thought to have occurred. However, in 1981 melting occurred early,
and the field measurements late, and there was little snow on the lowest parts
of some glaciers when they were visited. A lower limit on the winter season
balance was therefore all that was obtained from that year. Attempts to
estimate the magnitude of the effect on winter balance from snow course data
are described in R & H and Harrif?n (1981) but no corrections are included in
this compilation.
Superimposed ice forms only in the lower, essentially impermeable areas
of a glacier, but an analogous pheno~enon called internal accumulation occurs
in higher areas. Its physical basis was discussed earlier. Because the
higher areas ar a permeable, freezing can take place to considerable depth. If
the fre~zin~ was confined to the snow above the most recent summer surface,
and if the density of this snow was monitored, the effect should lead to no
serious errors in balance determination. However, freezing may occur deeper,
within the firn accumulated from previous years, where it may be impractical
to make density measurements . Thi.J quantity of interna l accumulation was
estimated by the method of Trabant and Mayo (198 5), which e mploys an empirical
relationship between internal accumulation and the late spring te~perature at
the summer surface. This temperature was determined from snow pits. The data
and results are summarized in Table II.
(3) Balance-Time and Balance-Elevation Curves
The evolution of water equivalent thi c kness was estimated at each
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measurement point from the snow and ice accumulation and ablation data, the
density data, and the internal accumulation estimates. The results permit
balance-time curves to be drawn for each measurement point from spring 1981 to
fall 1983 (Figure 4). From these curves balance-elevation curves can be
constructed for any desired time interval. Summer season (15 May to 30
September), winter season (l October to May 14) and annual (l October to 30
September) balance-elevation curv~s are shown in Figure 5.
(4) Average Balances
Average balances were found by integrating the point balance measurements
over the glacier surfaces, and dividing by their total surface areas. A
standard method was used, The balance elevation relations of Figure 5 were
multiplied by an areal distribution function describing the distribution of
glacier area with elevation, surface elevation then becoming the single
variable of integration. The areal distribution functions were obtained by
planimetry from l:63,~o v USGS topographic maps, using a 152 m (500 feet )
contour interval, At low elevations, the areas were divided into debris
covered and clean sections. Ablation under the debris covered areas was
assumed to be one half that of clean ice at the same elevation (Na k awo and
Young, 1981; Fujii, 1977; ~strem, 1959). Percent debris cover for each
glacier and elevation found is given in Table III. At high elevations only
the areas shown in white on the maps were included. The result s are shown in
Figure 5 next to the balance-elevation curves. The procedure is open to
criticism as discussed later. The results of these integ rations a re g iven in
Table IV.
All tributaries, surrounding small glaciers and perennial snow patche s
were assumed to behave in a manner similar to that of the main glaciers. The y
were divided according to river drainage (Figure 2). The area-elevation
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relations of each were determined and added to the areas of the main glaciers;
these areas are included in the Figure 5 plots. Susitna Glacier, with its
more complex tributary system, was the only exception to this procedure.
Complete accumulation/ablation data were obtained on its Turkey and Horthwest
tributaries in 1981 and on Turkey tributary in 1982. The balance-elevation
relationships on those tributaries were used to calculate their individual
mass balances.
The accuracy of the mass balance of Susitna Glacier in 1983 is limited
because the only reliable accumulation data for that year were collected in
the large north facing basin on the main tributary. If this stake, and the
other two on the main glacier are used to calculate annual balance for 1981,
which we were forced to do for 1983, the result is -0.09 m water equivalent
rather than -0.30 m. This is a difference of 0.21 m water equivalent or 57 x
106 m3 of water. It should be noted that this uncertainty has little effect
on estimated glacier runoff for that year. The balance relations on the lower
reaches of the glacier, where nearly all the melt and therefore from which
most of the runoff occurs, are relatively unaffected by this lack of
accumulation data. Rather, the amount of replenishment in the upper reaches
is affected, which is a reflection of the general health of the glacier that
year, not the melt or runoff.
(5) Eureka Glacier
Eureka Glacier presents a problem because it straddles the drainage
divide between the Susitna and Delta River basins (Figure 2). Its balance
characteristics were assumed to be similar to those of Maclaren Glacier. It
was further assumed that 60% (24 km 2 ) of its area lies within the Susitna
basin.
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(6) Error
There are many uncertainties in a mass balance investigation such as
this. They fall into two categories, those associated with the measurements
at each point, such as density, snowpack thickness, superimposed ice,
elevation, etc. and those associated with extrapolations of the points over
the glacier area. The former, point b a lance error, was estimated by squaring
the uncertainty in each measurement that contributed to a balance, summing the
squares and taking the square root. The results are given as error bars on
the balance-time data points in Figure 4. These errors were transferred to
the balance-altitude plots of Figure 5. In general the errors are small
compared to the balance changes between measurements .
The elevation of each stake and the distribution of area with altitude
were taken from USGS 1:63,360 series topographic maps, which are based on
1949, 1954 and 1956 aerial photography. Based on the photogrammetrically
determined surface elevation change on East Fork Glacier (R & M and Harrison,
1981) and the work by Post (1960) or. Susitna and other surging-type glaciers
in the region, a 100 m elevation error was assigned to the lower elevation
stakes, 75 m to the middle stakes and 50 m to the upper stakes . These errors
are shown as vertical bars on the balance-elevation plots of Figure 5.
Although the glaciers have lost considerable mass since 1949, it is not safe
to assume that elevation uncertainty can only be in the form of an elevation
loss. Surge type glaciers build a reservoir of ice at high elevation prior to
a surge, and deplete it during a surge. Both Susitna and West Fork Glaciers
surged before the maps were made, so their upper basin elevations should be
higher than the map elevations. As noted earlier, at high elevations only the
areas shown as white on the USGS maps were counted as glacierized. This must
lead to a small underestimate in effectively glacierized areas, since mos t of
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the snow falling on the steep slopes at high elevations is probably avalanched
onto the glaciers.
