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upstream from habitat historically occupied by the endangered fishes, its operation changed the flow regime of the Gunnison and Colorado rivers within what is now critical habitat for these species. The effect of these dams on the endangered fishes was not considered before they were built; however, operation of these reservoirs continues to affect the four endangered fishes and consultation under section 7 of ESA is therefore required. The Aspinall Unit investigations were a group of studies designed to determine the effects of operation of these reservoirs on the endangered fishes and to provide the biological basis for flow recommendations in the Colorado and Gunnison rivers to benefit these species. These studies were designed to: 1. Track the reproductive success of endangered fish and other species in the Colorado and Gunnison rivers, and determine the relationship between physical variables, biological variables, and spawning success of endangered fish and other fish species; 2. Evaluate recruitment of endangered fish from age 0 to subsequent life history stages and determine the relationship between seasonal parameters and survival of young fish; 3. Monitor the relative abundance and population structure of endangered fishes and sympatric species to acquire information on interactions among species, and how, various physical parameters may differentially affect species; 4. Determine the relationship between quality and quantity of important habitat -types and seasonal flows of various levels; and 5. Establish the relationship between geomorphology and fish habitat and how these factors influence the distribution of the endangered fishes. - Because bonytails are extirpated from the upper basin, studies concentrated on humpback chub, Colorado pikeminnow, and razorback sucker. The Aspinall Unit investigations were conducted in the Gunnison and Colorado rivers, but relevant information on relationships among the endangered fishes and their habitat requirements were also available from the San Juan and Green River basins. This report summarizes and integrates results from studies conducted as part of the Aspinall Unit investigations and other relevant information to produce flow recommendations for the Gunnison River and the Colorado River downstream from their confluence that will benefit the four endangered fishes. Monitoring population responses of the endangered fishes to different flow regimes is difficult because of their long generation times. However, relationships between reproductive success of Colorado pikeminnow and humpback chub and river flow were established for the Colorado River. Reproductive success of both species was highest in years when peak spring flows in the Colorado River downstream of the Gunnison confluence ranged from 30,000-40,000 cfs. Antecedent flows were also important predictors of Colorado Final Report xix July 2003