North American Journal of FisheriesManagement 5:330-339, 1985 ¸ Copyrightby the AmericanFisheries Society 1985

Review of Ecological Effects of Rapidly Varying Flows Downstreamfrom Hydroelectric Facilities1,2

ROBERT M. CUSHMAN

Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, Tennessee37831

ABSTRACT Rapid changesin flow below hydroelectricfacilities result from peaking operations,where water is typicallystored in a reservoirat night and releasedthrough turbines to satisfyincreased electrical demandduring the day. Potential impactsof these short-term,recnrring disturbances of aquatic systemsbelow are important considerationsin hydropower development.Rednced biotic productivityin tailwaters may be due directly to flow variationsor indirectly to a variety of factors relatedto flow variations,such as changesin water depth or temperature,or scouringof sediments. Many riverine fish and invertebratespecies have a limited range of conditionsto which they are adapted.The relativelyrecent pattern of daily fluctuationsin flow is not oneto which mostspecies are adapted;thus, such conditionscan reducethe abundance,diversity, and productivityof these riverine organisms.Information needsfor site-specificevaluations of potential impacts at hydro- electric peaking projectsare outlined,along with managementand mitigation optionsto reduce anticipated adverse effects.

In typical hydroelectric peaking operations, irrigation (Sartoriset al. 1981), flushingof res- water is storedin reservoirsat night when elec- ervoir sediments(Gray and Ward 1982; Hesse trical demand is relatively low and then is re- and Newcomb 1982), spillsduring springfloods leasedthrough turbines during the day to satisfy (Rugglesand Watt 1975), and shutdownsfor re- increasedelectrical demand. Generally, there are pairs (Gore 1977). one or two releaseseach weekday, and discharge In this paper, I will summarize (1) observed at other times is essentiallyzero (leakageonly) effects on fisheries and fish-food organismsin or at a regulatedminimum (Fig. 1). These large tailwatersbelow peaking facilities, (2) underlying and rapid (within minutes)changes in discharge mechanismsresponsible for thoseeffects, (3) fac- result in correspondingchanges in flows in tail- torsto be consideredin an evaluationof potential waters(streams below the dams).Associated with adverseeffects at a given site, and (4) some pos- changesin flow are changesin other variables sible designor operationalchanges that may re- (e.g., depth, width, velocity, water temperature, duce those effects. and quality). Potential impacts of these short- term, recurring disturbancesof aquatic systems OBSERVED EFFECTS below dams are important considerationsin hy- dropower development (Hildebrand and Goss Published studiesof biological effectsof rap- 1981). There are other causesof rapid changes idly varying flow below hydroelectricfacilities in dischargefrom dams in addition to hydro- are summarized in Table 1. It is apparent that electric peaking, although they may not occur the authors differ in how they have chosento repeatedlyin a given season,such as releasesfor quantifythe extentof variation. Flow fluctuation may be describedon the basisof changesin flow, riffle area, velocity, depth, or wetted substrate. Specificityas to the time periods over which the • Publication No. 2362, Environmental SciencesDi- fluctuationoccurs or the recurrenceperiodicity vision, Oak Ridge National Laboratory, Oak Ridge, likewise varies among authors. Tennessee. 2 A condensedversion of this paper appearedin Vol- Fluctuationsin flow resultingfrom peakingop- ume 3 of the Waterpower '83 conferenceproceedings erations have been associated with reductions in (1983), pages1274-1283 (TennesseeValley Authority, river productivity(Radford and Hartland-Rowe Knoxville, Tennessee). 1971), specificallyin terms of tailwater fisheries

330 EFFECTS OF RAPIDLY VARYING FLOWS 331

