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.......v( New Storage Facili- ties, On-Channel (Alternative II-A) The proposed Geer Canyon, Coffin- top, and Grey Mountain' Reservoirs were selected to illustrate water re- sources development in this alterna- tive. Geer Canyon Reservoir, with a storage capacity of 25.000 acre-feet, would be located about 5 miles north of Boulder on Left Hand Creek witbin the St, Vrain Creek sub-basin. This reservoir would be designed to sture the waters of St. Vrain Creek as well as to provide alternative storage for existing direcl flow and reservoir decrees within Ihe Left Hand Creek sub-basin, The Coffinlop Reservoir, with a stor- age capacity of 103.000 acre-feet, would be localed in Boulder County on South SI. Vrain Creek. approxi- mately 1 'I, miles southwest of Lyons. The project would be constructed for waler supply. hydroelectric power generation, flood control. water qual- ity, recreation, and fishery enhance- menl. Flows of North and South St. Vrain Creeks would be integrated with a lunnel connecling the existing Butlonrock Reservoir on North St. Vrain Creek with the proposed Cof- finlop Reservoir on South St. Vrain Creek. This lunnel would divert water to Coffin top Reservoir 10 pro- vide for power genera lion for a pro- posed 156 MW pumped storage project. The proposed Grey Mountain Reser- voir on the Cache la Poudre River, west of Fort Collins should have an aclive slorage capacity of 200,000 acre-feel. A small hydroelectric power plant associaled with the pro- jecl would have a capacity of 2 MW. Energy Production and Use The project design for this configur- alion of reservoirs includes two hydro- electric planls with a total of 158 MW rated capacity,which operating at a 24 percenl plant factor, would produce 468,000 MWH of peaking power and 2 MW of reserve capacity annually, The average annual net value of this power output would be $13.2 million, Supply The average dependable municipal water supply provided by the design project would be 39,000 acre-feet. The relurn flows from the municipal use of the addilional water would be 26,100 acre-feet annually, Agricul- lural reuse of return flows four limes downstream would provide an addi- tional 9,600 acre-feel of irrigation water deliveries in the upper and lower South Platte River Basin, Costs The 1980 cost estimale for construc- tion of the design projecl is $352 million. The interest costs during construction would be $146 million by the time construction is completed in 1992. The annual costs for opera- tion, maintenance, and replacement would be $2 million. Impacts The investment required would total $498 million in construction cosls and interest during construction. The nel economic benefit would be aboul -$131 million, The development of these projects would probably result in a moderate positive change from present water quality cunditions. The eslimated impacts on fish and wildlife may be a large negative change from pres en I conditions. New Storage Facili- ties, Off-Channel (Alternative II-B) The proposed Cactus Hill Reservoir was chosen to illustrate this type of water resources development. Cactus Hill Reservoir, with a total capacity of 291,000 acre-feet, would be located on Black Hollow Creek. which is a tributary of Lone Tree Creek in Weld County. Approximately 20,000 acre feet would be allocated for an active storage conservation pool. Supply The design project produces 13,60C acre-feet of direct irrigalion watel deliveries upstream from Sterling, The relurn flows from the use of thi, water and from the 3,400 acre-feet 01 conveyance losses would make a total of 2,500 addtional acre-feet 01 irrigation water available below thE design project. Costs The 1980 cost estimate for construc- lion of the design project is $108 million. The inleresl cost during conslruction would be about $45 million, The annual costs for opera- tion, maintenance, and replacement of the design project would be over $640,000. Impacts The investmenl required would total $153 million in conslruction costs and interest during construction. The net economic benefit would be -$73 million. The estimated impacts on fish and wildlife would be a moderate nega- tive change from present conditions, specifically on Black Hollow Creek where the Greenback cutthroat trout may be affected, 'Parls of the Cache la Poudre River are being considered for potential designation as a Wild and Scenic River. Detailed sludies of the im- pacts on Grey Mountain Reservoir by the Wild and Scenic River desig- nation have not been made to date, 5