U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3106

September 2005

Prepared in Cooperation with the California Geological Survey

Printed on recycled paper

Introduction

Wildland fires are inevitable in 

the western United States. Expansion of man-made developments 

into fire-prone wildlands has created situations where wildfires can 

destroy lives and property, as can 

the flooding and debris flows that 

are common in the aftermath of the 

fires. Fast-moving, highly destructive debris flows triggered by 

intense rainfall are one of the most 

dangerous post-fire hazards. Such 

debris flows are particularly dangerous because they tend to occur with 

little warning. Their mass and speed 

make them particularly destructive: 

debris flows can strip vegetation, 

block drainages, damage structures, 

and endanger human life. 

The U.S. Geological Survey’s 

Landslide Hazards Program is 

participating in a multi-agency cooperative effort to 

investigate debris flows in burned areas of southern 

California and other parts of the western United 

States. Participating agencies are the U.S. Department of Agriculture (USDA) Forest Service, the 

Natural Resources Conservation Service, and the 

California, Colorado, and Montana Geological Surveys. The objective of this project is to develop 

methods needed to estimate the locations, probability of occurrence, and size of potentially destructive 

debris flows. Public officials can use this information 

to plan and execute emergency response and post-fire 

rehabilitation.

Analysis of data collected from studies of debris 

flows following wildfires can answer many of the 

questions fundamental to post-fire hazard assessments—what and why, where, when, how big, and 

how often? 

Southern California—Wildfires and Debris Flows

Photograph of a typical burned hillslope in southern California. Photograph by 

Sue Cannon.

What We Know About Post-Fire Debris Flows

What and Why? Post-fire debris flows generally are triggered by one of two processes: surface 

erosion caused by rainfall runoff, and landsliding 

caused by infiltration of rainfall into the ground. 

Runoff-dominated processes are by far the most 

prevalent because fires commonly reduce the infiltration capacity of soils, which increases runoff and erosion. Infiltration processes are much less common, 

but prolonged heavy rains may increase soil moisture 

even after a wildfire. The wetted soil then may fail, 

producing infiltration-triggered landslides.

Where? Although debris flows can occur in 

areas underlain by nearly any rock type, the areas 

most likely to produce debris flows are those underlain by sedimentary or metamorphic rocks that have 

more than about 65 percent of the area burned at 

moderate to high severities. In addition, debris flows 

are most frequently produced from steep (> 20°), 
Campground in Cable Canyon, southern California, where a debris flow on 

December 25, 2003, killed two people. A wildfire during the previous October 

burned hillslopes in the area, and heavy rains triggered the deadly debris flows. 

Photograph by Sue Cannon.

tightly confined drainage basins 

with abundant stored material, 

and are unlikely to extend beyond 

the mouths of basins larger than 

about 25 square kilometers (~10 

square miles) in area.

When? Post-fire debris flows 

are most common in the 2 years 

after a fire; they are usually triggered by heavy rainfall. Flooding 

and increased runoff may continue for several years, but it is 

unusual for post-fire debris flows 

to be produced beyond the second 

rainy season. Some of the largest 

debris-flow events have been triggered by the first intense rainstorm of the storm season. It takes 

much less rainfall to trigger debris 

flows from burned basins than 

from unburned areas. In southern 

California, as little as 7 millimeters (0.3 inches) of rainfall in 

30 minutes has triggered debris 

flows, and any storm that has intensities greater than 

about 10 millimeters/hour (0.4 inches/hour) is at risk 

of producing debris flows.

How big? Documented debris flows from 

burned areas in southern California and the western 

United States have ranged in volume from as small as 

600 cubic meters to as much as about 300,000 cubic 

meters. This larger volume is enough material to 

cover a football field with mud, rocks, and debris to 

about 65 meters deep.

How often? Basins slopes covered only by a 

thin layer of soil and loose rock and channels that 

contain only minimal deposits generally produce 

debris flows only in response to the first intense rainfall of the season. Basins that contain thick 

channel-fill deposits and slopes mantled with thick 

layers of soil and loose rock rubble commonly 

produce numerous debris flows throughout the rainy 

season. Once the sediment supply has been depleted, 

debris-flow activity wanes, but water floods are still 

possible.

Debris-flow hazard maps: In 2003, large 

areas in San Bernardino, San Diego, and Ventura Counties burned; debris flows are now a hazard in these 

burned areas. The U.S. Geological Survey (USGS) has 

published a series of maps of these counties that show 

the basins most likely to produce the largest debris 

flows. These maps show the potential for debris flows 

that might occur in response to 25-year, 10-year, and 

2-year rainstorm events; the maps are based on models 

derived from data collected from recently burned basins 

throughout the western United States. The models take 

into account the combined effects of the area of the 

basin burned, basin gradient, soil properties, and storm 

rainfall conditions. USGS scientists continue to refine 

ways to assess the risk of debris flows in recently burned 

areas, so that more timely information can be provided 

to emergency response agencies.

Rapid-Deployment Data-Collection Networks (RDNs)

RDN rain gages installed immediately after a wildfire 

provide valuable data for researchers studying the erosive 

response of burned basins, and they form the basis for 

post-fire flood and debris-flow warnings in some areas. 

