Big Thompson Canyon Flood of 1976

On July 31, 1976, a huge flood rushed through the Big Thompson Canyon and River in Denver, Colorado. There were a total of 144 deaths, many homes lost, and most of the US-34 in that alley destroyed and washed away. This flood was formed over the course of the day by  warm easterly winds being pushed up towards the peak of the canyon, and the rain clouds formed from these winds and water vapor moved at less than 20 mph, causing these clouds to pour torrential rain over the area for several hours.