I

Dayld R, Eutler, Departmentof Geography,Un versityof Georgia,Athens, Georgia 30602

Snow -Damsand ResultantHazards in GlacierNational Park, Montana

Abstract

Snowavalanches fonn natural dams to creek and.iv€r flow in Glacier National Park, Montana. Dams are produced primarily by densewet snowavalanches. Because such dams are unstableand so posea seriousthreat to doanstreamhabirals, I anallzed them in areasof Glacier National Pa.k. Dan fornation occursunder conditionsof either zonal or meridional flow. Dans may be depositedduring winters of high avalanchefrequency, but such conditions are not essential.Ternporary reservoirshave b€eninpounded by ayalanche-dams,causins stream impoundmentand inundation ofhighways. Calculatedpotenrial naxinun discharsesillustrate a hazard of s€riousproportions, in sone casesexceeding all but one known hisro cal spring flood. Three areasin the Park are ofspeciat concernbecaus€ of the history of avalanche-damdevelopnent acrossr.ansporrarion lines. Future avalanche-dansand resultant outburst could darnageroadways and a major bridge.

lntroduction snowavalanche-dans and resultantfloods in the mountainousregions of North America:excep- Natural damsform in many ways,Barriers can tions include the study by Mokievsky-Zubok be formed by , ice or , glacial (1975)in an uninhabited portion of the Coast moraines,volcanic eruptions, fluvial deposition, Mountain Range of British Columbia, and the eoliandeposition, coastal sedimentation, and by recentwork of Campbell(1988) in the vicinity of organicaccumulations (Costa and Schuster1988). McBride and Valemount in the RobsonValley Most natural dams are inherently unstable,and of Bdtish Columbia.As populationpressure and can lead to widespreaddevastation mountain and loss of life (Costa l9BB). developmentscontinue throughout the North Americanwest, it becomesincreasingly im- Snowavalanches form natural damsin aloine portart to identify areassusceptible to potential cnvironmentsaround the world;outburst floods flooding from snow avalanche-damoutbursts andassociated devastation caused by failureof (Campbell1988). The purposeof thispaper is to snowavalanche-dams have been reDortedfrom describethe hazardsassociated with avalanche- the Andes of Argentina (King 1934J,the Euro- dam failure and alsoto documentthe geographic pean AIps (Allix 1924, Peattie 1936, Tricart et extentand climaticcharactedstics responsible for cl l96l). the northern Scandinavianregion the formation of snowavalanche-dams in a tvoi- (Rapp 1960),and the Torlesse Range of New cal North Americanalpine environment. Zealand(Ackroyd 1987).One such tragedy was describedby Peattie(1936, p.59): "On the 20th Snow and Avalanche-Dams of February, 1720, the Swiss village of Ober- gestelenwas destroyed and 84 of its 200 inhabi- Two major types of snol{ avalanchesare recog- tants were killed by an avalanchewhich leaped nized in avalancheclassifications: the slab ava- an intervening forest and vrecked one-third of lanche, where a large area of cohesivesnow the village.....The snowfrom the avalanche acrossa slope begins to slide all at once; and blocked the river Rhone, which quickly cut loosesnorr avalanches,which start at a point or through it and then flooded a portion of the over a smallarea and increasein sizeas they des- settlement."The failureof the avalanchedam cend (LaChapelle 1985, p. 19-20).Loose snow and subsequentflooding exacerbatedan already avalanchesare frequently subdivided into dry desperate situation, and occurred during snow and wet snow avalanches.Of these three recovery operatlons, rypes(slab, dry loose,and wet loose snow),wet Despitethe widespreadoccurrence of sno\{ snowavalanches are the primary mechanismfor avalanchesand avalancheaccidents in Canada creating snowavalanche-dams. Slab avalanches and the United States (Stetham and Schaerer frequentlydisintegrate into smallerma6ses upon l9?9, 1980;Schaerer 1987; Williams and Arm- descent(LaChapelle 1985), and dry loose snow strong 1984),there has been very little focus on avalanchesfrequently do not possesssufficient