(Powell1958; Fraser 1972; Trotzky and Gregory 20,000 1974; Beckeret al. 1981), aquaticplants and bot- tom-dwelling invertebrates on which the fish populationsdepend (Powell 1958; Fisher and ,•o 15,000 LaVoy 1972;Trotzky and Gregory 1974; PNRBC 1974; Covich et al. 1978), and wildlife that de- pend on the biologicalproductivity of the river O 10,000 for food or cover (PNRBC 1974). DAILYMEAN A numberof variableshave beenused by bi- ologiststo documentthe reducedproductivity or • 5,000 O carryingcapacity of affectedtailwaters. Trotzky and Gregory(1974), citingunpublished data, re- ported a decline in the rainbow trout (Salmo 0 5 10 15 20 gairdneri) sport fishery in the upper Kennebec PERIOD DF TIME (hours) River (Maine) from rapid flow variations.Powell (1958) found that tailwaterbrown trout (Salmo Figure 1. Typical daily dischargeregime from trutta) had a low mean conditionfactor. Fraley a hydroelectricpeaking facility (from Hilde- and Graham (1982) reported that fluctuating brand and Goss 1981). flows interfered with reproductionof kokanee salmon(Oncorhynchus nerka) by reducingegg and alevin survival.Fraser (1972) alsonoted that to colonizehabitats in rapidly varyingflows than fluctuatingflows reduced the survival of sal- in unregulatedflows. monid eggs. UNDERLYING MECHANISMS Benthicorganisms have beenreported to dem- onstratereduced species diversity, density,bio- Interaction of Hydraulic Variables mass,mean individual weight, and "quality" (as When flow varies, a number of other stream trout food) as a result of rapidly varying flows variablesmay be affected,including velocity, (Powell 1958; Fisher and LaVoy 1972; Trotzky depth,width, and wettedperimeter (the distance and Gregory 1974; Abbott and Morgan 1975; along the stream bottom from one shorelineto Covich et al. 1978; Williams and Winger 1979). the other). Cross-sectionalgeometry will be the On the other hand, Abbott and Morgan (1975) primary determinant of the interaction among found that a fluctuating flow regime causeda thesevariables (Brusven and Trihey 1978);thus, relative increasein the importanceof "tolerant" the empiricalrelationship between discharge and (not defined)species. Other researchersalso have velocityis site-specific(PNRBC 1974). For ex- foundcertain species to be affectedselectively by ample,Kraft (1972) reportedthat, whenflow was fluctuatingflows. Trotzky and Gregory (1974) reduced90% in a well-definedstream channel, reportedreduced populations of insectssuch as velocitydecreased 71 to 85% while surfacearea Rhyacophila, Chimarra, Iron, Periidac, Elmi- and averagedepth decreasedless than 42%. Wil- dac, Blepharicera,and Rhithrogena,while Par- liams and Winger(1979) alsofound that velocity aleptophlebia,Alloperla, and Chironomidae in- wasaffected more by a given changein flow than creased. Similar, Williams and Winger (1979) was width or depth. The percent of stream bot- found that certain mayflies, stoneflies,and cad- tom characterizedas "deep fast," "pool," "rif- disflies(Epeorus, Acroneuria, Glossosoma, Arc- fle," "flow shallow," or "exposedbottom" also topsyche,Rhyacophila, Dolophilodes, and Phil- will vary with flow (Brusvenand Trihey 1978). opotamidae) were adversely affected while A number of authorshave observedthat pro- chironomids were favored. Powell (1958) re- ductive riffle areas are particularly affected porteddecreased populations of mayflies,stone- (floodingor drying)by changesin flow (Briggs flies, and caddisflieswhile Gislason(1980) also 1948; Neel 1963; Abbott and Morgan 1975). In found that the relative abundanceof mayflies an extreme case, a stream that at normal flow decreased,but that dipterans(primarily chiron- containsmostly fast-waterhabitats may at low omids) representeda greaterpercentage of the flow consistmostly of pools; runs can be more community. Gersich and Brusven (1981) ob- affected(in terms of surfacearea, depth, and cov- servedthat it took benthoslonger (66 vs. 47 days) er) than poolsby a drop in flow (Kraft 1972). 332 CUSHMAN

Table 1. Some published studiesof effects of rapidly varying flows below hydroelectricpeaking facilities.

