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resulting changes in stage would be approximately 1.5 m at the Greendale, Utah gage (located <br />0.8 km downstream of Flaming Gorge Dam), approximately 1.3 m at the Gates of Lodore (77 km <br />downstream) and approximately 0.6 m at the Jensen, Utah gage (150 km downstream) (Yin et al. <br />1995). Because this release pattern represents the extreme of powerplant operations, daily water <br />stage fluctuations from other operational patterns would be less pronounced. Low flow releases <br />from the dam typically occur at night. Flows usually increase rapidly between 0600 and 0800 <br />hours to the maximum flow rate and then remain approximately constant throughout the day. <br />The flow rate usually decreases between 1800 and 2200 hours back to the minimum flow. The <br />actual release pattern reflects power demand which may vary on any given day. <br />Hourly releases from Flaming Gorge Dam and the resulting flows recorded at the Jensen <br />gage during the 1997 portion of the field study are presented in Figure 18. During the three <br />weeks before the peaking period, Flaming Gorge Dam maintained a constant outflow of 1,900 <br />cfs, resulting in a relatively steady discharge of approximately 2,440 cfs at the Jensen Gage. <br />Diurnally peaking releases from Flaming Gorge Dam were initiated at 0700 hours on 25 January <br />1997, with a range of 800 to 3,000 cfs and a mean outflow of 1,900 cfs. Approximately 1.5 days <br />later, fluctuations related to this peaking cycle were detected at the Jensen gage and resulted in <br />flows that ranged from 1,700 cfs to 3,500 cfs during 26-31 January 1997 (Figure 18). <br />3.1.2 Water and Air Temperatures <br />The mean daily air temperature for Vernal, Utah from 1945-1997 was below 0 °C by <br />about the middle of November and remained below 0 °C through the winter and until early <br />March (Figure 19). From mid-December until early February, the mean daily air temperature <br />generally remained below -6 °C. The maximum number of freezing degree days for a number of <br />winters are listed in Table 2 and shown in Figure 20. <br />The mean daily air temperature at Vernal, the temperature of the water released from <br />Flaming Gorge Dam, and the water temperature recorded at the Jensen gage are shown in Figures <br />21 through 28 for the winters of 1989 to 1997 (USGS discontinued collection of water <br />temperature data at the Jensen gage prior to the winter of 1996-1997). Also shown in these <br />figures is the daily average discharge of the Green River at the Jensen gage for the same period. <br />It can be seen that the water temperature at Jensen was influenced both by the Flaming Gorge <br />release water temperature and the air temperature. The Green River water temperature at Jensen <br />reached 0 °C for at least a brief period during each of these winter periods and remained at 0 °C <br />for considerable periods of time for several winters. During the periods when the water <br />temperature was at or near 0 °C, the daily average air temperature was continuously below 0 °C, <br />often reaching minimums of -12 °C or less. It is during periods of very cold air temperatures and <br />with the water temperature entering the study reach at or near 0 °C that maximum ice production <br />in the study reach would be expected to occur. <br />-15-