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In addition, several surveys of ice cover distribution in the study reach of the Green River were conducted during this study. The first survey was conducted on 18-19 December 1997 before consolidated ice cover was present in most of the study reach. The second survey was conducted on 23-25 January 1997, after a consolidated stationary ice cover had formed on the river and just prior to the arrival of fluctuating flows in the study reach. The third survey was conducted on 28-29 January 1997, after several days of fluctuating flows within the study reach. These surveys were conducted by field teams that drove to multiple locations along the river and documented the presence or absence of ice cover and noted various characteristics of the ice cover (e.g., stationary vs. moving ice cover; frazil pans vs. consolidated ice cover; locations of ice jams) for the entire river reach. Brief inspections of the extent of ice cover were also conducted from small airplanes on 28 January and 20 February 1997. A summary of the ice observations from each of these sources is presented in Section 3.1.3. 2.3 1996-1997 FIELD MEASUREMENTS Releases from Flaming Gorge Dam were manipulated during the winter of 1996-1997 to allow comparison of ice conditions in the study reach under steady flows with conditions after peaking flows. After ambient air temperatures, water temperatures, and weather conditions became conducive to formation of ice cover (water temperatures near 0 °C and a weather forecast of consistent sub-zero air temperatures), releases from Flaming Gorge Dam were held steady for a 3-week period while ice cover formed within the study reach. This steady flow period, which began on 29 December 1996, was followed by a period of fluctuating flows; both periods had a mean release of 1,900 cfs from Flaming Gorge Dam. A field team observed and measured ice conditions on the Green River before, during, and after the peaking period. Goals of the field investigation were to document ice conditions before and after the wave from the peaking cycle passed through the study reach, and to record water and ice stage changes at cross-section locations as a hydropower-induced wave passed through the study reach. In addition to providing on-the-ground observations, this information was also needed to complete and calibrate modeling of ice processes in the Green River. During 25-28 January 1997, two teams worked alternating 8-hour shifts, to measure the relative water and ice stage at seven cross-sections (Figure 3) at approximately 1 hour intervals from just prior to the passage of the first peak release until changes in the measured stage indicated the first trough of the release had passed through each cross-section location. At the Chew (RM 316), Jensen, Bonanza (RM 290), and Ouray Bridges, the distances from the ice or water surface under the bridge to fixed reference points on the bridge rails were measured. At the remaining three sites (RM 254.5, 279.0, and 307.0; see Figure 3), changes in stage of the ice cover were measured by using shoreline-based surveying instruments to observe vertical -8-