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New Storage Facili- ties, On-Channel (Alternative II-A) The proposed Geer Canyon, Coffin- top, and Grey Mountain' Reservoirs were selected to illustrate water reo sources development in this alterna- tive. Geer Canyon Reservoir, with a storage capacity of 25,000 acre-feet, would he located about 5 miles north of Boulder on Left Hand Creek within the St. Vrain Creek sub-hasin. This reservoir would be designed to store the waters of St. Vrain Creek as well as to provide alternative storage for existing direct flow and reservoir decrees within the Left Hand Creek sub-basin. The Coffinlop Reservoir, with a slor- age capacity of 103,000 acre-feet, would he located in Boulder County on South St. Vrain Creek, approxi- malely 1'/, miles soulhwest of Lyons. The project would be constructed for water supply, hydroelectric power generation. flood control, water qual- ity, recreation. and fishery enhance- ment. Flows of North and South S1. Vrain Creeks would be integrated with a tunnel connecting the existing Buttonrock Reservoir on North S1. Vrain Creek with the proposed Cof- fintop Reservoir on South St. Vrain Creek. This tunnel would divert water to Coffin top Reservoir to pro- vide for power generation 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 active storage capacity of 200,000 acre.feet. A small hydroelectric power plant associated with the pro- ject would have a capacity of 2 MW. Energy Production and Use The project design for this configur- ation of reservoirs includes two hydro- electric plants with a total of 158 MW rated capacity. which operating at a 24 percent 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 lhe municipal use of the additional water would be 26.100 acre-feet annually. Agricul- tural reuse of return flows four times downslream would provide an addi- tional 9,600 acre-feet of irrigation water deliveries in the upper and lower South Platte River Basin. Costs The 1980 cost estimate for construe. tion of the design project is $352 million. The interest costs during construction would be $146 million by the time construction is completed in 1992. The annual cosls for opera- tion. maintenance, and replacement would be $2 million. Impacts The investment required would total $498 million in construction costs and interest during construction. The net economic benefit would be about -$131 million. The development of these projects would prohably result in a moderate positive change from present water quality conditions. The estimated impacls on fish and wildlife may be a large negative change from present 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 LoneTree Creek in Weld County. Approximately 20,000 acre feet would be allocated for an active storage conservation pool. Supply The design project produces 13,600 acre-feet of direct irrigation water deliveries upstream from Sterling. The relurn flows from the use of this water and from the 3,400 acre-feet of conveyance losses would make a total of 2,500 addtional acre-feet of irrigation water available below the design project. Costs The 1980 cost estimate for construc- tion of the design project is $108 million. The interest cost during construction would be about $45 million. The annual costs for opera- tion. maintenance, and replacement of the design project would he over $640,000. Impacts The investment required would lolal $153 million in construction costs and interest during construction. The net economic benefit would he -$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 cutthroattroul may be affected. 'Parts of Ihe Cache la Poudre River are being considered for potential designation as a Wild and Scenic River. Detailed studies of the im- pacts on Grey Mountain Reservoir by the Wild and Scenic River desig- nation have not been made to dale. 5