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remnant stocks may lose genetic diversity. Ultimately, extinction could result because the loss of genetic diversity may make populations more susceptible to environmental change. The historical ranges of the four endangered species have been fragmented by construction of dams and water diversions throughout the Basin (Carlson and Muth 1989). The Service believ~s that it is important to the survival and recovery of these species to reestablish populations in several geographically distinct areas within their former range. Providing geographically distinct areas that contain varying thermal, chemical, geological, and physical parameters will encourage maintenance of the current genetic pool. These parameters influence important life history characteristics such as time of spawning, recruitment, growth, mortality rates, and longevity. HABITATS AND STATUS OF ENDANGERED FISHES General--The four endangered Colorado River fishes evolved in the Colorado River and were adapted to the natural environment that existed prior to the beginning of large-scale water development. Thus, they were adapted to a system of fluctuating seasonal and annual flows influenced by wet, average, and dry climatic periods. Recent population declines and disappearances of endemic fish species in much of their former range have been associated with relatively rapid and widespread anthropogenic changes. These changes have altered the physical and biological characteristics of many mainstream rivers in the Basin and occurred so rapidly that the fishes have not had time to adapt to them (Carlson and Muth 1989). Dams and diversions have fragmented former fish habitat by restricting fish movement. As a result, genetic interchange (emigration and immigration of individuals) between some fish populations is no longer possible. Large floods were once normal in the Basin and provided food and nutrient exchange between river channels and shallow-water floodplain habitats. These floods are now controlled by numerous dams. As a result of these dams, major changes also have occurred in water quality, quantity, temperature, sediment and nutrient transport, and other characteristics of the aquatic environment (Carlson and Muth 1989). The altered habitats that have resulted are now more suitable for introduced, nonnative fishes, some of which have flourished (Minckley et al, 1982; Tyus et al. 1982; Carlson and Muth 1989). These changes have greatly altered the river environment and no unaltered habitat remains in the Basin for the four Colorado River endangered fish species. Razorback Sucker-- This species was abundant and widely distributed in mainstream rivers of the Colorado River (Jordan and Evermann 1896; Minckley 1973). A relatively large stock of . razorback suckers remain in Lake Mohave (Minckley et al. 1991). However, the formerly large Lower Basin populations have been virtually extirpated from all natural riverine environments, and recruitment is virtually nonexistent in the remnant stocks (Minckley et al. 1991). In the Upper Basin, the fish persists in the lower Yampa and Green rivers, mainstream Colorado River, and lower San Juan River (Tyus et al. 1982; Minckley et al. 1991; Platania et al. 1991), but there is little indication of recruitment in these remnant stocks. The largest extant riverine population occurs in the upper Green River Basin, but it consists 13