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Introduction Similaz to other lazge rivers, tributaries to the Green River aze likely important for maintaining the functional integrity of the overall giver system dynamics. Studies conducted on the Yampa River since the mid 1970's have demonstrated the essential role this tributary plays in maintenance and recovery of the endangered fishes in the Green River (Tyus and Karp 1989). Fish surveys on the White River downstream of Taylor Draw Dam have documented the yeaz- round occurrence of adult (in relatively high numbers), and sub-adult Colorado pikemianow Ptychocheitus lucius; and that resident adults spawned at sites in the Green and Yampa rivers (Irving and Modde in' press). Colorado pikeminnow and razorback sucker Xyrauchen texanus regularly utilize habitats at the mouth of the Duchesne River, especially during spring~runoff (Modde and Irving 1998), and cursory sampling in May 1993 documented some use of the lower 15 miles of the Duchesne River by adult Colorado pikeminnow and razorback sucker (Cranney 1993). However, existing data on seasonal use of the Duchesne River by the endangered fishes and on the river's fish community and physical conditions are inadequate for sound validation or refinement of year-round flow recommendations established by the U.S. Fish and Wildlife Service (USFWS 1997). The lower 2.5 miles of the Duchesne River was designated as critical habitat for razorback sucker in 1994 (Fed. Reg./Voi. 59, No. 54/Monday, March 21,1994), and Colorado pikeminnow and razorback sucker have been recently collected in the lower 33 miles of the river. Natural flows of the Duchesne River have been substantially altered due to agricultural and municipal development; direct diversions of water have occurred since the late 1800's. The result has been a reduction in the mean annual flow from approximately 768KAF to 220KAF (Duchesne River Draft Biological Opinion, 1997). In January 1995, the U.S. Fish and Wildlife Service (Service) provided preliminary flow recommendations for the Duchesne River. These preliminary. flow recommendations will be used to test hypotheses advanced in this ongoing revised study. Research results will be used to establish final flow recommendations that restore and maintain natural ecosystem components and functions (physical and biological) that will facilitate recovery of the endangered fishes. Historic biological information on the Duchesne River, although somewhat limited, indicates that Colorado pikeminnow and razorback suckers inhabited lower reaches of the Duchesne and Uintah rivers (Cranney, Utah Division of Wildlife Resources, UDWR, 1993). Cranney (1993) reported that the presence of Colorado pikeminnow in the Duchesne River was first documented in a 1956 unpublished UDWR stream survey report; eight Colorado pikeminnow (1-15 inches total length (TL); 25-380 mm) were collected by electrofishing during August in a reach from Bridgeland (upstream of the town of Myton, Utah, RM 49) to the Green River confluence. He also noted that Mullan (1975) collected a 3 inch (75 mm) long Colorado pikeminnow in the Uintah River upstream of its confluence with the Duchesne River (RM 14.3) in June 1970, and that Seethaler (1978) reported captures of five Colorado pikeminnow by Ute tribe members at the confluence of the Uintah and Duchesne rivers in spring 1975. The earliest record of razorback suckers in the Duchesne River was May 1978 in a BioWest, Inc. fish survey 6