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dOc9~ b. Castlewood Dam and Reservoir. For the BUthorized Castlewood Reservoir, an allowance of 7.000 acre-feet was made for the 100-year sediment accumulation. Because of the limited water yield from the basin upstream from the Castlewood damsite and because of the over-appropriated condition of existing flows, the 100-year sedim~nt pool was considered to be the maximum water surface ava~lable for recreation. In order to size the reservoir, studies were made to determine the level of flood control storage which would, when all factors were con- sidered. provide the best solution to the flood hazards and problems of the Cherry Creek basin. Several levels of flood control storage were analyzed at the Castlewood Reservoir, rang- ing from incidental or no flood control to complete or standard project plus probable maximum flood control. Construction of the Castlewood Reservoir without flood control storage would necessitate constructing a 260-foot wide concrete spillway at the Cherry Creek Reservoir, the least costly method of correct- ing that project's deficiencies. Such a solution would have undesirable environmental impacts upon the Cherry Creek Dam area and upon downstream reaches along Cherry Creek. The resi- dents of the area along Cherry Creek downstream from the dam have grown accustomed to living in a reach devoid of possible spillway outflows. The imposition of such a possibility on them after nearly 20 years, would have the effect of decreasing their standard of living and would impose a threat where one does not now exist. The installation of the vast expanse of concrete necessary for a large spillway would tend to degrade the visual environment of a highlY developed area. By increas- ing the amount of flood control storage in the Castlewood Reservoir, the spillway requirements for the Cherry Creek Reservoir could be decreased. The study results showed that a reservoir at the Castlewood site capable of storing the stand- ard project flood volume plus 53 percent of the probable maximum flood volume below the spillway crest would obviate the need for a spillway at the Cherry Creek project. In view of the intangible considerations, such as loss of life in the event of Cherry Creek spillway operation and the social and aesthetic impacts of the spillway, the flood control volume of the Castlewood Reservoir was selected so as to eliminate the Cherry Creek spillway. c. Cherry Creek Dam and Reservoir. With Castlewood Dam and Reservoir designed to control 53 percent of the probable maximum flood discharge, the Cherry Creek Reservoir can be modi- fied without the acquisition of additional reservoir lends or the construction of a spillway. Construction at the Cherry 32