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.

The Alluvial Fan

Besides the loss of life and the monetary cost, a flash flood of that magnitude leaves its mark on the landscape. A deep, five mile long scar marks the path of the Lawn Lake flood down the Roaring River. At Horseshoe Park, the bottom of this first descent, it ends in a huge fan-shaped deposit of silt, boulders, and broken trees - a geologic feature called an alluvial fan. Debris in this fan dammed Fall River, creating a shallow lake and wetland that is known as Fan Lake. An outlet channel along the margin allowed drainage into Fall River. The photograph above was take from Trail Ridge Road. The top of the channel is not visible in this photo.

The road to Endo Valley goes across the Alluvial Fan. At first, I drove through this devastated area without stopping, but one day I got out of the car and started looking around. Since then,I have stopped many times, still in awe of the force that created it. A few years after the flood the piles of broken trees began to weather in the sun, wind, and rain, exposing lovely grains and patterns of wood. Water flows down the middle of the deep scar and, depending on the season, cascades over the displaced rocks or emerges here and there from under layers of ice. Many trees were broken off four or five feet from the ground and the stumps stand like silent statues. This scar has a stark beauty that draws me back to see it in different light and at different seasons. It is hard to describe the size of the channel in any meaningful way. The photograph above doesn't cover the width of the area just above where the fan widens out.

If you look closely, you can see a little red dot slightly above and to the right of the center of the picture above. I cropped and enlarged this part and now you can see that the red dot is the coat on one of two people who are sitting among boulders the size of large trucks. They are dwarfed by the the gouged-out channel.

A few wild flowers now bloom among the weathered logs and piles of rocks. This scar that was created so suddenly will continue to change at a much slower pace. Fan Lake will eventually drain away, probably leaving a wetland area for some time. Heavy run-off into Fan Lake in 1995 caused changes in the outlet area and work had to be done to avert a sudden release of water that might have caused more flooding in Estes Park. This is a fascinating area, and well worth the time to stop and explore. Lawn Lake still exists (without a dam) and energetic hikers can climb the trail along the Roaring Fork channel to that lake and beyond. This trail is described in one of the following references.

References:

This USGS article is short, but informative. Note that the figures in this article are slightly different from those I quoted. I found numerous accounts of the flood with varying estimates and some out and out errors, so I used the best I could find.