WATER RESOURCES RESEARCH, VOL. 32, NO. 9, PAGES 2911-2921, SEPTEMBER 1996

Specification of maintenance flows for a large gravel-bed

Peter R. Wilcock Departmentof Geographyand EnvironmentalEngineering, The JohnsHopkins University, Baltimore, Maryland

G. Mathias Kondolf, and W. V. Graham Matthews Center for EnvironmentalDesign Research, University of California,Berkeley

Alan F. Barta Intermountain Research Station, U.S. Forest Service, Boise, Idaho

Abstract. Reservoirreleases may be specifiedto flushinterstitial fine sedimentfrom gravelbeds in the river downstream.Choice of an effectiveflow dependson trade-offs amongdischarge, flow duration,and pool dredgingas they determinerates of bed mobilization,sand removal, and gravel loss.A basisfor evaluatingthese trade-offs is developedwith an approximatemethod appropriateto the sparsedata typicallyavailable. Sand and graveltransport are representedwith rating curves.Approximate methods are introducedfor estimatingeffective gravel entrainment,subsurface sand supply,and pool sedimenttrapping. These are combinedin a sandrouting algorithmto evaluateflushing alternativesfor the Trinity River, California. A sedimentmaintenance flow of moderate size,just sufficientto entrainthe bed surfaceover the durationof the release,limits gravel lossand maximizessand trapping by pools.Larger dischargesproduce more finesremoval but at the costof greater gravelloss and reducedselective transport of fines.Dredged poolsincrease sand removal efficiency by providingmultiple exitsfrom the channeland minimizegravel loss if dredgedsediment is screenedand gravel returned to the river.

Introduction terms of measurablechanges to the physicalhabitat that may be producedby a flushingrelease, rather than the abundance River channelsimmediately downstream of reservoirstypi- of organisms[Ligon et al., 1995;Kondolf and Wilcock,1996]. cally experiencea decreasein magnitudeand sediment Flushingflow goalsinclude removing fine sedimentfrom pools transportcapacity; if flow diversionsare made at the reservoir, usedfor rearing habitat and from graveland cobblesubstrates total dischargeis also reduced.Supply of coarsesediment to used for spawning,juvenile cover, and invertebratefood pro- the downstreamchannel is typicallyeliminated by trappingat duction. A flushing release may also be needed to entrain the reservoir,whereas fine sedimentsmay be introducedto the coarse sediment on the bed surface,permitting removal of downstream either from the reservoir or from down- subsurfacefine sedimentand producinga looserstructure that streamtributaries. If the transportcapacity of the downstream facilitates salmonidredd construction[Beschta and Jackson, channel is sufciently reduced,the finer sedimentmay accu- 1979;Milhous, 1990; Diplas and Parker, 1985].Entrainment of mulate on the bed of the river. Controlled releases of reservoir sedimentthroughout the activechannel section may be spec- water can be used to mimic the action of natural in ified to preventestablishment of maturevegetation within the removingaccumulated fine sedimentsfrom the channeland activechannel, with a correspondingloss in aquatichabitat and looseningthe gravelbed. Sucha sediment-maintenanceflush- channelcapacity [Kondolf and Wilcock,1996]. of the ing flow is similar to, but typicallysmaller than, a channel- river banksand floodplainmay be desirableto maintain topo- maintenanceflow intended to maintain erosion and deposi- graphicdiversity and providea supplyof coarsesediment [Li- tionalprocesses throughout the channeland [Hill et gon et al., 1995]. al., 1991;Ligon et al., 1995;Milhous, 1982; Reiser et al., 1989]. There is a clear need to specifyflushing flows as accurately The two types of flushingflow are broadly complementary, as possible.Released water is typicallynot availablefor stor- althoughtheir specificobjectives may conflict. age, diversion,and power generation,so the financialcost of a Flushingflows are frequentlyspecified to restoreor main- flushingflow can be very large. Becausethe rate and efficiency tain aquatic habitat, especiallyfor salmonids(salmon and of sand removal increasewith dischargeQ, cost constraints trout). Becausethe ecologicalresponse to both reservoirop- suggestthat Q shouldbe aslarge aspossible. The rate of gravel erationsand flushingflows is complex,dependent on external transportincreases with Q, typicallymore rapidlythan that for sand,and a flushingflow can producea net decreaseof gravel factors, and often evident only over periods of years or de- in the channelif gravelsupply is limited by reservoirtrapping. cades,the goalsof flushingflows are most usefullystated in Becausegravel is an important componentof fluvial habitat, Copyright1996 by the American Geophysical Union. gravelloss, or its artificialreplacement, represents an environ- Paper number96WR01627. mental and financial cost of flushingflows that arguesfor a 0043-1397/96/96WR-01627509.00 flushingQ that is a smallas possible. A minimumQ may be set

