July 31, 1976: It was a Saturday, the eve of Colorado’s 100th anniversary of statehood. The weather was reported to have been partly cloudy that afternoon, but conditions began to shift in the early evening. Around 5:30 p.m. local time, a large group of nearly stationary thunderstorms began forming just east of Estes Park. That sudden surge of water triggered a fast-moving flash flood along the Big Thompson River and steep adjoining canyon. There has not been a deadlier flash flood in the U.S. since that event, and it remains one of the deadliest weather disasters in Colorado’s history.