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Some Lmplicalions of Sediment Studies for Glacial Erosion On

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Some Lmplicalions of Sediment Studies for Glacial Erosion On HughH. Mills Department of Earth Sciences TennesseeTechnological University Cookeville,Tennessee 38501 Somelmplicalions of SedimentStudies for GlacialErosion on MountRainier, Washington Abstracl A study o-f-glacial sedimeots throws light on rates a_ddmechanisms of glacial erosion on Mount Rainier, Itrashington.. Suspended-sedimEnttraosport measuremen$suggesr that most of the Nis- quarly,Krverquallyqualiv River'sRirers s suspendect-sedLmentsuspended-sedimentsusDend ed -se.l imcn r loa,l ha< h""" p.rf,ina.l h" .1. *;;;,A- io34load lashas b-ee_nbeen)een entraioedenrrainedentlaioed - by the time the stream emefges from benearh.rhe.benearhrhe terminusrerminus !tof NisquallvNisqually Glxcier.Clxcier. calculationsCalcularionsCalculations ofo the englacial- and superglacial-debrissup roads or Nrsqually Gtacrer rndicare that more than two_thitds of the stream sedimint must be derr'ved subglacially. lirhologic composition of outwash and theoretical considerarioos also sug- gcs( that val(ey gldcierson Mounr Rainier hare exteorionallvexceptionally hiehhigh subglacialsubslacial erosione.osi.n mtes.rrre( Introduction Duting a study of glacial sedimentson and near Mount Raioier, Washington,I made a number of observationsthat bear on the rares and mechanismsof slacial erosion.Some of thesehave been discussedin a previous paper (Mills, 1976;; additional findings are d iscussedherein. Mount Rainier (lat 46" 52'N, long 121' 46'W) is a euaternary andesiticsttato- volcano in the CascadeRange that rises to an elevation of 4392 m. The main coqe resrs upon granodiorire of the Miocene Tatoosh pluton and to a lesser extent upon older Tertiary volcanic and volcaniclasticrocks. Nisqually Glacier, a 7-km loog valley glacier on the south flank of the mountain, is by far the most intenselystudied glacier on Mount Rainier. Its steeplongitudinal gradient averages30o abovethe equilibrium line (I. L.) (elevation about 2300 m) and 14' belovz.Ablation ratesare high, reachingg m/yr rlear the terminus. Hodge (1!72, 1974) showedrhar the glacie! is thin and fast-moving.He found the ice thicknessto averageabout 70 m in the ablation zone; a steepersurface slope indicatesrhe ice is even thinner in the accumularionzone. He determinedthe cen_ terline surface velocity to range from 160 m/yt rlear rhe E. L, to about 30 m/yr near the terminus. He calculatedthat basalsliding accountedfor 90 percent of the movement near the E. L. and nowhere accountedfor less than 50 percent.He also found that the bedrock floor of the glacier is quite irregular; coupled with the steepnessof the ice surface,this fact explainsthe highly crevassedoarure of rhe glacier.Other valley glaciers on Mount Raider probably have similar characteristics. Nisqually River headsar the rerminus of Nisqually Glacier, emerging from beneath the ice as a swift, extremely turbulent meltwater stream.The sourcelnd orisin of the sediment in this stream coostiturethe main subiecrof this paper. Sourceof thc StreamSedimenl Nelson (1974) estimatedthe annual suspended-sedimentloid of Nisqually River at a gauging station near National, $Tashington (31 krn below the rerminus of Nisquallv 190 Northr\estScience. Vol. s3. 11o 1, 1970 Glacier) to be about 300,000t (tonnes) yr. In order to usethese data rc calculateerosioq mreson Mouot Rainier, it is important to know how much of this sedimentis produced by modern efosiooalplocesses on the volca[o. The problem is summarizedin Figure 1. Basically,there are three Possiblesources of sediment First, sediment derived from present-dayerosion of Mount Rainier may be carried directly to the gauging pojnt via the meltwater sueam (path A in Fig. 1). An individual Particle may be depositedon the floodplain and subsequently re-entrained sevelal times during its iourfley, but the averagetransit time should be relatively short,probably on the order of decades.Second, sediment may be caried away from the volcaoo by processessuch as glacial advances, lahars,and rock falls. These processesoften dePosit sediment some distance from the floodplain, so that a given particle may not find its way into the sfeam and theoce to rhe gauging poirt for thousandsor tens of thousandsof years (path B in Fig. 1). Third, sedimentmay be derived ftom sourcesuffelated to Mount Rainier (path C in Fig. 1). Basedupon certain assumptioos(Mills, 1976), it has been estimatedthat of the 300,000 t/yr of suspendedsediment reaching the station at Na.ional, 278,0O0t/yt can be attrib- uted to modern-dayerosion on the volcaoo (path A) or to erosion of Neoglacial drift within 2 km or so of the glacier termini (path B). Becausethis 278,000 t/yr is con- uibuted by .hree meltwater stleamsof roughly comparablesize (Nisqually River, Kautz Creek,and Tahoma Creek,the later two being tributaries of the former), approximately ooe-rhird of this amounr,or abou. 90,000 t/yr, is probably conffibuted by the uppermost Nisqually River itself. In the preseAtstudy an attempt was made to checkthis 90,000t/yr estimateby acElal measurement,and also to seewhether path A or B is the more