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PRE- AND POST-DAM CONDITIONS Hydrology And Ice Processes Flow Pattern, Magnitude. And Variation Literature on ice physics from various rivers indicates that the operation of Flaming Gorge Dam has substantially altered ice processes in the Green River. High post-dam winter releases of relatively wane hypolimnetic water, and daily fluctuations likely contnbute to delayed formation of ice cover, greater ice instability, and a higher incidence of &azil and ice block jams. This results in less available low velocity habitat, and increased stress and mortality of fishes. Mean monthly pre-dam flows ranged from 597 cfs (16.9 cros) in January to 1,410 cfs (39.9 cros) in March, as compared to a post-dam range of 1,780 c£s (50.4 cros) in March to 2,260 cfs (b4.0 cros) in December. Mean daily flows of the Green River near Greendale, Utah (0.5 miles downstream of Flaming Gorge Dam), before (pre-dam, 1951-62) and after (post-dam, 1963-84) construction of Flaming Gorge Dam, reveal a substantially altered hydrograph, in terms of seasonal pattern, IIow magnitude, and variation (Figure 15). Pre-dam flows were typically low with little variation from October to early March, followed by increasing flows and increased variation from April through July, and low IIow with moderate variation from August through September. Average daily post-dam flows from October to early March were 2 to 2.5 times higher than pre-dam flows, while post-darn flows from April through July were 2 to 6 times lower than pre-dam flows. Variation in mean daily flows, as indicated by one standard deviation above, and one standard deviation below the mean (includes 68% of observations), was greater in post-dam releases (Figure 15). The band of variation (+\- 1 standard deviation) around mean daily flows, from October through March, for post-dam releases was 400 to 600 percent greater than for pre-dam flows. Most of this variation was attributed to differences in daily hydropower operation of the dam between years. Mean monthly winter flows of the Green River, pre- and post-dam, compared to flows of the Yampa River, illustrate the historic similarity in flow regimes of the two rivers (Figure 16). Flow of both rivers was typically lowest in December and January, with a sharply rising pattern between February and March. Peak mean monthly flow of the Green River was in June, about one month later than the Yampa River. Historic mean monthly flows of the Green River were about double the flows of the Yampa River. In contrast, winter releases from Flaming Gorge Dam were 2 to 2.5 times higher than mean monthly historic flows, with highest flows in December and January, and lowest flows in March. 40