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1 forms in the Green River study reach is composed largely of this frazil ice. The bridging location at the downstream limit of the ice cover 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 ice observations are summarized in Table 3. 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 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 31b) even though the maximum AFDDs recorded (486.5-1333.0) during the winters varied widely. There are two reasons for this. The first reason is that the ice cover consistently bridges at or near the Ouray Bridge each winter (RM 248}. The second reason is that the ice cover can progress 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 1987-88, layers of frazil ice were observed beneath the stationary ice cover in the reach from RM 305 to RM 316. This is the only winter season when such frazil ice deposits were observed. This was the coldest winter, as measured by the accumulated freezing degree days, for which ice observation 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. Frazil ice layers beneath river ice have been observed in many rivers throughout the world. It is interesting to note that the daily release schedule of Flaming Gorge Dam does not seem to have a very large influence on the overall pattern of ice formation in the study reach of the Green River except for the maximum extent of the ice cover. Even this difference is relatively small, and varied only between River Mile 302 and 316 (Table 3). It is important to note that the maximum extent of the ice cover occurred during the winter of 1987-88, when it was observed that the iee cover progressed upstream as far as Chew Bridge (RM 316). It was also during this winter that exceptionally thick ice covers were observed in the reach from RM 305 to RM 316 due to frazil ice deposition beneath the cover. It is likely that the daily fluctuating flows produced velocities capable of transporting the frazil ice produced upstream beneath the ice cover in this reach. During winters when the flow was held steady, such as 1996- 97, the flow velocities were not great enough to cause frazil ice transport beneath the ice cover. However, when the flows are held steady ice cover can only progress upstream as far as River Mile 310.8. Apparently this is due to the fact that flow velocity is too high upstream of this location to allow the ice cover to progress unless it is exceptionally strong. The deposition of frazil ice beneath the ice cover causes the ice cover to be thicker and ultimately stronger. In addition, the very thick ice covers formed through frazil deposition would tend to cause a "backwater effect" and "drown out" the high velocity areas. For these reasons, ice cover formed 10 1 1 1 1 1 1 1 'I J ri