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23 I I -I I I 1 I I 1 I 1 I :1 .1 I I I I I releases. Despite different operational scenarios, ice processes were similar for both years, with ice covering most shorelines and backwaters, but rarely extending as a canopy over the entire river. Although ice processes were similar, ice persisted for a considerably longer period in year 2--the year oflowest flow--as a result of a smaller mass of water and hence, a smaller heat budget. Ice jams occurred in both years, which led to geomorphic changes and eventual loss of a similar proportion of backwaters studied in both years (Le., 21-22%). Also, natural geomorphic processes lead to the loss ofan additional 11-14% of backwaters studied. Hence, nearly one-third of backwaters were lost during winter as a result of natural channel dynamics and ice jams, while no new backwaters were formed. We concluded that the resulting ice conditions in both years were more a function of the presence of the dam and relatively warmer releases than of the operation of the dam. Although we did not observe a loss of nursery habitat as a direct result of ice conditions--other than the indirect effect of darning caused by ice jams-these relationships may not hold true in more severe winters. In years of colder and more prolonged air temperatures, ice development is more extensive and prolonged and may be more closely linked to dam operations. 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 to 2.10 C below normal and an ice canopy persisted from December to mid- February. 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 jamed 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 this investigation, it is critical to understand linkages between severe winter ice conditions and operations. This investigation showed, as have others (Ashton 1979), that ice processes are greatly influenced by water volume. In the second year of this study (1994-95), when release volumes from December through February were 39 to 49% lower, ice persisted for much longer than in the first year of the study, despite similar weather conditions. We attlib~e this to a lower heat budget and hence more rapid cooling of the water mass leaving Flaming Gorge Dam. Relatively low water volume in a cold winter could lead to more rapid development of an ice canopy, which would insulate the river. Furthermore, if canopy thickness could quickly reach and exceed the critical breaking point, there would be a lower frequency of ice jams and frazil ice development; i.e., the vertical rise in river level necessary to break an ice layer is approximately three times the thickness of that layer (Donchenko 1978). According to calculations by Valdez (1995), in a severe winter, a critical ice thickness of 15 em would have to be reached at Jensen to resist anticipated stage changes of 46 em under maximum dam operations. At maximum ice development potential, this ice thickness could be reached in about 3 days (Calkins 1979). Although these calculations provide a possible management scenario--stabilize dam releases for 3 days to allow an ice canopy to develop-other unpredictable variables preclude such decisions, primarily air and water temperature.