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densest at approximately 4 °C) throughout the winter. This warmer water comprises part or all of the wintertime releases and, as a result, the release temperature rarely drops below 4 °C in the winter. The net effect of these warm releases is to keep the river immediately downstream of the dam free of ice throughout the winter. Water released during the winter cools as it travels downstream and the influence of the release of warmer water lessens with increasing downstream distance. The length of the reach influenced by Flaming Gorge Dam depends on the rate of the heat transfer from the water surface, with the distance inversely proportional to the heat transfer rate. If the temperature of water entering the study reach is at 0 °C, as is often the case throughout the winter, it is clear that the influence of Flaming Gorge Dam on the river water temperature is no longer evident at this point. Frazil ice was reported in the Green River study reach during every year for which records are available. This ice was observed at the water surface, in the form of slush, flocs, and pancake ice that was transported downstream. The stationary ice cover that forms in the Green River study reach is composed largely of this frazil ice. The ice cover bridging location at the downstream limit of the study area was consistently observed to be in the area of Ouray Bridge or beyond. The stationary ice cover progressed upstream from this point during each winter, consistently reaching between RM 302 and about RM 316. The extreme upstream limit of the stationary ice covers was at the Chew Bridge (RM 316). This is the downstream end of a steep gradient reach and it is unlikely that ice-cover in the study reach would progress upstream of this point in most mild and moderate winters due to the high flow velocity in the channel. It is interesting to note that the maximum upstream ice cover extent on the Green River only varied by about 14 miles (RM 302-RM 316) even though the maximum AFDDs recorded during the winters varied widely. There are two reasons for this: (1) the ice cover consistently bridges at or near the Ouray Bridge each winter (RM 248), and (2) the ice cover progresses upstream very quickly during periods of cold weather. The ice cover progresses upstream largely through juxtaposition from the Ouray Bridge (RM 248) to the Bonanza Bridge (RM 290). Upstream of Bonanza Bridge, the ice cover progresses largely through juxtaposition with some underturning of the ice floes. The tendency of the floes to underturn increases as the ice cover progresses further upstream from Bonanza Bridge because flow velocity and Froude number increase. During the winter of 1987-1988, layers of frazil ice were observed beneath the stationary ice cover in the reach from RM 305 to RM 316 (Valdez and Masslich 1989). This is the only winter season for which such extensive frazil ice deposits were reported. This was also the harshest winter, as measured by AFDDs, for which ice observations are available. It is likely that the intense cold of this winter season resulted in tremendous amounts of frazil ice being produced upstream of the study reach. The frazil ice was probably carried beneath the stationary ice cover which was prevented from progressing upstream beyond the Chew Bridge due to the steep gradient of the river. This frazil ice was deposited beneath the ice cover throughout the reach immediately downstream of the leading edge of the cover, RM 305 to RM 316. Under the meteorologic and flow conditions that occurred during the 1997 field study, there was no -23-