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. ".J,. THE EFFECTS Of DAMS . I n~~ construction on th~ Lower Colorado River greatly altered the physiochemical character- istics of the system. Laguna Dam. built in 1909. had only a slight effect on the iower river because of its small size and downstream location (Table 1. Figure 1). .Comp1etion of Hoover Dam (about 108 I<m upstream from the more recently constructed Davis Dam) in 1935 allowed the storage of large quantities of flood water in Lake Mead; regular releases during the dry season made possible a more nearly constant river flow. Fluctuations in salinity concentrations decreased and the water of the lowet river cleared (figure 5). later, Parker, Imperial. Davis. Headgate. and Palo Verde darns further stabilized discharge rates and increased salinities and transparencies in the downstream river water. & c TABLE 1. Major dams and reservoirs along the Lower Colorado River.a & b Dam Height Reservoir Area Year Reservoir Dam type (m) (h~) Completed Laguna ER 3.0 1909 Imperi a 1 SB 14.3 2.957 .1938 Palo Verde Diversion ER 12.5 1958 Havasu (Parker Dam) VRA 25.9 8.262 1938 noovaiya (Headgate Rock Dam) E . 1":l1 7 1941 .: ~~ ~ Mohave (Davis Dam) ER 42.1 1,134 1953 aCompiled from California Dept. of Water Resources (1974). Douglas (1947). & Slawson. (1972). bE = earth; ER = earth and rock; SB = slab and buttress; VRA = variable radius arch. .' ~ .~.>.: "";:~~ The regulated and clarified waters were distinctly different-from those of the pre- impounpment period. Immediately below dams, the mainstream was clear and currents were swift (Borland and Miller 1960; Dill 1944; Moffett 1942). The currents scoured the river bottom. leaving it rocky and gravelly. Farther downstream from the dam, turbidities were higher and current velocities lower; certain downstream areas had braided and meandering channels and resembled the original river. Diurnal water level fluctuations resulting from irrigation and hydroelectric needs replaced the seasonal fluctuations of the original ri ver. Static water habitats during the immediate post-impoundment period are poorly described. Backwaters were usually clear and had little or no current. In many respects, the back- waters probably resembled those present before dam-construction. One obvious difference was that more of the backwaters were permanent because seasonal floods were eliminated. Perhaps the.greatest single influence of dams on the river system was the creation of large r~servoirs that were not present in the original river. The reservoirs had clear water and reduced currents. and some - such as Lake Havasu and Imperial Reservoir -.had relatively stable water levels. Because rapid water passage thrOugh all of the reservoirs prevented prolonged thermal stratification, dissolved oxygen concentrations remained high from surface to bottom (Beland 1954; Dill 1944; Kimsey 1957; Ponder 1971). After impoundments were built, the biota in the remaining stretches of mainstream between darns responded to their altered environment in several ways. Some species were able to live and reproduce in the new environment but many less adaptable species became locally extinct. Exotic species entered the mainstream through natural immigration and stocking. In areas with sandy and muddy soils, some carrizo cane, cattails, and bulrushes survived. The paucity of submergent aquatic plants continued except for attached algae in clear, rocky portions of the river (Dill 1944; Moffett 1942). Ditch grass (Ruppiasp.) spiny naiad (Najas marina), and sago pondweed (potamogeton pec~inatus) were common rooted sub- mergent plants. although submergents were not abundant enough to be important 1;0 fish, except in a few areas (Dill 1944). Production of invertebrates in the river was low because CAL-NEVAWILDLIFE TRANSACTIONS 1989 104