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Objective 2: Availability of Overwinter Nursery Backwaters Loss of backwaters as a result of ice jams occurred in all three reaches, but were most common in Reaches 2 and 3, where fluctuating dam releases were largely ameliorated. We believe that ice jam formation was more extensive in these downstream reaches of the study area because of the relatively low gradient, the meandering nature of the river channel, and frequent channel constrictions from sand bars. Ice jams have been observed in past years (Valdez 1995) at the same locations as reported in this study, suggesting a link to local geomorphic channel features that cause channel constrictions. The phenomenon of ice jams occurred throughout the winter when ice conditions were suitable; hence, backwaters were lost at various times during winter. This study showed no difference in loss of backwaters due to ice jams between high fluctuating flows and low steady releases during the two mild winters, suggesting no link between the incidence of ice jam formation and dam operations during mild winters. Objective 3: Condition and Longevity of Backwaters The numbers of backwaters that became unsuitable from primarily ice jams was about the same (56% and 50%) in the 2 years of study. With the known link between dam operations and surface ice development and persistence, it appears that in a year of colder air temperatures, greater surface ice development would result in less frazil ice development and fewer ice jams, and presumably lower losses of backwaters from these processes. The effect of high-volume fluctuating releases on backwater stability may be exacerbated in years of colder air temperatures when ice development is more extensive and prolonged. Evidence of this is presented in observations of river conditions in winter of 1987-88, when monthly air temperatures in January and February were 4.2- 1.1 oC below normal and an ice canopy persisted from December to mid-February (Valdez and Masslich 1989). During that period, frazil and jam ice were common from Echo Park to Split Mountain, and the area from Split Mountain to Bonanza Bridge (upper end of the primary nursery area) was continually jammed with ice blocks and underlaid with encroaching frazil ice. The river below Bonanza Bridge was covered with a solid canopy of ice. Holes drilled in the river near Jensen and the resulting cross-section of ice conditions following breakup indicated that many shoreline areas, including backwaters, had filled with frazil and jam ice, leaving little free water, and possibly establishing super-cooled water conditions. While these conditions did not occur during the present investigation, it is critical to understand linkages between severe winter ice conditions and . operations. This investigation showed, as have others (Ashton 1980), that ice processes are greatly influenced by water volume, stage changes, and air temperatures. CONCLUSIONS · The annual frequency of ice cover on the middle Green River has been reduced by 52% following the completion of Flaming Gorge Dam. · River surface ice on the middle Green River persisted for a longer time period under low, steady dam releases than under high, fluctuating releases, under similar mild winter temperatures. 23