2911 2912 WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS

Area of detail with confidence.More importantly,the analogydepends crit- LEWISTONDAM ically on the assumptionthat each channelhas adjustedto a RushCreek steadystate geometryfor the water and sedimentsupplied to it. This assumptionis not likely to hold for channelsdown- streamof existingreservoirs, for whichthe water and sediment supplyhave been altered. Specificationof the magnitude,duration, and timing of a STEELBRIDGE • ]/• • - LEWIST•Nflushingflow requires definingquantifiably achievable objec- tives,developing a set of simple,but representative,functions STUDYx•x:j• /GrassValley Creek N representinggravel and sandtransport and sedimenttrapping ////'--.._ % LEGEND by pools,and combiningthese functions in a sedimentrouting DOUGLASCITY(•c-'5•-x'N X• POKERk•'u•Y •iTEBAR • dredgedpools algorithm, so that the trade-offsamong different flushing op- • WeaverCreek tions can be evaluated.A method for evaluatingflushing flow • ...... • • majortributaries options is developed in this paper for the Trinity River in northernCalifornia. Although developed for a particularriver, I I we believethese methods have applicationto other sites,not Figure 1. Location map. Flushing estimate prepared for for the meritsof anyindividual step, many of whichare obvious reach from Grass Creek to Steelbridge.Sand routing approximationsof more completetreatments, but for the man- usessubreaches bounded by the major pools. ner in which the steps,in combination,permit a quantitative evaluationof the differentflushing options that is appropriate to the level of data typicallyavailable. by the needto entrainthe gravelon the bed surfacein orderto remove fine sediment from the bed subsurface and loosen the gravel bed. History of Channel Change The size of a flushingflow may also be constrainedby the on the Trinity River release capacityat the , the financial and legal liability The TrinityRiver drains7640 km 2 of steepterrain in the associatedwith an artificial flood, and the availabilityof water Klamath Mountains of northwesternCalifornia (Figure 1). at the appropriatetime. Moreover, transportobservations in Runofffrom the uppermost1860 km 2 of the basinwas im- both the field and the laboratorysuggest that only a narrow pounded by Trinity Dam (and its reregulating reservoir, rangeof flow producesentrainment of mostof the bed surface LewistonDam) beginningin 1961, as part of the U.S. Bureau (allowinggravel loosening and subsurfacesand removal), while of ReclamationCentral Valley project. From 1963 to 1995, maintainingthe selectivetransport of fine sedimentnecessary about75% of the averagenatural runoff of 47 m3/sfrom the to reduce the fines content of the bed [Wilcock,1995]. The upper basin has been exportedvia a seriesof hydroelectric variousgoals and constraintsfor flushingflows imposeboth plantsto the SacramentoRiver basin,where it is divertedfor minimumand maximumconstraints, suggesting that the range irrigation.Floods have been virtually eliminated on the Trinity of effectiveflushing flows may be quite narrow. River in the reachdirectly below the reservoir.Flow regulation Accuratespecification of flushingflows is hamperedby the hasreduced the meanannual flood Q ma from 525 tO 73 m3/s complexityof the flow and transportsystem and the sparse andthe 2-yearflood Q2 from 484 to 30 m3/s,based on the data typicallyavailable. Both problemsarise from the large continuousdischarge record from 1911 at the U.S. Geological scaleof river reach typicallyconsidered, the spatialand tem- Survey(USGS) gage at Lewiston(Figure 2). In the 35 years poral variabilityin flow and transportwithin the reach,and the followingdam closure,the largestsingle daily mean nonlinear nature of the flow-sediment interaction. At the reach Q hasbeen 391 m3/s, and there have been only 13 dayswith scale,transport estimates are typicallymade with highly sim- Q > 240 m3/s,which is one-halfthe predam 2-year flood. plified models;field calibrationusing spot observations is nec- essaryfor any useful accuracy.The trade-off betweenmodel accuracyand data availabilityis of immediateconcern in this paper, becausethe need existsfor flushingflow estimatesthat !! = MaximumDailyDischarge are both efficient(requiring a minimumof observation)and ;i:! ---o-.-Mean Values:1911-60Annual Discharge &1964-90 ',• i11/60 Tdnity DamCloses i sufficientlyaccurate to permit evaluationof differentflushing ...... - ' ! I ! : •vers•onueg•ns I alternatives. 1,000i _ _'_s27 j• In view of the difficultiesinvolved in accuratelyspecifying a flushingflow, it is not surprisingthat flushingflows are often prescribedusing broad rules basedon simpleanalogy. The • 100 ?3:0 mostcommon approach is to specifya flow whosefrequency is that observed(or assumed)to produce desirableflow and transport conditionson other channels.For example,a dis- 10' chargewith a prescribedrecurrence interval (e.g., 2 years, based on the prereservoirhydrologic record) may be pre- scribed because such floods have been taken to correlate, on average,with channel-formingconditions of flow and transport in self-adjustedchannels [e.g., Leopold et al., 1964;Andrews, 1010 1020 1030 1040 1050 10t30 1070 1080 1000 1980].Such a rule, however,represents only the meanbehavior Figure 2. Mean annualdischarge and annualmaximum daily of many channelsand cannotbe appliedto any particularsite discharge,USGS gage,Trinity River at Lewiston,1911-1990. WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS 2913

