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Fact Sheet 2004-3142

Distinguishing between debris flows and floods from field evidence in small watersheds

By Thomas C. Pierson

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Post-flood indirect measurement techniques to back-calculate flood magnitude are not valid for debris flows, which commonly occur in small steep watersheds during intense rainstorms. This is because debris flows can move much faster than floods in steep channel reaches and much slower than floods in low-gradient reaches. In addition, debris-flow deposition may drastically alter channel geometry in reaches where slope-area surveys are applied. Because high-discharge flows are seldom witnessed and automated samplers are commonly plugged or destroyed, determination of flow type often must be made on the basis of field evidence preserved at the site.

First posted April, 4, 2005

For additional information:
Contact CVO 
Volcano Science Center, Cascades Volcano Observatory 
U.S. Geological Survey 
1300 SE Cardinal Court, Building 10, Suite 100 
Vancouver, WA 98683-9589

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Suggested citation:

Pierson, T. C., 2005, Distinguishing between debris flows and floods from field evidence in small watersheds:  U.S. Geological Survey Fact Sheet 2004-3142, 4 p., (Available at:

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