NorthwestScience, Vol. 63, No. 3, 1989 109 cohesionto act as efficient dams.The wet snow and March (Butler 1986a).Meteorological tig- avalanches,because of their weight and density, gering mechanismsfor avalanchesthere include: possessenough stength to temporarily dam l) heary snow,2) heary snov followedby a rapid creeksand riversand createshofi-term natural rise in temperatureto abovefreezing, 3) a rise resetvorts. in air temperature to above freezing without The period of time in which an avalanche- precipitation, and 4) rain in associationwith dam may exist will vary with the size, cohesive- above-freezing temperatures (Butler l9B6a). ness,and densityof the snowdeposit, Estimates Avalanchetlpes presentalong the major trans- of longevityof knownavalanche-dams range fiom portarionlinks includedry loosesnow avalanches, about one hour (Allix 1924)to over eight days wet snowavalanches, and slab avalanches(Butler (Mokievsky-Zubok1975). 0ne exampleof the den- and Malanson 1985a, 1985b, Butler 1986a, sity of a wet snow deposit which formed an 1986b).Wet snowavalanches are the most com- avalanche-damin the presentstudy area is pro- mon type to affect the major roads,with 80 per- vided by Martinelli (1984). He examined an ce\l ol 223 studied avalanchesresultins from avalanchedeposit which temporadlydammed the weather conditionswith temperaturesabove Middle Fork of the Flathead River in north- freezing (Butler 1986a, p.B2). Wet snow ava- westernMontana. The deposit,examined four lanchesare alsothe predominanttype of avalan- days after deposition, still had many large air che in other regional study areas such as the spaceswithin and was composedlargely of wet Kananaskis region of southwestern Alberta icy balls of snow. density,measured with (McPherson et al. l9B4), Because wer snow a Federalsrow sampler,averaged 414 + 37 avalanchesare the primary avalanchetype in kg/m3 for eight samples. the area,it is not surprising that a hazardexists from avalanche-damsand associatedoutburst The StudyArea floods. Glacier National Park, located in the Rocky Methodology Mountains of north$estern Montana, is typical of many North American alpine areas.Increas- In recent studies of avalanche in ing winter usageof the Park for recreationalpur- GlacierNational Park, Butler (1986a, 1986b) corn- po6esoccurs in heavily glaciated terrain prone piled statisticson the locationand natureof snow to snow avalanches.Major transportation links avalanche incidences for rhe 75-year period also pass through this avalanche-proneterrain 1910-1985.Data sourcesincluded Clacier Na- (Figure l). U.S. Highway Number 2 (US2) is a tional Park Ranger daily logbook entries and major east-westtralrsportation link in Montana monthly summaryreports (Unpublished Monthly which parallelsthe Middle Fork of the Flathead Ranger Reportr);U,S. Governmentmonthly River along the southernmostboundary of the weathersummaries for all yearssince l9l0; un- (Figure Park l). A daily averageof at least 714 published letters, files, photographs,and slides vehicles (Butler use this highway 1986b),which on file in the Glacier National Park library; in- wascompleted through this area in 1930.The formation from the Montana Depsrtmentof only road that penetrates deeply into, and Highways;and all Decemberthrough May issues, crosses,the Park is Going-to-the'Sun(GS) Road, 1946-1988,of the Hungry Horse Neus, a local a narrow,two-lane road which wasopened to traf- weekly newspaperpublished since 1946 in Co- fic in 1933.GS Road usageis seasonal, typically lumbia Falls,Montana. This newspaperprovides open only from late May to late October. Ava- local accountsof avalancheoccurrences of a size lanchepotential is high along GS Road in winter or timing significant enough to hinder local and spring, and the avalancheparhs which oc- transportationand impact local tourism. cur in this area penetiate through the mature forest to impinge upon the valley bottom (Butler This studyuses the samehistorical data set, 1980).McDonald Creek is a major permanent and is supplementedby additionalsources cover- streamthat parellels GS Roadfor about l0 km ing the winters of 1985-1988.Information was abovethe head of Lake McDonald. recordedon the locationof snowavalanche dams, The peaksnow avalanche month in the Park weather conditions associated with the ava- is February,with secondarymaxima in January lanching,and the typeof avalanchewhich formed

I l0 Butler British Columbia Alberta CANADA

.Babb

GLACIE R

St. Mary

Nr/NENr-{l

West la Glacrer

Figure l. Locations suscepriblero avalanche'dan hazards,Glacier National Park, Monrana. Abbreviations as follows: GSR, Coing'to the-SunRoad; U52, U.S. Highray 2; MFFR, Middl€ Forl of the Flathead River; McD Cr, McDonald Creek; Lale McD, Lake McDonald. Shaded area west of Summit is Bear Creek. Snall open circl€s, other dam sites. the dam.Dam longevityand damagecaused by Costa(l9BB), designed for usewith jdkulhlaups. dam formation and subsequentfailure werealso These formula relate peak potential discharge recorded.The areaof avalanche-dammedreser- (Q-",) to reservoirvolume (V) basedon known voirs wasdelineated (based on historicaldescip- historicaloutburst fl oods, tions),plolted on 1:24,000scale topographic maps,and multipliedby depthof inundationto Results deriverudimentary volume estimates. Because Yearsof Avalanche-DamFormation no empirically-derivedformula exists for cal- culatingpeak discharge of floodsfrom avalanche- SincePark establishmentin 1910,major snow damfailure, estimates were calculated using the avalanches have been numerous and methodsof Clagueand Mathews(1973), and geographicallywidespread in 1910,1929, 1933,