Studylocation Characterizationof flow variationa Reference North Fork of the Clearwater Flow 30 to >300 m3/second;A depth 0.3-0.6 Gersich and Brusven 1981; Brus- River, Idaho m/day over a 1-2 hour period; A wetted pe- yen and Trihey 1978 rimeter 68 m ConnecticutRiver, Massachusetts A depth 1.0 m, 10 km below Fisher and LaVoy 1972 KennebecRiver, Maine Flow <8.5-170 m3/second; -'/4 of bottom de- Trotzky and Gregory 1974 watered; velocity 0.1-0.5 m/secondat bot- tom, with a 4-fold increasein < 1 hour Grand River, Oklahoma A depth -<2 m, "rapid" Covich et al. 1978 Lower KananaskisRiver, Alberta A depth 32 cm in 0.5-4 hours Radford and Hartland-Rowe 1971 Green River, Utah-Colorado A depth 10-65 cm in 24 hours Pearson and Franklin 1968 SnakeRiver, Idaho-Oregon- A depth >0.3 m in 24 hours PNRBC 1974 Washington TennesseeRiver system /x flow 100x in "minutes" TvA 1978 , Washington A depth "several"m in 24 hours,"rapid" Becker et al. 1981 Blue River, Colorado /x flow 52 mVs in < 1 minute;/xdepth > 1.2 m Powell 1958 TennesseeRiver tributaries, Ten- "Extreme fluctuationin velocity and volume of Pfitzer 1954 flow" Obey River, Tennessee A depth 3 m, increasingin a "few minutes";de- Parsons 1955 creasingin "severalhours" St. John River, New Brunswick "Rapid alterationsin flow" Ruggiesand Watt 1975 CaneyFork River, Tennessee A depth 9 m; -<63% lossof riflte habitat Abbott and Morgan 1975 system, Diurnal flow 7.5-300 m3/second Hauer and Stanford 1982 Flathead River system,Montana A depth 2.1 m Appert 1980 FlatheadRiver system,Montana A flow 5-258 mVsecond;A depth -<2.5 m daily Fraley and Graham 1982 SturgeonRiver, Michigan Flow 0.4-17.5 m3/second,twice daily; 67% of Evans 1979 streambed dewatered SavannahRiver, Georgia-South < 10-688 m3/secondin -3 hours;A depth 1.7 Matter etal. 1983a, 1983b Carolina m SkagitRiver, Washington Monthly mean daily A depth -<0.9 m Gislason 1980 change.

One direct and obvious consequenceof in- Alternating Torrent and Pond Conditions creasedvariability is that the daily rangebetween Many specieshave evolved to tolerate, and minimum and maximum flow, velocity, depth, indeed require, either torrential flows of well- width, et cetera may increase over the corre- oxygenatedwater or low flowsof perhapspoorly spondingunregulated range. Thus, the minimum oxygenatedwarm water but not both. Behavior- depthbelow a peakingfacility may be lower than al, physiological,and morphologicaladaptations the normal unregulatedminimum and the reg- to theseconditions are all involved.Many stream ulated maximum depth may be greaterthan the insects,for example, require water currents for unregulatedmaximum (Fig. 2). The range of renewal of their oxygen supplies(Hynes 1970). physicalhabitat conditionsexperienced by the In a similar manner,maytties of the family Hep- biota may be greaterin regulatedthan in unreg- tageniidaecan withstand torrents but not very ulated streams over short periods of time. This low flows(Ward and Short 1978), while fivehhne could pose a particular threat to net-spinning fishesmay experiencethermal and oxygenstress caddisflies,for example,that require specificve- in pools formed by dewatering (Becker et al. locitiesfor food capture(Radford and Hartland- 1981). Conversely,pool speciessuch as the drag- Rowe 1971; Alstad 1982). Lack of a hydraulic onfly Ophiogomphusand the caddisfly Pycno- equilibrium violatesa major assumptionof the psycheare adapted for low-flow conditionsbut InstreamFlow IncrementalMethodology (Bovee may be unable to maintain their position in a 1982) developedby the U.S. Fish and Wildlife strongcurrent (Trotzky and Gregory 1974). Sta- Service to evaluate instream flow requirements bility of ttow at a certain time of year may be of aquaticbiota and could complicatethe use of critical. Fish in the Columbia River evolved to suchmethodologies in protectingtailwater fish- toleratea springspate, followed by relativelysta- eries(Loar and Sale 1981). ble late-summerand fall flows;the hydroelectric EFFECTS OF RAPIDLY VARYING FLOWS 