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I j ~ t INTRODUCTION The large rivers of the Upper Colorado River Basin provide habitat for four native fish species that are considered endangered by the U. S. Fish and Wildlife Service: Colorado squawfish Ptychocheilus lucius, razorback sucker Xyrauchen texanus (to be listed as endangered in May, 1491), humpback chub Gila cypha, ar~d bonytail G. elegans. Beginning in the late 1800's and continuing today, technologic man's activities have dramatically altered the Colorado River ecosystem. These man-induced changes are generally considered to be responsible for the decline of the four endangered fishes (Miller 1961, Minckley and Deacon 1968, Holden and Stalnaker 1975). The most dramatic physical change has been the alteration of natural flow regimes by water development projects on the tributaries and mainstem rivers of the basin (e.g. Vanicek et al. 1970). Other changes that may have affected the native species include the introduction of fishes that may compete with or prey upon the native fishes, and poor watershed management practices that affect the quality and quantity of water in the basin. Although many private irrigation projects already existed, development of water storage in the Upper Colorado River Basin began in earnest during the 1930's with the construction of several projects by the Bureau of Reclamation. Reservoir construction in the upper basin increased dramatically in the 1950's and 1960's with the Colorado River Storage Project (CRSP). This project built dams on large rivers in the Upper Colorado River Basin. Three of these dams (Glen Canyon, Flaming Gorge, and Navajo) were built within habitat historically inhabited by the four listed species. Three additional dams (the Aspinall Unit--Blue Mesa, Morrow Point, and Crystal) were built on the upper Gunnison River, upstream from reaches historically inhabited by the endangered 1