How accurately a given stake represents the area it samples can be
estimated qualitatively by comparing stake data to the more extensiv~ probe
data. The point balance data from probes for the winters of 1980-81, 1981-82
and 1982-83 are shown in Figure 6. In general the probe data agree well with
stake data up to about 2000 m (Figure 5). Above 2000 m probe data generally
show greater winter balance than stake data. Often at high elevation there is
no hard summer surface to probe to, which results in less accurate estimat~s
of winter snowpack. Consequently, stake data were considered to provide more
reliable data for these high elevation areas.
The error introduced by extrapolating the balance-elevation curve to high
elevation is probably large. The curves were drawn in such a way as to peak
and level off at the elevation of the topographic saddles in each glacier's
basin. This error is buffered by the fact that very little glacier area lies
above about 2500 m, where the error due to extrapolation is greatest
(Figure 5).
The stake and elevation errors, when combined, allow the balance-
elevation curves, as a whole, to shift both with respect to elevation and with
respect to balance. If these curves are shifted to the extreme end of both
the elevation error bars and balance error bars the effect is to increase or
decrease the balance at any point by an amount that is greater than just the
uncertainty in the balance at that point. For the winter season balance this
overall error is about 0.20 m water equivalent; for the annual balance it is
about 0.40 m water equivalent, and for the summer season balance it is about
0.50 m water equivalent. If the winter, annual and summer balance-elevation
curv~s are shifted, as a whole, by these respective amounts, the average
13
balances shift by approximately the same amount. It is felt that these error
estimates are conservative for the point balances and realistic for the
average balances, the latter of which are subject to the extrapolation errors
discussed previously.
Ill. GLACIER RUNOFF
When compared to nearby unglacierized areas, glaciers tend to produce
large quantities of runoff. The Susitna glaciers are no exception. During
1981, 1982 and 1983 the runoff per glacierized area was roughly 2.5 times tha t
of the unglacierized basin above Gold Creek and 1.5 times that of the
surrounding basin above the Denali Highway. Glacier runoff comes from three
sources: snow melt, firn and ice melt, and rain. A comparison of each of
these components to the total water flow through each gauge site during 1981,
1982 and 19 6 3 (USGS) can be found in Tables V and VI and Figure 7.
Before discussing each of these water sources, we emphasize that glacier
runoff is not the sum of the summer balance and summer precipitation because
the high elevation precipitation falls as snow. This summer snow has two
effects. First, it makes the average summer ablation less negative and
second, not all the precipitation that falls in a given summer leaves the
basin that same year.
(1) Snow Melt
By the definitions discussed earlier, all winter snow that falls below a
glacier's equilibrium line melts the following summer. This is the average
winter balance below the equilibrium line . Some snow melt also oc c urs above
the equilibrium line, but the quantity is obscured by summer accumulation at
the higher elevation stakes. This higher elevation snow melt had to be
estimated from melt rates at lower elevation stakes where summer accumulation
does not occur.
14
Melt rate decreases with increasing elevation to the point where all melt
is absorbed by the glacier as internal accumulation. Trabant and Mayo (1984 )
place this melt/internal accumulation equality at roughly 2100 m in the
central Alaska Range. Using this elevation as the point above which no runoff
comes, and all low and mid elevation ablation data, the melt rate versus
elevation plot of Figure 8 was developed. From this curve the average melt
per year, in excess of that absorbed by internal accumulation, can be
estimated at any elevation. The total snow melt above the equilibrium line is
approximated by integrating this balance-elevation relation over the area
between the equilibrium line and 2100 m.
The total snow melt is the sum of the winter balance below the
equilibrium line and the calculated amount of melt froc above. This total,
for each stream gauge and year is listed in Table V.
(2) Firn and Ice Melt
The firn and ice melt is the amount of melt produced below the
equilibrium line in excess of the past winter's snowpack. It is the total
melt below the equilibrium line minus the average winter balance below the
equilibrium line. This quantity for each year and stream gauge is listed in
Table V. It should be noted that significant firn melt only occurs durin&
years with exceptionally hot summers and/or low winter accumulation. Under
these conditions the equilibrium line is pushed to an unusually high
elevation, thereby exposing previous years' firn.
The firn and ice melt is the water that ma l es glacierized basins
different from unglacierized basins. It is the precipitation that fell in
decades past, metamorphosed and was then transported to lower elevation. It
provides, at least on the short term, a very large reservoir of solid water
available for melt. The quantity of melt depends almost entirely upon summer,
15
rather than winter meteorological c onditions and, therefore, has about t he
same predictability as summer rain.
(3) Rain
A rain gauge has been maintained by R & M Consultants on a west facing
~lope above the confluence of the Horthwes t tributary and the main Susi tna
Glacier at 1430 m elevation since 20 July 1981. The data from this gauge are
listed in Table VIIa. The data in Table V!Ia were supplemented by linear
regression using precipitation data at Talkeetna Airport. The 9 months for
which there are complete data on Susitna Glacier were used in the regression
(r2 • 0.86). The resulting regression equation is
?s • 1.65 PT-36.5 mm
where
Ps • precipitation on Susitna Glacier(s) in mm
PT • precipitation measured at Talkeetna Airport in mm
The supplemented data set appears in Table VIIb. Comparison of field notes to
the dates that precipitation fell at Talkeetna Airport allowed ~he
establishment, with reasonable certainty, that the calculated precipitation
did indeed fall as liquid on at least part of the glacier area. For example,
if the Septembe r 1982 (a month for which rainfall was calcu• ed) data are
exmained in Figures 4a and 4d it is reasonably clear ~hat no snow had
accumulated by late September at 1460 m (Figu re 4a) but some definitely had
accumulated at 1670 m (Figure 4d ).
Above 1600 m, summer precipitation almost invariably falls as snow on
nearby Gulkana Glacier (Mayo, pers. comm.). Assuming, for the case of the
Susitna Glaciers, that all summer precipitation below 1600 m falls as rain,
and all summer precipitation above 1600 m falls as snow, and assuming the
catch efficiency of the R & M rain gauge to be 1007., and ignoring
16
precipitation-elevation gradients, the average li q uid preci pi tation on t h e
glaciers can be determined by multiplying the rainfall in Table VIIb by 0.37
since only 37 7. of the basin's glacier area lies below 16 0 0 m. The results of
this calculation are shown graphically in Figure 7. It should be pointed out
~~t this is probably a lower limit on rainfall runoff since the catch
efficiency of the gauge is unknown.