3 3 3 flow regime in that river is an unnatural modi- fication(Becker et al. 1981). Fish migrationmay 1.0• • ABOVERESERVOIR be disrupted by very high or low flows (Neel 0 BELOW RESERVOIR 1963; Fraser 1972). Fisher and LaVoy (1972) have likened fluctuatingflow regimesto an in- tertidal situation (cf. river-ocean estuaries)to which freshwater biota have not evolved. .... _ _ Stream organismscan be strandedas waters recede.A number of authors(Powell 1958; Neel 1963;Pearson and Franklin 1968;Coming 1970; Fisher and LaVoy 1972; Kroger 1973; PNRBC 1974; Bauersfeld 1978a, 1978b; Becker et al. 1981;Extence 1981) have described stranding of invertebratesand fish. The accompanyingmor- tality may be due to a number of factorsinclud- Figure 2. Water depth aboveand below a peak- ing dewateringof isolatedpools and desiccation, ing facility on the Blue River, Colorado(mod- lack of food, low dissolvedoxygen, high tem- ified from Powell 1958). peratures,and predation by birds and mammals (Powell 1958;Nee11963; Coming 1970;PNRBC 1974; Beckeret al. 1981). High mortality of sal- the native chinooksalmon (Oncorhynchus tsha- toohid eggsin dewateredredds may be due to wytscha)fry in the Cowlitz River could be thermalstress, insufficient oxygen, or desiccation strandedand die from flow fluctuationsduring (Fraley and Graham 1982). Strandingis more of the rearingseason. Becker et al. (1981) foundthat a problemwhen there are gentlysloping shores the possibilityof strandingfish was increased or bars (PNRBC 1974; Brusven and MacPhee when (1) flow decreasedat night (becauseescape 1976; Bauersfeld 1978a), but substratecompo- wasmore difficu10,(2) flow decreasedafter a high sition,elevation above a fiver, bank storage,and discharge(because flooded shore areas provided flow from springsall determine the dewatering pools),(3) therewas a rapid decreasein flow after potential of isolatedpools (Beckeret al. 1981). a peak (becausethere was less escape time), and Strandingmay occureven when licenserequire- (4) flow was very low (becauseof more depres- ments such as minimum flow and "ramping" sionsand potholesto trap fish). (discussedlater in this paper)rules are satisfied (Bauersfeld1978a; Nelson et al. 1978). Stranding Stimulation of Drift actually may limit the carryingcapacity of tail- Many invertebratesdrift downstream,leaving waters(Neel 1963). Sloughsoften representim- the substrateby activeand passivemechanisms. portant spawningsites (Becker et al. 1981) and Changesin the flow or water level have been are particularlysusceptible to dewatering. observedto increasedrift rates (Minshall and Not all taxa are equally vulnerable to being Winger 1968; Pearsonand Franklin 1968; Rad- stranded.For example, in a study of benthic in- ford and Hartland-Rowe 1971; PNRBC 1974; vertebrates,Extence (1981) found that Gam- Brusven and MacPhee 1976; Gore 1977; Cibo- marus pulex, Potamopyrgusjenkinsœ Valvata rowski et al. 1977; Beckerr and Miller 1982). piscinalis,Bithynia spp., Athripsodesaterrimus, Similarly, Brusvenand MacPhee(1976) found Hydroptila tineoides,and (citingpreviously pub- that increasingor decreasingthe flow in a di- fishedresearch) Heptagenia sulphurea were rel- versionchannel caused juvenile chinooksalmon atively sensitiveto stranding,while Lymnaea to emigrate.At a given site, the responseof in- and (citingpreviously published research) Elmis vertebratesto a changein flow may vary among aenea were resistant. Similarly, Pearson and differenttaxa; thus, Minshall and Winger(1968) Franklin (1968) found Baetis sp. to be resistant found that drift of Baetis, Ephemerella colora- and Simuliidae to be sensitive to stranding. densis,Oligochaeta, Dugesia, Nemoura cinc- Speciessuch as the stoneflyPteronarcella badia tipes,Simulium, Cinygmula,Rhyacophila, and which migratesinto shallow,fiver-edge areas to Chironomidae increased, while drift of Neo- emergemay be particularly vulnerable(Kroger thremma,Dixa, Pericoma,and Stratiomyidae 1973). Bauersfeld(1978a) estimatedthat 60% of decreased.Increased