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 gldciers on 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 , a 7-km loog 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. 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 ; 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 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 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 , 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

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. Rockfall is exceedinglycommoo on the high, unstablewalls that flank the upper end of the glacier; the efficacy of subglacialerosion is more difficult to determine. The answer to the secondquestion depends to a large extent orr the thermal regime of the glacier.Accord- ing to Boulton ( 1972) , in warm-basedglaciers the debris producedby subglacialerosion is confined mainly to a basal layer of ice no thicker than 1.0 m; it rarely finds its way into englacialor superglacialpositions, even in the ablation zone where flow lines have an upward compooeot.If deposited,such debris is depositedas lodgment tilt. In cold' based glaciers,however, patticulady where water can reach the bottom aod refreeze there, subglaciallyderived debris may be carried to high levels in the ice and much of it may end up as englacialor superglacialdebris. BecauseMount Rainier lies in a maritime climatic zone,one would exPectits glaciers

Implications of SedimentStudies 193 to be essentially warm based. This expectation would imply, according to Boulton's (1972) model, that the impressivesuperglacial-debris mantles visible in the ablation zonesof Rainier valley glacierslate in the summer are probably derived from rockfall. Apparently much of this rockfall takesplace abovethe E. L. and is incorporatedioto the ice as eoglacialdebris, eventually melting out below the E. L. ro form superglacialdebris. In at least somecases, however, it appearsthat a substaqtialportion of the superglacial debris must be subglaciallyderived. For example,\finthrop Glacier on the north slope of Raioier has no headwall and is flanked by relatively low walls, yet even the center of the glacier displaysa large amount of superglacialdebris ar its lower end; this debris, at least, must be subglaciallyderived. This observation probably does not contradict (1pl2) Boulton's model, however.Becar.rse of Mounr Rainier.s high elevatioo,it seems quite likely that rhe upper ends of the valley glaciersare cold based.Miller (as cited in Kiver and Sreele,1975) measuredtemperarures of -10" C in a snowpit nea! the sum_ mit, and the upper ends of rhe glaciersmust be neady as cold. Furthermore,the rhio, higlly crevassedice in the upper teachesof the glaciers probably allows significant meltwater to reach the glacier basevrhere it refreezesand thus promotesbasal . Cold ice is probably limited to the highest reachesof the glaciersand is almost cer- tainly not fouod below the I. L. Hence, although subglaciallydetived

194 Mills pafi of rhe rotal glacier width are thus likely to be oot greatly less thao the centerline sulface velocity. Second,concerning rhe decreaseof velocity at depth, basal sliding is estimated to accouqt for 90 percent or more of the surface velocity in the upper ablation zone (Hodge, 1972) , so that the differenceberween surface velocity and velocity abladon over depth musr be small. Lven iq the lower ablation zone basal slidirrg accounts for at least 50 percent of the surface velocity (Hodge, 1972), w that surface aod average velocity should still be oI the same order of magnitude. In any event, transPort fates cal- culatedon rhe basisof centerlinesurface velocities carr be regalded as maximum values. Given the above assumprions,the followiog procedure was used to calculate the fiaos- port rate of superglacial aod englacial debris. In order to determine rates fo! suPerglacial debris alone, the thicknessof the debris maotle was first determined at a number of poins alongprofiles C8 aod C2 ( Fig. 3 ) by digging pits; the averagethicknesses obtained v/ere 21 cm and 48 cm, respecdvely.By multiplying the avelagedebris thicknessby the glacier width at each profile and by theq multiplying this product by the velocity in m/yr (centedine surfacevelocities are given in Fig. 3), the annual dischargeof super- glacial debris past each plofile was calculated.Results show that 7560 mi of super- glacial debris is carried past C8 annually and 4320 ms past C2. The latter value must be coffected for the fact rhar rhe glacier bifurcates above C2 as showq in Figure 3. Cross sectioo C6 shows rhat only about rwo-thirds of the glacier is representedby the west ------t CONTOURIN1TRVAL MO FT N

cr9 ?, AREA = 22.4x l0- M' VELOCITY = 140MlYB - FLUX = 31.4x l0- NI-/YR AREA = lz.g x lo3 rtt? = VELOCITY 60 I\1/YR5 ? FLUX = 19.7x l0- M-ryR

