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Home My WebLink AboutAPA4142 This document is copyrighted material. Permission for online posting was granted to Alaska Resources Library and Information Services (ARLIS) by the copyright holder. Permission to post was received via e-mail by Celia Rozen, Collection Development Coordinator on December 16, 2013, from Kenneth D. Reid, Executive Vice President, American Water Resources Association, through Christopher Estes, Chalk Board Enterprises, LLC. Five chapters of this symposium are directly relevant to the Susitna-Watana Hydroelectric Project, as they are about the Susitna Hydroelectric Project or about the Susitna River. This PDF file contains the following chapter: Some aspects of glacier hydrology in the upper Susitna and Maclaren River basins, Alaska by Theodore S. Clarke, Douglas Johnson, and William D. Harrison ........................ pages 329-337 Assigned number: APA 4142 American Water Resources Association PROCEEDINGS of the Symposium: Cold Regions Hydrology UNIVERSITY OF ALASKA-FAIRBANKS, FAIRBANKS, ALASKA Edited by DOUGLASL.KANE Water Research Center Institute of Northern Engineering University of Alaska-Fairbanks Fairbanks, Alaska Co-Sponsored by UNIVERSITY OF ALASKA-FAIRBANKS FAIRBANKS, ALASKA AMERICAN SOCIETY OF CIVIL ENGINEERS fECHNICAL COUNCIL ON COLD REGIONS ENGINEERING NATIONAL SCIENCE FOUNDATION STATE OF ALASKA, ALASKA POWER AUTHORITY STATE OF ALASKA, DEPARTMENT OF NATURAL RESOURCES U.S. ARMY, COLD REGIONS RESEARCH AND ENGINEERING LABORATORY Host Section ALASKA SECTION OF THE AMERICAN WATER RESOURCES ASSOCIATION The American Water Resources Association wishes to express appreciation to the U.S. Army, Cold Regions Research and Engineering Laboratory, the Alaska Department of Natural Resources, and the Alaska Power Authority for their co-sponsorship of the publication of the proceedings. American Water Resources Association 5410 Grosvenor Lane, Suite 220 Bethesda, Maryland 20814 JULY COLD REGIONS HYDROLOGY SYMPOSIUM AMERICAN WATER RESOURCES ASSOCIATION 1986 SOME ASPECTS OF GLACIER HYDROLOGY IN THE UPPER SUSITNA AND MACLAREN RIVER BASINS, ALASKA Theodore s. Clarke, Douglas Johnson and William D. Harrison 1 ABSTRACT: Proposed hydroelectric development on the Susltna River, Alaska, has raised Interest In the glacIers that form Its headwaters. Three separate aspects of the hydrology of these glaciers are addressed here. First, long-term glacier shrinkage, which releases water that Is not renewable In the norma I sense, appears to have produced on the order of 3-4% of the total Susltna River flow above the Gold Creek gauge site since stream gauging began. Second, the major glaciers of the basin are surge- type and have the potent I a I to produce, In a few ronths, up to 30 times the estimated annual sediment Input Into the proposed Watana Reservoir. The next surge of one of the glaciers, Susltna, Is predicted In the fIrst decade of the next century. ThIrd, wl nter precIpItatIon varIes by a factor of two among the glaciers, Maclaren Glacier receiving the most. (KEY TERMS: Glacier shrinkage, glacier surges, sediment supply, precipitation variations.) I NTROD U::T I ON This paper describes, In part, the results of a study of the g I ac I ers that head the Sus I tna and Maclaren rivers (Figures 1 and 2). It addresses three separate topics: ( 1) whether the glaciers have changed In volume since stream gauging began on the Sus I tna RIver, ( 2) If and when any of th.e g I ac I ers In the area may be expected to surge, and h~ surges might affect the Susltna River, and (3) how precipitation varies throughout th~ area. A previous paper provides glacier runoff and mass balances estimates (Clarke and others, 1985). Early phases of the work are descrIbed by R & M and HarrIson ( 1981 ) and R & M and HarrIson ( 1982) and summarIzed by HarrIson and others ( 1983). The mater I a I presented here shou I d be C:Jns I dered an u~ate to these three early papers. Glaciers cover about 790 square kilometers or 5,9% of the bas In area above the proposed Watana dam site, 5.2% of the area above the proposed Devil Canyon sIte, and 4. 