Debris Flows-Fans / Mudslides

Debris flows are a common hazard in many hillside areas of Colorado. Heavy rainfalls commonly trigger flash floods on steep slopes. These torrents pick up anything in their paths and may contain more solid material than liquid. They tear into the hillsides and deposit accumulated material when they reach flatter ground, creating an alluvial fan.

Debris flow. Photo by TC Wait, 2007

An exceptional example of an alluvial-fan deposit may be seen in Horseshoe Park in Rocky Mountain National Park. This deposit formed on a single day in July 1982, when the Lawn Lake Dam burst, sending a huge surge of water and debris down the Roaring River Valley.

Although alluvial fans may appear to be attractive building sites, it is prudent to remember that another debris flow may follow the same course that created the area to begin with. If it does, your new home may well end up a bit farther down the valley and flatter than you planned.

“Debris Flow Events in Colorado” produced by the Colorado Geological Survey:

Mudslides are debris flows or mudflows that are often triggered by storms in the mountains. Debris flows (mudslides) are shallow landslides, saturated with water, that travel rapidly downslope as muddy slurries. The flowing mud carries rocks, trees, and other debris as it pours down the slopes. Mountainous areas that have had recent forest fires are especially prone to mudslides because they have lost stabilizing trees and vegetation. Recurring mudslides can prolong the revegetation of an area for years.

Head of mudslide with water problem

Head of a mudslide with water problem.

The Colorado Geological Survey has created a guide to assist citizens in dealing with post-wildfire hazards such as mudslides:
Post-wildfire Hazards

7-10-2012 Mudslides generated by Wildfire

Check this video at about 1:24 to see a mudslide entering the Cache La Poudre River from the burn area to the north (left).