Long- Term Dynamics of an Invertebrate Assemblage Downstream from a Large Dam

LONG- TERM DYNAMICS OF AN INVERTEBRATE ASSEMBLAGE DOWNSTREAM FROM A LARGE DAM

MARK R, VINSON!

National Aquatic Monitoring Center, Department of Fisheries and Wildlife, Utah State University, .. Logan, Utah 84322-5210 USA

Abstract. A century of hydrologic data (1895~1999) and 50 yr of aquatic macroin• vertebrate assemblage data (1947-1999) were examined for two tailwater reaches of the Green River downstream from Flaming Gorge Dam in northeastern Utah, USA (40°54' N, 109°25' W). One reach was located upstream of an intermittent tributary, and the other downstream. The purpose of the study was to chronicle long-term dynamics and the effect of partial thermal restoration on invertebrate assemblages. The immediate hydrologic effect of the dam was a large, decrease in annual maximum daily discharges, water temperatures, and sediment transport. Upstream of the intermittent tributary, macroinvertebrate genera declined from >70 to <30, and the mean macroinvertebrate density increased from 1000 to 10 000/m2 after dam closure. In 1978, a multilevel water intake structure was installed on the dam. Mean summer water temperatures increased from 6° to 12°C, and the number of annual degree days warmed from 2340 to 3200, which was similar to pre-dam conditions; but the rate and timing of warming remained different than before the dam, In contrast to an expected increase in taxon richness, the number of taxa routinely collected after partial thermal restoration was similar to or lower than that observed before thermal restoration. Downstream of the intermittent tributary, taxon richness was about twice that found up• stream after dam closure, and the mean annual per-sample taxon richness increased from 7.6 to 11.0 following partial thermal restoration, In both reaches, invertebrate densities were near 10000/m2 before and after thermal restoration. The lack of an appreciable increase in insect taxon richness upstream from an inten,nittent tributary following partial thermal restoration is likely due to the combined effects of three factors: (I) the competitive dominance of insect taxa byamphipods, (2) low rates of im• migration and colonization, and (3) low reproductive success of insects due to a few degrees difference in the water temperatures between the regulated river and natural streams in the area. These results suggest that we should not only evaluate traditional habitat attributes, but biological interactions as well, when determining or monitoring the effects of river regulation on aquatic biota. Key words: Amphipoda; aquatic insects; aquatic invertebrates; Diptera; Ephemeroptera; Gam• marus lacustris; Hyallela azteca; invertebrate biodiversity; regulated river ecology; river restoration; water temperature.

INTRODUCTION life span we need data on how ecological communities Most large rivers in the northern hemisphere (>30° respond over longer time periods. Data from short-term. studies almost universally N latitude) are no longer free flowing (Dynesius and Nilsson 1994), and Petts (1989) estimated that 60% of point to changes in physical habitat, most often water the world's total stream flow is likely to be regulated temperature, as being primarily responsible for the de• cline in invertebrate biodiversity that normally follows by the year 2000. Heightened public awareness of the dam construction (Lehmkuhl 1972, Ward 1974, 1982, effects of dams on aquatic biodiversity and on eco• Ward and Stanford 1979, 1982, Brittain and Saltveit system function, has made rehabilitation of regulated 1989). The role of limited dispersal capabilities of rivers a research priority (Naiman et al. 1995, Poff et aquatic invertebrates, colonization dynamics, and bi• al. 1997). Although the life span of most dams is > 100 otic interactions in limiting aquatic insect diversity in yr, most of our knowledge of the effects of dams on regulated rivers have received little attention; although stream ecosystems is from studies typically lasting 1• Power et al. (1995, 1996), Stanford et al. (1996), and 3 yr, These short-term studies are inappropriate for Wootton et al. (1996) have described some ways that long-term planning purposes (Petts 1980). To predict these factors may influence biota in regulated rivers. and mitigate the effects of dams throughout their long In this paper, I summarize long-term trends in the discharge, temperature, and assemblage structure of Manuscriptreceived4 JaQuary1999;revised23 December 1999;accepted28 February2000;finalversionreceived13April aquatic invertebrates in the Green River at a site cur• 2000, rently influenced by Flaming Gorge Dam. Hydrologic

1 E-mail: aqua@cc,usu.edu data were collected nearly continuously from 1895.

711 712 MARK R. VINSON Ecological Applications Vol. 11, No.3

TABLE I. Summary of aquatic macro invertebrate sampling in the Green River near Flaming Gorge Dam.

summerspring,HessSurbermonthlyallspring,HessSurberSurberallspring,summerallpermonthsqualitative,Surbersummersumnier,months(timing)tripsperSamplingmonthsnetSamplesDRC;methodunknownnet:!:Samplingsum-stationnetPgenus/species,familyfamily/genus/spe-ciesgenus,oE(no./yr)locationstripsnetURC;DRC;netfallresolutiontsum-Sampling4-6date1-31-2fallTaxonomicspecies'speciesspeciesunknown438Streamnet:!:I12-20IC264-52-81-3Bosleyunpublished§BinnsVVhiteDibbleEdmundsVVoodburyPearsonMusserthisHodenJohnson12Sessionsor3familyI2574,326C0-25I121,I,176So'urcekbd2kbdfamily6,study12kadkbdreach105,kad(1965)et12,andkadsummer(1960)kbd(1960)(1960)(1967),etetDCal.andmer,andal.(1963)al.Cristunknown(1966)MusserGaulinfall family(1960)(1960) larvae and(1987) subfamilylarvaeadults and (1981) 1978-198019811977-19941954,1963-1966196219671950,1959,1952, DRC;1993-1999131,16Yearskbd2614kbdkbd 19591958-19591962-1964DRC;1960 26 kbdmer, fall stationadultslarvaelarvae and family1964,adults 1965,. 1959SaltInGorge§adults(1968)1993,R.:!:tLakeForTaxonomicDenton,DamsamplesdescriptionCity,(seeunpublishedUtah,Fig.wereresolutionseeUSA.1).collectedMerrittdataabbreviations:inonandDecember,benthicCumminsCmacro=andChironomidae;1996.invertebratesin 1998-1999Dfromsamples= Diptera;the GreenwereE =collectedRiverEphemeroptera;downstreamin January,0from=April,Odonata;FlamingJuly, PandGorge= Plecoptera.October.Dam, 1947,1948, DRC; 27 kbd Notes: URC or DRC = upstream or downstream from Red Creek; kad or kbd = distance in km above or below Flaming