9% of the area above the Sus I tna River gauge located at Gold Creek (Figure 1). Field measure~ents of precipitation, snow accumulation, Ice melt, glacier speed, and surface elevation were made on most of the major g I ac I ers In the bas In during 1981, 1982 and 1983. I. LONG-TERM GLACIER VOLUME CHANGE Long-term glacier volume change Is an Important part of any hydrologic feasibility or planning study because It may have a significant Impact on project- ed water SUI>Piy. In general, glaciers have decreas- ed In size during the last half century. Conse- quently, water to their basins has been supplied out of Ice storage. As the glaciers approach equlllb- rl um wIth the present c II mate, the amount <>f water from storage approaches zero. ThIs has I ed, In some Instances, to an overestimation of water supply (Bezlnge, 1979). It seems that before long-term water availability Is predicted from stream gauge records, smoothed trends of glacier release or storage of water over the per I od of record ,; holll d always be subtracted. This reduces the problem to a conventional on~ Jf I J:Jg-term prediction for !In unglaclerlzed basin, although, of course, even the conventional approach Is susceptible to errors caused by c II mate change. Mayo and Trabant ( 1986) present evidence that a definable climate change took place In the Alaska Range In the Gulkana Glacier region, starting about 1976. Yo I ume change est I mates for the Sus I tna bas In are based on measurements on an unnamed g I ac I er, commonly referred to as East Fork Glacier (Fig- ure 2), which makes up only 5% of the total glacler- lzed surface. Pre.vlous estimates of Its volume change over the period 1949 to 1980 were made from photogrammetrlc data (R & M and Harrison, 1981; Harrison and others, 1983). These estimates sug- gested an average change In thickness of -50 ± 18 m. If this were typical of the other glaciers, then 13% of the Susltna River flow at the Gold Creek gauge site would have been from glacier storage. Since this seems unreasonably large, two other methods for estimation of volume change were 1 Theodore s. Clarke, Douglas Johnson and William D. Harrison, Geophysical Institute, University of Alaska- Fairbanks, FaIrbanks, AI aska 99775-0800. 329 0'\ \ \ \ \ \ \ \ --- \ \ \ \ \ \ \ --....._ ---------- Figure 1. Location map. (From Acres American, 1982.) applied. The first used direct measurement of glacier surface altitude; the second used the runoff precipitation model of Tangborn (1980). Direct Measurement of Glacier Surface Elevation In 1982 surface el evatlons were measured at several points on East Fork Glacier as a check of those estimated photogrammetrlcal ly from 1980 photos In the earlier work. Elevations were measured with a he I I copter and Its altimeter. Measurement points were located either by Brunton co""ass bearings to map I dent If I ab I e features or by theodo II te and establIshed control points. The altimeter was ca I I brated per I od I ca I I y on rock poInts of known elevation. The results are shown In Table 1. The results agree with those from the 1980 photos except at the highest point. According to the altimeter data, this point has remained at roughly the same elevation since 1949 when the u.s. Geological Survey maps were made, but the data provided by the photogrammetrlc method show this point to have lost 40 m of elevation. This discrep- ancy might be explained by the fact that the 1980 330 aerial photographs of East Fork Glacier show almost no contrast In Its accumulation area. This makes It difficult to Identify the surface accurately In these smooth snowy areas. Also, one might expect the accumu I at I on area of a "norma I" (non surge-type) glacier In retreat to remain at roughly the same elevation because a decrease In annual balance over the surface of a glacier affects the volume of Ice transported by the glacier In a way that accumulates down-glacier. The change In volume of the glacier was obtain· ed by co111>arlng the altimetry data with elevations obtained from 1949 photos. Unlike the 1980 photos, the 1949 photos are of very good qual tty. The elevations obtained from these early photos agree wl th pub I I shed map e I evat Ions and are therefore probably accurate. In practice, the volume change was co111>uted by determl n I ng a thIckness change versus elevation relationship, multiplying It by the area per e I evat I on I nterva I determl ned from the nap, and finally, by Integrating over the elevation Interval spanned by the glacier. Taking the altimetry data as the more reliable, the average thickness change of East Fork Glacier comes out to -13 m water equivalent for the 1949 to 5 10 MILES 0 5 10 KILOMETERS I::::::==::::E;;;;;;;;;;;;;a if. VELOCITY POINT Figure 2. Glacier names, locations and drainage divides. Glacier center line velocity was measured where Indicated. The points on the figure were placed next to the glaciers tor clarity. (M:>dlfled from Harrison and others, 1983.