Arizona Spillway.

 spillway, interesting place

A controlled drum-gate spillway with a 200,000 ft^3/s flow capacity (400,000 combined). Both spillways (see also Nevada Spillway) handle overflow water and divert the water around Hoover Dam through the canyon rock and to the Colorado River downstream. The 75-foot opening tapers to a 50-diversion tunnel which terminates approximately 0.75 miles downriver. The Spillways are 27 feet below the crest of the dam and drop 600 feet to the Colorado River rapidly - in fact, so rapidly that in 1941 the force of the water during its highest recorded flow of the spillway (due to a failed drum gate) caused significant erosion. Engineering of the materials of both tunnels immediately afterward aimed at preventing future similar problems, though in the 1983 overflow, some erosion damage did occur again.

To put the amount of potential water flow in perspective, if the spillways were operated at full capacity, the energy of the falling water would be about 25,000,000 horsepower. The flow over each spillway would be about the same as the flow over Niagara Falls, and the drop from the top of the raised spillway gates to the river level would be approximately three times as great. The maximum water velocity in the spillway tunnels at this flow rate is about 175 feet per second, or 120 miles per hour.

Overflow has gone over these spillways only twice in the dam's history, in 1941-2 and again in 1983.