Concurrentwith the reductionof sedimenttransport capac- pearsto have occurredover a cumulativeduration of the order ity in the mainstem,sediment yields from tributarywatersheds of only one to severalweeks. This rapid depositionimposes an increased as a result of road construction and timber harvest. important constraint on the timing of flushing releasesfor Most notable among these tributariesis Grass Valley Creek, in-channel sediment maintenance: future flushing releases which flowsinto the Trinity River about 13 km downstreamof shouldbe timed to avoid floodsin order to minimize LewistonDam (Figure1). GrassValley Creek drains a 98 km2 furtherbank depositionand lossof within-channeltopographic basinunderlain principally by the ShastaBally Batholith,which diversity. weathersto producedecomposed granitic soils that are readily eroded and producelarge yieldsof sedimentfiner than 8 mm. From 1950 to 1960, 90% of the GrassValley Creek basinwas Flushing Objectives and Options logged, resulting in estimated 'annual sediment yields of Different flushingobjectives may be definedfor the fine and 102,000m 3 in the 1970s[Frederiksen, Kamine, and Associates, coarse portions of the bed material. One clear objectiveis to 1 •hl IVouJ. removeas much sandas possible.When the value of the water The primary postdamchanges to the Trinity River channel is considered,a related objectiveis to minimize the volume of have resultedfrom depositionof tributary-derivedsediments water used or to maximize the sand removal per volume of within the channelbed and in steep,fine-grained banks along water. For the gravel,one objectiveis to entrain the bed sur- the channelmargins. Although the sedimentin both deposits face in order to permit removal of subsurfacesand and to comesfrom the sametributary source, there is little overlapin maintain some looseness in the bed structure. Because reser- grain size' fine iediment within the channelbed is predomi- voir trappinghas severelyreduced the supplyof new gravelto nantly 1 mm to 8 mm in size,whereas the sedimentsare the reach,a flushingrelease should also produce a minimumof predominantlyfiner than 1 mm (J. Pitlick,p•ersonal communi- downstreamgravel transport to limit gravelloss. Further, be- cation,January 1995). Little transportof bed materialoccurs at causethe ratio of sand to gravel transportincreases with de- Q < 85 m3/s,and essentially no transportof materialcoarser creasingdischarge, a smallerdischarge produces stronger se- than 1 mm occursat the typicalpostdam in- minimum lectiveremoval of sand.The net resultis a flushingrelease that flowsof 4 m3/s(1961-1978) and 8.5 m3/s (1978 to present). is tightlydefined: it shouldbe no largerthan that just sufficient The absence of sediment finer than 1 mm in the channel bed to causeentrainment of most of the bed surface,thereby per- suggeststhat most,!f not all, mainstem flows are capableof mitting gravel loosening,subsurface flushing, and selective preventingits depositionon the bed and that much of this sandtransport, while minimizingdownstream gravel loss. sedimentis transporteddirectly through the reach.Some dep- The availableflow optionsare the volumeof water usedand osition of this sedimentdoes occur in low-velocityoverbank the rate at whichit is released.A third option,dredging pools regions•under favorabledepositional conditions of high river to act as sedimenttraps, is alsoconsidered because sand traps stage and high sedimentconcentration. Encroachment of ri- can provide a larger and more even distribution of sand re- parianvegetation within the predam active channel plays a moval for a specifiedvolume of water. Further, if the dredged centralrole in this bank buildingby slowingthe overbankflow, material is screened,transported gravel can be returnedto the inducingdeposition of suspendedsediment, and stabilizingthe channel,which directlyincreases the selectiveremoval of sand banks. These banks have narrowed the channel to 20-60% of and may allow larger, more etficientsand-removal discharges its predam width, producing a straightenedchannel course to be used. with decreasedtopographic variability. The bank depositsare There is no combinationof releasevolume and discharge now anchoredby matureriparian vegetation (principally alder that optimizesall objectives.Some of the objectivesevidently and willow) and cannotbe removedby the largestlikely res- conflict.A dischargecannot both minimize gravel transport ervoirrelease of 240 m3/s,which is the legallyrecognized and maximizesand transport; a releasethat is just sufficientto postdam100-year flood. Thus bank removalcannot be accom- produceentrainment of most of the bed surfacewill maximize plishedby flushingflows on the Trinity River. However, the neither selectivetransport nor sand removal efficiency.Be- controlsof bank depositiondo influencethe timingof flushing causeno flushingrelease can satisfyall flushingobjectives, a flows,if they are to not causefurther bank building. satisfactoryrelease must be a compromiseamong the various High flow releasesfrom Trinity Dam containlittle sediment objectives.Because the relations among releasevolume, dis- and have typicallylagged behind tributary peak flows,which charge,pool trapping,gravel entrainment, and sedimenttrans- are the principalpostdam source of suspendedsediment. Thus port are nonlinear,and in most cases,rapidly varying,quanti- most postdamhigh flows are unlikely to have contributedto tative estimatesof entrainment and are aggradationof the inset banks. Our analysisof water and essentialin evaluatingthe trade-offsamong the differentflush- sedimentrecords for USGS gagesin the Trinity and adjacent ing options. basinsfor the 35-yearperiod after dam closure(1961-1995) indicatesthat,of the 208 daysfor which flowshave Methods exceeded85 m3/s (the threshold for nonnegligibletransport of the channelbed), only 46 dayshave coincided with large rates Overview of tributary sedimentsupply (defined as runoff exceeding10 Methodsfor estimatingthe gravelentrainment, gravel trans- mm/d, which is the runoff associatedwith 81% of the total port, and sand removal during a flushingrelease necessarily suspendedsediment yield from Grass Valley Creek over a representa trade-off among systemcomplexity, data limita- 15-yearperiod). Of these46 days,3 daysin 1974 and 6 daysin tions, and the need for quantitativeestimates of sand and 1983had exceptionallylarge tributaryfloods (62% of the total gravel removal. The approachtaken here is to use simple sedimentyield for the 15-yearrecord on GrassValley Creek functionsthat representthe essentialsystem response to flush- was deliveredduring the 6 daysin 1983). Thus depositionof ing flowsand that canbe either directlycalibrated using limited the inset fine-grainedbanks of the presentriver channelap- field observations or evaluated relative to functions found to be 2914 WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS broadlyapplicable in gravel-bedrivers. The basisfor evaluating entrainment. The bed thickness that could be flushed with flushingflows is a massbalance of fine sedimentin the reach, active gravel entrainmentwas taken to be 0.15 m, which is whichrequires estimates of an initial quantityof sandand its slightlylarger than the limit of plane-bed gravel scour of rate of transportas a functionof discharge.Estimates of gravel 1.7D9o estimatedfrom local observationsof gravel entrain- entrainmentand transportrate are necessaryto determinethe ment [Wilcocket al., thisissue], implying that sandremoval can amountof gravelloss and the degreeof sandflushing from the proceedto a depth slightlygreater than the depth of gravel bed subsurface.New relations, with limited field calibration, entrainment[Beschta and Jackson,1979; Diplas and Parker, are required to estimate the rate of sedimenttrapping in 1985].A finescontent of 25% wasassumed for the subsurface dredgedpools and sandentrainment from the bed subsurface. layer, basedon the percentfiner than 8 mm observedin bulk An importantand necessarysimplification is the treatment samplestaken at Poker and Steelbridge. of the sedimentas a two-part size distribution.This is the Sedimenttrapping in the five major pools along the reach minimumnecessary to addressthe objectivesof preferentially (Figure 1) was measuredby surveysbefore and after trial removing fine-grained sediment. Calculations of sediment reservoir releasesin 1991, 1992, and 1993. Identical points transportfor a larger numberof size fractionsrequire local were surveyedat 1.5-m intervalsalong networksof parallel information on sediment content that is not available. Because crosssections or multiple rays extendingfrom monumented the fine sedimentforms the matrix of a clast-supportedgravel/ points along the banks.Bed elevationwas measuredto the cobblebed, it maybe arguedthat it is more transientin content nearest 0.025 m with a fiberglasssurvey rod. Contour maps and that it will exhibittransport behavior that differsfrom the were prepared of the bed elevationand the net changein framework gravel and cobbles.This is supportedby observa- sedimentstorage in each pool. tionson a wide rangeof gravelbed [e.g., Carling, 1988; Church et al., 1991; Kuhnle, 1992; Jacksonand Beschta, 1982; Sediment Transport Rates Leopold,1992; Lisle, 1995; Wathenet al., 1995]. Such an as- Sediment transport rates for sand and gravel are repre- sumptionis also implicit in the basic concept of sediment sentedusing a rating curve maintenanceflows. Different transportrelations are observed for sandand gravelalong our studyreach of the Trinity River, Qi = (Fi/cri)(Q- Qci)t3' (1) and the variationsin transportrate within the sandand gravel fractionsare evidentlymuch smallerthan the differencesbe- where the subscripti representseither sand(s) or gravel(g), tweenthe two fractions[Wilcock et al., this issue]. Q i is sedimenttransport rate in metric tonsper day,Fi is the proportionof sand or gravel on the bed surface,Q ci is the Sand Content in Study Reach dischargeat the onsetof substantialtransport, cr i and /3i are The studyreach extends 8.0 km from the GrassValley Creek fitted coefficientsand exponents,respectively, and both Q and , 13 km downstream of Lewiston Dam, to the Steel- Qcgare in unitsof m3/s.The factor F i reducesthe transport bridge studysite (Figure 1). For flushingestimates the reach capacityby the amountof sandor gravelavailable for transport was dividedinto five subreaches,separated by major pools.In on the bed surface,which becomesimportant as a reach be- the studyreach, sedimentfiner than 8 mm is generallylight comesflushed of sandand Fs becomessmall. coloredand derivedfrom decomposedgranitic terrain, partic- Althoughthe ratingcurves are obvioussimplifications of the ularlyin the GrassValley Creekwatershed. This materialcom- actual transportfield throughoutthe reach, they do incorpo- prises20-30% of the bed and hasa mediangrain sizeof 2 mm, rate dominantfeatures of the process,including a nonlinear with ---75% in the 1 to 8 mm range and 90% coarserthan 0.5 increasein transportrate with dischargeand the effect of mm. Sedimentcoarser than 8 mm is predominantlydark col- surfaceconcentration on the transportrate of the finer frac- ored rock fragmentsof metamorphicand volcanicorigin. The tions.The ratingcurves require calibration with field data and mediangrain sizeof the coarsefraction is 36 mm at Poker Bar are therefore subjectto considerableuncertainty arising from studysite and 56 mm at Steelbridge(Figure 1). the largeamount of scattertypically found in measurementsof The distinct color difference between fine and coarse frac- in large rivers.Because flushing results may be sen- tions enabledvisual estimatesof the proportionof fine sedi- sitive to choice of rating curve, two sets of sedimentrating mentFs (<8 mm) on the bed surface.Estimates were madeat curvesare developedto evaluatethe sensitivityof the flushing low flow, usingthe samepersonnel to avoid systematicdown- calculationsto uncertaintyin the estimatedtransport rates. streambias. For eachsubreach an approximateestimate of the One set of rating curveswas developedfrom transportob- volumeof sedimentrequiring flushing was developedby map- servationsmade at Poker Bar duringtrial reservoirreleases in ping regionsof uniform Fs on enlargedaerial photographs, 1991,1992, and 1993(Figure 1; labeledas Poker Bar in Table from whicharea-weighted averages were determined.Compar- 1 and Figure 3 [Wilcocket al., this issue]).The sand rating ison with pebblecounts at the Poker Bar studysite suggests curve is more sensitiveto F i than the gravel rating curve, +0.1 uncertaintyin visualestimates of F s,with slightlygreater becausethe proportionalvariation in F s (0 < Fs < 0.3) is accuracyfor changesin Fs producedby the flushingreleases or muchlarger than that of Fg (0.7 < Fg < 1.0). BecauseF s from reachto reach.Uncertainty in the total volumeof sandin variesin both spaceand time and is generallyunknown at the each subreachprimarily influencesthe durationor numberof time of sampling,a larger scattermay be expectedin the sand flushingreleases required to reduce the fines content to a transportrates (Figures 3a and 3b). The plotted data and specifiedlevel and has a second-ordereffect on the tradeoffs ratingcurves use a value of F s --- 0.22,which is a typicalvalue amongflushing alternatives. In practice,regular monitoring is for the Poker Bar subreach.In the sand routing performed neededto updateFs estimatesand flushingrecommendations. later in the paper,F s varieswith both locationand time. A surfacelayer thicknessof 0.075 m (•D9o of the bed The alternativerating curves were selectedto providea fit to frameworkgravel) was assumedfor the volume of fine sedi- both the Poker Bar observationsand Helley-Smith bed load ment on the bed surfacethat could be flushedwith no gravel samplestaken at a discontinuedUSGS gageat Limekiln, im- WILCOCK ET AL.' SEDIMENT MAINTENANCE FLOWS 2915