(4) Evaporation
It is known from surface energy balance studies that "net" condensation,
the difference between condensation and evaporation, plays a significant role
in the sut'f..tCt: energy budget of a glacier (Paterson, 1969; Sharp, 1960). Data
from a number of glaciers indicate that the energy input from "net"
condensation varies from near zero to about 307. of the total energy used for
summer melt. However, because the ratio of the heat of vaporization to the
heat of fusion is about 7.5, the upper limit of 307. in energy converts to one
of 47. in mass. In other words, the ratio of total melt water to condensed
water is usually less than 4 7., which was considered negligible. What is
interesting is that condensation almost invariably exceeds evaporation in
glacierized areas.
(5) Timing of Runoff
On the average, runoff from glacierized basins in Alaska. peaks in late
July or early August (Chapman, 1 982). This is when the air is war m, most
precipitation falls as liquid, insolation is still relatively high, and a
large amount of low-albedo glacier ice is exposed. If storage of earl y summer
melt water by the glaciers is ignored, the proportional monthly melt runoff
can be approximated by adding the water equivalent melt at all stakes fo = a
given month and dividing by the melt at all stakes for the summer as a
whole. This could not be done for each year owin~ to lack of mid-summer data,
17
especially in 1983, Rather, all stakes that had enough data to allow
resolution of monthly proportions were used in the analysis. Stakes that
showed net accumulation were omitted.
The melt distribution as calculated by this met.hod comes out to 47., ~07.,
427., 307. and 47. for May, June, July, August and September respectively. For
comparison, the average monthly flows at Phelan Creek, a 707. glacierized basia
40 km east of Susitna basin, were 17., 157., 407., 337., 97. and 27. for May , June,
July, August, September and October during the 1967-1978 period of record.
Comparison of these percentages shows a larger Susitna spring melt than Phelan
Creek runoff, which is probably at least partly due to spring melt storage in
the Phelan Creek glaciers. Since such storage is a well-known and documented
fact from other glaciers (Patterson, 1981; Tangborn and others, 1975;
Stenborg, 1970), we have used the Phelan Creek data to distribute the monthly
melt from the Susitna Glaciers, even though it is a different basin and the
data are for different years. The results are shown in Figure 7.
IV. DISCUSSION AND CONCLUSIONS
The primary objective of the work described in this report has been to
assess the impact of high-elevation glacierized areas on the flow of the
Susitna River above Gold Creek. Melt and snow accumulation data obtained on
the glacier surfaces in 1981, 1982 and 1983 were used for the analysis. The
interpretation of the data serves L\ree purposes; first, to produce an
estimate of the amount of water produced by different sources in the
glacierized areas; second, to provide an estimate of the timing of its runo i f;
and third, at least in principle, to assess glacier volume change over the
three year period. Volume change estim~tes over a longer period, from 1949 to
1980, have been crudely estimated by Clarke (1985) and R & M and Harrison
(1981). The most recent estimates by Clarke (1985) indicate that on the order
18
of 3-47. of the Susitna River discharge at Gold Creek has been from secular
decrease of glacier volume for this period.
The conclusions of this report can be listed as follows:
(1) During 1981, 1982 and 1983 roughly 347. of the flow from above the Denali
Highway originated on the 257. (790 km 2 ) glacier cover, and about 137. of
the Susitna River flow at Gold Creek originated on the 4.9% glacier cover
above that gauge (exclusive of the glaciers in the Talkeetna Mountains)
(Table VI). Of the approximately 1.5 m/yr flow from the glaciers, 0.49
m/yr came from snow melt, 0.79 m/yr came from ice and firn melt and about
0.25 m/yr came from rain. Runoff from the rest of the basin above the
Denali Highway was about 0.9 T 0.2 m/yr; runoff from the basin above the
Denali Highway as a whole was 1.1 T 0.2 m/yr (Table VI); flows through
the Susitna at Denali, Maclaren near Paxson and Susitna at Gold Creek
gauges were 1.1 = 0.2 m/yr, 1.2 T 0.2 m/yr and 0.59 T 0.06 m/yr (Table
VI), respectively.
For comparison, the smaller and better-studied Phelan Creek
drainage, 70% g~acierized and 40 km to the east, produced about 2.02 m/yr
from 1967 to 1979 (Mayo, 1984).
(2) If the average monthly runoff from 1967-1978 for Phelan Creek is taken as
representative for the 1981-1983 melt runoff from the Susitna Glaciers,
the resulting flow distribution is li., 157., 407., 337., 97. and 27. tor May,
June, July, August, September and October (Figure 7).
(3) For 1981, 1982 and 1983 the average annual glacier balances (in m water
equivalent) were -0.05 T 0.40 m, -0.15 T 0.40 m and+ 0.26 T 0.40 m,
respectively (Table IV). Based on these data, which average to a gain of
+ 0.02 m/yr, it is tempting to say the glaciers were in approximate
19
equilibrium for these years, but the error is so large that this cannot
be said with much confidence.
(4) Accumulation varies considerably from glacier to glacier. Generally the
winter precipitation gradients are the same throughout the basin, about
1.2 t 0.1 mm water equivalent/m elevation, based on winter accumulation,
but each glacier's accumulation-elevation line is shifted vertically with
respect to the accumulation axis (Figure 6 a-c). This shift ranges over
about 0.5 m water equivalent, Maclaren Glacier being invariably the
highest, Turkey tributary the lowest, East Fork and Susitna main branch
close to Turkey and West Fork closer to Maclaren. Upon closer
examination East Fork and the main tributary of Susitna have nearly
identical winter precipitation gradients, even down to local accumulation
fluctuations (Figure 6c). This is probably due to similarities in basin
geometry (Figure 2). Also, as might be expected, this variability in
accumulation is reflected in the equilibrium line elevations (Table
VIII). The greater the accumulation, the lower the equilibrium line.
