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<br />CrltIca1 and SupercrltIca1 PlowIIIn two UIl8table.
<br />Mountain Rivera. Toutle River Sya\:em. Wublngton
<br />
<br />An__' andJ.H._m'
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<br />Iltr04D.ctioD
<br />
<br />Critical and lIupeItritical flols i1le glllerall! COlIsidereG to be rare actlnlDees 11
<br />Diltural rifel chanoels IDatbie and ilolf, n~3; Jarrett, nat}. Discharge .eaBulueats at tbree
<br />9a9119 statiols 1u tbe !ootle Binr systel draiDialj Moolt St. Delus, i1ashiagtoll, bUlIeteI. han
<br />docUlented tbe occurreoee of critical aId aupeledUcal flol.. As part of an uteDsile
<br />1001tor119 prograa u.ndertaken by tile U.S. Geological Sune" gaging statioas were established DB
<br />all of tbe .ajnr tributary atrlillS affected by the Kay 18, 1980, catastrophic erupUoD of the
<br />'olcaDo. Hydraulic, bydrolt\9ic, aDd Illli.eot-transport data were rOltiBely collected at these
<br />stations for use in research studies aDd to provide data for reporting nd predicting sedllellt
<br />distharges.
<br />Ihis paper presents data and results pertaiDing to the eaisteace of leasured critical
<br />and supercritical flows at gagiog StatiOIS 01 the lortb Fori loutle Riler (In) aod the loutle
<br />River laill stu (TR). The data set iueludes 930 discharge leaSll[eJeuts lade by tbe Itaff of the
<br />U.S. Geological Suney, Cascades YolcilIo Obsenatory, between 1980 aod 1989.
<br />
<br />Ducrilttioo of Studt Area lid CuuDel cnditio..
<br />
<br />lbe 2.5- cubic tilOJeter (kI3) debris avalaucbe associated with the lIay 18, 1980,
<br />eruptioD of 1I00ot S1. Beleos buried tbe valley of the upper Iff to iD ,average deptb of 40 letm
<br />(I) lIitb uocoltsolidated latedal raltgilg iashe frol silt to boulders. Lahars elallated frOl
<br />.ater.satDnted parts of the debris anlancbe and swept down the lower IFI no the tI depositing
<br />op to 5 I of &aDd. Ihe opper 30 tl of the In draioage basin liS initially de,oid of throDgb-
<br />flovillg atreus. Drairlage illtegritioD oc:corred by lake breakouts frol closed depressillDs aid
<br />ilpollldiG tributaries, additioDilllahars, and by storlflows. By Decelber 1982, 100 percllt of tbe
<br />draiDage area of tbe )In was cOltributiDg water to the TR. The let result was that tbe loderatdy
<br />SilllOOS grnel.cobble sUeau of tbe Toutle Riter syst" were tnllsforJed iota reliti1tly
<br />straigbt sand-bed &treals (SiJOI, }992). Chauel gradientl rHged frol 0.009 to 0.081 Jeter per
<br />eter (.,.) OR the Iff! aad frOl 0.0006 to 0.001 1'1 01 tbe '!I.
<br />G&gilg stations .ere established after the eruptioD OD the lover I" (at lid 'lalley) and
<br />tbe TH, tint at Castle Bact, aDd il 1982, at Tower Road (table I). Because of logistical
<br />probllll, initial atteJPtl to establish a perlaneot Itreuflo. 9age 00 the debris-avalanche
<br />deposit lIere largely IIlsuccessful. By late 1984, bOlever, a staHol wal established at Elk Roct
<br />for tile (oBectiol of ptriodic data aDd rllaiDed il operation throuljb 1987.
<br />
<br />IResearcblydrologist, U.S. Geological Suney, P.O. BOI l0128, Raleigh, IC, 21622
<br />21ydrologic !eclllliciau, U.S. Geological Suney, 5400 lIaclrtbur Bodened, 'laICOUler, VI, 98&61
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<br />742
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<br />743
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<br />CRmCAL & SUPERCRITICAL FLOWS
<br />
<br />Station"''''''' U.S.G.S. River Kllomem' Drllln. .........f
<br /> stat10nnUlllber ..... I~='-ara
<br /> '"'
<br />TouUefllverat
<br />Hlohway99. near 14242690 1,' 1,326 1980-1982
<br />CaaUe Rock,
<br />Wash.'
