Hoover Dam is protected against an uncontrolled overflow by two flood spillways, one on the Nevada side and the other one the Arizona side. The spillways are controlled by drum gates and the overflow goes into side channels and huge 15 m wide tunnels to the tailwater. The total capacity is 11000 m³/s. The large spillway tunnels have only been used twice, for testing in 1941 and because of flooding in 1983.
scanned slides, Minolta X700