The limitations of this study need to be borne in mind. With only one
measurement point per SO km 2 , it can at best be considered a reconnaissance
level study compared with the mass balance studies done on many other
glaciers. An even more serious problem may be its short (3 year ) duration,
which has given but little perspective into the year-to-year variability of
the water supply from glaciers. Based on experience elsewhere, it s eems safe
to assume that in a drought year such a s 19 69 , water from ice and firn melt is
much more important, both in relative and absolute terms, than over the period
of this study. Finally, no attention has been given to the problem of
understanding, or seeking a correlation with, the meteorological factors
responsible for glacier water supply.
20
REFERENCES
Acres American Inc., 1982. Susitna hydroelectric project; feasibility
report. Final draft report for the Alaska Power Authority, Anchorage,
AK. 8 Vols.
Ahlman, H. W., 1953. Glacier variation and climate fluctuations. American
Geographical Society, New York, New York.
Anonymous, 1969. Mass balance terms. J. Glaciol., V. 8, no. 52, p. 3-7.
Bazhev, A. B., 1973. Infiltration and runoff of meltwater on glaciers. IASH
95, p. 245-250.
Benson, C. S., 1962. Stratigraphic studies in the snow and firn of the
Greenland Ice Sheet. SIPRE Research Report 70, 93 p.
Bezinge, A., 1979. Grande Dixence et son .. ydrologie, la collection de
donne~s hydrologiques de base en Suisse, Association suisse l'amenagement
des eaux. Service hydrologique national, 19 p.
Bjornsson, H., 1977. The cause of jokulhlaups in the Skafta1River,
Vatnajokull. JoKull, v. 27, p. 71-78.
Chapman, D. L., 1982. Daily flow statistics of Alaskan streams. NOAA
Technical !iemorandum NWS AR-35, 57 p.
Clarke, T. S., 1985. Glacier Runoff, Balance and Dynamics in the Upper
Susitna River Basin, Alaska. M. S. thesis, University of Alaska,
Fairbanks, in preparation.
Fountain, A. G., and W. V. Tangborn, 1985. The effect of glaciers on
stream flow variations. Water Resources Research. V. 21, no. 4,
p. 579-586.
Fujii, Y., 1977. Field experiment on glacier ablation under a layer of
debris cover. J. Japanese Soc. of Snow and Ice. V. 39 (Special Issue)
p. 20-21.
Harrison, W. D., L. R. Mayo and D. C. Trabant, 1975. Te mperature
measurements on Black Rapids Glacier, Alaska, 1973. In: G. Weller and
S. A. Bowling, eds., Climate of the Arctic. Geophysical Institute,
University of Alaska, Fairbanks, Alaska, p. 350-352.
Harrison, W. D., B. T. Drage, S. Bredthauer, D. Johnsoc, C. Schoch and A. B.
Follett, 1983. Reconnaissance of the glaciers of the Susitna basin in
connection with proposed hydroelectric developoent. Annals of Glaciol.,
V. 4, p. 99-104.
21
Hodge, S. H., 1974. Variation ln the sliding of a temperate glacier. J.
Glaclol., V. 13, no. 68, p. 205-218.
Krimmel, R. H. and ~. V. Tangborn, 1974. South Cascade Glacier, the
moderating effect of glaciers on runoff. ~estern Snow Conference, 1974,
p. 9-13.
Msyo, L. R., 1984. Glacier mass balance and runoff research ln the U.S.A .
Geogr. Ann., V. 66A, no. 3, p. 215-227.
Mayo, L. R., H. F. Heier, and W. V. Tangborn, 1972. A systeQ to combine
strati&raphlc and annual mass-balance systems: A contribution to the
International Hydrological Decade. J. Glaclol., V. 11, no. 61, p. 2-14.
Meier, M. F., 1965. Glaciers and climate. In:
eds., The Quaternary of the Unl ted States.
Princeton, New Jersey, p. 795-805.
H. E. Wright and D. G. Frey,
Princeton University Press,
rakawo, !1. and G. J. Young, 1981. Field experiments to determine the effect
of a debris layer on ablation of glacier lee. Annals of Glaciol. V. 2,
p. 82-91.
~strem G., 1959. Ice melting under a thin layer of moraine, and the existance
of lee cores ln moraine ridges. Geogr. Ann., V. 41, no. 4, p. 228-230.
Paterson, W. S. B., 1969. The Physics of Glaciers. Per~amon Press, New York
N.Y., 250 p.
Paterson, W. S. B., 1981. The Physics of Glaciers, 2nd ed. Per~amon Press,
New York, N.Y., 380 p.
Post, A. S., 1960. The exceptional advances of the Muldrow, Black Rapids and
Susitna Glaciers. J. Geophys. Res., V. 65, p. 3703-3712.
R & M Consultants and W. D. Harrison, 1981. Alaska Power Authority Susitna
Hydroelectric Project; task 3 -hydrology; glacier studies. Report for
Acres American, Inc., Buffalo, N.Y., 30 p.
R & M Consultants and W. D. Harrison, 1982. Alaska Power Authority Susitna
Hydroelectric Project; task 3 -hydrology; glacier studies. Report for
Acres American, Inc., Buffalo, N.Y., 22 p.
Sharp, R. P., 1960. glaciers. Univtrsity of Oregon Press, Eugene, Oregon.
78 p.
Stenborg, T., 1970. Delay of runoff from a glacier basin. Geogr. Ann., V.
52A, p. 1-30.
Tangborn, W. V., 1980. Contribution of glacier runoff to hydroelectric power
generation on the Columbia River. Acad. Sci., USSR, Section of
Glaciology, Data of Glacier Studies, Pub. 39, p. 62-67 and 140-143.
22
Trabant, D. C. ~d L. R. Mayo, 1985. Effects of intecnal accumulation on five
glaciers in Alaska. Annals of Glaciol., V. 6, in press.
Tangborn, W. V., R. H. Krimmel, and M. F. Meier, 1975. A ccmparisQn of
glacier mass balance by glaciological, hydrological and mapping methods,
South Cascade Glacier, Washington. IAHS 104, p. 185-196.
U.S. Geological Survey. Water resources data, Alaska, 1981, 1982 and 1983.
