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0 DRAFr February25, 1998 For the period prior to 1963, the 1984 peak flow of 40,000 ft 3 at Jensen would have had a frequency of approximately a 50-year return period flood. Using the post-1963 data base, this discharge is estimated to have a return period of about 250 years (FLO Engineering, Inc., 1996). For approximately the first 29 years of Green River record, until 1930, the mean annual peak discharge was 39,700 W/s (excluding 6 years of missing record). The 1984 peak discharge of 48,300 ft /s at Green River, Utah was exceeded only seven other times during the period of record and all seven times occurred before 1921. Thus, a discharge that was equaled or exceeded in one third of the years of available record during the period from 1894 to 1921 has only been exceeded once since 1921. This observation agrees with the results for mean annual flow prior to 1947 described earlier. Development of water resource facilities also has played a role in the reduction of peak flows, most notably Flaming Gorge Dam. After the completion of the dam, historical spring flows that averaged about 7,000 ft3/s at Greendale, Utah between 1951 and 1962 were reduced to about 3,000 ft/s (Smith and Green, 1991). Water released from the dam for power generation has ranged from 800 to 4,200 ft3/s , although a recent upgrade of the generators allows power releases up to 4,950 ft3/s. The maximum power release is limited by the capacity of the turbines, whereas the lower limit is set by an agreement with the State of Utah to maintain a high-quality • cold-water fishery in the tailwater of the dam. An additional 4,000 ft/s can be releases through steel-lined jet tubes, and 28,800 ft3/s can be discharged over the spillway. Flows greater that 4,950 ft/s are referred to as spills and produce no electric power (Yin et al. 1995). Peak Flows - Tributary Streams Streamflow records for U.S. Geological Survey gauges on six tributary streams in the Green River basin were evaluated by Pick (1996) and the magnitude of flood flows for a range of occurrence frequencies was determined. The gauges used in this analysis were: Yampa River near Maybell, Colorado Little Snake River near Lily, Colorado Duchesne River near Randlett, Utah White River near Watson, Utah Price River at Woodside, Utah San Rafeal River near Green River, Utah For the stations with major upstream reservoirs, especially the Duchesne River with four upstream reservoirs, no attempt was made to remove the effects of regulation from the record. Also, it was assumed that log-Pearson type III was the proper distribution to be used in analysis of the flood records for these gauges. The computed flows for a range of return periods are given in Table 2. 8