The Big Thompson Flood: Colorado's Worst Natural Disaster

It happened on July 31, 1976, in the middle of Colorado's Centennial celebration - a three day weekend when the canyon was filled with tourists and residents. Between 2500 and 3500 people were in the canyon, filling motels and camping spaces, relaxing in their summer retreats or retirement homes. The canyon is not wide anywhere along its length so everyone was close to the river. People were coming and going from Estes Park and Rocky Mountain National Park.

The Storm

There have been flash floods in the canyon before, but the flood of July 31, 1976 was something entirely different. I will try to summarize information later reported by the National Weather Service and the National Oceanic and Atmospheric Association (NOAA).

• Early in the morning, an unusually large amount of moisture in the atmosphere east of the mountains was reported, along with very slow movement of the system.
• This highly moist air pushed upslope against the mountains by a polar front, creating a temperature inversion, and a thunderhead began rising, eventually topping at about 60,000 feet.
• Usually, the prevailing winds from the west push storms to the east away from the mountains, but a moist low pressure system had moved in across the Continental Divide from the southwest and it lacked the power to move this storm.
• Instead, it added to the storm by pushing smaller cells from the southwest into the storm.
• The base of the storm clouds was at ground level.

This storm stalled over the Big Thompson Canyon and just to the north of it for more than four hours. During this time, 10"-14" of rain fell on the area, an amount equal to a whole year of normal precipitation. The river began rising unbelievably fast just below Olympus Dam as every small stream and dry wash filled with water from the moisture laden clouds. Lightning was intense, flashing through the black clouds from side to side and striking all around, while thunder rolled continuously. And the rain! It fell in sheets, in pounding torrents strong enough to be painful to fleeing survivors.

The Flood

Water rose so fast! A woman helping to move some guinea hens to higher ground found water around her ankles and then, as she ran toward safety, it was up to her waist. Others were bowled over suddenly when a huge wave of water struck them. People in cars were isolated when mud slides and trees blocked their way and chunks

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