> 1982 period, rather than the -50 m tor the 1949 to 1980 period estimated by the previous work. It this 13m of water equivalent loss Is again extrapolated over the remaining 95% of the Ice In the basin (with suitable caution) then, on the average, about 3 or 4% of the Susttna River flow at Gold Q-eek has been due to glacier recession as opposed to the 13% of the earlier estimates. This estimate has very large errors associated with It since It Is based on tour points on a glacier that makes up only 5% of the Ice In the basin. However, It does seem more reasonable considering that the glacier runoff over the 1981 to 1983 period, when the glaciers were In approximate equilibrium, totaled only about 13% of the flow at the Gold Creek gauge site (Clarke and others, 1985). Tang born Runott-Precl p ltatlon M:>de I Tangborn ( 1980) has suggested a model tor determining long-term historical glacier balances by co~arlson of adjacent glacterlzed and unglaclerlzed basins. The model works by determining differences In runoff that do not correspond to precipitation changes, and these differences are assumed to be 331 caused by changes In storage of water as glacier Ice. The annual precipitation In each basin Is determined by using a representative precipitation station and determining a coefficient that corrects for precipitation differences between the basins and the precipitation station. The sum of evaporation, transpiration and condensation, per unit area, Is assumed to be the same tor both basins. The coef- ficient can be determined It runoff from both basins and glacier volume change are known tor a period of at least 1 year and If a suitable precipitation station exists. The model was tested against published mass balances of nearby Gulkana Glacier tor the period from 1967 to 1977 (Meter and others, 1980). Six different precipitation stations and three different unglactertzed basins were checked for the best possible tit of the model. Phelan Q-eek was used as the glaclertzed runoff station since this drains Gulkana Glacier. The best correlation between ca I cuI a ted and measured ba I ance occurred when Talkeetna precipitation station was used with ~he unglaclertzed basin Sh lp Creek near Anchorage (r = 0.77). Further datal Is are given by Clarke (1986). Table 1. Comparison of photogrammetrlc data (R & M and Harrison, 1981; Harrison and others, 1983) to helicopter altimetry data on East Fork Glacier. The surface elevation changes tor the altimetry data are for the period from 1949 to 1982; the surface elevation changes for the photogrammetrlc data are for the period from 1949 to 1980. A loss of elevation Is Indicated by a negative sign. East Fork Glacier Location on Elevation Change Elevation Change Glacier Center Line Altimeter Photogrammetry {1949 Map Elevation) (m) (m) ( 1949 to 1080 1390 1590 2050 In applying the model to the Susltna basin, there was a cons lderab I e uncertaInty In what the actual balance was for the period from 1981 to 1983. The measurements, tor all Ice In the basin, came out to +0.06 m water equ Iva I ent when summed over the 3-year period, but the cumulative uncer- tainty tor the 3-year period was 0.6 m (Clarke, 1986). In Tangborn 1 s model this uncertainty plays a large role In the resulting change In glacier mass tor the period from 1950 to 1983. These dates were chosen because 1950 Is the t lrst year from which complete runoff data are available tor the Susltna River at Gold Creek. It It Is assumed that balance tor the period from 1981 to 1983 was +0.06 m, then the average loss from the glaciers above the Susltna River at Gold Creek gauge site for the period from 1950 to 1983 was -16 m water equlva lent. It the balance was +0.66 m, then the average loss comes out to -9 m, and It the ba I ance was -0.54 m, then a calculated balance of -22m water equivalent results. The results of the two methods of volume loss estimation are summarl zed In Table 2. They are uncertain, but