<br />Tout1&Rlverat
<br />Tower Road, near ""''''' 10.4 1.284 1982-1992
<br />SUver~ke,
<br />Wash.'
<br />"'ortll Fork
<br />~~~~:~rW~b. 142011100 11.1(38.8) n, 1981-1992
<br />NoJ"thFork
<br />Tout1eRlver 14240400 41.0 (68.7) '" 1985-1987
<br />aboV<l!Bear
<br />Creek, neiV Kid
<br />~~t~':.~.
<br />
<br />I--f>>.ta combined because of min1nral ditfer-ence tn drainaQearea.
<br />2--Per1odicd6toonly.
<br />J--Rlver kIIom&ters are above mouth of specific stream. NWllber!; In parentheses are rlV.1II
<br />kilometer above IlIIQUth of Toutle River.
<br />
<br />Table L--Characteristics of stream-gage sites used in the study.
<br />ItdroloaiclffecuauCoII1iUau
<br />Gr01lJld-buH attribates of tu hydrologic cycle ill tbe tootle Riler Basil url
<br />profolllldlr altered by the eraptloll of lout St. Beleas. Incteases il stO[l ruloff, flow
<br />ulocities, aad peat dischargel were tile relult of (11 decrealed ilfiltratiOl ilId (2) redltcH
<br />brduulic rOllgbless alolg laur-affeded CHIIIls (Juda aId otbers, 198t). Sabsta_tial
<br />rtdllctiolls or elililatiol of illtereepUoI also occured 111 blast-affected basias.
<br />lllfiltratioa rates were i.tUaU, reduced frOl typical nlues of 50-100 lillietera pel
<br />hou lu/bl to 1-1 BIb ill thoSI areu affected by the directed blast, and il thict tephra
<br />deposita wbere a ClIlSted, silty tephra-layer bad. been deposited (Bertelratb and Leatesley, 1981;
<br />JOhUDB alld Bllchta, 1981; Leifesle, aad oten, 1989). lithil 18 IOltbs of tH er.ptioa,
<br />ullage lOl}giIl9, sheet aid rill Iroliol, frenl thaw actioR, iIlcl other distlIbalces dtsnptei the
<br />IUlace crust aDd caased infiltratiOl rates to reeOter SOJeVhat (to 4-29 ./h) by elpoaiag lOre
<br />ptnuble tepbra below (Svaalon ud otbers, 1983i Leavedey aad otben, 1989).
<br />Reductioss 10 bydndic I01Ighllls frOl pre-elllptiol cOlditioDS by '1Iooth1lg' of labar-
<br />dlecled cbalnels vere cauled .by:
<br />1. relotal of coarse-graited (graYel aId cobble) bed literial
<br />2. relOtal of acealalatiODI of coarse woody debris
<br />], destnctiOl of pDOl-riffle I!qUllce.
<br />f. destrvctioll of da.sl ripariu forests bordedag the streal challels
<br />5. deposUioe of fiaer-grailed (lUd-aiaed) bed aatedal and
<br />6. a guera! shorteail! of tbe flow paU by st[ligbteling leuders
<br />Then lil facton wlolbtuly coltribltH to the bcreased flasbiaesl of sarface nloff il the
<br />maadtbeTR.
<br />Tbe let resalt of tbe hydrologic effects 01 the I" ud !R .as to cause all initial
<br />lItIent ill tbe IilJlitllde aid (or) frequency of peat flols. The peat discbarge for the '~hour
<br />aut b,drograpb for the l_r !olUe Ri,er dudag tbe first nillf leaSOIl fol101lil1j the eluptioo
<br />iDcrused frol a pre-enptiol ulae of 351 cubic liters per secold (I:t/li) to 614 13/S (1.15 tileS
<br />luger), witb tbe tile-to-peak reduced frot 15 bours to 9 haus IOnig aDd IlatbilOl, 1982).
<br />lo.ner, a..rage aDlual discharge was 58.2 13/8 for tbe 55 Jears of bistorica1 record OD the
<br />10m Toutle River (Castle Rock gage), aid 58.6 l:t/s for tbe 12 Yllrs after tbe enptiOll (CalUe
<br />lod aId To.er ROi-dgi-ges).
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