SnowAvalanche-Dams lll 1936,1939, 1945, 1950, 1952, t954,1956, 1957, pected,given the preponderanceof wet snow 1963,1972,1975, 1979,1982, and l986 (Butler avalanchesin the overallpattern of avalanche 1986a,1986b). Discounting those years pior to typesin ClacierPark (Butler l9B6a).The remain- constructionof transportationlinks, sevenof fif- ing 4l percent have little data availablefor de- teenmajor avalancheyears have also produced termining type of avalancheand meteorological hazardoussnow avalanche-dams (number of dams conditions.The main exceptionis the deposition in parentheses):1933 (l), 1939(2), 1952 (l), 1954 of a powdersnow dam during a blizzardin 1956, (3), (3), 1956 1979(l), and 1982(l). Avalanche- whenover two feet (60 cm)of newloose snow was damshave also been deposited during 11inters depositedin a period of a few hours.6Another not particularly recognizedas major hazardyears dam wasdeposited in late January 1960afier an for snowavalanching; years these include 1935, unusually heavy snowfall.Temperatures at this 1951, 1960, 1970, and 1984, each with one time werebelow freezing.T avalanche-damoccurrence recorded in Parkand local records. The patternsof atmosphericflow conducive to major avalanchewinters in Glacier National Location of Snow Avalanche-Dams Park have been describedby Butler (l986a). Avalanche-damformation and the associated Briefly, two alternative synoptic pafierns exist hazards are concentratedin three locations in which haveproduced major avalanchecycles in GlacierPark (Figure l): alongMcDonald Creek, the Parki winters with strong zonal flow, heary from the headof McDonaldLake to the baseof snowfall,and rapidly fluctuating temperatures; the GardenWallz; along Bear Creek on the and winters with sustainedmeddional flow and southernboundary of the Parks; and along the catastrophicavalanching resulting from subse- Middle Fork of the FlatheadRiver near the quent inyasion and advection of Pacific air. southern tip of the Parka(Figure l). The latter Thesepatterns were shown to be similar to those two locations experienceavalanches which dis- affectingthe RogersPass area of Bdtish Colum- rupt traffic along U.S. Highway 2, whereasthe bia as describedby Fitzharris and Schaerer former createsconditions damaging to Going-to- (1980). the-SunRoad. Two other reportedcases from more isolatedparts of the Park are also illus- A comparisonof yearsof high frequency trated in Figure 1.5 avalanchingwith yearsin which manyavalanche damswere producedreveals that both atmos- There is no preferred slope direction pheric flow-patterns can produce avalanche associatedwith the formation of avalanche-dams. dams.The wintersof 1933and 1954,for exam- The avalanchesin the Bear Creek area form on ple, were years of strong zonal southto southeast-facingslopes, those along the flov and avalanche-damdeposition; MiddleFork ofthe Flatheadform on southwest- but, the winter of 1979 was facingslopes, and thosealong McDonald Creek strongly meridional in nature and also led to production. developon west-to northwest-facingslopes. dam The key to efficient ava- lanche-damdevelopment is apparentlynot so Meteorologcal ConditionsAssoctated with muchthe overall flow patterr asit is the tenden- Avalanche-DamFormation cy for warm (abovefreezing) temperatures and wet snow avalancheoccurrence. Precisetemperature and precipitation data are not availablefrom avalanchepaths in ClacierNa- Avalanche-DamDamage tional Park.However, general statements describ- and Potential Hazards ing weatherconditions and typesof avalanche in Glacier Park depositsprovide some insight into local meteoro- Depositionof avalanche-damshas produced tem- logicalconditions during avalanche-damdepo- porary road blockagesin and alongthe boun- sition. daries of the Park, as temporaryponds or Of the knownincidences of avalanche-dam redirected streamshave coveredthe highways. deposition,59 percentwere a resultof wet snow These blockagesproduce unaccountedbut dis- avalanchesoccurring while air temperaturewas tinct financiallosses for businesseslocated along abovefreezing. This conclusionis not unex- Highway2. tt2 Butler An avalanche-damdeposited in April 1952 whichalso contains large pieces of timber that blockedMcDonald Creek and completelyre- act asbattering rams along stream banks and at directedthe