Data on aquatic macroiiwertebrate assemblages were utary of the Colorado River. It originates in the Wind available before the construction of a large hypolim• River Range in Wyoming, and flows south to its con• netic release dam (1947-1962; Dibble 1960, Edmunds fluence with the Colorado River in southern Utah (Fig. and Musser 1960, Musser 1960, Sessions and Gaufin 1). The elevation of the river at the base of the dam is 1960, Woodbury 1963, Binns 1965), immediately after 1835 m above sea level. The contributing drainage ba• dam closure (1963-1968; Pearson 1967, Pearson et al. sin is 7500 km2. Construction of Flaming Gorge Dam 1968), and following the installation of a multilevel began in 1959 and was completed in December 1962 water intake structure on the dam (1978-1999) that (see Plate la). The dam rises 149 m above the original raised mean summer water temperatures 6°C (Table 1). river channel. The dam is principally used for hydro• The primary objective was to evaluate the long-term electric power generation and flood control. It creates responses of invertebrate assemblages to see if the data a 146-km long, 1.5 million-ha3 capacity reservoir. support current ideas developed from much shorter• Two small « 1 m3/s) perennial tributaries, and a term observations. I was specifically interested in de• large intermittent tributary, enter the Green River 2.4, termining if the aquatic insect diversity increased after 12, and 18 km downstream from the dam (Fig. 1). Red the 1978 partial restoration of thermal conditions. Creek is an intermittent tributary that causes several changes in the physical and biotic nature of the river. STUDY SITE One or more sediment mobilizing flash floods occur This study was conducted on the Green River near annually in Red Creek in response to summer thun• Flaming Gorge Dam in northeastern Utah (40°54' N, derstorms. Suspended sediment concentrations up to 109°25' W, Fig. 1). The Green River is the largest trib- 87 g/L have been measured in Red Creek, and its con- June 2001 LONG-TERM REGULATED RIVER ECOLOGY 713

PLATE 1. (a) Flaming Gorge Dam on the Green River in Daggett County, Utah, was completed in December 1962. The dam rises 149 m above the river bed and has a crest length of 392 m. (b) Gammarus lacustris (upper) and Hyallela azteca (lower). Maximum size of G. lacustris and H. azteca in the Green River downstreamfrom Flaming Gorge Dam are 21 and 12 mm, respectively.

tribution of sediment into the Green River is estimated the studies listed in Table 1. Between 1993 and 1999, to be 77 X 106 kg/yr (Andrews 1986). Upstream of water temperature was measured every 3 h with Hobo Red Creek, the Green River flows through a deep bed• Temp electronic temperature recorders (Onset, Pocas• rock canyon. Within the canyon, the river ranges in set, Massachusetts, USA) adjacent to the dam and 26 width from 70 to 150 m, and in depth from one to 10 km downstream from the dam. m. Bed material in riffles consists of coarse gravel, cobbles, and boulders. Downstream from Red Creek, Macroinvertebrate assemblages the river leaves the canyon and enters Brown's Park Methods and data sources used in the analysis are Valley. River width increases to 200 m, and the depth summarized in Table 1, and data are available through decreases to 0.5-2 m. Bed material in the upper reaches Ecological Archives (see Appendix). The number of of Brown's Park consists of gravel and cobble overlain samples collected each year varied among studies; from by seasonal deposits of sand and silts exported from one to four stations per stream reach, one to eight sam• Red Creek. In downstream reaches, bed material is ples per station per date, and one to 20 sampling dates composed almost entirely of sand. per year. Upstream of Red Creek, pre-dam data for METHODS Ephemeroptera (Edmunds and Musser 1960), Plecop• tera (Sessions and Gaufin 1960), and Odonata (Musser Hydrology 1960) were from a single site 25 km upstream from Streamflow, water temperature, fluvial sediment, and the dam, near Hideout Draw (Fig. 1). Edmunds sampled water chemistry data were obtained (1) from United this site repeatedly between 1947 and 1960 (Edmunds States Geological Survey stream gages at Green River, and Musser 1960). Other pre-dam data were from sev• Wyoming (Station 09216500), located 240 km up• eral sites located between Green River, Wyoming, and stream from Flaming Gorge Dam for the years 1895• Red Creek (Bosley 1960, Dibble 1960, Binns 1965, 1906 and 1916-1927; (2) at Bridgeport, Utah (Station and White et al. 1966). Post-dam data were collected 09235000), 28 km downstream from Flaming Gorge at five sites located 1-16 km downstream from the dam Dam for the years 1912-1915; (3) near Linwood, Utah (White et al. 1966, Pearson 1967, Pearson et al. 1968, (Station 09225500), 50 km upstream from the dam for Holden and Crist 1981, Johnson et al. 1987). Down• the years 1928-1950; and (4) near Greendale, Utah stream from Red Creek, pre-dam data were collected (Station 092345000), 0.8 km downstream from the dam' by Woodbury (1963) the summer before dam closure for the years 1950-1999 (Fig. 1). Only small tributaries in 1962, at locations 74, 105, 131, and 144 km down• enter the Green River between gages so differences in stream from the dam (Fig. 1). Post-dam macroinver• discharge among gages were slight «7% among years tebrate collections were made at two sites located 26 where data was collected at more than one location). and 27 km downstream from the dam (White et al. Instantaneous high, low, and mean annual discharges 1966, Pearson 1967, Holden and Crist 1981; and R. and temperatures were compiled for each water year Denton, unpublished data). of record where available. Additional water tempera• Data analysis ture and water chemistry data, and descriptions of the pre-dam environment were compiled from Madison The study was divided into the two study reaches and Waddell (1973), BoIke and Waddell (1975), and (upstream and downstream from Red Creek) and into 714 MARK R. VINSON Ecological Applications Vol. II, No.3

Flaming Wyoming Gorge Reservoir Utah

......

f Dinosaur j National

...... 144 ~.~.~JMonument

FIG.!. Location of pre-dam (open circle) and post-dam (solid circle) aquatic macroinvertebrate sampling stations, and USGS streamflow gaging stations (triangle) on the Green River near Flaming Gorge Dam. The abbreviations"kad" and "kbd" refer to locations km above the dam or km below the dam.

three hydrologic periods; (1) before the closure of RESULTS Flaming Gorge Dam (December 1962), (2) after dam closure and before the installation of multilevel water Physical environment intake structures on the dam (January 1963 to June Pre-dam environment.-The pre-dam Green River 1978), and (3) after installation of multilevel water exhibited the extremely variable hydrology typical of intake structures (July 1978 to December 1999). Only most unregulated Rocky Mountain rivers (Poff and insects and amphipods were included in the macroin• Ward 1989). Annual (Fig. 2) and seasonal (Figs. 3 and vertebrate assemblage estimates. Large variation in the 4) variation was high for all measured hydrologic at• completeness of the invertebrate assemblage collected, tributes. Annual peak discharges normally exceeded and in the level of taxonomic resolution among studies 300 m3/s during spring runoff, and minimum discharges warranted the exclusion of other non-insects (e.g., mi• were <10 m3/s during winter (Fig. 2). The mean annual crocrustaceans, annelids, and molluscs). Taxon rich• discharge was 56 m3/s for the years 1928-1962. A rapid ness was calculated as the mean number (± 1 SE) and vernal increase and autumnal decrease (Fig. 4) char• cumulative number of aquatic insect and amphipod taxa acterized pre-dam annual water temperatures. The an• that were collected for all sampling dates each calendar nual range in instantaneous daily water temperatures year within each stream reach. Changes in macroin• was 0°-26°C (Fig. 2). Mean monthly water tempera• vertebrate assemblage composition were evaluated by tures ranged from ~2°C in February to 18°C in August calculating the percent of the mean total abundance (Fig. 4). The mean annual water temperature was within the major taxonomic groups (Amphipoda, Co• 12.8°C, and the mean annual accumulated number of leoptera, Diptera, Ephemeroptera, Plecoptera, and Tri• degree days was 3140 for the years 1956-1963. There choptera) within each stream reach for each calendar were no data available on the diel range in water tem• year. peratures before the dam, but upstream of the dam at FIG. 2. Annual maximum (upper line), minimum (lower line), and mean (middle line) annual discharge and water tem• perature for the Green River near Flaming Gorge Dam. Maximum and minimum annual discharges and temperatures are instantaneous values. Annual means were calculated from mean daily values.

