Castlewood Dam

The Castlewood Dam was built in 1890, but failed in 1933, releasing a billion gallons of water, causing a 50-foot wall of water to rush down the canyon, and sending a 15-foot wave all the way into downtown Denver.  Luckily, only 2 persons were killed.

See also: http://www.atlasobscura.com/places/castlewood-canyon-dam-ruins