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" --- -. _W':j ..,,, UI CIIt' IlIdlnscream adverSely affected the sport fishery (Beland 1953). It attem;Jts to mitigate these effects, the Water and P.ower Resources Service deepened many shallow back~at~rs by dreooino them and constructed channels or installed CUlverts b~tween them and the ri~er. These backwaters now exhibit a wide range of environmental conditions and represent the most diverse habitat along the lower river. The physiochemical and bioloaical. conditions in some backwaters nearly duplicatie those in the mainstream, the main difference beina the reduced currents in the backwaters (Arizona Cooperative FiShery Research Unit 1975; Marshall 1976; Ponder 1975). Otherbaclc- waters resemble stagnant pools, devoid of game fish but harboring biota capable of sur- viving wide temperature fluctuation~, low dissolved o;r;ygen concentrations, and high salini- ties {Ponder 1975). Most backwaters range between these e;r;trerres. In general, biotic standing crops in the backwaters are higher than those in the mainstream (Arizona COopera- tive Fishery Research Unit 1975; Saiki 1976). Emergent and sub~rgent aquatic plants are abundant, as are immature forms of aquatic insects. There are substantial populations. of bottom organisms such as chironomids and oligochaetes. Zooplankton is abundant only in some of the more lentic types of backwaters. Major game fishes include channel catfish, largemouth bass, bluegill, redear sunfish, and black crappie (Arizona Cooperative Fishery Research Unit 1975; "',arshall 1976; Ponder 1975; Saiki 1976). Of the endemic fishes. only razorback sucker is still present in backwaters (Arizona Cooperative Fishery Res~arch Unit 1975; Marshall 1976). THE FUTURE .. . Over the next decade, significant further changes in the lower Colorado River system will _ probably enSue in response9to man's continuing attempts to control and use the hydrological resource. About 1.36 X 10 m3Jyear of water is now withdrawn from lake Havasu by the Southern California MetroPl?litan Water District (Valantine 1974). Under ~hi~ regime of water depletion, the annual discharge below Parker Dam averages 8.63 x 10 m (calculated from unpublished data of scheduled water releases at Parker Dam by the U.S. Water and Power Resources Service, 1 January 1973 to 31 December 1979).3 When the Central-Arizona Project begins operations in the mid-1980's, about 1.75 x 10 m of water will be diverted each year from lake Havasu; concurrently, the Southern California Metrop~1itan Water District. will cut ~ack. it~ u~e of Colorado Ri~er water to about 0.g2 x 109 m (year (V?lantine 1974). These estlmates lnd1cate that a net lnc"rease of 1.01 x 10 m3fyear w111 be wlthdrawn from the reservoir by 1990. Assuming that other factors (inflows of the Colorado River into lake Havasu, evaporative rates, precipitation rates, etc.) remain constant. this increased comsu~ption rate will reduce the annual discharge below Parker Dam by nearly 12 percent. The decreased rate of flow may reduce average river water levels and lower the surrounding water table. Consequently, some of the backwaters now serving as fish habitat. might be greatly reduced in-volume or eliminated. Reductions in the quality of water may also occur because reduced flows in the river below Parker Dam could lead to highar salinities and wider seasonal temperature ranges _ condi- tions that mighte;r;ceed the tolerance limits of many fishes. Skogerboe and Walker (1975) and Slawson (1972) reported that return flows from agricultural irrigation in the Lower Colorado .River basin are highly saline, presumably from the effects of evaporation and soil leaChing. These inf10\'IS cause the total dissolved solids content of the mainstream to increase. If agricultural activity intensifies in the futu.re, the salinity of the river will also increase. Valantine (1974) predicted that salinities in the mainstream oelow Parker and Imperial dams will be 880 and 1,080 mgfl, respectively, by 1990; when extrapola- ted to conditions in backwaters, much higher concentrations are expected. Annual ranges of water temper_atures may also increase because the reduced water volume of the system 't.Quld be subject to rapid equilibr.ium interactions with seasonal air temperatures. These altered water quality conditions could affect all biota, inClUding fishes, and cause species changes toward organisms tolerant of higher salinities and wider temperatures ranges. . lITERATURE CITED Arizona Cooperative Fishery Research Unit. 1975. Final report: Fisheries potential of dredged backwaters along the Lower Colorado Rover. Rept. submitted to USBR, Boulder City, NV 89 pp. Beland, R.D. 1953. The effect of channelization on the fishe~ of the Lower Colorado River: Ca 1"i f. Fish Game 39: 137-139. CAl-NEVA WILDLIFE TRANSACTIONS 1980 109 . - --'-'--"~,- '--,...--~-~ .-.- _. - .---; '.---.------------. -- -~~-------