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14 environments upstream of dams and diversions or in quiet, off-channel areas have soft, silt-dominated bottoms. Much of the bottom of Lake Havasu is cluttered with vegetative debris, remnants of the extensive riparian forest and desert scrub that existed and was not cleared at the time the reservoir was created. The same is less true of lakes Mead and Mohave, which are much deeper, and where the original riparian zone was narrower. Discharge and Currents -- Discharge of the lower Colorado River is controlled at every major structure beginning at Glen Canyon Dam upstream of Grand Canyon. That facility has capability to release up to a maximum of 256,000 cfs (combined powerplants, river outlet works, and spillway releases; USBR 1995); however, recent typical flows have varied from about 5,000 to 31,500 cfs with daily fluctuations of 12,000 (October) to 16,000 cfs (August and January). Withdrawals (and evaporative losses) occur along the river course to Morelos Dam where, in many years, all remaining water is diverted and the downstream channel is wetted only by seepage and irrigation returns. Currents are similarly varied, and in part are determinants of sediment character. Below dams, current velocity may exceed a meter per second at high discharge. Channelized reaches of river have currents of 0.5-1 m/s, while water movement in lentic habitats (reservoirs and backwaters) may be undetectable at the surface. However, substantial underflow may exist in reservoirs when both in- and out-put discharges are occurring. For example, sub-surface currents measured by Langhorst and Marsh (1986) in Lake Mohave ranged upward to 0.25 m/s at 5-15 m depth. Water Chemistry -- Nowhere in the lower Colorado River mainstream, except in extreme instances such as toxic chemical spills, is water chemistry inhospitable to aquatic life. Dissolved oxygen varies widely, seasonally and with location, but almost always exceeds 50% saturation. Lowest levels occur in quiet habitats with dense