mediatelydownstream of our studyreach (Figures 3a and 3b). lOOO 1000 The alternativerelations were chosento provide a clear dif- (a) Sa (b) Gravel ferenceinthe ratio of sand togravel transport uponwhich the •100 , '• 100 flushingresults may depend. Theratio of sand togravel trans- portdecreases with discharge forboth pair of ratingcurves, with the Poker Bar rating curvesdecreasing to a value of 0.34 • 10 • • 10 andthe alternate curves decreasing toa value of 3.4 at Q • 240m3/s (Figure 3d). Using single rating curves to represent sand and gravel transport throughoutthe studyreach implicitly assumesthat transport rates are spatiallyuniform. Although this clearly •-0.1 ? oo!!ir!rr I-' 0.1 cannotbe true in detail, the absenceof obviousaggradation or Alternate u•gradaaon.,A _,. within the •'•Luuy • reach (as required by ....6,..... n•lj 0.01 i 0.01 i nonuniformtransport) suggests that the error may fall within lOO 1000 10 100 1000 acceptablebounds, particularly over entire subreachesfor Discharge (cms) Discharge (crns) which some of the local error would cancel. A conventional 1000 (d) alternativeapproach, calculating transport rates from section- (c) ,'' averagedestimates of Zo from a one-dimensionalhydraulic • 100 modelof the study reach, islikely to give similar or larger errors.Local and section-averaged values of Zocan be very o different,[Wilcocketevenal., thisfor reachesissue]. Becauseand sectionsthe relation withsimplebetween geometryZoand • 10 transportrate is stronglynonlinear, particularly at smalltrans- portrates, error in Zo may produce errors incalculated sedi- • 1 mentload that are very large and difficult to constrain [Carson Poker Bar Sand and Griffiths,1987]. For the samereason, this approach is likely •' 0.1 ...... Alt. Sand to be more sensitive to flow conditions at a limited number of Poker Bar Gravel measuredcross sections than the rating curve approachused - - - Alt. Gravel --•ltkcCmraBt:r; here.Regardless, theinformation onchannel geometry and 0.011 o 1 oo 1 ooo 50 100 150 200 250 roughnessneeded for hydraulicmodeling is not available,as is Discharge (crns) Discharge (crns) often the case,and an approachusing sediment rating curves is likely to be useful at many sites. Figure 3. Transportobservations and sedimentrating curves for sand and gravel. Transport observationsfrom Poker Bar Gravel Entrainment studysite and the USGS gage, Trinity River below Limekiln .(a) Transportof sand(<8 mm). Transportobservations An estimateof the rate of gravelentrainment is requiredfor with gray symbolsare for 1981/1982,when Fs may have been two purposes.The first is to determinecombinations of Q and muchlarger. (b) Transportof gravel(>8 mm). Gray symbols flushvolume V that produceminimum adequate entrainment are transportrates recalculated after excludingthe largestand for bed surfaceloosening and subsurfaceflushing. The second smallest25% of samplesfrom the total. (c) Comparisonof is to provide a basisfor evaluatingthe frequencywith which sedimentrating curves for Q > 85 m3/s.(d) Ratioof sandto subsurfacesand is exposedand availablefor entrainment. gravel transportrates calculatedusing the Poker Bar and al- Becauseof the infrequent and stochasticnature of grain ternative rating curves. motion at low transportrates, the mobilizedproportion of the bed surface will increase with V or release duration. The nec- essaryduration for surfaceentrainment should decrease rap- usedto calculateother combinationsof Q and V that produce idly with increasingtransport rate, but the productof duration the samevolume of graveltransport. The duration producing and transportrate, transportvolume, should vary far less,sug- minimum satisfactoryentrainment, defined here as the ex- gestingthat a minimum flush durationfor differentQ may be changetime rex, is assigned a value of 5 daysfor Q = 164 m3/s. approximatedusing a volumeof transportedsediment known Using (1), other valuesof rex(in days)are determinedfor the to provideadequate surface entrainment [Wilcock, 1995]. sametransport volume as A reservoirrelease of 164m3/s for 5 daysin 1992produced nearlycomplete entrainment of the bed surface[Wilcock et al., this issue].Using this as a referencecombination of Q and V tex=5Qg]=5 •-Q-•gg] (2) for a minimumflushing release, the gravelrating curvesmay be whereQ g and Q• 92are the gravel discharge associated with Q andQ = 164m•s, respectively. Because V = t•xQand there is a uniquevalue of t•x for eachQ, a monotonicrelation exists Table 1. Parametersfor SedimentRating Curves(1) be•een V and Q that representsminimum satisfacto• bed entrainment.Equation (2) also providesa timescalethat is RatingCurve F i oti [3i Qci,m3/s usefulin estimatingthe upward rate of sandsupply from the Poker Bar gravel 1.0 2,000 3.0 77 bed subsurfaceas a function of gravel entrainment rate. Alternate gravel 1.0 600 2.5 77 Poker Bar sand 0.22 13.0 2.0 30 Pool Trapping Alternate sand 0.22 350,000 4 0 The storagevolume and trap efficiencyof eachpool mustbe Qi in metrictons per day,Q andQ ci in m3/s. specifiedin order to calculatethe sand removal that can be 2916 WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS

