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Final Report September 2000 To achieve these objectives, Flaming Gorge Dam releases should provide a wide range of peak and base flows, and daily fluctuations in downstream reaches due to hydropower generation should be moderated. Providing greater inter-annual flow variability would maintain and restore important geomorphic and biological processes and improve the spatial and temporal habitat complexity in the system, which is required by the endangered fishes. Such inter-annual variability should be achieved by providing flows consistent with hydrologic conditions in the upper Green River basin in a given year. Flow recommendations were based on the following information or assumptions: (1) populations of the endangered fishes and habitats required by all life stages are concentrated in Reaches 2 and 3 of the Green River; (2) habitat for endangered fishes in Reach 1 is limited to Lodore Canyon because the summer water temperatures upstream are too cold; (3) providing suitable habitat conditions through flow and temperature management at Flaming Gorge Dam will enhance endangered fish populations in the Green River; (4) current hydrology of the upper Green River basin, including inflows to Flaming Gorge Reservoir and available release volumes from Flaming Gorge Dam, will remain largely unaltered; and (5) changes in flow, temperature, and sediment regimes in Green River tributaries (particularly the Yampa and White Rivers) will be consistent with known or pending biological opinions. Specific peak- and base-flow target levels are recommended for each reach for five hydrologic conditions as defined by exceedance probability: wet (0-10% exceedance), moderately wet (10-30% exceedance), average (30-70% exceedance), moderately dry (70-90% exceedance), and dry (90-100% exceedance). Over the full range of hydrologic conditions, recommended peak releases from Flaming Gorge Dam range from full power-plant capacity (130 m3/s [4,600 cubic feet per second or cfs]) to greater than full bypass (244 m3/s [8,600 cfs]) as needed to achieve specific target flows in Reaches 2 and 3. No upper limits are placed on recommended peak-flow releases in any hydrologic condition. Onset of peak dam releases should be timed to coincide with peak and immediate post-peak spring flows in the Yampa River to produce higher peak flows and extend the duration of peak flows. Recommended peak flows in Reach 2 range from 235 m3/s (8,300 cfs) in dry years to greater than 748 m3/s (26,400 cfs) in wet years. In Reach 3, recommended peak flows range from 235 m3/s (8,300 cfs) in dry years to greater than 1,104 m3/s (39,000 cfs) in wet years. Relatively low base flows should be maintained for the summer through winter period (August through February). Base-flow releases from Flaming Gorge Dam should be based on the year's hydrologic condition (23-28 m3/s [800-1,000 cfs] in dry years to 50-76 m3/s [1,800-2,700 cfs] in wet years). Recommended annual mean base flows for Reach 2 range from 26-31 m3/s (900-1,100 cfs) in dry years to 79-85 m3/s (2,800-3,000 cfs) in wet years and, for Reach 3, they range from 38-72 m3/s (1,300-2,600 cfs) in dry years to 92-133 m3/s (3,200-4,700 cfs) in wet years. Variation in flow around the annual mean base flow for Reach 2 should be consistent with the variability that occurred in pre-dam flows. Mean daily flows should be kept within :t 40% of the annual mean base flow in summer-autumn (August through November) and within :t 25% of the annual mean base flow in winter (December through February); however, dam operations should not be adjusted to compensate for short-term increases in tributary inflow resulting from weather events that would exceed these thresholds. xxvi ~