drift, particularly if it oc- 334 CUSHMAN cursduring the daytime,could increase feeding inhibitsthe developmentofmacrophytes (Fisher activityby fish(Minshall and Winger 1968;Brus- and LaVoy 1972). In shallow meanderingchan- ven and MacPhee 1976). However, if this in- nels,especially, variations in water level will be creaseddrift were to continuefor a long time, associatedwith variations in wetted perimeter the benthoscould be depleted (Minshall and (Hauer and Stanford1982) sothat reductionsin Winger 1968; Brusvenand MacPhee 1976;Gore these variables may concentratestream organ- 1977), resultingultimately in lower fishproduc- ismsin a narrow channel(Radford and Hartland- tivity. Matter et al. (1983a) calculatedthat drift Rowe 1971), resultingin increasedmortality. lossescaused by peaking operationscould rep- Brusvenand Trihey (1978) found that only sub- resentalmost 14% of the benthicstanding crop stratesconsistently submerged for at least28 days in a month's time in a 12.5-km tailwater reach, would supporta productivebenthic community; but that inputs of zooplankton,Chaoborus, and while midgeswould recolonizeareas above the fishlarvae from the reservoirsupplemented the low-water mark when flow and river stage in- tailwaterfood base. Changes in depth,width, and creased,some rewettedareas still would not be velocityhave all been implicatedin the stimu- recolonized several hours later. lation of drift (Radfordand Hartland-Rowe 1971; Gore 1977; Ciborowskiet al. 1977), as has the Reservoir Phenomena erosionof newly depositedsilt with its resident Sometailwater effectsof rapidly varyingflows insects(Pearson and Franklin 1968). may be explained on the basis of processesoc- curringin the impoundmentthat affectthe char- Organic Materials acteristicsof dischargedwater. For example,rap- Changesin water level or flow may causeben- id changesin reservoirwater level associated with thic algae to die or break loose (Powell 1958; peakingoperations can increase mixing and pro- Neel 1963; PNRBC 1974; Hauer and Stanford ductivity (Paulsonet al. 1980) and reduce nu- 1982). Gislason(1980) found the amount ofpe- trient trap efficiency,thereby increasing nutrient dhphytonchlorophyll to be inversely related to inputsto tailwaters(Hildebrand 1980) and pos- exposureto desiccation.A significantchange in siblyincreasing downstream productivity. Peak- the type of organicmatter availablein the stream ing operationsalso can increaseoutflow of silt, could have direct effectson insectpopulations clay,organic matter, and (if at night)some aquat- (suchas caddisfly or mayfly larvae) that feed on ic insects(Hildebrand 1980). Thus, an under- the algae (Powell 1958; Radford and Hartland- standingof eventsin the impoundmentmay be Rowe 1971; Hauer and Stanford 1982), and in- necessaryfor an evaluationof potential effects direct effectson higher trophic levels. on tailwater biota. Hildebrand (1980) presented Coarseparticulate organic matter suchas leaf a thoroughsummary of the effectsof water-level packs, bark, and twigs provides important mi- fluctuations on reservoirs. crohabitatsand food sourcesfor many stream organisms.Rapid flow variations reduce the Sediments abundance of such matedhals(Ward and Short Retention of sediments by impoundments, 1978; Matter et al. 1983a)and of organismssuch coupledwith fluctuatingdownstream velocities, asthe stonefly Nemoura which inhabitsleaf packs can result in altered patterns of suspendedand (Radford and Hartland-Rowe 1971). bed sedimenttransport which, in turn, can affect tailwater biota. As a result, frequent flow vari- Fluctuating Water Level ationsmay delay the reachingof an equilibrium A number of changesin stream habitat and in the rearrangementof downstreambed mate- productivity may be attributed to fluctuating dhalsand combat the solidification of littoral areas water levels. For example, ice may be kept bro- (Neel 1963). There may be cyclesof deposition ken up and mineral coatingsmay build up on and erosion of sediment (Pearsonand Franklin