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significant thickening of the ice cover after fluctuating flows were initiated and after the <br />upstream portion of the ice cover broke up, indicating that there was no significant deposition of <br />frazil under the leading portion of the ice cover. <br />It is interesting to note that the daily release schedule of Flaming Gorge Dam does not <br />seem to have a very large influence on the overall pattern of ice formation in the study reach of <br />the Green River except for the maximum extent of the ice cover. Even this difference is <br />relatively small, and varied only between River Mile 302 and 316 (Table 11) for studies that <br />made reliable observations of ice cover extent. The maximum documented extent of the ice <br />cover occurred during the winter of 1987-1988 (ice cover progressed upstream as far as Chew <br />Bridge, RM 316), a winter during which fluctuating flows were in effect at Flaming Gorge Dam <br />(Valdez and Masslich 1989). It was also during this winter that exceptionally thick ice covers <br />were observed in the reach from RM 305 to RM 316 due to frazil ice deposition beneath the <br />cover. It is likely that the daily fluctuating flows produced velocities capable of transporting the <br />frazil ice produced upstream beneath the ice cover in this reach. The mean daily flow at the <br />Jensen gage during the winter of 1987-1988 was 2,701 cfs, with a range of 1,470 to 3,700 cfs. <br />During winters when the flow was held steady, such as 1994-1995 and 1996-1997, the flow <br />velocities were not great enough to cause substantial transport of frazil ice beneath the ice cover. <br />However, during years when the flows were held steady, ice cover progressed upstream only as <br />far as River Mile 310.8. Apparently flow velocity is too high upstream of this location to allow <br />the ice cover to progress unless the cover is exceptionally strong. The deposition of fi-azil ice <br />beneath the ice cover would cause the ice cover to be thicker and ultimately stronger. In <br />addition, the very thick ice covers formed through frazil deposition would reduce conveyance <br />and cause a "backwater effect," thereby reducing the velocity in areas directly upstream. For <br />these reasons, ice cover formed during fluctuating flows could progress upstream of River Mile <br />310.8 when large quantities of frazil ice are available. However, it appears unlikely that any <br />substantial ice cover can develop within the study reach upstream of Chew Bridge. Ice covers do <br />form in some upstream areas outside the study reach (e.g., Island Park and Rainbow Park) during <br />some winters. <br />The breakup of the stationary ice cover in the study reach of the Green River appears to <br />occur largely as the result of thermal meltout due to water temperatures above 0 °C and mild air <br />temperatures. For example, meltout rates of 1 river mile per day were observed downstream of <br />Bonanza Bridge during the 1987-1988 winter (Valdez and Masslich 1989). Often relatively short <br />sections of the ice cover were observed to break up and move out more or less simultaneously <br />during periods when melting was occurring. No ice covers have been reported to last past late <br />March. <br />The primary result of daily fluctuations would be to transport frazil ice beneath the ice <br />cover in the reach above the Jensen Bridge. As the cover progresses upstream above Jensen <br />Bridge, the deposition of frazil ice caused by the daily fluctuations would result in an ice cover <br />-24-