23
Table I. Sn ow pack density (kg/m3 ), averaged over both depth and .elevation,
for different times of the year on the Susitna glaciers. The
underlined number is the number of stations occupied during the
indicated time period. The number next to it is the total number of
samples used in calculating the mean density. A sample is one
complete snowpack density determination, either by core or snow
Hay
Late
July
Late
August
pit. The number of samples is always greater than or equal to the
number of stations because often several samples were taken at the
same station. Error shown is one standard deviation where the data
points are average snowpack density. That is, the error reflects
density variations over the glacier's area rather than density
variations with snow depth.
Snowpack Density in kg/m3
Snow Pit Data Core ~ata
1981 1982 1983 1981 1982 1983
370 ~ 20 420 i: 20 390 i: 10 410 i: 80 390 i: 40 390 i: 10
~10 4/4 1f5 35/65 ])3 3/12
---530 i: 40 --
7/13
---380 i: 50 --
3/7
24
Table II. Internal accumulation based on late spring firn surface temperature
(method from Trabant and Mayo, 198 5).
Glacier Date Elevation Firn Temeera ture Internal Accumulation -( m) (•c) (meters water equivalent)
May 1981 1950 -3 0.12*
West Fork 5/23/82 1950 -8 0.30
5/8/83 1980 -4 0.16
Susitna 5/26/81 2010 -3 0.12
Main 5/17/82 2010 -7.5 0 .2 8
Tributary 5/5/83 2010 -5.2 0.20
Turkey 5/23/81 2290 -6 0.23
Tributary 5/15/82 2200 -8 0.30
of Sus!tna 5/6/83 2040 -5.6 0.21
Northwest
Tributary May 1981 2350 -6 0.23*
of Susitna
5/28/81 1950 -l 0.04
East Fork 5/20/82 2050 -6 0.23
4/28/83 2060 -3.0 0.12
5/29/81 1950 -3 0.12
Maclaren May 1982 2010 -6 0.23*
5/l/83 2030 -2.0 0.08
*These firn temperatures were not measured. They had to be estimated from
other temperatures at similar altitudes and snow depths.
25
2 6
TABLE III. Percent debris cover in each elevation band on each glacier in the upper Susitna River
basin. Area for each elevation band is shown graphically in Figures 5a-f.
Susltnll Susitna Susltna
ELevation Interval \Jest Fork Haln Branch Turkel Tributarr Northwest Tributarl East Fork Maclaren
763-915 m 1007. 1007. 717.
915-1067 91 92 19 377.
1067-1220 63 48 10 18
1220-1372 42 21 20:Y. 10 23
1372-1525 22 3 277. 14 8 13
1525-1677 6 0 23 6 2 0
1677-1830 0 0 0 5 0 0
Table IV.
Average Yinter Season Balance (meters water equivalent )*
Glacier 1981 1982 1983
West Fork +0.86 +0. 78 +0.93
Susi tna +0.73 +0.65 +0. 78
East Fork +0.77 +0. 78
Maclaren +0.83 +1.14 +1.07
Average +0.80 +0 .81 +0.89
*l October-14 May
1981-1983 Average: +0.83 m/yr
Average Summer Season Balance (meters water equivalent)*
Glacier 1981 1982 1983
West Fork -0.87 -1.02 -0.81
Susi tna -1.03 -0.87 -0.38
East · ck -0.97 -0.69
Maclaren -0.52 -1.00 -0.70
Average -0.85 -0.96 -0.63
*15 May-30 September
1981-1983 Average: -0.81 m/yr
Avera~e Annual Balance (meters water equivalent)*
Glacier 1981 1982 1983
Yest Fork -0.01 -0.24 +0.12
Susitna -0.30 -0.22 +0.40
East Fork -0.20 +0.09
Maclaren +0.31 +0.14 +0.37
Average -0.05 -0.15 +0.26
*1 October-30 September
1981-1982 average: +0 .02 m/yr
27
Table V. Annual water yield from snow melt, firn and ice melt, and rain from
the Susitna basin glaciers during 1981, 1982 and 1983.
Glacier Firn Total Glacier To tal
Snow and Ice Glacier Rain Glacier
Melt Melt Melt Runoff Runoff
Year Stream Gau&e m/yr m/yr m/yr m/yr m/yr
1981 Maclaren River at 0.54 0.42 0.97 0.33 1.3
Denali Highway
1981 Susitna River at 0.43 0.93 1.2 0.33 1..7
Denali Highway
1981 Susitna River at 0.45 0.83 1.3 0.33 1.6
Gold Creek
1982 Maclaren River at 0.64 0,51 1.1 0.25 1.4
Denali Highway
1982 Susitna River at 0.45 0.95 1.4 0.25 1.7
Denali Highway
1982 Susitna River at 0.49 0.86 1.3 0.25 1.6
Gold Creek
1983 Maclaren River at 0.70 0.36 1.1 0.17 1.2
Denali Highway
1983 Susitna River at 0.49 0.77 1.3 0.17 1.4
Denali Highway
1983 Susitna River at 0.53 0.69 1.2 0.17 1.4
Gold Creek
Average Maclaren River at 0.63 0.43 1.1 0.25 1.3
Denali Highway
Average Susitna River at 0.46 0.88 1..3 0.25 1.6
Denali Highway
Average Susitna River at 0,49 0.79 1.3 0.25 1..5
Gold Creek
28
2 9
Table VI. This table gives a detailed break.down of how the runoff fro1n glaciers compares to total runoff. The
first four columna refer to total runoff and area above the given stream gauge . The middle three
columns refer to glacier melt runoff. The last three columns refer to both glacier melt runoff and
glacier rain runoff. The years in parenthes below each stream gauge refer to the time period over
which averages were taken. Runoff from the two dam sites, (5) and (6), do not strictly compare because
streamflow data are for a different time period than glacier data.
Basin Area
above Stream
Guage
km 2
Stream Guage
(l) Maclaren River at
Dena 11 Highway
(1981-1983)
(2) Susitna River at
Dena 11 Highway
(1981-1983)
(3) Total flow from
above Denali Highway
[sum of (1) and (2))
(1981-1983)
(4 ) Susitna River at
Gold Creek**
(1981-1983)
(5) Uatana Dam S ite**
(1949-1981 synthesized
flow)***
(6) Devil Canyon Dam Site**
(1949-1981 synthesizeu
flow ***
7 30
2460
3190
15,950
::::11,100
Average
Annual pow
m /s
28.3
83.6
112
299
224
258
Specific
Runoff
m/yr
l. 2 2
1.07
1.10
0.59
Glacier
Are ·~
km 2 /i.
