The Big Thompson Canyon Flood of 1976
It was the height of tourist season in Big Thompson Canyon, a popular camping area an hour west of the city of Denver, Colorado.
As several thousand people enjoyed hiking, fishing and relaxing at their campsites on the pleasant summer afternoon of July 31, 1976, they had no way of knowing that tragedy would strike in just a few short hours.
A weak but moist easterly flow was forming on the east side of the Rockies. The air rose up the mountain slopes, enhanced by daytime heating which caused cumulus clouds to spring up. As the afternoon wore on, benign cumulus clouds developed into cumulonimbus clouds, then into a thunderstorm accompanied by heavy rain.
The winds generally found at mountain crests of 10,000 feet are strong enough to push thunderstorms eastward and out of the area. On this particular afternoon, however, the upper winds were extremely weak, and the storm remained stationary over the mountain. Heavy rain continued to fall.
The slope leading into Big Thompson Canyon is sheer rock, unlike slopes with soil and vegetation which are able to absorb water. As the heavy rain fell, it plummeted straight down the walls of the canyon to the Big Thompson River below.
Eight inches of rain fell in one hour. The water in the river quickly rose over its banks. The weight of the charging water was so strong that it sent huge boulders hurtling with it downstream.
Residents and visitors had no idea that they were in the path of raging flood waters. Within two hours the Big Thompson Canyon Flood created over $30 million of property damage and killed at least 139 people.
[Some data courtesy of The Thunderstorm in Human Affairs, Edwin Kessler, Editor.]