National Research Program


Risk of floods and erosion

USGS investigators are looking at the effect that fire and its aftermath have on watersheds. In unburned settings, the tree canopy softens the impact of driving rain and most of the rainfall soaks into the soil or runs off gently into watercourses. Wildfires can greatly alter the hydrologic characteristics of stream basins, by removing the tree canopy, undergrowth, organic litter, shallow roots, and obstructions and by creating water-repellent soil conditions. As a result, fire damage is often followed by severe flooding and fire-related erosion, particularly when intense rain falls over small steep watersheds soon after a wildfire that has burned both the soil and canopy.

 Rill erosion on a burned slope after fire: Buffalo Creek, Colorado.[Rill erosion on a burned slope after the Buffalo Creek, Colorado fire.]

In the American West, this pattern of fire damage followed by severe flooding and erosion has been an integral and important part of regional ecosystems for thousands of years. In a study, funded in part by the Denver Water Board, US Geological Survey researchers found a sequence of characteristic fire-flood deposits preserved at the mouths of stream valleys in Buffalo Creek, Colorado. Sediment layers alternated with burnt soil or charcoal layers; the oldest charcoal was 3000 years old. Although people and property along river channels downstream of burned mountainous watersheds are often at great risk, the data also show that the risk of fire-flood events drops considerably after only a few years as new vegetation is reestablished and the soil infiltration is increased by wetting, frost action, and animal activity.

For additional information, see Hydrologic and Erosional Responses of Burned Watersheds, or contact:

Deborah A. Martin or John A. Moody,, U.S. Geological Survey, P.O. Box 25046, MS413, Denver Federal Center, Lakewood, CO

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