not Inconsistent. They Imply that 3 to 4% of the water t I ow at Go I d Creek between 1949 and 1980 came from Ice storage. This amount Is -74 ± -43 ± -51 ± +16 ± (m) 1982) ( 1949 to 1980) 18 -67 ± 18 18 -32 ± 18 18 -78 ± 18 18 -40 ± 18 wIthIn the stream gaugIng error and wou I d therefore probably not be s lgn It I cant In terms of projected water supply. II. GLACIER SURGES The major g I ac I ers of the Sus I tna bas In are West Fork, Sus ltna, 11 East Fork", Mac I aren, and Eureka (Figure 2). All except East Fork are listed by Post ( 1969) as beIng surge-type. Surges are sudden episodes of rapid glacier speed triggered by some Internal Instability, during which Ice movement may be hundreds or thousands of meters wl thIn a few months. The effects on sedIment and water supp I y, particularly the former, may' be substantial. There are some descriptive reports of high sediment production during glacier surges (Uskov and Kvachev, 1979; Shcheglova and Chlzhov, 1981) and two direct measurements. Humphrey (1986) reported that the 1982-1983 surge of Variegated Glacier, Alaska, released as suspended sed lment the equIvalent of about 0.3 m of eroded rock from the bed of that glacier. Bjljrnsson (1979) reported an erosion rate of 0.014 m/yr from the surge of Bruarj~kul I Glacier Table 2. Summary of glacier shrinkage estimates by two different methods. % Total Time Area Glacier I zed ThIckness Method Span Covered Area Loss Error Altimetry 1949-East Fork 5 13 (m) large, 1982 Glacier see text Runoff 1950-all 100 16 +6, -7 Precipitation 1983 glaciers If model Model In bas In applicable 332 In Iceland. The two measurements differ by more than an order of magnitude, but both are extremely high when col!l>ared to sediment production In non- surge years. Although Variegated Glacier Is consid- erably smaller than Susltna Glacier, both are narrow valley glaciers underlain by faults. It Variegated Glacier Is representa~lve of the Susltna basin, then a surge of the 250 ~ Susltna Glacier could release as much as 200 x 10 kg of suspended sed lment Into the Sus!tna ~lver, assuming a rock density of 2. 7 x 10 kg/m • ThIs Is 30 times the estimated annual ~edlment lntl~x,3 Including bed load, of 6.8 x 10 kg (5.8 x 10 m ) Into the proposed Watana Reservoir (R & M, 1982). There Is little direct evidence about the effect of surges on water supply. However, there are three potential effects. First, there should be a temporary Increase In me It water because of the Increase In ablation area that accol!l>anles some surges. Second, the extreme crevasslng that occurs during a surge temporarl ly Increases effective surface area, and therefore ablation. Third, surges release stored water (Kamb and others, 1985), a I though It Is not c I ear whether thIs water comes from long-term storage or merely from the most recent summer season. Given these effects of surges on sediment and water supply, It seems worthwhile to review the past hI story of surges In the Sus I tna bas In, and what It ~y Imply tor the future, particularly since surges tend to be per I odIc (MeIer and Post, 1969). West Fork Glacier Is known to have surged sometime shortly before 1940 when Bradford Washburn photo- graphed It, Susltna Glacier underwent a strong surge between 1949 and 1954 (Post, 1960); photos that we recent I y exam I ned IndIcate that the surge was COI!l>lete by July, 1952. Maclaren Glacier underwent a weak surge or strong 11 pu lse" In 1971 (Mayo, 1978). Surface speed measurements on West Fork, Susltna, and East Fork glaciers Indicate flow regimes that reflect the surge behavior of the first two. For both of these g I ac I ers the rate of Ice flow from the accumulation area Is considerably less than the rate of snow accumulation there (Table 3). This Indicates a thickening of the accumulation area that wl II probably be terminated by another surge. The velocity data and details of how accumu- lation and outflow were calculated are given by Clarke (1986). Wast Fork Glacier The disequilibrium of West Fork Glacier evident In Table 3 Is consistent with Its past behavior. Oblique aerial photographs of the terminus, taken by Bradford Washburn In 1940, show It to be extremely broken up and chaotic (see Clarke, 1986). This Information, along with the looped moraine pattern, Is conclusive evidence that a surge took place. Post (written comm. to Steven WII