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have the reproductive disadvantages of lower fecundities. no parental care for eggs and young. and sexual maturation at an older age than centrarchids. By the early 1940's all major species of fish in the mainstream were exotic (carp, Cyprinus cClr;,ic. channel catfish, 1cta!urua puncta=us; yellow bullhead. 1. natal-is; rrosquito- fish. G~husia afjinis; bluegill. Lepomis macrochirus; green sunfish. L. cuanellus; and largemouth bass. Microvterus saZmoides). Minor species included black bullhead (Ictaturus melas). brown bullhead' (1. nevuz.osuS), black crappie (pomo=is nicromacuZa'tus). white crap- pie (P. anr~~-s), razorback sucker. machete (Elops affinia). and striped mullet t~z cephalus}. Only the razorback sucker. machete. and striped mullet immigrated from the Gulf of California and. according to Miller (1961). probably survived in the river atthis time because of the clearer water and the local extinction of predatory squawfish. Although annual yield of-the sport fishery was unknown. fishing success was.considered satisfactory by Di 11 (1944). .. I lhe status of biota in the backwaters in the early 1940'5 is poorly known. Favorable limno1ogical conditions probably existed. and biological productivity in most backwaters may have been higher than that in the mainstream. Di11(1944) reported that oligochaetes, gastropods. ephemeropteran nymphs. and chironomid larvae were found in backwaters near Palo Verde. California. and Yuma. Arizona. These waters supported large populations of, fish. including carp. channel catfish. yellow bullhead, mosquitofish. bluegill, green sunfish. largemouth bass, black crappie. white crappie. razorback sucker, and striped mullet. The newly built reservoirs developed 1imnological conditions conducive to the establishment of emergent and submergent plants and phytop1ankton (Kimsey 1957), which provided food and cover for fish-food organisms. Microcrustacea were present in open waters, and various insect larvae. gastropods. and oligochaetes inhabited the bottom sediments and plants (Beland 1954; Dill 1944). Crayfish were not seen at Lake Havasu in 1953 (Beland 1954), although Proca:rrbraus cZa:l'kiwas established in Imperial Reservoir and the Yuma area by 1942 (Dill 1944). The greater diversity of plants and invertebrates in ;reservoirs provided a habitat for larger fish populations than those occurring in the remnants of mainstream between the reservoirs. Common reservoir fishes in the early 1950's were largemouth bass, bluegill. green sunfish, black crappie. channe1 catfish. carp. and mosquitofish. .Among the rare species were yellow bullhead and razorback sucker. White crappie. black bullhead. Colorado squawfish. and bony tail chub were reported. but the reports were not confirmed by Beland (1954). - THE EFFECTS OF CHANNELIZATION Since the dam-building era ended in the 1950's. channelization and dredging of the river have modified the mainstream and caused losses of backwater habitat. Only two portions of the mainstream remain relatively unaltered: a 19 km reach-above Lake Havasu and a 63 km reach above Imperial Reservoir (Ponder 1975). - - Modifications of the mainstream include reduction of stream length. reduction in width of some reaches. and deepening of the channel. These changes increased the water velocities and lowered the water table. However. water quality in the mainstream was not. substan- tially changed; average monthly discharges (84 to 504 m3/s) remained relatively stable (Figure 6) and daily discharges continued to fluctuate in response to water releases at upstream dams. In the reach adjacent to the Colorado River Indian Reservation near Parker. Arizona. the average and maximum daily river level fluctuations were about 0.2 and 0.5 m during the winter and 0.8 and 1.5 m during the summer (Krummes 1959). Average fluctuations differed in other reaches of the mainstream. Narrow monthly salinity ranges (650 to 1,050 mg/l) were maintained (Figure 7). Tot~l biotic.iroduction in the mainstream may be higher now than at any other previous perlod. However. much of the biomass is contributed by the recently introduced Asian clam '(CorbicuZa flaminea). Although this clam is eaten by carp and redear sunfish fLer:,omi.s - microtophus) (Saiki 1976). a large portion of the clam population consists of individuals too large for the fish to eat. Other invertebrates useful as fish forage are scarce (Federal Water Pollution Control Admin. 1968). Fishes in the mainstream include at least 16 species CAL-NEVA WILDLIFE TRANSACTIONS 1980 106