250

200 "if 1150

OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

FIG. 3. Mean monthly discharge (± I SE) for the Green River 0.8 km downstream from Flaming Gorge Dam before (circle) and after (triangle) closure of Flaming Gorge Dam in 1962. 716 15 MARK R. VINSON Ecological Applications Vol. II, No.3 :s Cii~(]) 0 ~E~ 105 25 020 - URC, Pre-dam, 1956-1962 - +- URC, Post-dam, 1963-1978 ..•.. URC, Post thermal restoration, 1979-1999 ..•.. URC, Post thermal restoration, 1993-1999

:0:: '.~ ''--'~~. 'Ie :£:" . ':sz:. : : .

OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

FIG. 4. Mean (:!:I SE) monthly water temperatures for the Green River for three time periods and two locations, 0.8 km (URC, upstream from Red Creek) and 26 km (DRC, downstream from Red Creek) downstream from Flaming Gorge Dam: 1957-1962 was before the closure of Flaming Gorge Dam; 1963-1978 was after dam closure and prior to partial thermal restoration of the dam's water intake structure; 1979-1999 was after installation of the selective water withdrawal structure. For the site downstream from Red Creek, data were only collected during 1993-1999.

Green River, Wyoming (USGS gaging Station 1984, 1986, 1997, and 1999, flows of 388,244,227, 09216500, Fig. 1), the diel range was 1°_2°C in the 244, and 297 m3/s were released through the dam's winter and 2°_4°C in the summer for the years 1994• bypass tubes because of high spring runoff (Fig. 2). 1999 (M. R. Vinson, unpublished data). The concen• Post-dam water temperatures were characterized by tration of major water chemistry constituents varied increased constancy. Between 1963 and 1978, instan• widely among seasons and years (e.g., conductivity, taneous maximum daily water temperatures were typ• Fig. 5; also see Irons et al. 1965). Before dam closure, ically 8°_10°C and minimums were generally 3°_4.5°C entrainment of coarse bed particles probably occurred (Fig. 2). The diel range in water temperature was <2°e. a few days a year in this section of the river (Graf Mean monthly temperatures during these years ranged 1980, Andrews 1986) and fine sediments (sands and from 4°C in March to 9°C in November (Fig. 4). Max• silts) likely overlaid coarser cobble substrates during imum water temperatures occurred three months later low flow periods (Topping et al. 2000). Aquatic mac• and minimum temperatures one month later than in the rophytes were scarce. pre-dam river. The mean annual number of degree days Post-dam environment.- The major effect of the dropped 25% from 3140 to 2340. Downstream from dam on hydrology was a large reduction in the amount Red Creek, near the Bridgeport stream gage, the annual of annual and seasonal variation in discharge, sediment range in water temperature was 0°_14°C between 1963 transport, and water temperature, and a several month and 1966. change in when high and low discharges and water The concentrations of all major water chemical con• temperatures occurred. Maximum daily discharges de• stituents increased immediately following dam closure clined from >300 m3/s to <140 m3/s, minimum flows (1962-1968; Madison and Waddell 1973), and then de• increased from < 10 m3/s to >20 m3/s (Figs. 2 and 3), clined rather rapidly between 1968 and 1972 (BoIke and their was no year-to-year consistency in when high and Waddell 1975). The long-term trend since 1972 and low flows occurred throughout the year (Fig. 6). has been a gradual decrease in concentration of all Prior to dam closur~, maximum flows generally oc• major chemical constituents, and a gradual increase in curred around mid-June and minimum flows in late pH (Fig. 5). winter; after dam closure, maximum or minimum flows After dam closure, coarse bed particle movements occurred with little seasonal regularity. Mean annual were mostly eliminated (Graf 1980, Grams 1997) and discharge was little affected by the dam, except for the suspended sediment discharge decreased from 3270 X first year after dam closure when the reservoir was 106 kg/yr to near zero from the dam downstream to being filled and the mean discharge dropped to 6.5 m3/s Red Creek (Andrews 1986). The size range of substrate (Fig. 2). For the first 16 yr following dam closure, particles appeared similar to pre-dam conditions (An• hourly fluctuations of 60 m3/s were common in re• drews 1986; M. R. Vinson, personal observation of sponse to electrical power demands. After 1973, max• U.S. Bureau of Reclamation pre-dam photographs ar• imum hourly discharge fluctuations were limited to 11 chived at Flaming Gorge Dam, and pre- and post-dam m3/s, daily fluctuations to 135 m3/s, and the minimum photographs in Stephens and Shoemaker 1987), but streamflow was increased from 11 to 23 m3/s to im• cobble sized particles were likely more armored (Wil-. prove habitat for rainbow trout. Five notable large dis• liams and Wolman 1984), and the seasonal fine sedi• charge events occurred after dam closure. In 1983, ment deposits thought to occur before dam closure 8.01200 25060 717 c ::i. :::JJune- 350200200130040 LONG-TERM REGULATED RIVER ECOLOGY .,0!§ro en01 150 '" 4001500100350 1955 ~~I E 8.57.06.5150208000 ~ 9.0 . I ...... ••••.••••••••••••• ...... ---.. - -. - .. -- ...... - --.------. -- -...... - - . - ...... 'J'ft:1~~~~·~'~/~·

., ~..~, _ ...... ,~ .-...... ---.-...-.-_ .- -. .. .- .. 1! .,..;-:-:-.. -I.;:. ---:.-:-:.=:.-- .••- • - - .:-:,"- .. .s: .

Magnesium ...... o . 0 Calcium fo~~~o5"09_~ : 0

1960 1965 1970 1975 :1980 1985 1990 1995

Post dam Post thermal FIG.5.. Trends in water chemistry concentrations in the Green River, 0.8 km downstream from Flaming Gorge Dam.

(Topping et al. 2000) were eliminated. In response to were common in sand deposits in pools and eddies. the increased streambed stability and high water clarity, Downstream from Red Creek, aquatic vegetation was the bryophyte Amblystegium, and the green algae much less abundant and consisted primarily of Chara. Chara and Cladophora covered most cobble and boul• Upstream of Red Creek, channel width has changed der substrates within a few years following dam clo• little since dam closure, but the elevation of the stream• sure. Gravel and smaller sized particles both upstream bed aggraded ~9 cm at the USGS gage site, 0.8 km and downstream from Red Creek generally lacked any downstream from the dam (1. Schmidt, personal com• substantial plant growth other than epilythic diatoms. munication). Downstream from Red Creek, some chan• The aquatic macrophytes Elodea and Potamogeton nel narrowing has occurred. Andrews (1986) found 718 15090 MARK R. VINSON Ecological Applications C/)== '">-'" 60 ...., :;0.c~(jj 120 Vol. II;No. 3 0 CD0<1J 30 1930 >- 0 180 Q Jan Q Feb or Dee

