Alluvial Fan: A Legacy of the Lawn Lake Flood

In 1902, a 25 foot earthen dam was built on Lawn Lake at an elevation of 11,000 feet in what is now Rocky Mountain National Park. The dam increased water storage capacity of the lake to about 800 acre feet, or roughly 257 million gallons of water. Water from the dam normally was released down the steep channel of the Roaring Fork River into Fall River, and into the Big Thompson River at the town of Estes Park.

Flash Flood

Jump forward 80 years to about 6:30 a.m. on the morning of July 15, 1982. The earthen dam broke, releasing with explosive force nearly 700 acre feet of water (about 228 million gallons) of water. A battering ram of water surged down the steep mountainside, overflowing the river channel, ripping out trees and boulders and carrying them along in the furious crest of the flood. The sound was tremendous and the earth shook.

A Park Service employee was out working early, emptying trash barrels in the Horseshooe Park area about five miles below the broken dam at about 8500 feet. He heard the roar of the flash flood and used the radio in his truck to notify his dispatcher. Park Rangers rushed to warn campers at Aspenglen Campground located along Fall River. The flood tore through Aspenglen, drowning two men in the campground and one other camper along the river.

A 17 foot high concrete gravity dam at Cascade Lake failed under the onslaught, releasing another 12 acre feet (almost 4 million gallons) of water. The flood was slowed slightly as it ran through the open meadow and wetlands of Horseshooe Park, then it picked up speed and power again as it continued down the mountain. At Estes Park, the flood surged into the Big Thompson River. People in the town had some warning and no lives were lost, but six feet of water ran down the main street of the town, destroying shops with all their merchandise, and restaurants, as well as some of the infrastructure of this small town. Residents and tourists stood on higher ground and watched helplessly as the water swirled by.

Below Estes Park, the channel of the Big Thompson river had been deepened and fortified after an even more devastating flood on that river in 1976 (but that's another story). It contained the Lawn Lake water with little additional damage. However, the monetary loss in Estes Park was over $30 million, and damage in Rocky Mountain National Park was $2.5 million.

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