im- potrant. NON.RAINIER SEDIMENTSOURCES A I MOUNT --+ MELTWATERSTREAM RAIN IER ANDFLOODPLAIN I EXTRA- FLOODPLAIN SEDIMENTSTORAGE (GLACIALDRIFT, LAHARS,ROCKFALL DEPOS ITS ) Figure 1. Possiblesoutces of suspendedsedimert iri Nisqually River. Implications of SedimentStudies 191 15 May and .Bets/een 1 August 1976,20 suspended_sedimentsamples were collected and the stream discharge measured at a point 1.S km belon the Nisqually Glacier rer- minus. A linear regressionof the log of sedimentconcenuarion on the log of discharge was performed_, giviog the regressionequatioo, \'-0.1gg Xr 6a (where y is in parts per million and X is in cubic feet per second). Sucha regressionline can be usedto esti- mate the aonual suspended-sediment yield from a streamhydrograph. Unfortunately, no facilities for conriorlous dischargerecording were available.Hoi,"ier, it _u, possibieto consffucr an approximate hydrograph,and from this descripdooan order-of_magnitude estimateof yield was made. On this basisit was estimareclthat the upper Nisqually River traosportedat least 50,000t during the ablation season(during whiJ most of the annual transportprobably occurs,as dischargeis low duriog the remainderof the year). ,Because of the lack of continuous dischargerecording, hoiever, peak dischargesprobably were omited, and thesemight havetransported large quantitiesof sediment.As an exampleof the importance of rare peak discharges,@strem and others ( 1967) found that, for the DecadeRiver ort Baffin Island,60 percent of the total suspended_sedimentyield lor the trj-.1,"f 1965 was transportedin a single day. :l:t: Thus, the upper Nisqually prob_ ably couldeasily accounr for 90.000r yr. A second approach to the - problem vras to determine the downstream variation iq suspended-sedimentload at a given iostant in time. This calculationwas done by near_ simultaneoussampling and measuring of sream dischargeat three pornrs, ooe ar rhe glacier terminus, one 1.8 km below the terminus (oear ,h. ,lo*n.rrau- limit of Neo- glacial drift), and another 4.9 km below. By their locationsir can be inferred that sedi_ ment collectedat the upper starion was derived mainly via path A (Fig. 1); that at the derived via path :iddl." :,i,t:i_*^ A plus B (the latter being confined to erosion of Neoglacial drift); and that sedimentat the lower sratioowas de-rivedvia path A, B, and Resultsare presented in Figure 2; points along a veJticalline representsimultaneous samples.Only three sampleswere taken at the upper station,and beiauseof extremetur- bulenceno dischargemeasurem"rrts *er" ,aud. ir-"re; instead,discharge was estimatedto be 0.75 times that at rhe middle starion. Figure 2 shows thar early i"n rhe ablation sea- son,sediment traospot rates4.9 km below the terminus were slighrly greaterthan at 1.g km below, although rates were very low in both cases.\fith tt e"b"gi;ing of high clis- chargesin early however,io July, evely casethe sedimentroad at thle lower sratronwas acnally less than rhe load ar the middle sratioo; in other words, not only was no addi_ tional sedimenteotrained in the reachbetween these stations, but sedimentwas acrually lost. This sedimeot appearedto be deposited in slack water, especiallyio the lee of bouldersat the edge of rhe srream.Since. thete is no evidenceof iong-t"r_ uggrud"tio., of fines,.it may be that this depositron rs temporary; perhaps <luring large floods this This result implies that most suspended 1,":",'1"1," :",:"ii*. sedimeniis supptiedeither Dy rne glacrerrtselt or by erosion of Neoglacialdrift, as suspectedearlier. A comparison of samplesat rhe upper and middle ,,u,io., ,ogg"r,. rhar of cheserwo later sources, that supplied by the glacier is by far the more i-ptt"nt, as the sedimentload either de_ creaseso: showsonly a slight increase ovet this l.g_km reach.Because only three samples ue invglv5f, however,this conclusion must remain highly rentative unril additional data are avatlable_ No mention of bedloadhas beeo made. Fahnestock(1963) demonsuatedthat bed- 192 Mills I l 25 tr NEAR TERMINUS I o qlrJ . I.8 KM BELOWTERMINUS (9 20 O 4.9 KM BELOWTERMINUS = lrl t5 E Ero a o (L ao o o 25 o o t Fo MAY JUNE JULY Rivet, as de- Fieure" 2. Susoended-sediment transport rates at three stations on upper Nisqually termined by near-simr.rllaneoussampling. load transport ratesbelow Emmons Glacier on Mount Rainier can be quite high, and no doubt during occasionalfloods they are high on the Nisqually River, too. However, ex- cept for immediatety below the rerminus,little movemeflt of bed material vras detected (either aurally or by contact white wading) during the summer of 1976. Thus, it ap- pearsthat suspendedsedimeot accouots for the bulk of sedimentuansport in the stfeam Origin of lhe Sediments The above evidencesuggests that the meltwate! stream has entrained most of its sus- pendedsediment by the time it emergesfrom beneaththe glacier.It is thereforenecessary to look abovethe glacier terminus for the origio of this sediment.Two quesrionsarise: the first involvesthe ultimate sourceof the rock debris; the second,the manner in v/hich this debris is transported by the glacier. In tespooseto the first question, there are basically two potential sourcesfor debris: rockfall anci subglacialerosion.
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