Mar or Nov

Apr or Oct

:0 Mayor Sep o 0 ;:-0 Jun or Aug 6 Jul 1940 1950 1960 1970 1980 1990 2000 FIG.6. Date of occurrence of the maximum(dotted line) and minimum(solid line) annual flowsfor the GreenRiver, 0.8 km downstream from Flaming Gorge Dam. Dates were standardized as the number of days before or after 1 July. Prior to 1962, maximum flows generally occurred within 30 d of I July, and minimum flows occurred in late winter. After dam closure, high and low flows occurred during all months.

bank full channel width decreased an average of 13% monthly water temperatures was 2.8°-13.6°C. The diel (171 to 148 m) at 24 cross sections located 30-65 km range in water temperature was 1°-2°C in the winter downstream from the dam between 1951 and 1980. and 2°_4°C in the summer. The mean annual temper• Further downstream, Lyons et al. (1992) found an ature was 8.3°C, which was nearly identical to that ~ 1%-3% change in channel width among aerial pho• measured 26 km upstream. The mean annual number tographs taken in 1952, 1963-1964, 1974, 1978, and of degree days was 2907, 9% less than that measured 1986, 161-193 km downstream from the dam, and near the dam. Grams (1997) found a 20% decrease in width, 225• 270 km downstream from the dam (Fig. 4). Pre-dam invertebrate assemblages Post-thermal restoration, 1978-1999.-In 1978, a multilevel water intake structure was installed on the Prior to dam closure, this section of the Green River dam to allow warmer waters to be withdrawn from the supported a diverse aquatic invertebrate fauna that was dominated by insects (Table 2). Edmunds described the reservoir during the summer to primarily enhance sal• Ephemeroptera fauna as being one of the most diverse monid growth. After 1978, mean summer water tem• to exist worldwide (Edmunds and Musser 1960, Ed• peratures increased from 6° to ]2°C and instantaneous munds 1973). Twenty-one genera and more than 30 maximum annual water temperatures increased from 9° to 15°C and occurred in July as compared to November species of mayflies were collected at a single site. Much less information is available on the diversity of other prior to partial thermal restoration. The number of an• nual degree days increased from 2340 to 3200 and was invertebrate groups prior to dam closure, but based on now similar to pre-dam conditions (3140), but the rate, collections in other streams in the region (reviewed by timing, and magnitude of warming remained different Ward et al. 1986), Ephemeroptera richness appears than before the dam. The similarity in annual degree much higher than the richness of other groups in these days to pre dam was only because of warmer winter mid-order, cold desert, streams. Data collect~d through• temperatures. Mean summer maximum temperatures out the 300-km study reach from Green River, Wyo• remained ~5°C less than pre dam. Minimum annual ming, to Dinosaur National Monument (Fig. 1) indi• water temperatures occurred in February and remained cated that there were likely few differences in inver• at 3°-4.5°C. The diel range in water temperature was tebrate assemblages upstream and downstream from < 1°C during the winter and 2°_3°C in the summer for Red Creek before dam closure. the years 1993-1999. The vernal increase and autumnal Few quantitative samples were collected before dam decrease in water temperatures were more rapid than closure. However, the macro invertebrate densities and before partial thermal restoration, but were still more composition reported for 1958-1959 (~1000 insects/ gradual than before dam closure. m2 and 56%-80% Ephemeroptera) were similar to those Water temperatures downstream from Red Creek (26 reported for other mid-order unregulated stream reach• km downstream from the dam) between 1993 and 1999, es in the Colorado River Basin (Ames 1977, Anneal' were generally warmer in the summer and slightly cool• 1980, Ward et al. 1986), and in the Green River, up• er in the winter than those measured just below the stream of Flaming Gorge Dam (Binns 1965) and down~ dam (Fig. 4). The annual range in instantaneous water stream from Flaming Gorge Dam's influence (Pearson temperatures was 1°-19°C and the range in mean et al. 1968). June 2001 Aquatic1963-19671981-1999tURCU+DDRCU+D1977-1981invertebrates<1962DRCU+DURC collectedLONG-TERMDRCURCURCURC'U+DDRCURCin the GreenREGULATEDURCU+DRiver near DRCU+DURCFlamingRIVERDRCU+DURC GorgeECOLOGYU+DURCDRCDamU+D719 during InsectaDryopidaeCollembolaElmidaeOstracodaIsopodaChironomidaeLumbriculidaeCrustaceaHeteroceridaeOligochaetaNaididaeCladoceraCulicidaeHemerodromiaLaraHyallelaZaitzeviaOrthocladiinaeBezziaOrdobreviaChironominaeaztecaparvulaDipteraLampyridaeTubificidaeProbezziaHeterlimnius MuscidaeAtherceridaeAtherixSimulium HaliplidaeHelodidaeCurculionidaeSimuliidaeHydrophilidaeHirudineaAgabusHelichusGyrinusHydrobius = Scirtidae AnthomGammarusColeopteraHydracarinayiidaelacustrisScathophagidae ArthropodaEphydridaeEmpididaePsychodidaeDytiscidaeCopepodaCeratopogonidaeBlephariceridaeGyrinidaeAnnelidaAtrichopogonCheliferaCoptotomusOptioservusTanypodinaePalpomyia websteri CleptelmisTaxa DolichopodidaeHydraenidae four majorAmphipodaChrysomelidaeLimnophoraBlepharicerastudy periods. = TABLE 2. Time period Ecological Applications 720 Continued.1963-1967ORC1981-1999t1977-1981U+OURC <1962ORCU+O ORCU+OURCURCORC MARKURC R. VINSONORCURCU+O ORCURCU+OORCURC EphemeropteraTabanidaeTipulidaeEphemeridaeAmetropodidaeOligoneuriidaePseudironidaeCaenidae.R.MolophilusD.HolorusiaB.EphemerellaE.PentageniaH.EpeorusDicranotaHexatomaDrunellaSerratellaR.TipulaHeptageniaTricorythodesCallibaetisTravereliainfrequensgrandisundulataalbertaeinsignificansinermisrobustagrandiselegantuladoddsispiniferalevisgrandisalbertanaEphemerellidaeminutusCloeon IsonychiidaeAntochaEphemeraCinygmulamonticolaLeptophlebiidaePo1ymitarcyidaeTricorythidaeBaetidaeHeptageniidaeSiphlonuridaeHexageniaB.RhithrogenaLeptophlebiaParaleptophlebiaE.H.CamelobaetidiusCaenistricaudatuslongimanuslimbata limbatagavasteliawarreni AmetropusLachlaniaAttenellaAme1etidaeStratiomyidaeEphoronSiphlonurusLeptohyphesBrachycercusIsonychiaBaetisAmeletusPseudironChoroterpesVol. albumIJ,'No.siccapowellialbrightialbiannulatacampestris3 Oiptera Taxa TABLE 2. Time period· June 2001 Continued.1963-19671981-1999t1977-1981DRCU+OURCU+DORC <1962U+DORCU+O LONG-TERMURCORCU+ODRCU+DURCU+DORCURC REGULATEDU+OURC ORCURCRIVERORC ECOLOGYU+OORCURCORCORCU+O721U+D OdonataTrichopteraMegalopteraPlecopteraPyralidaeGerridaeCoenagrionidaeTaeniopterygidaePteronaricidaePerlidaeHydropsychidaeChloroperlidaeArcynopteryxA.GerrisZapadaArgiaNemouraB.MicrasemaPerlestaI.AlloperlaIsogenoidesTaenionemaClaasseniaSkwalaI.ProtoptillaGlossosomaOligoplectrumelongatusebrialgrandisamericanusoccidentalisparalellaamericancinctipesplacidasabulosaPerlodidaeGlossosomatidaeIsogenusCapniaa CapniidaeMalenkaCultusGomphus LepidopteraNemouridaeNotonectidaeVeliidaeGomphidaeNotonectaArctopsycheMergarcysI.IsoperlaPetrophilaHesperoperlaOphiogomphusCuloptilapatriciamormona pacifica RhagoveliaAeshnidaeBrachycentrusCorixidaeOemeopteryxBrachycentridaePteronarcysCalineuriaCoCorydalidaeryda Ius californica Hemiptera Taxa TABLE 2. Time period Ecological Applications 722 Continued.1963-19671981-1999tDRCU+D1977-1981URC <1962U+DDRC URCU+DURCDRCDRC MARKURCU+DDRCDRCR. VINSONURCU+DDRCDRC DRCU+DURCURC MolluscaLepidostomatidaeLimnephilidaeHydroptilidaeLeptoceridaeRhyacophilidaePlanorbidaePhryganeidaePhysidaeHydropsycheAgrayleaMayatrichiaHesperophylaxR.PhysellaParapsycheNeotrichiaLeucotrichiaLepidostomaHydroptilaPsychoglyphaOchrotrichiaOecetisOnocosmoecusacropedesvepulsacoloradensisvagrita Pe1ecypodaelsisBasommatophoraAllocosmoecus SphaeriidaeAlisotrichia Po1ycentropodidaePhysaLeptocerusNectopsycheLymnaea however,ofLymnaeidaeRedRhyacophllaPolycentropusPisidiumtDuringGastropodaGyraulusCheumatopsycheCreek,Vol.onlythe11,duringU+DNo.3entire=1994-1999upstreamperiod 1981-1999,andweredownstreamcollectionscollectionsofmadeRedwereupstreamCreek.made ofdownstreamsRed Creek asof well.Red Creek; Notes:TrichopteraSee Table for sampling details.TaxaURC = upstream of Red Creek, DRC = downstream TABLE2. I Time period