streamonto the sudaceof CS Road. bridge abutments.The pastblockages of the Mid- The highwaywas completelysoaked, and the dle Fork of the FlatheadRiver have included springtimesnov removal had to be delayedun- large quantitiesof timber, suggestingthe future til the road dried out and could bear the weight possibilityof a damefficient enough to produce of heavy snow-clearingequipment.s Past an outburstflood of sufficientsize to damagethe blockagesof Bear Creek along the southern highwaybridge where US2 IeavesGlacier Park borderof the Park havealso temporarily caused and entersthe smallcommunity of Essex(Figure submergenceof Highrray2.o Future occurrences l). Destruction of a highway bridge by direct of avalanche-damsalong these creeks will un- avalancheimpact in 1979caused a month-long doubtedlylead to similar situalions closureof U52 and necessitateda 300 km detour; Estimatedpeak discharges(Q-"") of pust similarimpacts would be felt if the bridgespan- outburstfloods vary by location.Estinated Q-- ning the Middle Fork is damagedor destroyed valuesalong Bear Creek range from 39 cubic by an avalanche-damoutburst flood. metersper second(cms) utilizing Clagueand Mathews' (1973) equation, Q-", = 0.0075 Conclusions and Recommendations ut, V.",o to about 59 cms using the Costaand Avalanchesmade ofwet loosesnow are mostlike- (l9BB) (Q-., = o6{, Schuster equation ll3 V ly to producesnow avalanche-dams in ClacierNa- with V in l0o m' format).Application of the same tional Park. No preferred slope orientation ex- produces (Clague formulae rangesof 56 cms and ists for the occurrenceof avalancheswhich oro- (Costa Mathews1973) to about 105 cms and ducedams. Avalanche-dams hare develop.d aJong Schuster1988) for both McDonaldCreek and the McDonaldCreek, Bear Creek,and the Middle Middle Fork of the FlatheadRiver. The Bear Forkof theFlathead River. under synoptic con- value exceedsall historical spring flood Creek ditions of both zonal and meridional flow. dischargesexcept for the catastrophic 1964 June Avalanche-damshave dammed reservoirs which flood(more than 300 cms:Boner and Stermitz temporarilyinundate transportation lines in and 1967).Prior to 1964, the maximum spring adjacentto the Park. Calculatedvolume esti- dischargeflood along Bear Creek rras lessthan matesand peakdischarges from avalanche-dam 25 cms,a valueexceeded by estimatedavalanche reservoir formation and failure provide mute floodoutbursts. Calculated avalanche outburst testamentto the destructivecapabilities of these dischargesdo not approachthe historicalmax- natural hazards. Potential exists for outburst imum of greater than 2500 cms (June 1964; floodsfrom avalanche-damsvhich could destroy Bonerand Stermitz 1967)along the Middle Fork highwaytransportation along the southernedge andin excessof 700cms along McDonald Creek of the Park.Future avalanche-damsand resul- aboveLake McDonald.Nevertheless, the ava- tant hazardsin the areaare likely. As hasbeen lanche dischargevalues for these latter two shownelsewhere (Butler l9B7)local residents who slreamsmay occur approximatelyonce every 3-5 frequently drive on US2 are largely unawareof years,nh"reas lhe e\lrememaximum spring the extent of the snow avalanchehazard in rhe flood values appear but infrequently in the area. It is likely that future avalanche-damout- historicalrecord. It is not out of the questionfor bursr floods could produceserious damages and more than one avalanche-outburstflood to oc- possibleloss of life, cur at onelocation in onewinter, if severaltem- porary avalanche-damsdevelop in the sameloca- The National Park Serviceand Montana tion and subsequentlyfail. Departmentof Highwaysshould be rnadeaware Severalpast avalancheshave delivered large of thesepotential hazards, and road-crewem- quantities of downed timber to the damming ployeesshould be cognizantof conditionscon- deposit.'oThis timber can act to anchorand duciveto damformation and failure. Because of stabilizethe snowdam, prolonging its longevity the nature of its mission,the National Park Ser- and thereforeincreasing the size of the tem- vice is unlikely to engage in any widespread poraryreservoir upstream. Upon the inevitable engineeringpractices within the Park along failure of the dam, a greater flood is unleashed McDonaldCreek to mitigate the hazard.Rather,