160*/22
628/25
790*/25
790* /4.9
790*/7.1
790*/5.9
Glacier Snow, Firm
and Ice Helt Runoff
m/yr m3/a X ·
1.07 5,44 19
l. 34 26.8 32
1.29 32.2 29
0.06 32.2 ll
32.2 14
32.2
*Area is not known accura tely because Eureka Glacier straddles the drainage divide.
**Numbers do not include glaciers in the Talkeetna Mountains.
***From Acres American, 1982
Glacier Melt and
Glacier Rain Runoff
m/yr m3 /a 7.
1.32 6.7 24
l. 59 31.7 38
l. 54 38.4 34
0.08 38.4
38.4 17
38.4 1 5
3 0
Table VIIa. Rainfall collected by an R & M rain gauge during 1981, 1982 and
1983 at 1430 m elevation next to Sustt-ca Glacier . Data are
listed in mm.
1981 1982 1983
April N/A 16.6* 13.0
May N/A 26.0 2.6***
June N/A 103.8 18.8****
July N/A 194.2 50.8
August 300.2 78.6 242.0
September 66.7 0.4** 108.0
October N/A N/A 3.4
TOTAL 366.9 mm 419.6 mm 438.6 mm
*April 14-30 **September l-2 ***May l-10 ****June 14-30
Table VIIb. Summer precipitation on the Susitna Glaciers during 1981, 1982
and 1983. In general, summer precipitation abov->OO m
elevation falls as snow (Mayo, pers. comm.) ~ce only 377.
of the glacier area lies below 1600 m ation
quantities must be multiplied by ~ te
rainfall runoff from glacier~
1981 19
April N/S N/S
May ll* 26 _..,.
June l8J* 104 38*
July 330* 194 51
August 300 79 242
September 67 279* 108
October N/S N/S N/S
TOTAL 891 mm 682 mm 455 mm
*Precipitation approximated by linear regt·ession with Talkeetna Airport data
(r2 • 0.86 ).
Table VIII. Approximate equilibrium line elev~tions
1981 1982 1983
West Fork 1650 1675 1650
Susitna 1775 1850 1700
Hain Tributary
Turkey 1950 1825
Tributary
Northwest 19 25
Tributa.ry
East Fork 1825 1775
Maclaren 1625 t575 1625
31
w
t -.l
~\
' ' ' \
\
\
\
I
\
---
\
\
\
\
\
\
Figurt! l.
---------
Location rna p (From Acres Am erican , l 982).
10 MILES
0 10 ICILOMHfAS
• ACCUMULATIONtABLATIOH STAKES
Figure 2. Glacier names and locations, stake lbcations, and drainage divides
(Adapted from Harrison, 1983).
-E
::'i
0 a: u..
l: 3.0 -1-
11.
0.2
SPECIFIC GRAVITY
0.3 0.4 0.5
•
•
•
•
•
•
w
Q •
1980 Summer Surface . ··----__ ::__:_=..::,.~.--
-E
w
(,J
~ u.. a:
::1
(I)
4 .0
tJORTH'<'iEST TRIBUTARY,
SUS I TN/\ G LACIER
2350 m Elevation
22 May 1981
SPECIFIC GRAVITY
0 .2 0 .3 0.4 0.5
•
•
•
1.0
~ 1.0
0 1980 Summer Surtace
ll: u..
l: 2.0
1-
11. w
Q
TURKEY TRIBUTARY,
SUSITNA GLACIER
1670 m Elevation
13 May 1981
SPECIFIC GRAVITY
0.2 0.3 0 .4 0.5
•• • •
• !•
• • • • • • • • • .• •• • • •• • • • !'"·,mmer Surface • •
EAST FORK GLACIER
2060 m Elevation
2 8 April 1983
Figure 3. Snowpack density variations with depth and elevation.
Note that the top two figures depict accumulation area snowpacks,
and the bottom figure depicts an ablation area snowpack.
34
1-z w
..J
<(
>
3
0 w
a: w
1-
<(
~
(I)
[T
w w
1-\.J1 w
:i
UJ u z
<(
..J
<(
Q)
2 ------o-----o----o-:rr-n--~-------------~--------------~,s~s~o~m~-----------,
o o------a ~::;;:====~--~~~----------------------------------~ 0 ~~=-------------------------------9-9-~---2------~~~4~6~0~m~----------~
-.!
-·4
-
2
"' co 0 0>
~
-2
-4
~
M
;o 0 0>
-2
-
-~
-
0 0
0
0 SCS SNOW SURVEY DATA
0 STAKE DATA
15
FEB
I
1 5
MAR
15
APR .
·' ·'---------------~·~1~2~0~m~----------~
0~--u---<? 9
oa
n=-
1 5
MAY .
~ -----¢-9
¢
-~
0 ·-
----1460m ~. ~
15
JU'I .
I
15
JUl.
I
15
AUG .
I
15
SEP
WEST FORK GLACIER
I
1950m
1460m
I 120m
1950m
I 120m
15
uCT
j
15
~IOV
I
15
DEC
-
-
-
-
-
-
-
-
-
-
-
Figure 4a. Balance-time relations on West Fork Glacier.
:)
t-z
w
...J
<(
>
:::)
0 w
a: "' w .xl
t-Cll
<( -
:!:
(/)
w a: w a-t-
w :::
w u z
<(
...J
<( M
ID .xl
.;>
2 r-
r-
0 ~.3
t-
-2 r-
r-
-4 t-
r-
'f-
0
-2
-4
2
0
-2
-4
f-
~0
r-
f-
r-
r-
f-
1 5
JA N
I I I I I I I
2010m
1270m -----:a ~ -a -:J a a .Q, =:: --5 -===-
. --------'o .......... 6 15 3 0 m
' -----~-----~ .....
