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1 point, further upstream progression of the ice cover may be halted until the deposition of the floes somewhere downstream of the leading edge reduces channel conveyance enough to cause upstream water levels to rise and the flow velocities at the leading edge to be reduced. If flow velocities are high enough, the ice cover will stop progressing upstream. In this case, open water will remain upstream of the leading edge throughout the winter season. An intact stationary ice cover does not survive the spring of the year. Breakup transforms an ice-covered river into an open river. Two ideal forms of breakup bracket the types of breakup commonly found throughout most of North America. At one extreme is thermal meltout. During an ideal thermal meltout, the river ice cover deteriorates through warming and the absorption of solar radiation and melts in place, with no increase in flow and little or no ice movement. At the other extreme is the more complex and less understood mechanical breakup. Mechanical breakup requires no deterioration of the ice cover but rather results from the increase of flow entering the river. The increase in flow induces stresses in the cover, and the stresses in turn cause cracks and the ultimate fragmentation of the ice cover into pieces that are transported by the channel flow. Ice jams occur at locations where the ice fragments stop; severe and sudden flooding can result when these ice jams form or when they release. Most river ice breakups actually fall somewhere in between the extremes of thermal meltout and mechanical breakup because breakup usually occurs during warming periods when the ice cover strength deteriorates to some degree and the flow entering the river increases due to snow melt or precipitation. As a general rule, the closer that a breakup is to being a mechanical breakup, the more dramatic and dangerous it is because of the sudden increase in flow and the large volume of fragmented ice produced. Historical Water Temperature and Air Temperature Measurements Periodic water temperature measurements of the Green River have been made by the United States Geologic Survey (USGS) at their Jensen gage site. Measurements of the Flaming Gorge Dam release temperatures were also available. Maximum and minimum daily air temperature measurements in the vicinity of the study reach were obtained from three Utah weather recording stations: Dinosaur Quarry in Dinosaur National Monument, Vernal, and the Ouray National Wildlife Refuge. The air temperature measurements from these sites were similar, differing by only about 1°F (0.6°C) on any given day. For the purposes of this study, it was decided to use the Vernal, Utah air temperature records. The daily average air temperature for Vernal was estimated for each day by taking the average of the maximum and minimum temperature reported for each day (Panofsky and Brier, 1968). A statistical evaluation of the daily average air temperature are shown in Figure 5. The average daily average air temperatures for the years evaluated was below 32°F (0°C) by late in November and remained below 32°F (0°C) through the winter and until late February. Throughout the winter, the average daily average air temperature generally remained within a few degrees of 20°F (-6,7°C). A good index of the severity of a winter is found by accumulating the number of freezing degree days that occur during the winter period. The number of freezing degree days that occur on any day is found by subtracting the daily average temperature from 32°F. For example, if the daily average temperature is 23°F, the number of freezing degree days for that day would be 9. If 6 ~I 1 1 1