watefiinerocks and sand(Neel 1963). Suchmin- 1968; Rugglesand Watt 1975), along with col- eral-coated stonesprovide a smaller number of lapse and erosion of banks (Hildebrand 1980) benthic microhabitats and hold less detritus than and erosion of sand bars (PNRBC 1974). Sedi- do substratescovered with filamentous algae ment-free dischargecan be aggressivelyerosive (Spenceand Hynes 1971). Fluctuatingwater level (Hildebrand1980). In additionto velocity,rate EFFECTS OF RAPIDLY VARYING FLOWS 335 of the rise in water levels has been cited as a causethey burrow into the substrateduring ad- determinantof the erosivecapacity of stream- verse periods (Trotzky and Gregory 1974; flow (Hildebrand 1980). Brusvenand MacPhee 1976; Brusvenand Trihey Slugsof turbid water can be abrasiveto biota 1978; Ward and Short 1978). Thus, Trotzky and (Radfordand Hartland-Rowe1971; Ruggles and Gregory(1974) foundthat a rapidly varyingflow Watt 1975), and benthosin downstreamreaches regime causedan increasein the abundanceof can be smothered when suspendedsediments Paraleptophlebia,Alloperla, and chironomids. settle out (TVA 1978). Standsof macrophytes Extence (1981) indicated that pulmonate (air- can increasetheir "hydraulicdrag" while grow- breathing)snails and uncased(relatively mobile) inguntil they are uprooted by increasingvelocity, caddisfly larvae were relatively resistant to resultingin a large amount of suspendedsedi- strandingmortality. Moth, caddisfly,and dip- ment as the disturbed bottom substrate and root- teran larvae were found to be more resistant to associatedsediments wash away (TVA 1978). strandingthan were mayflies(PNRBC 1974); the Even naturalpatterns of fishpredation on insects ability to burrow into algal mats or under cobble may be affectedby changesin substratecom- contributed to this resistance.Thus, the rapid positionand suspendedsediment loads (Brusven flow fluctuationsmay allow a communityof rel- and Rose 1981). atively resistant speciesto replace the natural speciesassemblage. Mature fish may be lesssu- Water Quality and Temperature ceptibleto strandingmortality becausetheir hab- Rapid flow variationsmay be accompaniedby itat preferencemay shift from shallow,shoreline rapid fluctuationsin water quality and temper- waters to riffle-pool areas in the main channel ature, particularlywhen there is a hypolimnial (Bauersfeld1978a). discharge.As a result, slugsof dischargedwater Ward (1976) notedthat adverseeffects of daily may be different from downstream water with flow fluctuationscould be overcome by more respect to not only temperature but also dis- seasonal-flowconstancy, attributing this phe- solvedoxygen, hydrogen sulfide, ammonia, iron, nomenon to a more stable substrate. Even the and manganese.Abbott and Morgan (1975) re- fluctuatingconditions themselves could promote ported "rapid thermal fluctuations"of up to 5 C an increaseddiversity if such conditions alter- below a hydroelectric dam in Tennessee,while nately favored different species,allowing in- Matter et al. (1983a) found that temperatureand creasedniche overlap(Ward and Short 1978). dissolvedoxygen dropped rapidly by 7 C and 4 rag/liter, respectively,4.5 km below a dam in INFORMATION NEEDS FOR SITE-SPECIFIC Georgia-SouthCarolina after a peakingsurge was EVALUATIONS released.Tailwater biota are affectedby both the Existinginformation does not permit a quan- magnitudeand rate of changeof suchwater qual- titative predictionof the effectsof rapidly vary- ity variables.The effectwill be moderatedby a ing flow at a given hydroelectricsite but it is return to equilibrium as the tailwater flow is possibleto list factors that cause tailwaters to warmed by sunlightand reaerated,and by trib- experiencegreater or lesserimpacts. These fac- utary inflow (TVA 1978). tors determine the minimum information needs for the site-specific evaluations that are sum- EXPLANATIONS FOR RESISTANCE TO OR marized below. MODERATION OF FLOW EFFECTS Certain groupsof organismsor their life stages DischargeRegimes are relatively