.......... ¢ ...... .........____
I 2 70m .• .r, ~ J
't ~ a --=~~ ~......_
~: 9 '----2 -~
~?
_.._I 2 70m
--'~ n :-:..---:..-,..-:_-;:-... "n ====.a
0 S C S S NOW SURV E Y DATA
0 S .. A K E DAT A
1
15
FEB
1
1 5
M AR
15
APR .
15
t-1 .\Y .
~
1
1 5
JUN .
I
1 5
J UL
I
SUSITNA GLACIE R
15
AUG .
15
:;e p
1110m
2 0 10m
1 5 30m
1 110m
-"} 2 0 10m
~ 1110m
15
0 CT
Fig ur e 4 b . Ba lan ce-time r e l a ti ons on S u si tna Gl acier.
I
15
NOV
I
15
DEC .
d
~
i
I .....
l
l
I
~
-
-
-
-
-
-
-
-
-
-
-
-
2 -
a)
.:> 0 -
2
t-z
UJ -4 ..J ...c
~
::::> 2 0 w
a: (\0
w a) 0 t-"' < -~
"""
en -2
-...J c.:
w
t-w -4 :::E
w u z 2
<
-I < M m J) 0 .:>
-2
-4
1 5
JAN
15
FE B .
I
' I I
---II ;_ -9-------2 350m -~-
----------t-~-~-·}
n -
~2 "-----~-2
:a: -------------:c:.
1 5
MAR
I
15
APR.
I
15
MAY .
I
15
JUN.
I
15
JUL .
I
-------~,
1 5
AUG .
I
15
S EP
I
NORTHWEST TRIBUTARY OF SUSITNA GLACIER
140 0m
1400m
t400m
15
OC T
J
I
15
N OV
I
I
I
-~c .
Figure 4c. Balance-time relation E on Northwest tributary of Susitna Glacier.
~
~
J
I
....J
-
-
-
l
~
-
-
-
-
-
al
Cl
..... z
w
...J
<(
::
:;)
0
w
a: N
w <XJ ..... Ol
<( -~
(/)
a: w UJ
CXl .....
UJ
~
u.: u
<(
...J
<( M m c:o
Cl
2
0
-2
-4
~
0
-2
-4
2
0
-:!
-4
t-
-
t-
t-
1-
t-
t-
t-
1-
-
t-
t-
t-
15
J AN
.s
FEB
I
15
MAR
I
15
APR .
I
---·9---y a-n.____
~~-?-~
< 9 <}
't
!I n---,J>---2-2
rr
15
MAY .
I
a 1670m
15
JUN .
I
1 5
JU L
I
TURKEY GLACIER
1:>
A U G .
I
15
S EP
J.. y
I
2200•
1670m
2200m
1670m
2200m
15
OCT
I
15
NOV
I
Figure 4d. Balance-time relations on Turkey Tributary of Susitna Glacier.
15
DEC
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
:0
:> -
... z
Ul
..J
~
>
3
0 w
a: "' w co ... Q)
c( -~
(/)
\,..) a:
\0 w ... w
::IE
w
0 z
<
..J
c(
ID M
Cl)
Q) -
2
·1
-2
-4
2
0
-2
-4
2
0
-2
-4
....
15
JAN
I . . I t I I I I I I I
~ 1950m
--~
0 0~~---.....
--------~----------t460m
2050m
X ~
0 0 0 :C:Il-1580m
~~ ~--~ t080m
~ ~~Om
0 n -o......._______
"-t080m~.
t590m
0 SCS SNOW SURVEY DATA ~,•oeom 2___.a
0 STAKE DATA
15
FEB
15
MAR
15
APR.
15
MAY .
15
JUN.
I
15
JUL
I
15
AUG
EAST FORK GLACIER
1
15
SEP
_l
15
OC!
Figure 4e. Balance-time relations on East Fork Glacier.
I I
'5
I•OV
I ~ -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
, 5'
CEC .
-~-::~ G~------------------------~~~~;:~----~~~----------------------------------~ ........ ""' ~c._ 1430m
t-z
~-------------
i --...J --------------~1~1=0=0~m~--------------
~ -4
< >
§ !t_ __________ ------------------~~~--------------------------~
~
2010m J
-
~ N '===============================~0~]----====~~~--------------------------------------------~ ~ ~ o F ------r -\ ~,, _____ 1:_1:..:0:.:0:.:m.:..:_ _____________ =_,
::E -4 1-1 -1
w -
~ 2 -1----------------?· 2030m
-~ -l------=--=--=--=-~~~-=--=--=--=--=--=--=--=-~~-=-~~=-0 -" ---m ~ o~====~~==::::=:=:=:==--------~:_~===~==~~~------------------------------------------4 ~ 11~0m -:? f-
-4
I 5
JA N
15
FEB
15
MAR
I
15
APR.
I
15
MAY .
I
15
JUN.
l
15
JUl
MACLAREN GLACIER
~~----'
l
15
AUG .
I
15
SEP
l
15
OC T
Figure 4f. Balance-time relations on Maclaren Glacier.
l
15
·IOV
I
:5
~ E C
-
-
-
-
4000
3500
E 300 J
z
0
1-
< > w
_j
w
500
\ \ \ I \ \ i\ \ \ \ I I \
I I I I
\ \ I I I I I I I I
I l I I I I I I I I I: I
-4 -2 0 2 -4 -2 0 2 -4 -2 0 2 10203040
A.r ea. km 2 Balance. m water equav .
1981
Balance . m water equav . B a lan ce . m water e quav .
1982 1983
WEST FORK GLACIER
Figure Sa. Balance-elevation and area-elevation relations for West Fork
Glacier.
I
I
I
D SUSITNA MAIN
4000 TRIBUTARY II I I ~ NORTHWEST
TRIBUTARY I I I I I
3500 -[£]TURKEY TRIBUTARY ,, t I I
PLUS BASIN TO EAST I I I I ,, I I I
E I I I I I I
z ,, I I I I I
0 ~ I I I
j: I I I I
~ II >
UJ
.J
1
UJ
~
N
500
10203040 -4 -2 0 2 -4 -2 0 2 -4 -2 0 .!
