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14C-; New Stor~ge Facili- ties, On-Channel (Alternative I-A) Potential water development under this alternative is illustrated by the Two Forks reservoir. which would be located just downstream from the confluence of the North Fork and mainstem Soulh Platte River, The total design capacity of the Two Forks project would be about 450,000 acre-feel to include flood control. sediment storage, waler supply, and surcharge storage. Supply The average dependable municipal water supply provided by such a project would be 30.800 acre-feel. The estimated return flows from the municipal use of this additional water would be about 20,600 acre-feet an- nually, Agricultural reuse of return flows four limes downstream would provide an additional 7 ,600 acre-feet of irrigation water deliveries, Costs The 1980 cost estimate for construc- tion of the assumed project is $239 million. The interest cost during the period would be over $99 million by the time construction would be com- pleted in 1992. The annual costs for operation, maintenance. and replace- ment would be $4.7 million, Impacts The investment required would total over $33B million in conslruction costs and interest during construc- tion, Net economic benefit would be about -$161 million. Some water qualily benefits would occur from releases of water in this part of the basin during periods of normal low flows downstream. The projected impacts on wildlife and fisheries would cause a large negative change from present conditions, The creation of lake fisheries could re- duce some of the large negative im- pacts caused by the loss of riverine fisheries, New Storage Facili- ties, Off-Channel (Alternative I-B) To illustrate water resources devel- opment under this alternative, the project considered was the proposed West Plum Creek Reservoir located in Douglas County, The proposed project would consist of a diversion dam on the South Platle River. a dam and reservoir on West Plum Creek to store South Platle River flows. and a tunnel connecting the Foolhills Fil- tration Plant to the West plum Creek Reservoir, Supply The average dependable municipal water supply provided by the design project would be 30,800 acre-feel. The return flows from the municipal use of the additional water would be 20,600 acre-feet annually. Agricul- tural reuse of return flows four times downstream would provide an addi- tional 7,600 acre-feet of irrigation water deliveries. Costs The 19BO cost estimate for conslruc- tion of the design project is $793 million, The interest cosl during con- struclion would be $329 million. The annllal costs for operation, main- tenance, and replacement would be $4.7 million. Impacts A total investment of$1.12 billion in construction costs and interest during construction would be necessary in this alternative. Net economic bene- fits would be nearly -$542 million. The reservoir site is on a stream where no threatened. endangered, or high interest species are found. Moderate positive change to the fish- ery could occur as a result of project construction, Wildlife impacts would be a negative change from present conditions. Management Changes of Existing Facilities (Alternative I-C) This alternative would consider the reallocation of flood control and waler conservation storage in Chatfield Reservoir to waler supply storage. Chatfield Reservoir is located on the mainstem South Platle River, about 2 miles south of the City of Denver, and is operated by the U. S, Army Corps of Engineers as a flood control project. The reservoir includes 20.000 acre-feet for sediment storage and 215.000 acre-feet for flood controL If operational procedures for flood control were changed, the reservoir could provide up to 54,000 acre-feet for project water supply, Supply The average dependable municipal water supply provided by Ihe design project would be 6.000 acre-feet. Tbe relurn flows from the municipal use of the 6,000 a~re-feet of water would be 4,000 acre-feel annually. Agricultural reuse of the return flows four times downstream would pro- vide an additional 1.500 acre-feet of irrigation water deliveries. Flood Control Additional use of these existing facil- ities for water supply slorage might be expected to reduce their flood control value, Costs New construction costs have not been estimated, as this alternative involves only changes in operation. Annual costs have not been esti- mated. for the facilities are now being operated and maintained. Impacts It was assumed that no additional investment costs would be required in order to implement this alter- native. The present value of project benefits less the present value of project costs would be $6,3 million. The projected impact on the fisheries and wildlife would be a moderate positive change from exisling con- ditions. 3