Post-dam invertebrate assemblages numbers, kinds, and richness of taxa as those found Upstream of Red Creek.-Invertebrate taxon rich• immediately following dam closure (Fig. 7). The ap• ness declined swiftly following dam closure (Table 2; parent increasing trend in cumulative annual taxon Pearson et al. 1968). Ephemeroptera were decimated; richness over time is probably related to the increased richness declined from more than 30 species to a single number of samples collected by each consecutive study and an increase in taxonomic resolution for some common species, Baetis tricaudatus, and two rare spe• cies, Ephemerella inermis and Paraleptophlebia pal• groups, particularly for the years 1993-1999. lipes. The only other common insect taxa were Chi• Macroinvertebrate abundance upstream of Red roriomidae and Simuliidae. Just a few individuals Creek increased almost immediately after dam closure (Pearson et al. 1968) represented other taxa collected and then leveled off. Between 1963 and 1967, mean during these years (Table 2). annual invertebrate densities were 4280/m2. Between Samples taken 15-18 (1978-1981) and 30-36 1978 and 1981, mean annual densities ranged from (1993-1999) years after dam closure contained similar 10700 to 24400/m2. Between 1993 and 1999, mean June 2001 LONG-TERM REGULATED RIVER ECOLOGY 723

a .•. ·4070J'7

20 ..•..•..•..•. ..•. .•. ...: .•. ~ 10 ..•.. ~ 301 :o~o.o o~o~ ~oooooo J::: 0 . ()

~ b .••. ~ ~ .•. ~~I ...... 20+ ~:'" .••.•.. .•. "'oQ Q .•.

101 ~Q~ •• ~:oQ .•..•..•..•..•. 0 0 0OOQO 1 I' 1°, ....-' -' 1 -' -' -' I' 1 -' -' -' -' 1·,: -' I' I F I -' -' -' -' -' I' 1 -' -' -' -' -' I -' a -' -' -' • I 1960 1970 :1980 1990 2000

100000 T_ o :2 a 10000 -1= 9. Q a g; 2 0000000 1000-L n_ 0

~ 100 o .s 10 OJ g 100000

..6 10000 n Q a 00 000000 ~ 12 00 00 1000 :2 . 0 0 :0 0 0 OQ a ~100 id 10 I I 1960 1970 :1980 1990 2000 Post dam Post thermal

FIG. 7. Mean (:±: I SE, open circle) and cumulative (triangle) annual taxon richness (as given in Table 2) of aquatic insects and amphipods in the Green River near Flaming Gorge Dam, (a) upstream and (b) downstream from Red Creek; and mean annual abundance (:±: I SE) of aquatic insects and amphipods in the Green River near Flaming Gorge Dam, (c) upstream and (d) downstream from Red Creek. annual densities ranged from 8100 to 11 800/m2 (Fig. densities of G. lacustris were l31/m2 in 1978, 15971 7). m2 in 1979, 1830/rri2 in 1980, and 2930/m2 in 1981. The most dramatic change in the macroinvertebrate No benthic samples were collected upstream of Red assemblage following the initial adjustment to post• Creek between 1981 and 1992. When sampling re• dam conditions was a change in relative abundance sumed in 1993, the invertebrate assemblage was quite among the major taxon groups (Fig. 8). Immediately different (Fig. 8). Between 1993 and 1999, Amphipoda after dam closure (1963-1967), Diptera, principally accounted for 61%, Diptera for 32%, Ephemeroptera Chironomidae and Simuliidae, accounted for ~90% for 4%, and Coleoptera for 3% of the total invertebrate and Ephemeroptera, primarily B. tricaudatus, for 10% abundance. The majority of the amphipods were Hy• of the total macroinvertebrate abundance. Between ailela azteca, a taxon that had not been collected up• 1978 and 1981, Diptera accounted for ~65%, Ephem• stream of Red Creek before 1993, and was not found eroptera for 29%, and the amphipod, G. lacustris, for at the sampling site downstream from Red Creek until 6% .of the total macroinvertebrate abundance. Gam• 1988 (see Plate Ib). Gammarus lacustris densities de• marus lacustris, was the only taxon that initially ap• clined from 1354/m2 in 1978-1981 to 371m2 in 1993• peared to benefit from thermal restoration. Mean annual 1999, while H. azteca densities increased from zero to Post thermal MARKEcologicalR,ApplicationsVINSON '.." 'u)00..724() 2080 :1970198020001990 _rn±mESS:IAmphipodaP.lecopteraDipteraTrichoptera (]) 0... 0()EQj0 100 t b D c Vol. 11, No, 3 Post dam~ Coleoptera c 60 1960 ~ Ephemeroptera100 406020800

FIG, 8, Relativ'e mean annual contributions of Ephemeroptera, Coleoptera, Diptera, Plecoptera, Trichoptera, and Am• phipoda in the Green River near Flaming Gorge Dam, (a) upstream and (b) downstream from Red Creek, Data for 1960 are from Dibble (1960) and qualitative collections made from ~1947 to 1960,

