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Changes in Physical Environment Construction of water development projects beginning after 1900 has had a major impact on the physical habitat of the native fishes of the Colorado River basin (Fradkin 1984, Carlson and Muth 1989). More than 20 major dams have been constructed on mainstream rivers beginning with Roosevelt Dam on the Salt River in 1911 and ending with closure of Glen Canyon Dam in 1963. By 1963, much of the mainstream river had been converted into a system of dams and diversions. Extensive flow regulations altered the timing, duration, and magnitude of annual flood flows. Modification by impoundment resulted in increasing water clarity and lower water temperatures in downstream sections. In addition, peak discharges in many areas of the Colorado River system have been reduced by about 50% since 1942, and base flows have been increased by 21 % (Fradkin 1984). Diversions from the river systems begin at or above tree line in most sub-basins. In the upper Colorado basin, transmountain diversions take water out of the basin. This water is diverted for agricultural, municipal, and industrial uses, and is also lost from the system by evaporation from reservoirs. Consumptive use data for the river system suggest that, if water usage equates to habitat, fish have to survive on 60% less basinwide, and much less than 60% at specific locations within the system (Brookshire 1993; Maddux et al. 1993).. Flow depletions constitute a major threat to endangered fishes in some areas. For example, historic aquatic habitats once maintained in important tributaries, such as the Salt and Gila, may now be dry. Construction of large impoundments has changed the distribution of riverine and lacustrine habitats in the basin over a very short time period. The effect is more pronounced in the lower basin. Historically, portions of the Colorado River system have had extensive flood plains that were inundated seasonally. The seasonally flooded bottomland, marshes, and oxbow lakes once were a normal feature of the river system, and presumably were important habitats in the life cycle of the razorback sucker. By reducing the magnitude and duration of peak flows, impoundments have greatly reduced the extent and duration of seasonal flooding. Channelization and construction of dikes, especially in valleys near human population centers and agricultural areas, also have reduced seasonal inundation of the floodplain (Bestgen 1990). For example, many of the flooded pastures or oxbow lakes in the Grand Valley near Grand Junction have been filled or access has been blocked with dikes (Osmondson and Kaeding 1989). Access has also been blocked on the Green River, where several waterfowl management units have been created by constructing levees along the river and filling the wetlands with water from the river or from irrigation ditches. Through channelization, dams, and diversions, the Gila River drainage has lost much of the habitat that once supported razorback suckers. Changes in the hydrologic regime also have had more subtle influences on physical habitats. Closure of mainstream impoundments has altered sediment transport and 17