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converted into a system of dams and diversions (Figure 1). As a result, the timing, duration, and magnitude of flows of most rivers of the Colorado River basin have been substantially altered. Today, most mainstream areas little resemble their natural condition with respect to native fish habitats. These habitat changes were associated with the decline of native species and proliferation of non-native forms introduced by man (Minckley 1982; Tyus et al. 1982; Carlson and Muth 1989). The disappearance of native fishes from greatly altered habitats were associated with an invasion by other species, and these changes have ostensibly occurred too quickly for native forms to adapt and recover (Minckley and Deacon 1968; Moller 1980). The causes for declines of endangered fish stocks in less-altered habitats are not as obvious and they are even more difficult to determine. The U.S. fish and Wildlife Service (USFWS) has been in consultation with other federal- agencies in the upper Colorado River basin under provisions of the Endangered Species Act of 1973 as amended, and has issued over 100 Biological Opinions pursuant to Section 7 of that Act (Rose and Hamiil 1988). In general, USFWS has determined that water depletions and dam operations would likely jeopardize the continued existence of some listed fishes. ,As..a- -~=est~-kt; ~n i nteragency program ( i . e. , Recovery Implementation Program) ~ra~s-1~a S ~~ f''- establisFied in the upper Colorado River basin in an effort to recover listed fishes without violating existing state and federal water agreements (USFWS . 1987; Rose and Hamill 1988). This program oversees recovery activities in the upper Colorado River, provides funds for evaluating habitat requirements of the fishes, and seeks ways to obtain water needed by the fish. In this paper, I present information about endangered Colorado River fishes, and research and management activities for them. COLORADO RIVER FISHES Deci i ne Geographic isolation and extreme climatic and hydrologic conditions have resulted in a unique Colorado River fish fauna (Miller 1959, 1961; Molles 1980). This fauna can be separated into three categories: (1) fishes + h a ~ inhabit~i.ag~high or intermediate elevations that either share, or have closely allied forms in adjacent drainages, (2) endemic species of small streams at low to intermediate elevations, and (3) big river fishes, commonly called the Colorado River fishes, which are mostly endemic species of mainstream rivers (Minckley et al. 1986). Native big river fishes, consisted only of cyprinids (minnows) and catostomids (suckers), that were widely distributed in mainstream habitats of the historic Colorado River basin (Jordan and Evermann 1896). Although some fishes are found in restricted reaches, four of the big river fishes, the Colorado squawfish Ptychocheilus Lucius, razorback sucker Xyrauchen texanus, humpback chub Gila cyuha, and bonytail Gila elegans, once ranged throughout warmwater reaches of the mainstream rivers of the Colorado River basin from Wyoming to Mexico. The range of these fishes has been drastically reduced and they are now threatened with extinction. The endangerment of the big-river fishes of the Colorado River has been attributed to conversion of riverine habitat to artificial impoundments, 2