6657/m2, Ephemeroptera densities declined from 44891 taxon richness was 7.6 before thermal alteration and m2 in 1978-1981 to 414/m2 in 1993-1999,99% of the 11.0 following (Fig. 7). mayflies were B. tricaudatus. However, between 1993 Immediately following dam closure, 1963-1967, and 1999, Amphipoda densities steadily decreased macroinvertebrate densities were highly variable, and from 10 008/m2 in 1993 to 5764/m2 in 1999, while were about half those found upstream of Red Creek Ephemeroptera densities steadily increased from 231m2 (Fig. 7). Since 1977 mean annual macroinvertebrate in 1993 to 1314/m2 in 1999 (Fig. 9). densities have been relatively stable and similar to Downstream from Red Creek.-Since dam closure, those found upstream of Red Creek (~1O 000/m2). Ex• the mean annual number of taxa per collection and the ceptions occurred during the flood years of 1983-1986 cumulative number of taxa collected each year below when mean annual densities averaged 3520/m2• Den• Red Creek was about twice that found upstream of Red sities did not decrease following the 1997 and' 1999 Creek, and unlike what happened upstream of Red floods. Creek, restoration of thermal conditions appeared to Changes in the invertebrate assemblage composition have increased taxon richness. Mean annual per sample since dam closure downstream from Red Creek have

2000 8 (]) - Amphipoda c () 12000 (\j c -0- Ephemeroptera (\j 10000 1600 -g "'0C :J :J ~ .Q FIG, 9, Mean Amphipoda and Ephemerop• 8000 1200 (\j N""' ~~ ~.§ tera abundance (:+: I SE) between 1993 and 1999 Q) , (\j 6000 upstream of Red Creek. Note that the y-axis "'0 0C _ 0 o~ 800 g- .s scales (left and right) are different. 0.. 4000 :.c Qj 0.. 400 E E 2000 Q) « .!::. o o ~ 1993 1994 1995 1996 1997 1998 1999 June 200] LONG-TERM REGULATED RIVER ECOLOGY 725 been subtler than those that occurred upstream. Be• thermal restoration of the river, the number of aquatic tween 1963 and 1967, the assemblage was dominated insect taxa routinely collected upstream of Red Creek by Diptera. Since 1981, few changes in community was as low or lower than before partial thermal res• composition have occurred (Fig. 8); Diptera, primarily toration (Fig. 7). The reasons underlying the lack of Chironomidae and Simuliidae, accounted for 42%, an increase in insect diversity are thought to be due to Ephemeroptera for 37%, Coleoptera for 14%, Trichop• (I) remaining differences in the discharge and water tera for 5%, and Amphipoda for 2% of the total as• temperature regimes compared to the pre-dam river or semblage abundance. In contrast to upstream from Red unregulated rivers in the region; (2) low rates of im• Creek, Ephemeroptera (3 1471m2) were more abundant migration and colonization due to continued isolation; than H. azteca (750/m2) between 1993 and 1999. Gam• (3) low reproductive success of insects; and (4) inter• marus lacustris averaged <11m2 between 1993 and specific competition between aquatic insects and H. 1999. azteca.

