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The evaluation of flow in the study reach during the winter was based upon records from the Jensen gage (USGS Station No. 09261000), located approximately 93 river miles downstream of the dam at RM 316.6, in the upstream end of the study reach. There is a nearly continuous record for this gaging station for the period from October 1946 to the present. 2.2.2 Water and Air Temperature Measurements Periodic water temperature measurements of the Green River have been made by the USGS at their Jensen gage site. Measurements of the temperature of water released from Flaming Gorge Dam were also available for some years. Maximum and minimum daily air temperature measurements were obtained from three weather recording stations in the vicinity of the study reach: 1) Dinosaur Quarry in Dinosaur National Monument, 2) Vernal, Utah, and 3) the Ouray National Wildlife Refuge. The air temperature measurements from these sites were similar, differing by only about 0.6°C on any given day. For the purposes of this, study, it was decided to use the Vernal, Utah air temperature records, which were more extensive and complete than for the other recording stations. The daily average air temperature was estimated for each day by taking the average of the maximum and minimum temperature reported for each day (Panofsky and Brier 1968). A good index of the severity of a winter can be calculated by totaling the number of freezing degree days ("accumulated freezing degree days", abbreviated AFDD) throughout the winter period. The number of freezing degree days that occur on any day is found by subtracting the daily average air temperature from 0°C. For example, if the daily average temperature is -5 °C, the number of freezing degree days for that day would be 5. If the daily average temperature is above 0 °C, a negative number of freezing degree days is found for that day. The severity and length of winters in the study area were determined for each year from 1950 through 1997 by finding maximum AFDD for each winter period and by determining the date on which the maximum AFDD value occurred. 2.2.3 Historical Ice Observations There are only a limited number of historical ice observations available for the Green River in the vicinity of the study area. These include a summary of USGS observations made at irregular times throughout the winter2, and the BIO/WEST reports of Valdez and Masslich (1989) and Valdez and Cowdell (1999). 2The summary of USGS observations was based upon information in unpublished USGS logbooks obtained in 1992 by Dr. Jack Schmidt, University of Utah. -7-