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6 <br />first 32 km, which includes Black Canyon, is characterized primarily by <br />river conditions. Water temperatures within Black Canyon are virtuaiiy <br />constant at 12-13°C due to the hypolimnetic discharges from Hoover Dam. <br />Current velocities may reach 2.1 m/sec (6.5 ft/sec ) during periods of <br />high discharge. There is considerable seasonal and daily variation in <br />water levels (1-3 m) due to discharge cycles from Hoover Dam. <br />The only significant inflow to Lake Mohave is from the Colorado <br />River via discharges from Hoover Dam. There are some warm springs <br />located in Black Canyon, but these inflows are insignificant relative to <br />the Colorado River. The Willow Beach Trout Hatchery, located 18 km below <br />the dam, uses river water which is returned to the river. There are no <br />major diversions of water from Lake Mohave. <br />The river is bordered by the Black Mountains to the east and the <br />Eldorado Mountains on the west. The canyon walls extend several hundred <br />meters above the river and shade the water for most of the day, greatly <br />reducing incident solar radiation. The riparian community in Black <br />Canyon is sparse except for a few stands of saltcedar (Tamarix sp.) and <br />creosote (Larrea tridentata). <br />The transition between river and true reservoir conditions in Lake <br />Mohave occurs in Eldorado Canyon. This is especially evident in the <br />summer when a sharp interface (convergence) develops as cold, <br />nutrient-rich, river-water flows under warm lake-water. Mixing at the <br />interface promotes high phytoplankton productivity in Eldorado Canyon, <br />resulting in a marked change in water clarity between river and <br />lake-water (Paulson et al. 1980b). Below Eldorado Canyon, Lake Mohave <br />expands into Eldorado, Little, and Cottonwood Basins. Cottonwood Basin, <br />located 54 km downstream from Hoover Dam, is the widest point in the