resistantto effectsof rapid flow Historical data are neededon dischargein the variations. For example, some chironomidsare tailwaterreach, in termsof both long-termvari- opportunisticinvaders of suchsituations and are ations(monthly, annual)and short-termfluctua- able to tolerate changesin flow and water level; tions (hourly and daily). Thesenatural flowswill specieswith high fecundityand ability to dis- then be compared with the anticipated flow re- persewould be especiallyfavored (Covich et al. gimesfor correspondingtime periodsunder hy- 1978). Others,such as the stoneflyAlloperla, the droelectricregulation. As discussedearlier, while caddisflyCheumatopsyche, or the mayfly Para- rapid flow variations are expected to disrupt leptophlebia,may persist or even flourish be- stream biota and reduceproductivity, a reduc- 336 cusi•^•q tion in this variation on an annual time scale tions (e.g., burrowing into the substrate)of the may compensatefor this effect. tailwaterorganisms throughout the yearwill per- mit an evaluationof their susceptibilityto rap- Channel Morphometry idly varyingflow. Data on the cross-sectionalconfiguration(s) of the tailwatersare necessaryto derive a relation- MANAGEMENT OPTIONS shipbetween changing flow and changingveloc- If analysesof site-specificfactors indicate that ity, depth, width, and wettedperimeter. This in- rapidly varying flow may adverselyaffect tail- formation also is required to estimate the water biota or if impacts at a site have been variations in the extent of rifle and pool habitat observed,three major areasof managementare at differentflows, and the likelihoodof stranding availableto minimize theseimpacts: operational in isolated pools and sloughs. changes,structural changes, and habitat modi- fication. Discharge Quality Data are neededon the expectedquality (tem- OperationalChanges perature,dissolved oxygen, iron, manganese,hy- One approachto reducethe adverseeffects of drogensulfide, ammonia, suspended solids, and rapid flow variation is to specifyan upperlimit nutrients) of the water to be releasedfrom the to the amount of variability of one or more of hydroelectricfacility, and the variability of that the physicalor chemical characteristicsof the quality both seasonallyand as a function of dis- tailwaters.Among the rulesthat have been pro- chargerate. This information will require a pre- posedor adoptedare limitations on the change diction of the limnologicalcharacteristics (biotic in downstream water depth per unit time and abiotic)of the impoundmentcreated for hy- (PNRBC 1974; Nelson et al. 1978), percent droelectricregulation. changein wettedperimeter per unit time (Nelson et al. 1978), changein watertemperature per unit Tributary and Spring Inflow time (TVA 1978), and changein dischargeper Tributary inflow to the tailwaters will mod- unit time as a functionof preexistingdischarge eratethe variability and physico-chcmicaleffects or "ramping" (Table 2) (Bauersfeld 1978a). of flow changes(Appcrt 1980; Gislason 1980). Maintaining a small, instantaneousminimum Tributariesalso serveas both a sourceof organ- dischargeat facilities that otherwisewould have isms for rccolonization of stressed areas and a virtually zero dischargealso has been proposed refugefor tailwaterbiota duringadverse periods. as a means of maintaining a constanttempera- Consequently,biological, flow, and water-qual- ture,increasing reaeration, and maintaininga wet ity data are neededon any tributariesto the tail- substrate(TVA 1978). Althoughcontrol of flow waterreach. Similar data also are needed on spring fluctuationhas been implementedat a number flows near potentially isolated pools. of sites(Nelson et al. 1978), there may be una- voidableconflicts with hydroelectricgeneration, Channel Composition floodcontrol, irrigation, and navigation(Nelson Information on the physicalcomposition (es- et al. 1977; Nelson et al. 1978; TVA 1978). pecially particle size) of both streambedsub- Bauersfeld(1978b) found that increasingthe strates and