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future because there is virtually no recruitment to wild populations, despite sucxessful spawning and dispersal of larval razorback suckers throughout the system. The decline in abundance of the big rarer fishes has occurred at the same time that major changes occurred in their physical and biological environment. Ph sisal Chan e y gs were primarily a consequence of the operation of the many dams and diversions that have been constructed in the Colorado River basin since 1905. These structures deplete water alter flow re i g mes, change water quality, and fragment habitat. At the same time the physical environment was being altered extensively by human actions, the nature and composition of the fish community was altered dramatically by the introduction of nonnative species, because some of these did well in the changed environments. As a result, native fish species were confronted with competitors and predators with which they had had no evolutionary "experience" (Molles 1980, Johnson et al. 1997). Construction of water development projects beginning after 1900 has had a major impact on the physical habitat of the native fishes of the C I 0 orado River basin (Fradkin 1984, Carlson and Muth 1989). More than 20 major dams have been constructed on mainstem 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 26