= --lo .=----- AREA tl.2 r tol N2 -- V Yiif'": llnfTlo',,n* Figure 3. Ice fluxes through representativectoss sectionsof lower Nisqually Glacier (velocities and crosssections from Hodge, 19-12,. Goss sectionsare drawn looking downglaciet.

Implications of SedimentStudies 195 prong where C2 is located, so that the superglacialdebris dischargeat C2 should be multiplied by 1.5 to account lor clebris carried ioto the east prong, rhus making the resultsequivalent to rhosear C8; rhis calculariongives an annual dischargeof debris at C2 of 6.180m3. The fact that the dischargeof superglacialdebtis tJecrea:e:benveen C8 and C2 even after this correction has beeo applied may seem surprising, for much ed- ditional englacialdebris is releasedby ablation betrveenthese t*'o profiies. The explana- tioo of this pheoomenooprobably involves Ioss of superglacialdebris as the ice moves dowoglacier; some of the debris is depositedalong the siclesof the glacier becauseoI lateral ablation and somefalls ioto . In order to estimatethe combioed englacial./superglacialdebris discharge,ice fluxes rhrough C1! and C8 rvere firsr estimatedby multiplying velocitiesby the crosssectional areasat rheselocations (Fig. l). C19 is not far beloq'the E. L.; rhe ice flux through this crosssection (11.4 x 105m37'yr) representsrhe roral amouot of ice passiogthe E. I. annually.By C8 the ice flux has dropped to 19.7 x 105mr/yr, so that betweenC19 and C8, 11.7 x 105 m3/yr of ice is losr by ablation. The melting of rhis lnuch ice, then. re- leases7560 m3 of englacial debris per year (rhe annual superglaciaidebris dischargeat C8). This figure indicatesthat the debrisconcentration of the ablatedice is 0.65 percent by volume, or, assuminga deosity of 1700 kg,/m3for the superglacialdebris, a concentra- tion by weight of about 11 Lg,/m3.To estimarerhe maximum debris dischargeof the glacier,the totai ice dischargethrough the E. L. (v,here englacialclebris has not yet begun to melt out) was then mulriplied by this concentration,giving a dischargeof about 34,500 !/ yr. For reasonsdiscussed above, this estimateis probably a generousone, although the actual value is thought to be not less than half this amount. Certainly 34,500t,/yt can be regardedas a maximum ffaflsport fate. The abovecalcularions indicate, then, that lessthao one,third of the estimared90,000 t/yr of sediment supplied to Nisqually River can be accouotedfor by the englacial./ superglacialdebris transport system.This finding implies that unless the estimate of 90,000 r/yr is grosslyin error, a considerableamounr of sedimenrmusr be supplied by the subglacialriansFort sysremand,'or perhapsby subglaciatfluvial erosion.Additional evidencethat rhe subglacialload must be irnportant is furnished by a considerationof the particle-sizedistribution of the sediment transportedby the river and that of the superglacial/eoglacialdebris. The greater part of the former consists of suspended- sediment rvhich, as analysisof three samplesshowed, is confined almost completeiy to particles less rhan 2 mm in size.In contrasr,a number of size analysesof superglacial debris (performed by using field point-counting for patticles larger than 102 mm and sievingfor smallerparticles) showed rher only 16 percenrof tbis mrrerial consistedof particles lessrhan 2 mm. N(/heredoes all the fine material come frorn? one possibilitv iovestigated*'as that englacialdebris might contain a much higher percenrlge of fines than does the superglacialdebris, the fines being carried away to rhe mekwater sueam during the melt-out process.To tesr this possibJiry. wo samplesr,f ice rvere collected, allo*ed to melt, and the particle,sizedistribution of the releasedenglacial debris theo .I.he comparedwith thar of superglacialdebris samplescollecred ar rhe snm-esrtes. results showed englacial debris is _that ooly a few percent hieher in silt antl clay than super_ glacial debris. Another possibility is rhar rhe fines ate winoowed out of the glacial debris, leaving behind the larger paricies. Horvever, becausethe glacial debris contains only 16 per

