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two principal areas; the lower reaches of the Yampa River and in Desolation/Gray Canyon of the Green River (Tyus and Haines 1991). Newly-hatched larvae from the Yampa River drift to principal nursery backwaters between Split Mountain (River Kilometer [RK] 513; i.e., distance upstream from the confluence of the Green and Colorado rivers) and Sand Wash (RK 346), and larvae from Desolation/Gray Canyon drift to backwaters between Green River (RK 191) and the Colorado River contluence (RK 0; Tyus 1991). The densities of these young fish are often high in fall, but decrease by an average of 46-54% by spring (Haines and Modde 1996; Valdez and Cowdell1996; McAda and Rye! 1999; Valdez et a1. 1999), indicating substantial overwinter mortality, which we believe is abnormally high and contributes to reduced recruitment and continued endangerment of the species. The causes for these decreased densities are unknown, but may be related to unstable nursery backwaters from fluctuating dam releases; infringement of backwaters by ice processes; predation by non-native fishes; natural downstream dispersal by young Colorado pikeminnow, which ar~ potomadromous or long-distance migrators as adults (Tyus 1984); and condition or fat content of the young fish at the beginning of winter (Thompson et al. 1991). Purpose and Objectives The purpose of this study was to determine the effect of winter flow regulation from Flaming Gorge Dam on ice development and breakup on the Green River and the effect of these ice processes on nursery habitat of age-O Colorado pikeminnow. We hypothesized that the current pattern of ice development and breakup on the Green River downstream of Flaming Gorge Dam are linked to dam operations, and that these ice processes significantly reduce availability and persistence of backwater habitat for overwintering age-O Colorado pikeminnow in one of the primary nursery areas on the Green River. The objectives of the study were to: 1. Determine if there is a link between dam operations and ice development and breakup in the nursery area of Colorado pikeminnow, 2. Determine availability of overwinter nursery backwater habitat for age-O Colorado pikeminnow, 3. Determine condition and longevity of backwaters exposed to winter flow regulation and ice development and breakup, and 4. Recommend winter releases from Flaming Gorge Dam to protect nursery habitat of age-O Colorado pikeminnow. STUDY AREA The study was conducted on the upper Green River in eastern Utah, downstream from Flaming Gorge Dam (Figure 1). The dam is at an elevation of 1,708 m about 658 km upstream from the confluence with the Colorado River. The study area extended from Island Park (RK536), within 3