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On July 4th, 1976, the United States celebrated its bicentennial. Coloradoans were to celebrate their centennial on August 1st. Many celebrated that weekend by heading to the mountains to hike, camp, fish, and just hang out. Thousands were enjoying themselves in the Big Thompson Canyon area near Rocky Mountain National Park in the evening of July 31st when a most usual storm gathered overhead. The scale of the resulting flood caught everyone by surprise. The record flood waters rumbled down the narrow canyon (see picture below) taking out everything in is path. Officials would later report 145 dead and $41,000,000 in damage.

Here’s a report from NOAA on the occasion of the 25 anniversary of the tragedy:

At the height of the Colorado tourist season, several thousand people escaped city heat by traveling to a popular camping area an hour northwest of Denver for hiking, fishing, camping and relaxing in the cooler mountain air. By late afternoon, an estimated 2,500-3,500 people were enjoying themselves in one of Colorado’s most scenic river valleys. They had no way of knowing that unusual atmospheric conditions and the physical make up of the Big Thompson River valley were setting the stage for disaster.

The Big Thompson River basin is similar geologically to many river basins along the eastern side of the Continental Divide. Sheer rock forms the canyon walls, with little soil and vegetation to absorb runoff from storms. The river starts high in the Rocky Mountains near Estes Park in north-central Colorado and flows eastward through the rugged, steep-walled canyon. In some places, the canyon walls jut almost straight up. From top to bottom, the river drops vertically more than half a mile and exits the canyon into the rolling, forested plains west of Loveland. Dotted with homes, restaurants and other businesses, U.S. Highway 34 stretched the length of the canyon.

As campers frolicked, a witch’s brew began to develop in the atmosphere.

A weak but moist easterly flow began to develop on the east side of the Rockies. The moist air rose up the mountain slopes and combined with daytime heat to form thunderstorms. A thunderstorm lifted along the Front Range and began to dump heavy rain on the region about 6 p.m. Winds found at mountain crests of 10,000 feet are usually strong enough to push thunderstorms to the east and out of the area. On July 31, 1976, however, the upper winds were extremely weak and weren’t strong enough to push the storm away from the Big Thompson Valley.

Instead, the storm remained virtually stationary for more than three hours and dumped a foot of rain into the canyon. Eight inches of rain fell in one hour-long stretch, and turned the normally placid two-foot-deep trickle into a raging torrent of water 19 feet high. Sweeping 10-foot boulders in front of it, the wall of water sped down the canyon slope. Cars, campers, and buildings in its path had no chance of survival.

The wall of water moved so fast that, even had Highway 34 not been washed out, the only avenue of escape was up the canyon walls. Vehicles and buildings became death traps for unsuspecting campers.

In two hours, the Big Thompson Canyon flood killed 145 people (including six who were never found), destroyed 418 houses and damaged another 138, destroyed 152 businesses and caused more than $40 million in damages.

An aftermath of the flood was adoption of numerous regulations that limit building along the Big Thompson River and other, similar rivers throughout the United States. The tragedy was also a major impetus nationwide for creating early warning systems for flash floods in mountainous cities and recreation areas.

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