Snow Avalanche-Dams I 13 monitoring and temporary road closureswill prob- conducive to dam developmentarrd outburst ably be the most cost-effectivemethod of deal- flooding, and prepare for temporary road clo- ing with the hazardduring springsnow clearance. sures during time of high risk. Little can be done to preventavalanche deposits from damming Bear Creekand the Middle Fork, Acknowledgments and few oplionsexist for highwayrerouting in Accessto archival materialswas graciously pro- the confines of the narror,rcanyons where the vided by personnelof GlacierNational Park. The streamsand highway parallel eachother. Along commentsof two anonymousreviewers were very U.S. 2, the Montana Departmentof Transporta- helpfullresponsibility for the paperremains tion should also anticipate weather conditions mlne.

Notes 3. Hungry Horce Neus,2? January 1950. Report about snow slides fron Essex,p. 5. Hungry Horse Neus, 2 March 1. Unpublished Monrhly Ranger Reporr, 1935, LaLe 1956a.Avalanche blocks G.N. and No.2, p. l. McDonald Ranger report, March. UnpublishedMonthly 4. Hung.y Horse Neus,2 March 1956b.Reporr rncr€as€d Rang€r Report, 1939a,Clacier Natl. Park Chief Ranger snowdepths in GlacierPark,p.l. Huryry Hofte Neus, Reporr,February. Unpublished Monthly RangerReport, 9 March 1956. U.S. No. 2 re-opening Sarurday, p. l. 1939b, Lake McDonald Range. Reporr, February. Unpx!- Hungry Horse Nents,30March 1956.Find snow deprhs lishedMonthly RangerRepod, 1939c,Fish Cr€ekRang€r just over normal, p. L f/il ne.r Horse Net!)s,25Fehtt^ry Report, March. All on file ar G. Ruhle Library, Glacier 1982. This time, rhe bridse h€ld, p. 5. Hungry Ho6e Natl. Park, West Glacier. ffeos, 2 March 1956^, op cit. 2. HungryHorce Neus.l3 April lasl. Parkmoose isnores 5. Hungry Horse ?Vea.'s,23 April 1954. Park plows lind avalanche,plow, p. l.Iinsty Ho.se Nebs,It Apill952. recordsnow depth near C arden$l all, p. t. Hungry Horse Snow slide caus€screeL to take orer Goine-to-the,Sun lve,r 2l May 1954.Avalanches cause park destruction, Hiehqa|, p. L Hun|ry HoAp N?&s. lo Aprii t954. Sun p. l. Highway to Avalanche open, p. r. Hunqty Horce Neus, 6. Hungry Horse Neus, 2 March 1956a, op cit. l0 April 1970.Avalanche off Mt Cannon in Glacier Na, 7. Eungrr Horce Neus,5 February 1960. Here are 1960 tionaf Parl is deepe.,p. 12.Hunary Horse Nelr..s,5Apr;l avalanchesin Glacier National Park, p. 10. 1984.Snon barrier, p. t. HunAry Horce Neus, 12 Apil 8. Hungry Eo+",4"@s. ll April 1q52,op rit. lm4. Snowand ice Fon Going-tethe-SunRoad 0oat dovn 9. Hunsrt, Ho^e N"@r. 2 \4arch 1956a.b. op ct. McDonald Creel in Glacier National Park. D. l. 10. Hungry Horce Npws q ltarch 1c56.op .ir.

