Design of Optimum Selective Withdrawal Operation for Temperature Management at Round Butte Dam, Lake Billy Chinook, Oregon

As part of the ongoing efforts to restore downstream anadromous fish passage through Round Butte Dam, modifications to the reservoir operation and withdrawal structure were evaluated. A reservoir temperature model (BETTER) was calibrated, verified, and then applied to evaluate the effects of proposed reservoir modifications on the in-lake and downstream temperatures. An understanding of the effect of these modifications on temperature is critical due to the presence of Endangered Species Act (ESA)-listed chinook salmon and bull trout downstream of the project. One of the proposed modifications (Surface/Selective Withdrawal) would provide surface attraction flows for collection of downstream migrants in the forebay. The stored cooler bottom waters during winter and spring could be released during the summer to best re-create the predam downstream temperature regime. The reservoir temperature model was used to optimize the Surface/Selective withdrawal blending sequence through a number of iterative model runs. The final blending sequence (Blend 13) indicated that Surface/Selective Withdrawal operation would restore downstream temperature conditions to a state very similar to that predicted to have occurred before construction of the dam, while still providing spring surface flows toward the forebay to enhance fish passage.