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INTRODUCTION The Green River forms the major northwest section of the Colorado River basin (Fig 1), which occupies approximately eight percent of the continental land mass of the United States. Because the river basin was isolated from other river basins for millions of years, 74% of its fish fauna are C,:I y endemic species (Behnke and Benson 1983). Extreme variability in flow, t turbidity and temperature created a harsh aquatic ecosystem. In the upper portions of the Colorado River basin, above Lees Ferry, Arizona, only 13 species coevolved. The Colorado River is one of the most highly developed river systems in the world (Carlson and Muth 1989). Although fed by mountain snowmelt, most of the Colorado River drainage passes through and land. Increasing regional population has placed great demands on its water resources and stress on native biota and natural river function (Davies et al. 1994). Dams and diversion structures blocked migratory routes, altered flows and water temperatures, and diverted young and adult fishes from the main river into canal and irrigation systems. When high flows passed through levied areas on the Colorado River, channel beds degraded, or rivers broke through levees. Spawning and nursery habitats utilized by native fish were severely disrupted, causing fish displacement. emigration and recruitment failure. Native fishes evolved unique adaptations to the historic river flows and geomorphic structure. However. these adaptations became disadvantages for native fish as a result of altered flows. temperature, sediment discharge, blocked access to floodplain habitat, and introduction of nonnative species.