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of hydroelectric power and the implementation of water and fish management practices, the Colorado River basin began to change. These various impoundments acted as "river blocks", trapping the fish. This effectively fragmented the native fish habitat. The impoundments stopped fish migration as well as the passage of genetic material beyond them and changed the historical variations in flow, temperature, and turbidity that these fish evolved in. Seasonal and interannual variations in flow were greatly reduced while daily fluctuations increased. Mean annual temperatures were lowered, salinity increased, and the water in the tailwaters of the dam were made clear and colder. Some important fish habitat has either disappeared altogether or has been converted from lotic to lentic habitat (Hickman 1983). The basin has taken on a different form altogether and it appears as though some of the environmental factors which stimulate spawning reactions in the native fish no longer occur in today's Colorado River Basin (citation ). Water development was not the only change that has occurred in the Colorado River Basin. By the mid 1900's sport fishing became a popular pastime as well as an economic factor. States became eager to create lucrative sport fisheries, and introduced exotic sport fish that were favorites of anglers. The upper Green River was even poisoned with rotenone, to remove native "trash" fish and make way for sport fishes (Holden 1991). Ironically, many of these "trash" fish were endemic, and four are currently listed as either threatened or endangered. Many nonnatives were introduced and quickly made the basin their home. This created problems for the native fishes as they were now forced to compete with multiple species of nonnative fish for food and space, deal with predation pressures from a plethora of predatory fish new to the basin, and adapt to a very different habitat. The changes in the hydrograph related to river development coupled with the introduction of nonnative species has exaserbated the species decline. Minckley and Meffe (1987) found that nonnative fish gradually dominated native fish during low discharges, and during high flows, native fish were dominant in number. Valdez (1985) noted similar findings. Flooding may prevent 3