Losses and gains in the invertebrate fauna Physical controls Invertebrate genera believed to be extirpated from Though the number of annual degree days were sim• the reach of the river upstream of Red Creek include ilar between the pre-dam river and the post-thermal 15 mayfly genera (Ametropus, Brachycercus, Calli• restoration river, the annual maximum and minimum baetis, Camelobaetidius, Caenis, Ephemera, Pentagen• water temperatures, the rate of seasonal temperature ia, lsonychia, Choroterpes, Leptophlebia, Traverella, change, and the annual and diel variation in water tem• Lachlania, Ephoron, Pseudiron, and Siphlonun/s); five peratures remained different than the pre-dam river and stonefly genera (Alloperla, Claassenia, lsoperla, Per• unregulated rivers in the region. These remaining dif• lesta, lsogenoides); and three caddis fly genera (Lep• ferences in thermal conditions may be limiting some toeen/s, Nectopsyche, and an unidentified Phryganeid). insects that require specific thermal cues to complete Most taxa vanished from this reach soon after dam their development (Sweeney and Vannote 1978, Ward closure. Pearson et al. (1968) found all of these taxa and Stanford 1982, Newbold et al. 1994). Other phys• except Pentagenia, Pseudiron, and Claassenia down• ical factors commonly cited as playing a role in limiting stream from Red Creek (69-125 km below the dam) insects downstream from dams, such as discharge, sub• in 1964-1967. strate conditions, and water chemistry, do not appear New taxa found in the regulated river upstream of to directly explain the continued low diversity of in• Red Creek following dam closure include the amphipod sects in the reach between Flaming Gorge Dam and H. azteca, the mayfly Paraleptophlebia, the stoneflies Red Creek; however, the indirect effect streamflow has Arcynopteryx, Hesperoperla pacifica, Taneionema, Za• on other ecosystem properties, such as substrate qual• pada, and Oemeopteryx, and the caddis flies Hydroptila, ity, appears important. Leucotrichia, Oecetis, Hesperophylax, Psychoglypha, Warmer winter water temperatures.-Minimum and Rhyacophila. Except for Oemeopteryx, these taxa temperatures in this reach of the Green River remain have also been collected in cold water tributaries be• 2°_3°C warmer than before dam closure. These warmer tween the dam and Red Creek (Pearson 1967; M. R. temperatures can eliminate taxa through the loss of Vinson, unpublished data). Oemeopteryx has been col• physiological signals (Britt 1962, Hynes 1970, Le• lected further downstream and was likely part of the hmkuhl 1972), or by disrupting normal growth, fecun• pre-dam fauna (Baumann 1973). Except for H. azteca, dity, and emergence (Sweeney and Vannote 1978, Van• all of the new taxa were rare. Hyallela azteca likely note and Sweeney 1980, Newbold et al. 1994). The immigrated from downstream reaches of the Green Riv• need for a minimum winter chill to break insect egg er or alternatively could have migrated through the or larval diapause appears species specific and may be reservoir. Gammarus lacustris, H. azteca, Oligochatea, .a locally evolved trait dependent on the regional ten- and the insects Chironomidae, Simulium, and Baetis dencies of rivers to reach near freezing temperatures tricaudatus were the only taxa consistently found in all (Ward and Stanford 1982). For example, in a laboratory studies since dam closure. However, several taxa, such study, minimum temperatures < 1.3°C followed by a as Ephemerella inermis, Tricorthyodes minutus, Hes• period of lOoC were necessary for Ephoron album from perophylax, Brachycentrus, Rhyacophila, Hydroptili• the Great Lakes Region to complete its life cycle (Britt dae, Hesperoperla pacifica, and several genera of EI• 1962). Conversely, data for other insects and from other midae, and Tipulidae were represented by one or two parts of the world are more equivocal (Saltveit et al. individuals every couple of months, suggesting that 1994, Helesic and Sedlak 1995). The effect of 2°_3°C they also maintained resident populations in the river warmer winter water temperatures is likely an increase after dam closure. in the growth of aquatic insects throughout the winter, which might result in winter rather than spring or early DISCUSSION summer emergence (Ward and Stanford 1982). Winter The most interesting and significant finding of this daytime air temperatures at Flaming Gorge Dam are study was that after 20 yr of implementing a partial typically below or near freezing. Emergence during 726 MARK R. VINSON Ecological Applications Vol. II, No.3 most of the winter would either be lethal or impede probably not sufficient to directly restrict the estab• mating of aquatic insects (Nebeker 197 I), lishment of any insect taxa, the lack of high flows may Cooler summer water temperatures,-Similar to a be promoting non-insects, and the lack of seasonal pre• need for a specific minimum temperature, some insect dictability in high and low flows might be problematic taxa may require a maximum summer temperature to for taxa with well synchronized life histories, such as complete development (Newbold et al. 1994). Ward and mayflies (Vannote and Sweeney 1980). A typical prob• Stanford (1979) suggested that the lack of stoneflies lem in rivers that experience fluctuating discharges is downstream from Hungry Horse Reservoir, Montana, dewatering of emergence and oviposition sites. Up• was caused by the lack of attainment of a specific max• stream of Red Creek, the confined nature of the Green imum annual water temperature, and not due to a re• River channel limits the amount of channel that is ex• duction in annual degree days. Just after dam closure, posed during low flows, so dewatering is not likely to Pearson et a1. (1968) reported that lower summer water be a severe problem. temperatures associated with higher discharges in 1967 An important indirect effect of the reduced maximum caused a complete failure of a summer generation of flows and high water clarity has been the proliferation Baetis tricaudatus upstream of Red Creek. of aquatic plants. These plants provide refugia, detritus Slower warming of streams throughout the summer (Spence and Hynes 1971), and epiphytic diatoms can reduce fecundity of emerging adults (Clifford and (Shannon et al. 1994) for invertebrates capable of using Boerger 1974, Sweeney and Vannote 1978), exaggerate this type of habitat, such as amphipods, and may hinder the separation of male and female emergence (Nebeker invertebrates adapted to exposed inorganic substrates, 1971, VanGundy 1973), prolong the emergence period such as mayflies (Ward 1976, Ward and Stanford 1979, of individual generations (VanGundy 1973), and reduce Brittain and Saltveit 1989). The rate and magnitude of growth rates such that emergence might occur later in short-term flow fluctuations were much less in the years the year when air temperatures are suboptimal for mat• following partial thermal restoration, which may have ing. Prolonged emergence reduces the number of in• contributed to the dominance of amphipods. Following sects emerging at anyone time, which may increase floods in 1997 and 1999, amphipods decreased. their individual risk of predation by trout (Butler 1984, Limitations on dispersal and colonization Sweeney 1984). Thermal constancy,- The predictable annual and The native insect fauna of the Green River and fauna die I rise and fall of stream water temperatures appears presently found in its tributaries are adapted to warmer to strongly influence the life cycles of aquatic insects summer and colder winter temperatures, and more sea• (Sweeney and Vannote 1978, Newbold et al. 1994). sonally predictable flow patterns than those found in Thermal variance appears to maximize the number of the post-dam Green River. Insects that would likely species that can coexist (Vannote et al. 1980), with each find the post-dam Green River suitable habitat are those species able to optimize resource use during a partic• inhabiting constant temperature springs of which there ular subset of the annual or diel thermal cycle (Sweeney are few in the vicinity of Flaming Gorge Dam. How• 1978, Vannote and Sweeney 1980). In the absence of ever, in spite of the paucity of nearby springs, most of a variable temperature, increased competition for re• the taxa commonly found in springs in this region sources may occur to the detriment of some species. (VanGundy 1973, Gray et al. 1983; M. R. Vinson, un• Over the long term, these nonlethal processes that act published data) are present in the Green River, includ• to reduce fecundity and mating success may lead to ing Baetis, Ephemerella, Hesperoperla pacifica, Rhy• smaller populations, or even local extinction, where acophila, Hesperophylax, Hyallela azteca, Gammarus recruitment falls below a critical level (Sweeney 1978). lacustris, and flatworms. However, only Baetis and H. In contrast to insects, amphipods and other non-in• azteca developed robust populations downstream from sects do not leave the water to complete their life cycle Flaming Gorge Dam. and do not appear to require a minimum or maximum For insects, downstream drift is clearly the most im• critical temperature or a range of temperatures to com• portant pathway for colonizing downstream areas plete their development. They are commonly found in (Townsend and Hildrew 1976, Williams and Hynes high abundance in springs with nearly constant tem• 1976), and upstream aerial flights by adults are the most perature (Bousfield 1958, Pennak and Rosine 1976) and important mechanism for recolonizing upstream areas hypolimnetic release dams (Angradi and Kubly 1993). (Hershey et al. 1993). The reservoir unquestionably Smith (1973) found that G, lacustris was able to com• prevents downstream immigration of lotic insects into plete its life cycle and reproduce under constant tem• this reach of the river and small downstream population perature (18°C) in the laboratory, sizes may limit the amount of recruitment from up• Streamflow regime,-The role streamflow patterns stream flying adults. have played in influencing the post-dam aquatic ma• Biotic interactions croinvertebrate assemblage is not entirely clear. Al• though the magnitude of post-dam daily, seasonal, and The primary difference between invertebrate assem• annual discharge fluctuations are relatively low and are blages collected upstream of Red Creek in 1978-1981 June 2001 LONG-TERM REGULATED RIVER ECOLOGY 727 and those collected in 1993-1999 was the replacement Downstream from Red Creek of the mayfly, B. tricaudatus by the amphipod, H. az• The degree to which tributary streams lessen the in• teca. Downstream from Red Creek, H. azteca did not fluence of dams on mainstream rivers is probably a become a dominant member of the assemblage, and B. function of the size of the tributary and its distance tricaudatus densities remained high (>2000/m2). In from the dam (Stevens et al. 1997). The degree to which many temperate North American river systems, a re• tributary streams differ with respect to discharge, sed• duction in aquatic insect diversity and an increase in iment, and temperature patterns from the main stem amphipods have been observed following dam con• river is also important. Red Creek, though intermittent, struction (e.g., Hilsenhoff 1971, Spence and Hynes appears to cause several changes in the biota. Although 1971, Ward and Stanford 1979). The mechanism un• invertebrate abundance between the two stream reaches derlying these population shifts has been largely attri• was similar, taxon richness and evenness were higher buted to changes in water temperature. The hypothesis downstream from Red Creek where amphipods de• that amphipods may limit insects in some regulated creased and insects increased. rivers through competition or predation warrants fur• The factors underlying these assemblage differences ther investigation. Interspecific competition among in• are not entirely clear, but are likely related to the higher vertebrates in other streams is well documented (Allan disturbance frequency and reduced water clarity that 1995), and the Green River's combination of a highly limit aquatic macrophytes and filamentous algae, pro• predictable, low disturbance environment and low rates ducing conditions that do not support large numbers of of dispersal and colonization offers the greatest op• amphipods. In addition to fewer amphipods, slightly portunity for local population interactions among as• more natural environmental conditions may also allow semblage members (Palmer et al. 1996). Glazier (1991) insects to maintain populations in this reach. Differ• speculated that non-insect invertebrates likely limit in• ences in water temperatures between the two reaches sects in non-thermal springs. Amphipods are known to were mostly in the annual maximum water temperature, consume insect eggs and small instars (Embody 1912, ~ 14°C upstream and 19°C downstream -from Red Dick 1996), and Macan (1977) hypothesized that the Creek, and the rate of summer warming. Minimum an• absence of Ephemeroptera and Plecoptera from English nual temperatures still remain ~ 1°-2°C above freezing. lakes was a result of predation on their eggs by non• In the absence of large numbers of amphipods, warmer insect invertebrates. summer water temperatures did appear to benefit the Hyallela azteca' s dominance upstream of Red Creek insect assemblage, as the mean annual per sample taxon is probably attributable to its high reproductive capa• richness increased by 69% following partial thermal bilities and two major changes in the operation of the restoration in 1978. dam after 1978. Hyallela azteca typically has a 12-16 mo life span, may mate as often as every six days, Study implications produces between three and 30 young, and juveniles The findings of this study have implications for the reach maturity in ~ 1 mo (Cooper 1965, Strong 1972, design of monitoring projects and the management of de March 1977, Edwards and Cowell 1992). In this hypolimnetic release dams. Few changes in macroin• reach of the river, Baetis appears to have two genera• vertebrate assemblages were observed within each in• tions per year, a winter generation that emerges in June dividual 2-4-yr study, whereas large changes were ob• or July and a summer generation that emerges in Sep• served over the long term. For example, few changes tember (Pearson et al. 1968; M. R. Vinson, unpublished in macroinvertebrate assemblages were reported 1-3 yr data). Most of the other insects, excluding Chiron• following partial thermal restoration; however, data omidae, are univoltine. presented here show that warmer summer water tem• In 1978, mean summer water temperatures increased peratures dramatically changed the macroinvertebrate from 6° to 12°C (Fig. 4), and large variations in daily assemblage. discharge were curtailed. Hyallela azteca requires a My findings also support the ideas of Power et al. mean summer temperature> 10°C (Bousfield 1958) and (1995, 1996) that we should not only evaluate tradi• seems to prefer stable discharges. Since 1997, two tional habitat attributes (depth, velocity, and temper• floods have occurred and insect abundance has been ature), but ecological interactions as well. In the long substantially higher, and H. azteca abundance consid• term, it may be the relatively subtle and difficult-to• erably lower, than that observed between 1993-1996 measure nonlethal physical changes and biological in• (Fig. 9). This observation is consistent with the idea teractions that limit the ability of a species to maintain that flooding may have released insects from compe• robust or viable populations. tition with H. azteca. Gooch and Glazier (1991) also To reduce post-dam species losses it seems critical reported that following a disturbance in a Pennsylvania that flows and water temperatures be as close as pos• spring, Gammarus populations declined and the rela• sible to the natural riverine conditions soon after dam tive abundance of Ephemeroptera, Plecoptera, and Dip• completion (Poff et al. 1997). This seems especially tera increased. important in streams in arid landscapes where large, 728 MARK R. VINSON Ecological Applications Vol. II, No.3

