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1 Winter of 1996-97 Field Survey Two surveys of the ice cover conditions on the Green River were conducted during the ' winter of 1996-97. The observations of the first survey, December 18 and l9, are listed in Table 2. The extent of the ice cover during this survey is shown in Figure 12. Some observations were also available for December 28, 1996 and are shown in Figure 13 (personal observation, Bryan ' Cowden). The second survey was conducted on January 23-34, 1997. The observations of the river ice cover made at that time are described below. In addition, ice thickness measurements and observations of river stages are described in the section "Summary of 1496-97 Field Measurements." On January 23-24, 1997 a complete ice cover existed on the Green River from the Ouray Bridge (RM 248.2) to a location just downstream of Razor Island (RM 310.8) (Figure 14), with the following exceptions: continuous open leads, one-quarter to one-third of the aver width, were ' observed within the Ouray Wildlife Refuge (RM 251.3-254), along Hamacker Bottom (RM 270- 276.4), neaz Collier Draw (RM 286.5-287.2), and neaz Bonanza Bridge (RM 288.8-289.8). In the remaining reaches, isolated, discontinuous leads were fairly common, but they accounted for only a small portion of the total ice-covered area. Between RM 310.8 and 314.5 the river made a gradual transition from a complete ice cover to open water. A 300-ft-long ice jam was observed on January 24, 1997 in the bend below Chew Bridge at RM 316.3. No ice was observed from the ' Chew Bridge to the confluence of the Yampa River and it is likely that the Green River was open to the Flaming Gorge Dam. No floating frazil was observed from Split Mountain Campground to Chew Bridge during the January 23-30 period. Historical Ice Formation on the Green River ' The general trend of ice cover formation in the Green River can be outlined based on the historical observations summarized above, the 1996-97 field surveys, and the historical water and ' air temperature data. First, construction of Flaming Gorge Dam undoubtedly had an influence on the ice regime downstream of the dam. The primary influence of the dam was to increase the river water temperature immediately downstream of the dam during winter. Wintertime stratification of the reservoir causes colder, less dense water to overlay warmer, more dense water (water is densest at 39°F [4°C]) throughout the winter. This warmer water comprises part or all of the winter time releases and, as a result, the release temperature rarely drops below 39°F (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. However, in the winter the river water 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 32°F (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 apparent at this point. ' Frazil ice was recorded in the Green River during every year for which observations are available. This ice was observed at the water surface, in the form of slush, flocs, and pancake ice, transported in the downstream direction by the river current. The stationary ice cover that ' 9