B•c np=0.044 @Q = 80 cms 1' • Rc= Bc+ 2hc (6) np= 0.040 @ Q = 164 cms Flowin the poolis assumedto be steady,but nonuniform,so thatthe simple form of momentumconservation in (4) maynot be usedand the pool roughness np cannotbe presumed,al- thoughthe form of the flow resistancerelation is assumedto hold.Sand transport in bothchannel and pool is assumedto followthe simpleproportionality Qs crFsu, 3 (equivalentto manytransport formulas for transportfar in excessof incipient motion).For comparablewidths of activetransport in the channeland pool, steady state requires that Qs is a constant. , , , Thisand the transportrelation allow the shearvelocity in the 2 3 4 5 6 pool and channelto be related as Depth Predicted(m) Figure 4. Observedand calculated steady state pool depths. u,p = (Fs/Fsp)1/3/•/*c (7) Observeddepths based on surveystaken before and after trial releases.Predicted depths from (7) to (11). Bestfit is achieved whereU,p is thepool shear velocity and Fsp is thesurface by allowingpool roughness rtp to varyslightly with discharge, proportion of sandin the pool.For steadystate conditions and althougha constantrtp -- 0.0415provides a trendthat falls specifiedvalues of Q, B•, Bp,h•, u,•, Fs,Fsp, and np, it is within the pool deptherror bars. possibleto findU,p andvalues of poolvelocity Up, cross- sectionarea Ap, hydraulicradius Rp, anddredge depth Ah (differencein bedelevation between channel and pool) from (7), togetherwith continuity achievedby a flushingrelease. Because little informationis Q =ApUp (8) availableon thegeometry of existingchannel/pool reaches and flow resistance becauseit is usefulto evaluatethe utility of addingnew pools, for whichneither location or configurationare specified,the 1 methodused to estimatestorage volume and trap efficiency Up-- (rip N/-•Rlp/6) /•*p (9) mustrely on relativelysimple input. Bothscour and fill wereobserved in dredgedpools during and the definitionsof pool hydraulicradius thetrial releases. In somecases, an individual pool aggraded at A oneQ andscoured at a higherQ. The largesettling velocity of __ P the fine bed material(1-8 mm) and the typicaldepth and Rp- Bc+ 2(hc+/Xh) (10) mean velocityin the poolsindicate that this sedimentmoves and cross-sectional area throughthe pools as bed load. The observedscour and depo- sitionsuggest that a steadystate pool depth exists for particular Ap = B•hc+ BpAh (11) combinationsof water discharge,sediment input, and pool geometry.A simplemethod is developedhere to estimatethe For specifiedQ, So,B•, Bp,nc, Fs, and Fsp , the only un- steadystate depth because it providesa useful reference depth knownparameter is thepool roughness rip, which is usedto for evaluatingthe relative benefit of differentdredging depths matchpredicted and observed steady state pool depths for trial in poolsof varyingwidth. reservoir releases in 1992 and 1993. Predicted values are cal- Calculationof the steadystate pool depthis basedon sedi- culatedusing the constant release Q, neglectingscour or dep- mentcontinuity, which requires that the sandtransport rate be ositionproduced by the relativelyshort ramping flows at the the samein both channeland pool.To simplifythe calcula- startand end of eachrelease [Wilcock et al., thisissue]. Ob- tions,the channelis assumedto havea rectangularcross sec- servedvalues are determinedbased on comparisonof contour tionof widthBc andthe pools are rectangular troughs of width maps of pool bed elevationbefore and after the releases.To Bp (Bp --

exceedsthe steadystate depth Ahss.The trap efficiencyT is betweenthe surfaceand subsurface.For the routing compu- defined as tationsdiscussed below, the value of the constantin (14) is taken to be 0.5. Qsc- Qsp T-- Qs½ (12) Sand Routing Algorithm Using the same proportional sand transport relation Q s oc The sand routing algorithm is based on sand massconser- Fsu,3,(12) maybe rearrangedas vation within the surfacelayer of the river bed. Sand is routed between subreachesbounded by major pools. Sand output T = 1 - (Fsp/Fs)(U,p/U,c)3 (13) from each subreachis calculatedusing the sandrating curves (1) (Table 1). Sandinput to a subreachis the sumof the output WithAh specified,(8)-(11) and (13) may be solvedfor T, U,p from the next reach upstream, reduced by trapping in the Up,Ap, andRp. This formulation is usedto solvefor T in the interveningpool, plus the upward sand supplyfrom the bed sandrouting calculations. subsurface(14). The amountof sandtrapped in eachpool is Sand Supply From the Bed Subsurface determined as a function of available depth h d below the steadystate depth (hd = Ah - Ahss),which determinesthe An expressionfor the upwardsupply of sandfrom the sub- trap efficiency(equations (8)-(11) and (13)) andthe remaining surfaceis needed to accountfor subsurfaceflushing during a storagevolume. The massof sand in the surfaceand subsur- release.The rate of upward entrainmentwill depend on the face is recalculatedat the end of each time step and used to frequencyof gravel entrainmentfrom the bed surface,which updatevalues of F sand Fss. Gravel transportout of eachreach determinesthe frequencywith whichsubsurface fine grainsare is calculatedusing the gravelrating curves(1) (Table 1). The subjectedto the flow. Existingformulations are basedon the pools are assumedto trap all gravelwhen hd > 0. Both sand incorporationof subsurfacesediments into the activetransport and gravel input to the pools reduce the remainingstorage, layer at a rate proportionalto the rate of bed which is recalculatedafter each time step.The time step used lAshidaand Egashira,1989; Hirano, 1971;Holly and Rahuel, in all of the calculations discussedhere is 1 hour, which is < 1% 1990;Parker and Sutherland,1990] and thereforecannot rep- of the duration of all simulated releases. resentsubsurface entrainment when the rate of degradationis negligiblysmall or zero. Under theseconditions, entrainment of subsurfacematerial may still occur but is limited to finer Evaluation of Flush and Dredging Alternatives sizesthat may be entrainedwhen a coarseroverlying clast is The method developedabove is usedhere to illustratethe entrained.This appearsto be the case on the Trinity River, trade-offsamong discharge Q, flushvolume ¾, sanddischarge where little net changein bed elevationwas observedduring Qs, gravel discharge Qg, andpool dredging depth hd for the the trial flushingflows. Because an effectiveflushing flow may study reach of the Trinity River. Although the results are often requirea compromisebetween maximizing selective sand directly applicableonly to that reach, someof the underlying transport and minimizing gravel transport, a formulation is trends are likely to be more general, and the presentation neededfor estimatingsubsurface sand entrainment in the pres- illustrates the kind of information necessaryfor selecting ence of small gravel transportrates and negligiblechange in amongdifferent flushing options. bed elevation. Becausewater costsare often the largestexpense in a flush- In addition to the rate of gravel entrainment,the rate of ing flow, many of the important trade-offsare evidentwhen subsurfacesand entrainment depends on the relative concen- flushingresults are calculatedfor a constant¾, whichprovides tration of sand in the surface and subsurface. When the two are an approximatecomparison of the relative efficiencyof differ- similar, rates of removal and depositionto the subsurface entQ andh d- Figure 5 presentsthese results for ¾ = 1.2x l0 s shouldalso be similar,so that net upwardentrainment is likely ms (100,000acre feet), which is comparable to thevolume used to be small.When the surfacelayer is relativelyclean of sand, in the 1992 and 1993 trial releases.Figures 5a and 5b present the concentrationof sandin transportwill be smallerand net the sandand gravelremoved as a functionof Q for the Poker entrainment from the subsurface should increase. Bar and alternatesediment rating curves,respectively. Figures The net rate with which sandwith massMu is removedfrom 5c and 5d presentthe massof sandtrapped in poolsfor the the subsurfacemay be expressedas same values of Q. A constantQ is used in the calculations,so the. infl•emce. e•f qhr•rt rarnnin• fiowq at the, .qtart and end of a d--•= const Fss M•s• (14) releaseis neglected.It is assumedthat sedimentdeposited in pools is removed, screened,and the gravel returned to the whereFss is the proportionof sandin the subsurface,Mss is the channel,so that only sandis removedat pools,whereas both massof sandin the subsurface,and rexis the exchangetime for sand and gravel are removed at the downstreamend of the spatiallycomplete gravel entrainment defined in (2). Valuesof reach. Resultsare shownfor dredgingthe five major pools in t exvary inverselywith Q, so that the larger entrainmentrates the study reach to hd = 0, 0.5, 1.0, 1.5, and 2.0 m. For ¾ = associatedwith higher dischargesproduce a smallert exand a 1.2 x 108m 3, the minimumdischarge Qmin required to mobi- more rapid dMu/dt. Becausetex is a scaledtransport volume, lize the bed surface(based on (2); shownas vertical dashed line dMu/dt hasan inverselinear relationwith the volumeof gravel on Figures5a and5b) is 147m3/s for the PokerBar rating transport.The subsurfaceentrainment rate in (14) depends curves(vertical dashed line on Figures 5a and 5b) and 143 m3/s directlyon the relativedifference in sandconcentration in the for the alternate rating curves. surfaceand subsurface,varying from zero when Fss = Fs to a Both sandand gravelremoval increase with Q for a constant constantmultiple of Mss/rexwhen F s = 0. When dt = tex(e.g., ¾ (Figures5a and5b), a resultof the nonlinearsediment rating 5 daysfor Q = 164 m3/s),the massof sandentrained is a curves.For the samereason, the quantity of sand removedby constantproportion of the relative differencein sand mass poolsalso increases with Q for smallQ but reachesa maximum 2918 WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS

10,000 PokerBarRating Curves ] 10,000 •[ Alternate RatingCurves 9,000 (a) ,: IMin(Q) (b) Min(Q)

I, 8,000 i' // 9,000 7,000 ---..... i-- ,ooot ll-•-- -•--- 6,0• 5,000

4,000 ---•--- 4,000 .....: 3,000 ,ooo 2,000 2,•

1,000• ...... -x_ ...... 1,oo.....

2,500

o (c) (d) o o_ 2,000 2,000 1,500

1,000 1,000

z 500 500

0 • • 0 • -+- 100 150 200 250 00 150 200 250 Water Discharge(cms) Water Discharge(cms)

No SandTrapping in Pools • Dredge 0.Sm •Dredge 1.0m ----o--Dredge 1.Sm ---A• Dredge 2.0m ------Gravel Lost Downstream - - -x- - Trappedsediment: %sand (x10) Figure5. Estimatedsand and gravel removed from the study reach using a flushvolume of 1.28x 108m 3 (100,000acre feet), asa functionof dischargeand dredge depth in existingpools. (top) Total sandand gravel removed;(bottom) net sandremoved by dredgedpools. Calculations based on PokerBar (Figures5a and5c) and alternative(Figures 5b and 5d) sedimentrating curves.Min(Q) is the minimumconstant Q that will entrainthe bed surface(tex = 1) for the specifiedwater volume. Although the total amountof sandremoved increaseswith Q, the amountof gravellost increasesmore rapidlyand the amountof sandtrapped in pools decreasesat higherQ becausean increasingproportion of pool volumeis takenup by gravel. and decreaseswith further increasesin Q (Figures5c and 5d) Figures6b and 6e), and the ratio of sandto gravelremoval becausepools fill and the proportion of sand (relative to (Figures6c and 60. Figures6a-6c use a constantQ = 150 gravel)in the trappedsediment decreases (shown as crosson m3/s;Figures 6d-6f useQmin, which, for ¾ of 0.1,0.2, and 0.3 Figures5a and 5b). With either ratingcurve, a moderatedis- km3, are 153,135, and 126m3/s for the PokerBar curvesand chargeof roughly150 m3/s minimizes gravel loss (subject to the 150,129, and 120m3/s, for the alternativerating curves, re- requirementQ -> Q min)and maximizesthe quantityof sand spectively.Both casesinclude dredging to hd = 0 m and 1.0 m trappedin pools.For a givenQ, additionalsand removal may in the five existingpools and two new pools (4100 m and be accomplishedwith eitherlarger ¾ or deeperdredging. The 5600 m downstreamof Grass Valley Creek) placed in the utility of the latter maybe judgedby the relativeseparation of largestsubreach with no existingpools. In all cases,the total the differentlines in Figures5c and 5d. For the specified¾ in volumeof sandremoved increases with ¾, but the efficiencyof Figure 5, h d of the order of 1.5-2.0 m is sufficientto trap most sandremoval decreases with ¾ becauseFs (and Q s) decrease of the sandand larger hd would not be useful. over a longer releaseand becausepools fill with sediment, The role of ¾ in increasingsand removal is shownin Figure causinga decreasein trap efficiency.At constantQ, reduced 6, whichpresents the variationwith ¾ of the total sandand sand trappingcauses sand removal to increasewith ¾ less gravel removed(Figures 6a and 6d), the efficiencyof sand rapidlythan gravelloss, causing the sand/gravelremoval ratio removal(in metrictons of sandper cubickilometer of water, to decreasewith ¾ (Figure 6c). For Q = Q minthe volumeof WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS 2919

8,000 --. • 50,000.... Q= 150cms •____ lO 150cms E (b) ß (c) 7,000 -- PBlm = 40,000 6,000 a,,,x. -o- AIt0m

5,000 =PB 0m i_• 30,000 • 6 AIt1rn 4,000

3,000 . I • • • 20,000 ..