Literatu.eCited 1985b. A reconstruction of snor-avalanche characierislicsin Montana, U.S.A.,using vegetariv€ Actroyd, P. 198?.E.osion by snow avalancheand inplica. indicaiors. J. Glaciol. 3l:185-187. tions for g€omorphicstability, TorlesseRange, New Canpbell, P. A. 1988.Debis iorents in rhe RobsonVa ey, Z€aland. Arct. Alp. Res. t9:65-70. east central 8.C., Canada. Zeitschrift fiir Ceonor- Allix, A. 1924.Avalanches. Geogr. Rev. 14:519-560. phologie SB 67:??. Boner, F. C., and F. Sternirz. 196?.Floods ofJune 1964in Clague, and V. northwestern Montana. U.S. Geol. Sur.rey Warer, J. J., H. Mathews.1973. Th€ magnitude of jokulhlaups. Supply Paper I840-B. U.S. covt. Printing off., J. Glaciot. 12:501-504. Vashingon. 242 p. Costa,J. [. 1988.Floods ftom dan failures.Itr V. R. Baler, Butler, D. R. 1980. Teminal eleyationsof snow ayalanche R. C. Kochel, and P. C. Patton (eds.)Flood Ceonor, paths,Glacier National ParL,Montana. North'. c€ol. phology, John Wiley and Sons,Nev York. Pp. 439-463. 9:5944. Costa,J. E., and R. L. Schuster.l9BB. The fornation and -. 1986a.Snow-avalanche hazards in Glacier Na- failure of natural dans. Geol. Soc. Aner. Bull. tional Park, Montana:meteorologic and climarologic 100:1054- I068. asp€cts.Phys. Ceog. 7172.a7. Fitzhanis,B. B., and P. A. Schaerer.l9m. Frequencyof najor -. 1986b. Spatial and tenporal aspectsof the snow avalanche winters. J. Glaciol. 26143-52 avalanchehazard, Glacier National Park, Montana, King, V. D. V. O. 1934.The Mendoza River flood of l0-tl U.S.A. ln L. Hepood (eds.) and D. Marks Proc. In- January 1934-Argentina. ceosr. J. 84:321-326. tenad. SnowSci. Vorlshop 22-25October t986, Lake Lachapelle, E. R. 1985.The ABC ofAvalanche Safery.The Tahoe, California. Pp. 223.230. Mountaineers,Seaitle. -. 1987.Snow-ayalanche hazards, southern Clacier Martinelli, M., Jr. 1984.Th€ Goar Lick Bridge avalanches NationalPark, Montana: the natureoflocal Lnowledee oI 1979 ^nd \9a2. In M. Marrinelli, M. Boyn€, and and individualresponbes. Disasrers I t12l4-220. R. Newcomb (€ds.) Prcc. Internatl. Snov Sci. Butler, D. R., and C. P. Malanaon.1985a. A history of high- Vorkshop 24-27October I984, Aspen, magnitudesnow avalanches, south€m Clacier National Colorado.Pp. Park,Montana, U.S.A. Mtn. Res.Devel.5:175-182. t98-207.

114 Butler McPherson,H. J., F. deScally,and J. S. Gardner. 1984.The Stethen,C. J., and P. A. Scha€rer.I979. Avalanche Accidenrs useof time lapsephotography to noniror ayalanche in Csnada I. A Selection of Case Histories of Ac' activit/ and snowbehavior.lnM. Maninelli, H. Boyne, cidents,1955 to 1976.Natl. Res.Coun. CanadaDBR and R. Newconb (€ds.) Proc. Internad. Snos Sci. 834. Div. BuildingRes., ottara. ll4 pp. Vorkshop 24.27October 1984,Aspen, Colorado. Pp. 1980. Avalanche Accidents in Canada IL A l6t-171. SelectionofCase Historiesof Accidents1943 to 1978. Mokievsky-Zubok,O. l9?5. Suddenflood and sorted debris Natl. Res.Coun. Canada DBR926. Div. Building Res., over the winrer snowpack within Sentinel Clacier Ottawa. 75 pp. basin,British Columbia.Can. J. Earth Sci. l2:8?3J79. Tricart, J., S. Rinbert, J. Marbach, R. Obermuller, A. R. Peattie,R. 1936.Mountain Geonorphology.Harvard Univer' Hirsch, F. LeBourdiec, and A. R. Hirsch. I961. sitl Pr". Cambrrdge.Ma"sachusptt'. M6canismesnornaw el ph6nominescatastrophiques Rapp, A. 1960.Recent developmentof mountain slopesin dansl'6volution d€sv€rsants du bassindu Guil (Htes- Karkevaggeand surroundings,northern Scandinavia. Alpes, France). Zeitschrifi fft Geonorphologie GeografiskaAnnaler 42:?3-200. 5:2??-301. I Scha€rer,P. A. 1987.Avalanche Accidents in CanadaIII. A WiUians, K., and B. Arrnstrong.1984. The SnowyTorrena- SelectionofCase Histories I97&1984. Natl. R€s.Coun. AvalancheAccidents in the United States 1972-79. CanadaNRCC 27950. Inst. Res. in Consrruction,0t- Teton BookshopPublishing Co., Jackson, Wloming. tawa. 138 pp.

Receioed,31 October 1988 Accepted.for publication 13 January 1989

SnowAvalanche-Dams l15