196 Mills ceot of material fine enough to be carried as suspendedsediment, io ofder to obtaio the estimatedaonual load of 90,000 t/yr, it woutd be necessaryto wionow 560,000t/yr ol this debris,everl if 100 percent of Particlesless rhao 2 mm were entlained ln actuality, size-distributionanaiyses of outv,'ashshow that winnowing is somewhat less efficient rhan this, so thac it vrould be necessaryto winnow vell over 1,000,000t/yr to obmin the 90,000 t/yr of suspendedmaterial Obviously,winnowing can explaio only a small par. of the suspendedload. The trost plobable origin of the fine material is that it is producedby abrasionat the baseof the glacier. This conclusionis suggestedby two considerations.First, many glaciersdeposit lodgment till that is subglaciallyderived and containsa large percentage of material lessrhan 2 mm in size (Mills, 1977). For example,lodgment till of nearby ParadiseGlacier cootains 65 percerlt particles less thao 2 mm. Nisqually Glacier and other large valley glacierson Morlnt Raioier, however,display no such tills. A possible explanation for this absenceis provided by Boulton (1975), whose calculationsshow rhat lodgment should not take place beneath glaciers similar to Nisqually Glacier in thickness and basal sliding velocity. Presumably,matetial similar to lodgment till is produced beneath Nisqually Glacier, but is releasedo subglacial meltwater sueams rarher rhan being deposited. In addition, theoretical consideratiols suggestthat the rate of abrasion should be very high beneath Nisqually Glacier. Several factors affect the efficacy of subglacial abrasion.First, the velocity of basaisliding is quite important (Boulron, 1974), so rhat NisquaLlyGlacier, with a basalsliding velocity of up to 144 m,/yr, should be highly ef- fective in this regard. Second,Rcithlisberger (1968) has pointed ollt lhat contioual orovementof rock particles toward the bed is important in order to renew the abrasive material constantly,arrd coocludedthat the most effective way for this tene*'al to occur in a warm-basedglacier was by melting of the basal ice. Severalcooditioos should pro- duce exceptionallyhigh rates of basalmelting beneathNisqually Glacier. First, the geo- thermai heat-flop'rates have been showo to be quite hiSh neaf the summit of Mount Rainier (Kiver and Steele,1!/J);possibly they are also high, at ieast locally, beoeath the glaciersoo the mountain's flanls. Second,basal stiding produceshert that resultsio basal meltiog (Sugden and Joho, 1976); heoce,a high rate of basal sliding produces high rates o[ basal melting. Third, becauseof the high mte of surfacemelting in the ablation zone (8 m/yr) and becauseof the thir, highly crevassedoature of the glacier, exceptiooalamounts of meltwater undoubtedlyleach the baseof the giacier and theleby grearly inqease basalmeltiog. A third factor ptomotiflg abrasioriis a thin ice cover.According to Boulton's ( 1975) calculations,for example,wheo the basalsliding veiocity is 100 m./yr, abrasioorate is at a maximum when effective nolmal Pressureis betweeo 10 and 30 bats (ice thickness becveeo90 m and 270 n, assumiogzelo \r'ater pressure).Above 30 bars,abrasion rate decreasesrapidly becausefriction bett'een the bed and rock Particiesembedded in the glacier sole becomessufficiently gtear to retard the movemeot of the patticles Below a pressureof 10 bars abrasionrate also decreases,but even at a Pressureof 5 bars (45 m of ice) the abrasionrate is near maximum. For most glaciers,theo, abrasionis effective only near the ice margin. For Nisqually Glacier,which probably is lalely much thinner than 45 m and oever thicker than about 90 m, abtasionrates should be oear maximum beoeathmuch of rhe glacier leogth.