. dams such as Flaming Gorge Dam on the Green River, Brittain, J. E., and S. J. Saltveit. 1989. A review of the effect Navajo Dam on the San Juan River, and Glen Canyon of river regulation on mayflies (Ephemeroptera). Regulated Rivers: Research and Management 3:191-204. Dam on the Colorado River (Blinn and Cole 1991, Butler, M. G. 1984. Life histories of aquatic insects. Pages Stevens et al. 1997, Schmidt et al. 1998) create highly 24-55 in V. H. Resh and D. M. Rosenberg, editors. The atypical habitat for most of the native fauna. These ecology of aquatic insects. Praeger, New York, New York, USA. lower basin dams have caused greater species losses Clifford, H. F, and H. Boerger. 1974. Effects of food supply than dams located higher in the basin (Ward 1976, Ward on the life history of Simuliidae (Diptera). Canadian Jour• and Short 1978, Stanford and Ward 1986, Rader and nal of Zoology 57:301-306. Ward 1988, Voelz and Ward 1991). Changes in dam Cooper, W. E. 1965. Dynamics and reproduction of a natural operations done long after dam construction may not population of a fresh-water amphipod, Hyallela azteca. be effective in restoring native species because the new Ecological Monographs 35:377-394. de March, B. G. E. 1977. The effects of constant and variable community may have considerable tolerance to new temperatures on the size, growth, and reproduction of the operations. freshwater amphipod Hyallela azteca (Sassure). Canadian ACKNOWLEDGMENTS Journal of Zoology 56:1801-1806. Dibble, C. E., editor. 1960. Ecological studies of the flora Financial and logistic support for this study was provided and fauna of Flaming Gorge Reservoir basin, Utah and by the Utah Division of Wildlife Resources, the U.S. Bureau Wyoming. University of Utah Anthropological Papers 48: of Reclamation, and the U.S. Bureau of Land Management. 1-243. I thank Ted Angradi, Don Archer, Dave Axford, Steve Bray• Dick, J. T. A. 1996. Post-invasion amphipod communities of ton, Todd Crowl, Charles Hawkins, Jeff Kershner, and Jack Lough Neagh, Northern Ireland: influences of habitat se• Schmidt for their assistance throughout the study, and Russell lection and mutual predation. Journal of Animal Ecology Rader, Jack Stanford, and anonymous reviewers for their ed• 65:756-767. itorial direction. Dean Blinn confirmed my amphipod iden• Dynesius, M., and C. Nilsson. 1994. Fragmentation and flow tifications, and Mary Barkworth identified the moss, Am• regulation of river systems in the northern third of the blystegium riparium. world. Science 202:629-631. LITERATURE CITED Edmunds, G. F, Jr. 1973. Trends and priorities in mayfly research. Pages 7-11 in W. L. Peters and J. G. Peters, ed• Allan, J. D. '1995. Stream ecology: structure and function of itors. Proceedings of the First International Conference on running waters. Chapman & Hall, London, England, UK. Ephemeroptera. Brill, Leiden, The Netherlands. Ames, E. L. 1977. Aquatic insects of two western slope riv• Edmunds, G. F, Jr., and G. G. Musser. 1960. The mayfly ers, Colorado. Thesis. Colorado State University, Fort Col• lins, Colorado, USA. fauna of Green River in the Flaming Gorge Reservoir basin Wyoming and Utah. Pages 111-123 in C. E. Dibble, editor. Andrews, E. D. 1986. Downstream effects of Flaming Gorge Reservoir on the Green River, Colorado and Utah. Geo-' Ecological studies of the flora and fauna of Flaming Gorge logical Society of America Bulletin 97:1012-1023. Reservoir basin, Utah and Wyoming. University of Utah Angradi, T. A., and D. M. Kubly. 1993. Effects of atmo• Anthropological Papers 48: 1-243. Edwards, T. D., and B. C. Cowell. 1992. Population dynamics spheric exposure on chlorophyll a biomass and productivity of the epilithon of a tailwater river. Regulated Rivers: Re• and secondary production of Hyallela azteca (Amphipoda) search and Management 8:345-358. in Typa stands of a subtropical lake. Journal of the North Annear, T. 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APPENDIX All of the aquatic macroinvertebrate data used in the analyses and the water temperature data for the site located 26 km downstream from Flaming Gorge Dam are available through ESA's Electronic Data Archive: Ecological Archives AOII-013. The water chemistry data and other water temperature data used in analyses are available from the U.S. Geological Survey (http://h20.usgs.gov/).