Salmo,
on the Clackamas in Oregon, they have at least one dam that works on a diversion tactic. For flood control, would it not be much cheaper in the long run to just put some type of over flow system and allow the main river to continue to flow and just allow the dam to fill as needed. Maybe a channel through the main dam with a out flow that only allows a certain amount out. During low water it would be completely open, and during high water times the out flow lowered.