John Day Lock and Dam, Portland District, U.S. Army Corps of Engineers
The Corps manages water levels to allow it to hold or release water, primarily to provide appropriate water levels for spawning salmon downstream of Bonneville Dam. (Only John Day Dam can store water; The Dalles and Bonneville dams are run-of-river dams, meaning the flow coming into the dam is the same as is going past it.)

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Photo by: U.S. Army Corps of Engineers |  VIRIN: 130308-A-AH115-025.JPG