Implicatioosof SedirnentStudies 197 A considerationof the lithologic compositionof clasrsin the outwashprovides evi- dence that erosion beneath Rainier valley glaciers also produces a substantial amount of coarsematerial. The lovrer ends of the valley glaciers on Mount Rainier have cut rhrough the Rainier andesite and now rest upon pre-Rainier formations; hence, clasts composed of pre-Rainier lithologies presumably are basally derived. Thus, by making lithology counts in a deposit, it is possible to place a mioimum on the percenrage of material that has a subglacial origin. More than 90 such counts were made. They showed that superglacial debris con- tained virrually no pre-Rainier rocks at any glacier. Till depositshad little more, thus indicating thar fo! the mosr part they consist of material derived from the superglacial or englacialenvironment. Outwash, on the other hand, containedlarge numbers of pre- Rainier clasts,thus demonsratiog the effectivenessof subglacialerosiofl, ar least in the ablation zone. The percentages of such clasts in ourwash varied somewhat from glacier to glacier,however. In an effort to determinethe reasol for this variation, the maximum pelcentageof pre-Rainier clastsin four meltwater steams (as measuredjust below rhe glacier termini) were related ro sevelal possible determining factors (Table 1). The {irst factor is rype of pre-Raider bedrock; theorerically,the less-resistaotTertiary vol- canics might produce more clasrs rhan the more resistant granodiorite. The results (Table 1) indicate this factor is probably not significant, for although Cowlitz-In- graham Glacier overliesa large areaof Tertiary volcanicsand has the highest percentage of pre-Rainier clasts,Emrnons Glacier also ovediesa large areaof Tertiary volcanicsbut has the lowest pefcenrage.

TABI,I] taciors aflecling percentage of pre-Rainier rock in Rainier vatjey-gtacier

Percent pr€-Rainier L holo8y oi Horizontal disLnnce Thickness of clasis in oulvash pre-Rainier Del\{eeD presenl superglacial near presenr ouicrops lerminus anal hishest debris cover Dre-Rainier ouicrop

Nisqually 10 1.1 km 48 cm 2Q 5.3 km 100+ cm

Tertil|y volcanics Tertiary volcanics 2.3 km plus small amount

2.4 km creaier tha.n Terliary volcanics Carbon

A secood factor is rhe total area of pre-Raioier bedrock beneath the glacier. Although this area cannor be determined directly, the distanceabove the terminus of the most upglacier pre-Rainier outcrop adjacent to the glacier margin can be determined,and shouldcorrelate roughly with this area.Table 1 suggeststhat this factor exerciseslittle in- fluence.A third factor is the superglacialdebris load of rhe glacier, which will tend to dilute the basallyderived pre-Rainier material in the outwash becausein all casesit is neady 100 percent andesite.As an index of this load, ihe averagethickness of rhe supet- glacial debris maflde ar a poiot severalhundred meters above the glacier rerminus was measuled.No measurementof this thicknesswas made on Emmons Glacier, but as productsof the 1963 lirtle Tahoma Peak rockfall (Crandell and Fahnestock,196j) were stili being deliveredto the rerminus,ir is likely that this glacier had the highest super-

198 Mills glacial debris load. Table 1 suggeststhat this factor may be the mosl imPoltant, for the order of increasiogpercefltage of pre-Raioier clasts in the outwash corlespondsto the order of decreasingsuperglacial debtis thickness.