bur, 1984) places the surge In 1937. Close Inspection of 1981 NASA color Infrared aerial photographs shows at least three successIve term Ina I moraInes, each of whIch was very II ke ly caused by a success Iva ly weaker surge. Unfortunately, the periodicity of the surges cannot be estimated quantitatively because little Table 3. Comparison of annual lee flow through several cross sections to the annual accumulation above the sections. The location of each cross section Is shown as a velocity point on Figure 2. Surface center line velocity Is assumed to be caused by 50% Internal deformation and 50% basal sliding. AI I quantities are given In water equivalents. The cross sections are slightly below the accumulation areas and are shown as velocity points on Figure 2. Average Annual Ice Flow May 1981- Glacier June 1983 Name <m 3 /yr x 10 6 > West Fork 54 t 21 Sus I tna, MaIn Branch 14 t 6 Sus ltna NW Tr I b. 36 t 14 Sus ltna Turkey Tr I b. 72 t 28 East Fork 31 ± 12 1981 98 t 33 50 t 19 21 t 15 89 t 15 Annual Accumulation Above the Cross Section (m 3 /yr x 106 > 1982 1983 82 t 33 113 t 33 34 t 19 71 ± 19 70 t 15 20 ± 13 25 ± 13 333 Volume Change Above Cross Section ( 1981-1983 average) (m 3 /yr x 10 6 ) +44 t 39 +38 ± 20 Information exists for West Fork Glacier prior to the Washburn photographs. tJofflt (1915) gives a brief description of the glacier as It was In 1913 but nothing to Indicate a surge had occurred recent- ly. If Its recurrence period Is similar to the 50 or so years for Susltna Glacier, discussed below, a surge may be expected fairly soon. Susltna Glacier Susltna Glacier, unlike West Fork, has a complex set of tributaries that were studied Indivi- dually, as summarized In Table 3. It can be seen that the main branch of Susltna Glacier Is trans- porting only a fraction of the accumulated snow down-glacier. This would Indicate that either this branch of the glacier Is the one causing the surges, or It Is at least a reservoir that depletes during a surge. Altimetry data collected In the accumulation area of Susltna Glacier also show this branch to be accumulating mass. A gain of 56± 18 m of elevation from 1956 to 1982 was meas urad by comparIng 1982 a ltlmetry data to 1956 map elevation data (Clarke, 19~6)~ This translates to a gain of 93 ± 30 x 10 m /yr, which Is reasonably consltfte_rt with the average rate of gain of 38 ± 20 x 10 m /yr tor the 1981 to 1983 period (Table 3). Examination of moraine patterns confirms that this basin did Indeed contribute a large quantity of Ice to the last surge. Figure 3 depicts the moraine patterns on Susltna Glacier before and after the early 1950's surge. Before the surge, Ice motion In the main trunk above Turkey trl butary appeared to be very smal I, with relatively vigorous flow from Turkey pinching It oft. After the surge, a large volume of Ice had clearly advanced from the basin of the main branch. A large volume of Ice appears to have come from Turkey trIbutary a I so, and Northwest trIbutary appears to have contributed very little Ice, If any, to the surge. These observations Indicate that flow and accumu I at I on In Northwest trIbutary were prob- ably In equilibrium before the surge, the meln branch was far out of equilibrium, and Turkey tributary was somewhere In between. There are two reasonably quantitative approach- es to determining Susltna Glacier's surge period. First, the lobe created In the moraines of the main glacle~ by Northwest tributary had an area of about 4.0 km In 1949. A surge of the main glacier took place about 1951, as already noted. By 198~ the new lobe had grown to an area of about 2.0 km (Figure 3). Assuming the surge occurred In 1951, and assuming the present glacier speeds to be similar to those In the past, a period of roughly 60 years Is Indicated. Second, close Inspection of the same lobe In 1949 aerial photographs shows about 47 oglves to have passed from Northwest tributary Into the main glacier trunk (see Clarke, 1986). Oglves, or Forbes bands, are known to form on an annua I basts (Nye, 1958). Again assuming the surge occur- red In 1951, a surge return per I od of 49 years Is Indicated. It could be argued that Northwest tributary surges Independently, but the slow growth 334 SHORTLY IE FORE IS 52 SURGE SHORTLY AFTER SURGE 1110 Main Br&nch Turkey Tr I b. NW Trlb. Figure 3. Evolution of moraine patterns