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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Duchesne River Adult fish monitoring Introduction The Duchesne River is the primary tributary receiving flows from the southeastern watersheds of the Uintah Mountains and empties into the Green River near Ouray, Utah. Historically, the annual water yield of the Duchesne River was comparable to those of the Yampa and White rivers, both of which support resident populations of endangered fishes (Tyus and Karp 1989, Lenstch et al. 2000). Whereas average annual flows ofthe Yampa and White Rivers have not been drastically altered, the flows of the Duchesne River have been reduced from historical flows of over 768,470 acre-ft to approximately 399,694 acre-ft (CHM2 Hill 997). Following reduced flows, the morphology of the lower Duchesne River has changed dramatically since the late nineteenth century becoming narrower and less sinuous (Brink and Schmidt 1996). In the last 20 years, razorback sucker and Colorado pikeminnow have been captured regularly in the lower Duchesne River (Archer et al. 1986, Cranney 1994). Based on the presence of "ripe females with strippable eggs", Archer et al. (1986) suggested that razorback sucker may spawn in the lower 2.5 mi of the Duchesne River. Cranney (1994) observed razorback sucker and Colorado pikeminnow as far upstream as river miles (rmi)12.0 and 13.6, respectively, and reported angler catches of pike minnow as far upstream as rmi 41.2. The presence of fishes upstream of the confluence implied that the Duchesne River, despite alterations in flow and channel complexity, remains an important environment for endangered fishes. The one year survey by Cranney (1994), and the anecdotal information he cites in his report are the only data that reference distribution of endangered fish in the Duchesne River above critical habitat. The undefined role of the Duchesne River to endangered fishes, together with the knowledge that Colorado pikeminnow reside in rivers of similar size gave impetus for the Recovery Implementation Program to initiate this study. In addition, proposed development of the Central Utah Projects, Uintah-Upalco and Uinta Basin Replacement Project, required a biological opinion relative to flow needs for endangered fishes in the Duchesne River. The purpose of this study is to provide information on the distribution, spawning, and temporal use of the Duchesne River by endangered fishes. The specific objectives of this study were to determine: 1) the seasonal use of adult and subadult Colorado pikeminnow and razorback sucker in the Duchesne River, and 2) the extent to which adult and subadult Colorado pikeminnow and razorback sucker 1