2,000 I

1,000

0

8,000 = PB0m -- PBlrn

7,000 fi !•e••-•--Qmln-• •PB ----.--Air 6,000 i---o-- Air0m---[:}-- Air 1rn (d) 5,000 ._•3o, ooo -• ....;.-- i--' 4,000 20,000 3,000 2,000 1,000 lO,OOO

0 0.1 0.2 o.3 0.1 0.2 0.3 0 0.1 0.2 0.3 water Volume (kin^3) Water Volume (km^3) Water Volume (km^3) Figure6. Totalsand and gravel removed, sand removal etficiency, and ratio of sandto gravelremoval, as a functionof flushvolume and dredge depth. Figures 6a, 6b, and 6c use constant Q = 150 m3/s;Figures 6d, 6e, and 6f useminimum Q requiredfor t ex -- 1 for the specifiedwater volume. Sand removal etficiency calculatedas tons of sandremoved per cubickilometer of water.The totalvolume of bothsand and gravel removed increaseswith water volume, but sand removal efficiencydecreases because Fs and pool trap efficiencydecrease over a longerrelease. See text for furtherexplanation. gravellost is constant,so the increaseof sandremoval with ¾ tion for the flushingflow needed to reducethe sandcontent in causesthe sand/gravelremoval ratio to increasewith ¾ (Figure the reachand predict a morefavorable ratio of sandto gravel 60. For Qmin< 150m3/s the totalsand removed and sand transport.Several important conclusions are independentof removalefficiency are slightly smaller than for Q - 150 m3/s, the choice of sedimentrating curve. Both casespoint to a eventhough the ratio of sandto gravelremoval is larger. superiorflushing discharge of moderatesize, of the order of The influenceof dredgedpools on the downstreamdistri- 150 m3/s,in order to minimizegravel loss and maximizethe butionof sandremoval is shownin Figure7 for the casesh d -- amountof sandtrapped in dredgedpools. In both cases,the 0.0 m and 1.0 m in the existingfive poolsand the two pools total quantityof sandremoved increases with flushvolume but addedin the downstreamhalf of the studyreach to providea is most efficient at smaller volumes. more uniform distribution of sand removal. The case shown usesthe Poker Bar rating curves and ¾ of 0.1,0.2, and 0.3 km 3 Conclusions at Q = 150 m3/s.Figures 7a-7b andFigures 7c-7d show the proportionof sandin the surfaceFs andsubsurface Fss layers, Numerousobjectives may be definedfor a sedimentmain- respectively.Without pool dredging, sand removal occurs only tenanceflushing flow. Among the most importantand com- at the downstream end of the reach, and there is a clear monlyneeded are to maximizethe total sandremoval and the bottleneck effect: the decrease in sand content at the down- ratio of sandto graveltransport, to minimizethe watervolume streamend is far lessfor hd = 0 (Figures7a and 7c) than for usedand gravelloss, and to require a minimumamount of h d = 1.0 m (Figures7b and 7d). Dredgedpools not only gravelentrainment to loosenthe bed surfaceand entrain sub- increasethe total amountof sandremoved but providea more surfacesand. Some of theseobjectives clearly conflict: a dis- uniform distribution of sand removal. Uniform sand removal is chargecannot both minimizegravel transport and maximize most evident at the smallestvalue of ¾. At larger ¾, more sandtransport; a releasethat is just sufficientto entrainmost extensivepool filling and diminishedpool trap etficiencydi- of the bed surfaceand providesome subsurface flushing will minish the decrease in sand content at the downstream end. A maximizeneither selectivetransport nor sand removal flushvolume of 0.2 km3 wouldbe moreeffectively used in two ciency.Specification of a flushingflow necessarily represents a separatereleases with interimpool dredgingto maintainthe compromiseamong gravel loss, sand removal, and water vol- pool trap efficiency. ume. The alternativesediment rating curves produce a largersand The optionsfor sedimentmaintenance flows considered in transportand smallergravel transport than the Poker Bar thispaper are the volumeand releaserate for flushingwater curves.As a result,the alternativecurves give a smallerdura- and the numberand depthof dredgedpools. Artificial gravel 2920 WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS

