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New Stora:Ji4acili- ties, On-Channel (Alternative III-A) The proposed Narrows Reservoir, with an active capacity of 973,000 acre-feet. was utilized to demonstrate downstream mainstem storage, A downstream mainstem reservoir on the South Platte River would serve as a flood control, irrigation. and recreation project. Supply The design project produces 34.000 acre-feel of direct irrigation water deliveries upstream and downstream from Sterling. The return flows from the use of this waler and from the 68.000 acre-feet of conveyance losses would make a lotal of 18,250 addi- tional acre-feel of irrigation waler available downstream from the de- sign project. Flood Control The proposed Narrows Reservoir would contain 475.000 acre-feet of storage capacity for flood control and 75,000 acre-feet of joint-use (ir- rigalion and flood conlrol) capacity to be reserved when Ihe potential for flooding is high. Costs The 1980 cost eslimate for construc- tion of the design project is $226 million. The interest cost during construction would he $94 million, The annual costs for operation. main- tenance. and replacement would be $1.4 million, Impacts The investment required would total $320 million in conslruction costs and interest during construction. The net economic benefit would be -$147 million, The estimated impact on water qual- ity may represent a small positive change from present conditions for al least a part of the year, Studies have, however. presented conflicting evidence regarding impacts of a downstream mainstem reservoir on water quality conditions in the South Platte River. The anticipated change from flowing stream habitat to lake habitat also would be a small positive change from existing fishery condi- tions. The anticipated change in wild- life would represent a negative change from existing conditions resulting from the inundation of 14,900 acres, New Storage Facili- ties, Off-Channel (Alternative III-B) Off-channel reservoirs in the plains zone would be utilized largely to store water for irrigation. The pro- posed Hudson and Wildcat Reser- voirs demons Ira te this variety of water resources development. The proposed Hudson Reservoir would be located on Box Elder Creek north- east of Denver, The reservoir would be sized with an active capacity of 120.000 acre-feet. The Public Service Company of Colorado and the River- side Irrigation Company have pro- posed that the Wildcat Reservoir be constructed to store 60.000 acre-feet of Soulh Platte River water for irri- gation and power generation. Supply The design project is assumed to produce 33.300 acre-feet of direct irrigation water deliveries upstream from Sterling, The return flows from the use of this water and from the 38,500 acre-feet of conveyance losses would make a total of 13.200 addi- tional acre-feet of irrigation water available below the design project. Costs The 1980 cost estimate for construc- tion of Ihe design project is $122 million. The interest cost during construction would be $51 million. The annual costs for operation. main- tenance. and replacement of the design proj ect are estima ted to be $730,000, Impaets The inveslment required would lotal $173 million in construction and in- terest during construction. The net economic benefit would be -$75 mil- lion. The eslimaled impacts on fisheries would represent a moderate positive change from existing conditions, The estimated impacts on wildlife would be a small negalive change from exisling conditions. Expansion of Existing Facilities (Alternative III-D) Several existing plains reservoirs could be expanded to provide addi- tional storage of mainstem flows. The Prewitt Reservoir is an example which would provide addilional agri- cultural water supply for Ihe lower South Platte River. The reservoir presently slores 27,500 acre-feet. and its capacity could be increased by 26.700 acre-feet. Supply The design project produces 1.500 acre-feet of direct irrigalion waler deliveries upstream from Sterling. The return flows from the use of this water and from the 3.000 acre-feet of conveyance losses would make a total of 830 acre-feet of additional irrigation water available below the design project. Costs The 1980 cost estimate for construc- tion of the design project is $15 million. The interest cost during construction would be over$6.2 mil- lion. The additional annual costs have not been estimated for this alternative because the facilities in question are now being operated and maintained. Impacts The investment required would total over $21 million in construction costs and interest during construction, The net economic benefit would be -$9.6 million, The estimated impacts on fish and wildlife would represent an overall small favorable change from existing conditions. 7