Conclusion Measurementsof suspended-sedimeottraflsPoft in the upper reachesof Nisqually River indicate that the buik of suspendedsedin.rent in the river is entrained at or aboYethe glacial terminus. Calculations of the englacial/superglacialdebris load of Nisqually Glacier suggestthat lessthan one-third of the sffeamsediment can be attlibuted to these soulces,so thac the remaioder must be derived subglacially.Natutal uacets in the out- $/ashas well as theoreticalconsiderations support the idea that subglacialelosion rates are exceptiorally high beneathvalley glacierson Mount Rainier. Although more data are neededbefore firm conclusionscan be dra$/o, the results bear on the question of what magnitude of suspended-sedimentloads can be expected duriog the next several decadesio the lowe! reachesof melrwater streams draining Mount Rainier. In an earlier paper (MiLis, 1976),I suggestedthat Presentsuspended- sedimentloads may be abnotmally high becauserweltieth ceotury glacier recessioohas exposedlarge amouo$ of loose,unstable drift that is readily eotrained by sueams; as recessionhas ceasedand oewly exposeddrift is becoming stabilized,sediment supplied to the stleamsvia path B (Fig. 1) cao be expectedto dedine. However,the preseotstudy suggesrsrhar sedimeotderived via path A (modern erosionalprocesses on Mount Rain- ier) is much more important than that derived via path B. Hence, suspended-sediment loadsare unlikeLyro decreasesigniiicanrly in rhe nearfurure.

LiteratureCited Boulton,G. S. 1972.The role o{ thetmalregime in glacialsedimentation, pp. 1'19 in R. J. Price aodD. E. Sugden(eds.), Polar Geomorphology. Iostrtute of BritishGeographers Special Pub.

Glacial Geology. Srate University of New York, Binghamton. 189 pp. 1975. Processcsand patterns of subglacial sedimentationi a theoretical approach, pp. 1-42;n A.E. Wright and !. Mosely (eds),Ice Agesi Ancient and Modern. Seel House Press, liverpool. 320 pp. Crandell. D. R.. and R. K. Iahoestock. 1965. Rockfalls and Avalanches from litde Tahoma Peak on Mount Rainier, lfashington. U.S. Geol. Sr.rrveyBull. 1221-4. 30 pp. Fahnestock, R. K. 1961. Morphology and Hydrology of a -White Rive!, Mount Rainier, Vashington. U.S. Geol. Survey Bu11. 422'4. 61 pp. Hodge, S. M. 1972. The Movement and Basal Sliding of the Nisqually Glacier, Mount Rainiet. University of $(ashington, Seattle, Ph.D. thesis.410 pp. 1974. Variations in the sliding of a temperate glacie!. Jour. 1.): 149-169. Kiver, E. P., and W. K. Stcele. 1975. Firrl caves in the volcanic craters of Moudtain Rainier, $fash' ington. National Speleological Society Bulletin 4: 45-55. Mills, H. H. 1976. Estimated erosion tates on Mount Rainier, rxrashington. Geology 4: 401-406. 1977. Textural chafacteristics of drift ftom some teplesentative Cordilleran glaciers. Geol. Soc. America Bu1l. 88: 1135-1143. Nelson, l. R. 1974. Sedimeot Transpon by Streams in the Deschutes a$d Nisquauy River Basins, . Vashington: November, 1!71-June, 1973. U.S. Geol. Survey Open-file Report. 3, pp. @strem, G., C. W. Bridge, and I(. F. Rannief. 1967. Glacio-hydroloey, discharae, and sediment rransporr in rhe Decade Glacier area, Baffin Island, N.V.T. Geog. Anr'alet 49At 268'282. R,irhlisbirger, H. 1o68. Erosive processeswhich are likely to accentuate or reduce the bottom relief of valley glaciers. Int. Ass- Scierrt. Hydrol.79: 87-97. Sueden, D. L, and B. S. John. 1976. Glaciers and Landscapes. John V/iley & Sons, Nes' York. 376 PP.

Receited Decetnber24, 1977 Acceptedlor pubLicationMay 1, 1978

Implications of Sediment Studies 199