on Susltna Glacier. Left and center diagrams are from Meter and Post (1959). Right dl agram Is sketched from National flero- nautlcs and Space Administration photo- graphs. (tJodlfled from Harrison and others, 1983. > of Its new lobe and the balance between accumulation and flow makes this seem unlikely (Table 3>. The next surge wou I d therefore be expected wIthIn the first decade of the next century. East Fork, Maclaren and Eureka Glaciers East Fork Glacier Is probably not a surge-type glacier, as suggested by the approximate balance In Table 3, and by evidence from the displacement of surface features that the speed has not changed much since 1949 <R & M and Harrison, 1982). Both Maclaren and Eureka glaciers are thought to be weak surge-type glaciers; they do not surge on the order of k II ometers II ke Sus f tna and West Fork. As noted previously, Maclaren Glacier under- went a "pulse" In 1971 (Mayo, 1978). No speed measurements were made on these glaciers. II I. PRECIPITATION VARIATIONS Another Interesting aspect of glacier hydrology In this basin Is the large difference In winter precipitation among the dl fferent glaciers. In the late winter of 1981, 1982 and 1983, snowpack thick- -;; w c: u~ z ~ <{ ·-...J§. <{ Q) cc ~ a:; w 3:: 1-"' z ... -Q) ;=~ E 2.0 1.0 2.0 1.0 0.0 2.0 1.0 1000 & MACLAREN c WEST FORK A SUSITNA 0 EAST FORK • TURKEY • NORTHWEST 1000 1000 ELEVATION (meters) 1500 1500 1500 2000 2000 2000 2500 MAY 1981 2500 MAY 1982 2500 MAY 1983 0.0~---------------------------------------------------------------------...J Figure 4. Winter accumulation versus elevation as determined from snow probe data. (Top figure Is modified from R & M and Harrison, 1981; mlddl~ figure Is from R & M and Harrison, 1982.> 335 ness was measured by probing at several points along the center line of each glacier, and snowpack density was measured at representative points on each glacier. The water equivalent thickness at each point Is plotted In Figure 4. These data are reasonably consistent wlth more accurate snow depths measured at a few sites where stakes were maintain- ed, Generally the winter precipitation gradients are the same from glacier to glacier, about 1,2 mm water equivalent per meter of elevation, but the a bso I ute amount of water varIes cons I dera b 1 y from glacier to glacier. Maclaren Glacier consistently received the most precipitation, and the two steep south-facing tributaries of Susltna Glacier consis- tent I y receIved the I east. An orographIc effect created by the Clearwater Mountains, which divide the tributary Maclaren River basin from the Susltna River basin, may direct moisture toward Maclaren Glacier and reduce precipitation In the Susltna basin to the west, It Is worthwhile to note that because Maclaren Glacier had a positive mass balance of nearly 0,3 m/yr and the others had generally negative balances, It produced less runoff over the study period even though It received considerably more preclpltat~n (Clarke and others, 1985), IV, DISCUSSION AND CONCLUSIONS An attempt has been made here to ( 1) determl ne whether the glaciers that head the Susltna and Maclaren rivers have changed In volume since stream gauging began on the Susltna River, (2) determine when these surge-type glaciers may surge again, and what the effects of surges are II ke I y to be, and (3) describe variation In winter precipitation throughout the area, The conclusions are as fol- lows: I, The elevation change due to glacier wastl!lg seems to be on the order of -10 to -15 m water equivalent tor the 1949 to 1983 period tor East Fork Glacier rather than the -50 m estimated by R & M and Harrison (1981) and Harrison and others (1983) tor the 1949 to 1980 period, ThIs amounts to 3 or 4% of the tot a I t I ow of the Susltna River at Gold Creek rather than 13%. This quantity seems more consistent with the tact that during 1981, 1982, and 1983, when the glaciers were In approximate equilibrium, the average runoff from the Susltna basin glaciers was about 13% of the total Susltna River flow at Gold Creek (Clarke and others, 1985), 2. West Fork and Susltna are surge-type gla- ciers, It sediment output during a surge of Susltna Glacier, tor example, Is simi tar to that of Variegated Glacier, a single surge may produce about 30 times the estimated average annual sediment Influx Into the proposed watana reservoIr. The