bank materials is needed. This in- number of hours when no more than a minimum formation will contribute to an understandingof flow was maintainedduring the peak salmon (1) erosionand sedimentationpotential and (2) spawningseason reduced the number of redds the role of bank storageboth in facilitatingbank establishedin areasthat eventuallywould be de- erosionand moderatingthe phenomenonof pool watered. This action apparently reduced later dewatering. mortality but it may have dewatered existing reddsand reducedtotal spawningarea, thus lim- Present Biota iting production. Informationwill be needed,also, on the species compositionof the tailwatercommunity. In par- Structural Changes ticular, data on environmentalrequirements (e.g., Re-regulating dams (smaller dams down- velocity, temperature, dissolved oxygen), mo- stream from the primary hydroelectricfacility bility (includingdrift), and behavioral adapta- that releasewater at a fairly constantrate com- EFFECTSOF RAPIDLY VARYING FLOWS 337 pared with flow into their relatively small im- Table 2. Example of "ramping" (from Bauers- poundments)can stabilize flows farther down- feld 1978a). stream, thus benefitting fish and wildlife Maximum permissible populations (Nelson et al. 1978; Anon. 1983). Dischargebefore change changeper 30 minutes For example,Gray Reef Dam on the North Platte (mVsecond) (m3/second) River, Wyoming,increases minimum flow below 210-280 27 Alcova Dam more than three-fold (Nelson et al. 160-210 21 1978). As an added benefit, re-regulatingdams 110-160 17 80-110 10 also can generateadditional electricity (Nelson 60-80 5 et al. 1978). Iron Gate Dam in California was built to protect salmon and steelhead(Salmo gairdneri)from flow fluctuationsbelow two hy- droelectric dams and also it can generate 19 ies of rapid flow variation are neededthat relate megawatts(Nelson et al. 1978). Effectsof rapidly varying flow attributable to pulsesof deoxygen- ecologicaleffects (from aquatic plants and in- ated releases from stratified reservoirs can be vertebratesthrough fish and wildlife) to quan- minimized by destratification,aeration of either titative characterizationof the hydraulic vari- ables. Data also are needed on the habitat the reservoiritself or the discharge,or by the use requirementsof many species(if not already of a submergedweir, flexible curtain barrier, or multilevelintakes(TVA 1978). Structuralchanges available) and on the effectsof rapid flow vari- may involve a major capital expenditure,how- ation on suchbehavioral aspectsof streambiota ever. suchas drift, colonization,and migration. Ulti- mately, it shouldbe possibleto protect and en- Habitat Modification hance downstream productivity while utilizing The extent of continuously wetted substrate available hydroelectricpotential. can be increasedby manipulatingthe cross-sec- ACKNOWLEDGMENTS tional geometry of the stream channel (TVA I thank Glenn F. Cada, Charles C. Coutant, 1978). However, suchmodifications likely would be achievedonly with a concomitantloss in hab- James M. Loar, and two anonymousreviewers itat diversity if, for example, productive riffle- for their review of this manuscript.Research was pool sequenceswere replacedwith more uniform sponsoredby the Geothermal and Hydropower substratecontours. Shirvell and Dungey (1983), TechnologyDivision, U.S. Department of En- for example, demonstrated the importance of ergy,under Contract No. DE-AC05-840R21400 microhabitat diversity to brown trout in six New with Martin Marietta Energy Systems,Inc. Zealand streams. REFERENCES CONCLUSIONS ABBOTT,T. m., AND E. L. MORGAN. 1975. Effects of an hydroelectricdam operationon benthic macro- Increasingnational attentionis beingdirected invertebrate communities of a tailwater stream. towardsthe installationof new hydropowerfa- Associationof SoutheasternBiologists Bulletin 22: cilitiesor the retrofittingof existingdame for 38. hydroelectricgeneration. Peaking operations can ALSTAD,D.N. 1982. 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