0.35 No Pool Trapping PoolsDredged 1.0 rn

o 0.3

(/) o.25

,_

c 0.2

'• 0.15

o ß1• 0.1 o P 0.05

o I I 0.35

I:: 0.3

:3 0.25

'o 0.2

._ 0.15 o

.o 0.1 .... 0.2 km^3 o p 0.05 :. := •0.3 km^3 I PoolLocation 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 DownstreamDistance (m) DownstreamDistance (m) Figure 7. Along-streamdistribution of sandin the bed surfaceand subsurfaceas a functionof water volume andpool dredging for • = 150 m3/s.Estimates use the Poker Bar rating curves. Sand proportion in the surfaceand subsurface laye,r s given in the top andbottom panels, respectively. (left) the casewith no pool dredging;(right) dredging to a depthof 1 m belowsteady state depth. A strongbottleneck effect is evident for the no dredgingcase, for whichsand removal occurs only at the downstreamend. replacementis implicitlyconsidered by calculatingthe gravel providingmultiple exits from the channeland minimize gravel lossassociated with the otheroptions. Evaluation of the trade- lossif dredgedsediment is screenedand the gravelreturned to offsamong these options requires quantitative estimates of the the river.The locationof dredgedpools determines the spatial gravelentrainment, gravel transport, and sandremoval pro- distribution of sand removal. If an even distribution of sand ducedby a flushingrelease. removalis desired,pools must be locatedthroughout the To be useful,a methodfor evaluatingthe effectivenessof reach. differentflushing options must balance system complexity with For the Trinity River casea dischargemagnitude and dura- the sparsedata typicallyavailable for largegravel-bed rivers. tion just sufficientto entrain the coarseportion of the bed Sandand gravel transport are representedwith ratingcurves. surfaceis foundto maximizesand removal by dredgedpools Approximatemethods are introducedfor estimatingpool sed- and minimize gravel lossbut at the cost of reducedsand re- iment trapping,upward supply of sandfrom the bed subsur- movalmagnitude and efficiency.Final specificationof a flush- face, and the rate of gravelentrainment. These are combined ing releasemust balance the relativecosts of water, pool in a sandrouting algorithm to estimatesand removal, which, dredging,and artificialgravel replacement in the trade-offbe- together.with an estimateof gravelloss, provide the basisfor tween sand removal efficiencyand gravel loss.The method evaluatingthe costand effectivenessof differentcombinations developedhere provides a basisfor thiscomparison. of flushvolume, discharge, and pool dredging.Application of Alternativesets of sedimentrating curves were used to eval- thesemethods is developedfor the Trinity River, California. uate the sensitivityof the flushingresults to the estimatedsand The optimum magnitudeof a sedimentmaintenance dis- and graveltransport rates, to whichconsiderable uncertainty chargeis a compromise.Larger dischargesproduce more ef- may often be assigned.The total amountof sandand gravel ficientsand transport and allowfiner-grained sediment to be removedand thereforethe volume of the flushingrelease entrained from the bed subsurface but also increase the down- requiredto reducethe sandcontent by a specifiedamount, is streamloss of gravel,reduce the trap efficiencyof dredged sensitiveto the estimateof sandand graveltransport rates. pools,and cause a largerproportion of poolstorage to be filled Other resultsare lesssensitive, including the observationthat with gravel. Smaller dischargesproduce favorable selective a flushingdischarge of moderatesize minimizes gravel loss, transportof fine sedimentbut do not producethe entrainment maximizesthe amountof sandtrapped in dredgedpools, and of the gravel surfaceneeded to loosenthe bed structureand maximizesthe efficiencyof sandremoval. flush sand from the subsurface. A superiorsolution may be obtainedif poolsare dredgedto Acknowledgments.This studywas funded by the U.S. Fish and actas sediment traps. Pools increase sand removal efficiency by WildlifeService (USFWS), Trinity River Flow Study, under coopera- WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS 2921 tiveagreements with the Johns Hopkins University (14-16-0001-91514) Hey, C. R. Thorne,and P. Tacconi,pp. 141-155,John Wiley, New andthe University of California(14-16-0001-91515), the latter admin- York, 1992. isteredby the Center for EnvironmentalDesign Research. Additional Leopold,L. B., Sedimentsize that determines channel morphology, in fundingwas provided by the Southern California Edison Company and Dynamicsof Gravel-BedRivers, edited by P. Billi, R. D. Hey, C. R. byan award for ResearchExcellence from the Pacific Gas and Electric Thorne,and P. Tacconi,pp. 297-311, John Wiley, New York, 1992. Companyadministered by the Universityof CaliforniaCenter for Leopold,L. B., M. G. Wolman,and J.P. Miller, in Water and Wildland Resources.The developmentof the methods Geomorphology,W. H. Freeman,New York, 1964. presentedhere benefited from discussions with Andy Hamilton, Mark Ligon,F. K., W. E. Dietrich,and W. J. Trush,Downstream ecological Hampton,Scott McBain, John Pitlick, and Bill Trush.The USFWS effectsof ,a geomorphicperspective, Bioscience, 45(3), 183- officein Weavervilleprovided useful information for ourstudy, as did 192, 1995. the TrinityRestoration Associates in Arcata,California. Review com- Lisle,T. E., Particlesize variations between bed load and bed material mentsby RogerKuhnle and Tom Lisle improvedthe clarityand in naturalgravel bed channels,Water Resour. Res., 31, 1107-1118, accuracyof the text. 1995. Milhous,R. T., Effectof sedimenttransport and flow regulation on the ecologyof gravelbed rivers,in Gravel-BedRivers, edited by R. D. References Hey, J. C. Bathurst,and C. R. Thorne,pp. 819-841,John Wiley, New York, 1982. Andrews,E. D., Effectiveand bankfulldischarges of streamsin the Milhous,R. T., Calculationof flushingflows for graveland cobble bed YampaRiver basin, Colorado and Wyoming, J. Hydrol.,46, 311-330, rivers,in HydraulicEngineering, vol. 1, Proceedingsof the 1990 Na- 1980. tionalConference, edited by H. H. Changand J. C. Hill, pp.598-603, Ashida,K., and S. Egashira,Mechanisms of armoringphenomena, Am. Soc.of Civ. Eng., New York, 1990. paperpresented at Seminar4, Armoringand Grain Sorting, 23rd Parker,G., andA. J. Sutherland,Fluvial , J. Hydraul.Res., 28(5), InternationalAssociation for HydraulicResearch Congress, Ottawa, 529-544, 1990. Canada, 1989. Reiser,D. W., M.P. Ramey,and T. A. Wesche,Flushing flows, in Beschta,R. L., andW. L. Jackson.The intrusionof fine sedimentsinto Alternativesin Regulated River Management, edited by J. A. Goreand a stablegravel bed, J. Fish.Res. Board Can., 36, 204-210,1979. G. E. Petts,pp. 91-135,CRC Press,Boca RatOn, Fla., 1989. Carling,P., The concept of dominantdischarge applied to twogravel- Wathen,S. J., R. I. Ferguson,T. B. Hoey,and A. Werrity,Unequal bed streamsin relationto channelstability thresholds, Earth Su• mobilityof graveland sand in weaklybimodal river , Water ProcessesLandforms, 13, 355-367,1988. Resour.Res., 31, 2087-2096, 1995. Carson,M. A., and G. A. Griffiths,Bedload transport in gravelchan- Wilcock,P. R., Sedimentmaintenance flows: Feasibility and basisfor nels,J. Hydrol.N. Z., 26(1), 1-151, 1987. prescription,paper presented at the FourthGravel-Bed Rivers Church,M., J. F. Wolcott,and W. K. Fletcher,A testof equalmobility Workshop,U.S. Fishand Wildlife Serv., Oreg. State Univ., Gold in fluvialsediment transport: behavior of the sandfraction, Water Bar, Wash.,Aug. 1995. Resour.Res., 27, 2941-2951, 1991. Wilcock,P. R., G. M. Kondolf,A. F. Barta,W. V. G. Matthews,and Diplas,P., and G. Parker,Pollution of gravelspawning grounds due to C. C. Shea,Spawning gravel flushing during trial reservoirreleases finesediment, Proj. Rep. 240, St. Anthony Falls Hydraul. Lab., Univ. on the TrinityRiver: Field observationsand recommendationsfor of Minn., Minneapolis,1985. sedimentmaintenance flushing flows, final report submitted to U.S. Frederiksen,Kamine, and Associates, Proposed Trinity River basin Fishand Wildlife Serv. Trinity River Flow Study, 222 pp., Lewiston, fishand wildlife manag6•ment program, Appendix B, Sedimentand Calif., Feb. 1995. relatedanalysis, report prepared for the U.S. Departmentof Inte- Wilcock,P. R., A. F. Barta, C. C. Shea,G. M. Kondolf,W. V. G. rior Water and Power ResourcesService, Sacramento, Calif., 1980. Matthews,and J. C. Pitlick, Observationsof flow and sediment Hill, M. T., W. S. Platts,and R. L. Beschta,Ecological and geomor- entrainmenton a largegravel-bed river, WaterResour. Res., this phologicconcepts for instreamand out-of-channelflow require- issue. ments,Rivers, 2, 198'210, 1991. Hirano,M., Riverbed degradation with armoring, Proc. Jpn. Soc. Civ. A. F. Barta, IntermountainResearch Station, U.S. ForestService, Eng., 195, 55-65, 1971. 316E. MyrtleSt., Boise, ID 83702.(e.mail: [email protected]) Holly,F. M., andJ.-L. Rahuel, New numerical/physical framework for G. M. Kondolf and W. V. G. Matthews, Center for Environmental mobile-bedarmoring, 1, Numericaland physicalprinciples, J. Hy- DesignResearch, University ofCaliforiiia, 390 Wurster Hall, Berkeley, draul.Res., 28(4), 401-415, 1990. CA 94720.(e-mail: køndølf@ced'berkeley'edu) Jackson,W. L., and R. L. Beschta,A model of two-phasebedload P. R. Wilcock,Department of Geographyand Environmental En- transportin an Oregoncoast range stream, Earth Su• Processesgineering, The JohnsHopkins University, Baltimore, MD 21218. Landforms,9, 517-527, 1982. (e-mail:wilcøck@j hu'ms'hcf'jhu'edu) Kondolf,G. M., and P. R. Wilcock,The flushingflow problem:De- finingand evaluating objectives, Water Resour. Res., 32, 2589-2599, 1996. Kuhnle,R. A., Fractionaltransport rates of bedloadon Goodwin (ReceivedFebruary 5, 1996;revised May 20, 1996; Creek,in Dynamicsof Gravel-BedRivers, edited by P. Billi, R. D. acceptedMay 24, 1996.)