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Background The Green River is regulated year around by Flaming Gorge Dam for flood control, water delivery, and power generation. The dam was authorized by the Colorado River Storage Project Act of 1956 (43 U .S.C. 620). Construction began in 1956 and was completed in 1964. Flaming Gorge Reservoir began filling in November 1962, and continued through 1966, with full operation in 1967. The reservoir impounds water for about 145 kIn upstream to Green River, Wyoming, with a total surface area of about 17,000 ha and a live storage capacity of about 3.75 million acre-feet. Releases from Flaming Gorge Dam can vary during a 24-hour period, primarily because of hydropower generation. Releases also vary seasonally primarily because of storage and downstream water demands, which have decreased spring peaks and increased fall and winter flows. Minimum releases of 800 cfs (23 m3/s) have been established by the Utah Division of Wildlife Resources (l !OWR) to provide flows necessary for the tailwater trout fishery, and maximum releases of 4,700 efs (133 m3/s) are constrained by power plant capacity and reservoir elevation. In addition to the power plant capacity, releases of 4,000 cfs (113 m3/s) can be made through two river outlets (jet tubes), and 28,800 cfs (816 m3/s) can be released through the spillway. River stage changes from flow regulation are ameliorated with distance downstream from the dam and by inflow from several major tributaries, including the Yampa, White, Duchesne, San Rafael, and Price rivers. The Green River and its tributaries in the upper Colorado River basin historically supported 13 species of native fishes (Tyus et a1. 1982). Eight warm-water species are native to the river below Flaming Gorge Dam, including Colorado pikeminnow (Ptychocheilus lucius), humpback chub (Gila (:lpha). bonytail (G. elegans), roundtail chub (G. robusta), speckled dace (Rhinichthys osculus), razorback sucker (Xyrauchen texanus), flannel mouth sucker (Catostomus latipinnis), and bluehead sucker (c. discobolus) (Holden and Stalnaker 1975). Following completion of the dam in 1964, numbers and distribution of these warm-water species were reduced by altered flow regimes., inundated habitat, blocked passage, and modified water chemistry and temperature, along with increasing numbers of non-native fishes (Vanicek 1967, U.S. Fish and Wildlife Service 1982). Colorado pikeminnow, humpback chub, bonytail, and razorback sucker are classified as endangered under the Endangered Species Act of 1973, as amended, and roundtail chub and flannelmouth sucker are special status species. Opportunities for recovery of these native fishes are related to understanding their life history requirements for all seasons of the year. However, the majority of studies on these species have been conducted in spring, summer, and fall. The only direct winter investigations are of movement and habitat use of adult Colorado pikeminnow and razorback suckers in the upper Green River (Valdez and Masslich 1989) and in the Yampa River (Wick and Hawkins 1989). A synthesis of winter investigations in the Green River and an annotated bibliography of related winter conditions for fishes from other basins was prepared by Valdez (1995). These studies helped to establish winter flow recommendations for adults of these species in the Green River downstream of Flaming Gorge Dam, but information on winter conditions of nursery habitats and survival of age-O fish is lacking. The Green River in Utah has the highest densities of age-O Colorado pikeminnow of any region of the Colorado River Basin (McAda et a1. 1994). These fish are spawned in late summer in 2