If you can fish below the damn at what point on the river can you start?
Idaho Dept. of Fish and Game does not have any rules prohibiting anglers from fishing below the dam, however, the entity that operates the facility may have restrictions to address public safety issues. Typically, they post signs and have buoy lines to designate safety risk areas.
If you access the area to fish by crossing posted property, you run the risk of being cited for trespass.