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I I I I I I I I I I I I I I I I I I I Small Reservoir Feasibility Study S-6 Instream Flows To evaluate needs for instream flow maintenance, the modeled. streamflow conditions in critical reaches were compared to CWCB filings, negotiated curtailment and mitigation thresholds, and flow rates incorporated into NPDES discharge permits, Table S-5 lists the key reaches so examined, Table 8-5 Critical Instream Flow Reaches North Fork Snake River Snake River below North Fork Blue River above Swan River Blue River below Swan River West Tenmile Creek below West Lake Tenmile Creek above West Tenmile Tenmile Creek above Officers Gulch North Tenmile Creek below Frisco Town Ditch Blue River below Dillon Reservoir Blue River below Willow Creek Under existing demands, modeled instream flows on nearly all critical reaches exceeded the minimum requirements embodied in instream flow decrees, agreements, and discharge permits, The few occasions when flows fell below minimum requirements were in dry years when virgin hydrology was simply insufficient to meet the minimums, Under ultimate demands, flows on the Snake River below Keystone snowmaking diversions were frequently at the 2 cfs curtailment level and some snowmaking curtailments were induced, Flows below Dillon Reservoir fell below 50 cfs in 5 year of the study period, chiefly in November, when snowmaking exchanges are at their maximum levels, and in February, when Dillon inflows are at their minimum levels, Average monthly wintertime flows on Tenmile Creek above West Tenmile Creek regularly fall below the Ql-IO values used as the basis for the NPDES permit for Copper Mountain, Curtailment 0 snowmaking diversions via the Tenmile Creek pipeline would do little to alleviate this apparent problem, since virgin flows are also often below the Q7-10 level. Information obtained from the Colorado Department of Health shows that the Q7-10 flows for the Copper Mountain plant are higher in January than in June, This counter-intuitive finding, along with uncertainty about Climax operations which affect Tenmile Creek flows, suggest further study is needed to verify whether this problem is real, It should be noted that the basin model operates on a monthly time step. This simplification tends to mask instream flow deficiciencies of shorter duration than a month. Reservoir Levels Because of their importance to the County recreational economy, Dillon Reservoir levels under the baseline scenarios were computed from simulated storage contents, Generally speaking, Dillon levels are near the targets defined in the Summit County Agreement except in