Flash floods are much more dangerous and flow much faster than regular floods. They result from tropical storms, dam failures, or excessive rain and snow.
The Big Thompson River flows through the Big Thompson Canyon on its way down from the Rocky Mountains to the South Platte River. On July 31, 1976, the canyon was filled with vacationers in motels and campgrounds. As thunderstorms began to form late in the afternoon, no one was greatly worried. However, this line of storms began to weaken as it reached Big Thompson, leaving 12 miles of intense storms over the upper third of the canyon.
It soon began to sprinkle, and then to pour. By 6:30 in the evening, rain crashed down from the sky and continued for nearly five hours. A state trooper claimed raindrops were half an inch wide and dropped straight down.
At 8:00 p.m., news came that U.S. Highway 34 had washed out. When a trooper went to investigate, he realized that the situation was becoming extremely dangerous. He radioed for everyone to evacuate as quickly as possible. As his own patrol car began to wash away, he had to abandon his position and swim to higher ground. In some areas of the canyon, the river had risen more than 20 feet. The water tore down the banks, sweeping away everything in its path. Campers, tents, trailers, trucks, and cars were washed away. Buildings were torn apart, and many people disappeared.
The narrow walls of big Thompson Canyon forced the river and rainwater to flow dangerously fast down to the sea. Because of the strength and speed of the water, this flash flood was extremely hazardous.