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1 SUMMARY OF THE 1996-97 FIELD MEASUREMENTS In order to allow comparison of ice conditions in the study reach under steady flows with conditions after peaking flows, a controlled flow regime was implemented for Flaming Gorge Dam. After ambient air temperatures, water temperatures, and weather conditons became conducive to formation of ice cover (water temperatures near the freezing point for water and a weather forecast of consistent sub-zero air temperatures) releases from Flaming Gorge Dam were held steady fora 3-week period in order to allow ice cover to form within the steady reach. This steady flow period was followed by a period of fluctuating flows. Afield team observed ice conditions on the Green River before, during, and after the peaking period. During the three weeks before the peaking period, Flaming Gorge Dam maintained a constant outflow of 1900 cfs, resulting in a discharge of approximately 2440 cfs at the Jensen Gage, which is located approximately 90 miles downstream of the dam. Arrangements were made to begin a diurnal peaking cycle at 0700 hours on January 25, 1997, with a rang,P of 800 to 3,000 cfs. Goals 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 the wave passed. Two field teams spent January 23-24, 1997 measuring ice thickness and channel depth at 17 cross sections under the steady flows regime. These measurements were made by using a hand-operated ice auger to create a 2.25-in hole in the ice cover and a tape measure with a hinged weight to obtain the measurement of ice thickness following the procedures of White and Zufelt (1994). The depth of each sample location was measured using a stadia rod inserted through the hole in the ice. Measurements of ice thickness and channel depth were repeated at the same cross-section locations on January 29-30, after the waves from several peaking cycles had passed The locations of the cross sections are shown in Figure 15. The ice thickness was fairly uniform along the channel, with measurements ranging from 0.6 to 0.8 ft, both before and after the peaking period (Table 4 and Figure 16). Ice thickness and river depth data for the individual points on cross sections are shown in Appendix A. In the lower region of the study reach, from the Jensen Bridge to the Ouray Bridge, roughly half of the channel area was composed of juxtaposed frazil pans and floes while the remaining area was thermally grown sheet ice. From the Jensen Bridge to Razor Island the channel gradient was steeper, and the ice cover was rougher, consisting primarily of juxtaposed frazil pans and floes. In the event that stage fluctuations resulting from hydropower peaking caused release of the ice cover, ice motion detectors were installed at three locations within the study reach. Each detector consisted of a sensor unit and a wire circuit. The sensor unit, which contains an internal clock to record the date and time was installed on the shoreline and the wire connected to the unit was embedded into the ice cover. Any break in the wire, such as occurs when the ice cover moves, caused the sensor to record the date and time when the circuit was broken. The first ice motion detector was placed at RM 308.2 on January 25, 1997. A second ice motion detector was installed upstream of the Bonanza Bridge at RM 290.4 on January 26, and a third was installed within the Ouray Wildlife Refuge at RM 254.3 on January 30, 1997. 12 ~J ~i