rates of transport and d I sper- slon of such a large sediment Influx are unknown. A surge of Susltna seems likely 336 because about two-thirds of the snow accumulat- Ing In the basin of Its main branch Is not beIng transported out <Tab I e 3), and the accumulation area of this same branch has gained approximately 56 m of elevation since the last surge. It past history Is any Indica- tion, It appears that Susltna Glacier has a surge period of 50 to 60 years, which places the next surge sometime between the years 2000 and 2010. It Is also likely that West Fork Glacier will surge In the future, but no quantitatively determined period can be placed on It since no data are available tor the period prior to Its 1937(?) surge. 3, Accumulation varies considerably from glacier to glacier, with Maclaren Glacier receiving more winter precipitation than any of the other glaciers. Generally, the winter precipitation gradIents are the same throughout the bas Ins, about 1,2 ± 0,1 mm water equ Iva I ent/m e I eva· tlon, but each glacier's accumulation versus elevation curve Is shifted vertically with respect to the accumulation axis, The shift ranges over about 0,5 m water equivalent (Figure 4), REFERENCES Acres American Inc., 1982, Susltna hydroelectric project; feasibility report, Final draft re· port tor the Alaska Power Authority, Anchoraqe, Alaska, 8 vols. Bezlng, A,, 1979, Grande Dlxence et son hydrologle, Ia collection de donnees hydrologiques de base en Suisse, Association Suisse l'amenagement des eaux. Service Hydrologlque National, 19 pp. Bjornsson, H., 1979, Nine glaciers In Iceland, Jokul I 29:74-80, Clarke, T. s., 1986, Glacier runoff, balance and dynamics In the upper Susltna River basin, AI aska, M.S. ThesIs. Un Ivers tty of AI aska, Fairbanks, 98 pp. Clarke, T, s., D. Johnson and W, D. 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Science 227(4686):469-479. Mayo, L. R., 1978. Identification of unstable glaciers Intermediate between normal and surging glaciers. Academy of Science, USSR, Sect I on of G I ac I o I ogy, ProceedIngs of the International Workshop on Mechanism of Glacier Variations, Pub. 33, p. 47-55 and 133-135. Mayo, L. R. and D. c. Trabant, 1986. Recent growth of Gulkana Glacier, Alaska Range, and Its relation to glacier-fed river runoff. Short paper for u.s. Geological Survey Water Supply Series. In press. Meter, M. F. and A. s. Post, 1969. What are glacier surges? Canadian Journal of Earth Science 6:807-817. Meter, M. F., L. R. Mayo, D. c. Trabant and R. M. Krimmel, 1980. Comparison of mass balance and runoff at four glaciers In the United States, 1966 to 1977. Academy of Science, USSR, Section of Glaciology, Report of the Inter- national Symposium on the Computation and Prediction of Runoff from Glaciers and Glacial Areas, Pub. 38, p. 139-143 and 214-216. Moffit, F. H., 1915. The Broad Pass Region, Nye, Alaska. u.s. Geological Survey Bul I. 608. 80 pp. J. F •• glaciers. 1958. A theory of wave formation on lASH 47:139-154. 337 Post, A. s., 1960. The exceptional advances of the Muldrow, Black Rapids and Susltna glaciers. Journal of Geophysical Research 65:3703-3712. Post, A. s., 1969. Distribution of surging glaciers In western North America. Journal of Glaciol- ogy 8(53):229-240. R & M Consultants, 1982. Alaska Power Authority, Susltna Hydroelectric Project; Appendix B.S, Reservoir Sedimentation. Report for Acres American, Inc., Buffalo, NY, 49 pp. R & M Consultants and w. D. Harrison, 1981. Alaska Power Authority Susltna Hydroelectric Project; task 3 -hydrology; glacier studies. Report tor Acres American, Inc., Buffalo, NY, 30 pp. R & M Consultants and w. D. Harrison, 1982. Alaska Power 114Jthorlty Susltna Hydroelectric Project; task 3 -hydrology; glacier studies. Report for Acres American, Inc., Buffalo, NY, 22 pp. Shcheglova, o. P. and o. P. Chlzhov, 1981. Sediment transport from the g I ac I er zone, centra I Asia. Annals of Glaciology 2:103-108. Tangborn, w. v., 1980. Two models tor estimating climate-glacier relationships In the North Cascades, washington, USA, Journal of Glaciol- ogy 25(91) :3-21. Uskov, Ju. s. and v. 1. Kvachev, 1979. The dldal surging glacier. Data of Glaciological Studies, Chronicle, Discussion. Academy of Sciences of the USSR, Section of Glaciology of the Soviet Geophysical Committee and Institute of Geography, Pub. 36, p. 170-175.