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i ~ ~~ ., ~ 1 i .'f ;;1 ;j 1 ~ r ~ I} ij ~.L.o..er Dc", .' '1eoe;:lIe flock Com Pf.R/(ER { ') :,)0 n: ~:'::. ~ l AXe ......-"7'""- .~1O,"';Vf/ .' -.... /.. .~..,...~_...- J.~"r..;Q.' " ,\ ~~.. oJ ca.... \"OIC\letde [i'versic~ !)om ---"co ~-I"!REN:!ERG If/~r BLYTHE · ;~ r. ,~ 30 ~_---t ~u.~ 8' C'~r.'l.I.', . . ....1 ~;o IX. '. (; o,~ ':IW ;i:'2: ul I;~- FIGURE 1. Locations of dams and reservoirs along the Lower Colorado River. reservoirs, and small backwaters. The environmental conditions in each of these habitats are influenced by man's control and use of the.water. This modified system bears little reser.blance to its original state. The Arizona Cooperative Fishery Research Unit is jointly sponsored by the University of Arizona, and Arizona Gar.e and Fish Department and the U.S. Fish and Wildlife Service. We acknowledge their assistance. THE NATURAL RIVER SYSTEM cWr- Before the construction of dams and channelization, the Lower Colorado River had seasonally fluctuating flows and salinities, and high turbidities. Monthly discharges often varied from less than 200 m3/s during fall and winter to more than 1,500 tn3/s in sumner (Figure: (2); Grinnell (1914) reported a maximum of 2,800 m3/s. Salinities flucuated inversely with' discharges, and ranged from less than 350 mg/l in summer to more than 1,250 mg/l in fall ~ and winter (Figure 3). Turbidities typically ranged above 15,000 mg/l and occasionally. exceeded 30,000 mg/l (Figure 4). Under these natural conditions, the only significant aquatic habitats other than the main- stream were backwaters that were distinct entities during low river flow and became part of the mainstream durlng high flow. Although they resulted from frequent meandering of the r.~instream, the backwaters were few and small because of the rates of evaporation and siltation were rapid (Grinnell 1914). Consequently most of these natural backwaters were short-lived; Ohmari et a1. (1975) estimated that they lasted for only 50 to 75 years. ~.anybackwaters had recently been part of the river and had water qualities closely re- - sembling those of the mainstream; they differed from the'r.~instream mostly in having ~- duced current and decreased turbidities. The system was characterized by a low biotic diversity, which reflected the fluctuating, harsh environment. Since the floodplain was subject to continual erosion through channel meandering, predominant vegetation consisted of such hydrophytic, rapidly growing species as carrizo cane (Fi11"a01'ites c~~s), willow (Sari.: gooddingii), cottonwood (Populus fremontii), and arrowweed (?hucnea sericea). Plants commonly found in marsh soi~s were cattails (TYPha spp.), bulrushes (S~rpus spp.), and sa1tgrass (Distich lis stricta) (Grinnell 1914). Submergent vegetation was scarce or lacking in the mainstream, presumably because of the high turbidity; backwaters may have supported more submergent plants because they had clearer water. CAl-NEVA 'WILDLIFE TRANSACTIONS 1980 un ~ _____~ '--'_ _...,____.___'._ _____~__.,.-.n__-..--~___..__ - .-- ----.,-. ~..... ~.