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New Stora~e Facili- ties, On-Channel (Alternative III-A) The proposed N arrows 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-feet of direct irrigation water deliveries upstream and downstream from Sterling. The return flows from the use of this water and from the 68.000 acre-feet of conveyance losses would make a total of 18,250 addi- tional acre-feet of irrigation water availa ble 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- rigation and flood control) capacity to be reserved when the potential for flooding is high. Costs The 1980 cost estimate for construc- tion of the design project is $226 million. The interest cost during construction would be $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 construction 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 at 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 hahitat 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 demonstrate 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 he constructed to store 60,000 acre-feet of South 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 helow the design project. Costs The 1980 cost estimate for construc- tion of the 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 project are estimated to be $730,000. Impacts The investment required would total $173 million in construction and in. terest during construction. The net economic benefit would be -$75 mil- lion. The estimated impacts on fisheries would represent a moderate positive change from existing conditions. The estimated impacts on wildlife would be a small negative change from existing 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 additional agri- cultural water supply for the lower South Platte River. The reservoir presently stores 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 irrigation water 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 hecause 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