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with such a high damage potential and potential for loss of life is not adequate. Additional storage upstream is therefore required to provide a more reasonable degree of protection. . Plans for additional upstream storage included four alterna- tive plans which would store the lOO-year flood, each with a different spillway width ranging from 20 to 300 feet; one plan for storing 60 percent of the standard project flood; and one plan for storing the standard project flood. All of these plans would store floodwaters behind a potential dam which would be located on the upstream part of Lowry Air Force Base. A dam for storing the probable maximum flood would exceed the feasible site capacity. For the six plans that would not exceed site capacity, those with the higher level of storage would result in higher downstream benefits and lower spillway costs at Kelly Road and Upper Lowry Dams. Similarly, for the lOO-year plans, the one with the narrowest spillway would result in the highest down- stream benefits and the lowest spillway costs. These savings in spillway costs, however, are offset by increased land and embank- ment costs. Of the plans for 100-year storage, the plan which would store the lOa-year flood with a spillway width of 200 feet at the Upper Lowry Dam and a 400-foot spillway at Kelly Road Dam maximizes net benefits. The total first cost of this plan for both Upper Lowry and Kelly Road Dams would be $11,Ol3,000; $2,ll5,OOO would be for lands, $7,l74,OOO would be for construc- tion, and $l,724,OOO would be for the purchase of lands and buildings immediately downstream from the spillway. The average annual benefits would be $1,540,000 and net benefits would be $721,000 per year. The benefit-cost ratio would be l.9. Two additional plans for 100-year storage, with the same spillway size but with larger outlet works, were considered to determine l-4 .