Area . km 2 Balance , m water eQUIV . Balance . m water eQUIV . Balance . m water eQUIV .
1981 1982 1 ' 1
SUSITNA GLACIER
Figure 5b. Balance-elevation and area-elevation relations for Susltna Glacier.
~000
3500
E 3000
z
0
~
4(
> w
-1
Lll
1500
l>-w
1000
500
ANNUAL
I I I
I I I
1 I I
10 2 03040 -4 -2 0 2 -4 -2 0 2 -4 I )
A r ea. k m 2 Balance , m water equ1v .
1981
Elalance , m water e QUIY . Bal a n ce . m w a t e r t~ue v
19b 2 1983
NORTHWEST TRIBUTARY OF SUSITNA GLACIER
Figure 5c. Balance-elevation and area-elevation relations for llorthweat
tributary of Susitna Glacier,
4000
3500
E 3000
z
0
t= < > w
...J w
1500
~
~
1000
500
I I I I I
I I I I I
I I I I I
I I ,,
I I I I
I I 1/ I
I I
I
,,
I
10 20 30 40 -4 -2 0 2 -4 -2 0 2 -4 -2 0 2
Area , km 2 Balance. m water eQuiv .
1981
Balance , m water eQulv . Balance . m water e Qu1v .
1982 1983
TURKEY TRIBUTARY OF SUSITNA GLACIER
Figure 5d. nalance-elevation and area-elevation relations for Turkey tributary
of Susitna Glacier.
4000
3500
E 3000
z
0 2500
1-
c(
> w
...J w
1000
500
10 20 30 40
Area . km 2
-4 -2 0 2
Balanc e, m wat e r equ1v .
1081
" II /
-4 -2 0 2
Balance . m water eou1v
1982
EAST FORK GLACIER
I
-2 0
Balance . m water eou1v .
1983
Figure Se. Balance-elevation and area-elevation relations for East Fork
Glacier.
E
z
0
1-
4:
> w
_J
w
4000
3500
3000
10203040
Area . km 2
-4 -2 0 2
Balance . m water equ1v .
1981
J
/
-4 -2 0 2
Balance . m water c qutv
1982
MACLAREN GLACIER
-4 -2
Bal a n c e . m water P Qu tv
1983
Figure 5f. Balance-elevation and art!a-elevation relations for Maclaren
Glacier.
I
I
I
80
UJ § u-60 z ~
<( 3
...Jcr
<( ~ co ... ~ 40 a:;;
~ w ~
....... ~ "' z = ~"2 20
0
3000
E L E VATI O t ~ (metersl
1000 1500 2000
A MACLAREN 5-29 -81
D WEST FORK 5 -30-81
!:!. SUSITNA 5-26-81
0 EAST FORK 5 -28-81
• TURKEY 5 -24 -81
• NORTHWEST TRIBUTARY 5-30-81
4000 ~0 00 6000 7000
EL EVATION tf <?c tl
Figure 6a. Winter accumulation for 1980-1981 as determined from snow pack
data.
:.'5 00
2 .0
::
UJ ~ u -z ·~
<( = ...J cr
<( ~
co ~ 1.0 a: ;;;
w ~ ~ z c
~ ~ ~
8 000 0 .0
...
we:
u..!!:!
z ~ < ·:: ...1-<g-
Ill ... Cll
a:~
w ~ ...... z Cll
-~
p. ~ .s
00
80
60
40
20
1000
• MACLAREN 5 -2 1-82
o WEST FORK 5 -23. 24 -82
{). SUSITNA 5-17-82
0 EAST FOR K 5-18 , 20, 21 -8 2
e TURKEY 5-15-82
ELEVATION (metersl
1500 2000 2IDJ
2.0
1.0
0
3000 ~----------~4~ooo~----------~sooo~~--------~6~ooo~--------~7~oo~o-----------8oo~o_J 0 ·0
ELEVATION (feetl
F igure 6 b . \linter accumulation for 1981-1982 as determined from sn ow p a c k
data .
wE u-2
z ~ ce:;
....10" < Cll
Ill ... Cll a:~ w ~ ...... z ...
-Cll .., ...
;.... Cll
E
80
.... c: u~ 60 z ~
<( :::
....Jcr
<( "" CD ~ "" 40 a: iii
UJ ~
1-"' .,.. z ~
\0 ::u
~ c: 20
0
3000
ELEVATION (metersl
1000 1500 2000 :?500
.l MACLAREN 5 -1-83
D WEST FORK 5 -8-83
.6. SUSITNA 5-5-83
0 EAST FORK 4-30-83
• TURKEY 5-6-83
a NORTHWEST TRIBUTARY
4000 5()()( J 6000 7000 8000
ELEVATION (feetl
Figure 6c. Winter accumulation for 1982-1983 as determined from snow pack
data.
r
~ 10
00
...... c
:J -
q
~ -
<( c:,
CD ~
a: "' UJ ~
1-" ;;::: '-'
~ ~
.::.
GLACIER WATER AND TOTAL WATER AT STREAM GAUGES
:::i
~ :! 500
W" o e
:-. ~~ a: ~ 200
~:l: zu :-en ;;a
::I en
~ 1 w w a: • u_..
0 I!
..J 0 . 0~
;-II: ~~ ...:6 -en .:-zO
:-g;
::I
CoO
1981
200llllill~
ONOJFMAMJJAS
500
0 ,. 0 .J F M A M J J A S
1982
200
[,; I I I~
ONOJFMAMJJAS
E'J Total ru noff
• Glacier rain runoff
~ Glacier melt runoff
1983
Figure 7. Runoff from glaciers compared to total runoff at stream gauges on
the Susitna and Maclaren Rivers.
50
E
z
0 .,_
<: > w
-I w
I I I . I
2100 ' 2000 ~, -
' • ' •
1500 i-:.' -
• • • ' ............_
.4 • • • • • ----• 1000 f------
I I I I I
0.0 1.0 2.0 3 .0 4.0 5.0
ABLATION RATE, m/yr
Figure 8. Ablation per year, in excess of that usP 1 for internal
accumulati o n, versus elevation. Ablation is given in meters of
water equivalent.
51