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ladder does not reduce the original design capacity of the spillway. The for~bay water elevation for this alternate is approximately 4,575.0 feet. Again, this is the upstream water level desired by RWPC. From preliminary computations, tailwater was calculated as elevation 4,567.0 feet. The change in water surface elevation from below to above the dam is then approximately 8.0 feet. With the basic ladder design (23 weirs, 8-foot- wide by 10-foot-long ;pools:) and accounting for the backwater effect into the ladder, the head drop will gradually vary from 0.15 foot at the down- stream end of the ladder to 0.5 foot at the upstream end of the ladder. For this operational condition, the total ladder flow is approximately 18 cfs. c. For flows above 6,000 cfs and below 8,800 cfs, the ladder should be closed at the forebay entrance. The height of the upstream ladder walls (elevation 4,576.5 feet) will protect the ladder from high riverflows. If the ladder was open at the upstream end., the ladder structure would be overtopped by water and not be functional for fish passage. d. -Flows above 8,800 cfs can be passed by the existing spillway. The ladder should be protected from extremely high flows by temporarily restricting waterflow through the structure. e. For the standard operational conditions, auxiliary water must be added at the entrance diffuser to provide better fish attraction conditions. The total quantity of auxiliary water is three times the ladder flow quantity. Each entrance slide gate opening and amount of auxiliary flow can be adjusted to attain the desired head drop at the entrances. 3. Operational Flexibility. a. The number of fish ladder weirs can be reduced to 11 weirs, which form 8-foot-wide by 20-foot long pools. The result of this modified design is a 1-foot head drop per pool. For this operational condition, the total ladder flow is approximately 25 cfs and the available auxiliary flow is 75 cfs. At high riverflows (6,000 cfs), the head drop will gradually vary from 0.25 foot at the downstream end of the ladder to 1.0 foot at the upstream end of the ladder and ladder flow will be 25 cfs. b . The number of fish 1 adder weirs can also be i ncreased to 45 weirs, which form 8-foot-wide by 5-foot-long pools. The result of this modified design is an approximate head drop of 0.25 to 0.35 foot per pool. For this operational condition, the total ladder flow is approxi- mately 15 cfs and the available auxiliary flow is still 75 cfs. At high riverflows, the head drop will gradually vary from 0.10 foot at the • 28