Scientists use rainfall data from the RDNs to define rainfall 

intensity-duration thresholds that identify those conditions 

most likely to result in post-fire debris-flow activity. Rain 

gage data are used to develop models for estimating the 

probability and magnitude of debris-flow activity. Data 

collected by these RDNs are rapidly becoming an integral 

element of the Federal government mission of protection of 

life and property following wildfires. 
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Old Fire

Grand PrixFire

Fire Perimeter

Basin Outlet

None

1 to 33%

34 to 67%

68 to 100%

Map 1A. Probability of FireRelated Debris Flow in Response

to 25-Year, 1-Hour Storm of 1.12

Inches

Map 1A. Probability of FireRelated Debris Flow in Response

to 25-Year, 1-Hour Storm of 1.12

Inches

0 4 Miles

0 4 Kilometers

F

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Cajon Cyn.

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West Mojave Cyn.

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Devils Cyn.

Badger Cyn

Sycamor e

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Coldwater Cyn.

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Basin Outlet

Fire Perimeter

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1501-3000 cfs

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Map 1B. Estimated Peak Discharges

of Fire-Related Debris Flow in

Response to 25-Year, 1-Hour Storm of

1.12 Inches

Map 1B. Estimated Peak Discharges

of Fire-Related Debris Flow in

Response to 25-Year, 1-Hour Storm of

1.12 Inches

0 4 Miles

0 4 Kilometers

Maps of basins burned by the October 2003 Old and Grand Prix wildfires in San Bernardino County, California, showing 

the probability of debris-flows (Map 1A), and potential peak discharges that can issue from a basin outlet (Map 1B), in 

response to 25-year-recurrence, 1-hour-duration storms. Source given in reference 5 under “For more information.”
Before a Storm:

1. Watch the patterns of storm-water drainage near 

your home, and note the places where runoff water 

converges, increasing flow in channels. These are 

areas to avoid during a storm.

2. Contact your local authorities to learn about the 

emergency-response and evacuation plans for your 

area. Develop your own emergency plan for your 

family or business.

During Heavy Rainfall:

1. If you are in an area that is susceptible to flooding 

or debris flow (or has experienced flooding or debris 

flow in the past), consider leaving if it is safe to do so. 

Remember that driving during heavy rainstorms can 

be hazardous.

2. Stay alert and stay awake! Many debris-flow and 

flood fatalities occur when people are sleeping. 

Listen to the radio for warnings of intense rainfall. 

National Oceanic and Atmospheric Administration 

(NOAA) Weather Radio All Hazards tone alert will 

let you know of hazards in your area. Be aware that 

intense bursts of rain may be particularly dangerous, 

especially after longer periods of heavy rainfall.

3. If you are near a stream or a channel, listen for any 

unusual sounds that might indicate moving debris, 

such as trees cracking or boulders knocking together. 

A trickle of flowing mud or debris may precede larger 

flows. Be alert for any sudden increases or decreases 

in water flow and for a change from clear to muddy 

water. Such changes may indicate debris-flow activity 

upstream, so be prepared to move quickly. Don’t 

delay! Save yourself, not your belongings. 

4. Keep in mind that during flash floods and debris flows 

water may rise much faster and much higher than it 

would if the watershed were not burned.

5. Be particularly alert when driving. Bridges may be 

washed out, and culverts overtopped. Do not cross 

flooding streams!! Embankments along roadsides are 

particularly susceptible to landsliding. Watch the road 

for collapsed pavement, mud, fallen rocks, and other 

indications of debris flow.

What Can You Do If You Live Near A Recently Burned Area?

For more information

1. http://landslides.usgs.gov/research/wildfire/

2. Gartner, J.E., Bigio, E.R., and Cannon, S.H., 2004, Compilation 

of post-wildfire runoff-event data from the western United States: 

U.S. Geological Survey Open-File Report 2004–1085. 

http://pubs.usgs.gov/of/2004/1085/

3. Cannon, S.H., Gartner, J.E., Rupert, M., and Michael, J.A., 

2004, Emergency assessment of debris-flow hazards from basins 

burned by the Piru, Simi, and Verdale fires of 2003, southern 

California: U.S. Geological Survey Open-File Report 03–481.

http://pubs.usgs.gov/of/2003/ofr-03-481/

4. Cannon, S.H., Gartner, J.E., Rupert, M.G., and Michael, J.A., 

2004, Emergency assessment of debris-flow hazards from basins 

burned by the Cedar and Paradise fires of 2003, southern California: 

U.S. Geological Survey Open-File Report 2004–1011. 

http://pubs.usgs.gov/of/2004/1011/ 

5. Cannon, S.H., Gartner, J.E., Rupert, M.G., Michael, J.A., Djokic, 

Dean, and Sreedhar, Sreeresh, 2004, Emergency assessment of debrisflow hazards from basins burned by the Grand Prix and Old fires of 

2003, southern California: U.S. Geological Survey Open-File Report 

03–0475. http://pubs.usgs.gov/of/2003/ofr-03-475/

Sue Cannon, Project Chief, U.S. Geological Survey 

Phone: 303-273-8604, cannon@usgs.gov

Lynn Highland

National Landslide Information Center

U.S. Geological Survey

Federal Center, Box 25046, MS 966

Denver, CO 80225 USA

1-800-654-4966

http://landslides.usgs.gov

This fact sheet is available on-line at 

http://pubs.usgs.gov/fs/2005/3106

Publication layout by Margo L. Johnson

Debris-flow deposits produced during a Christmas day 

2003 storm in Waterman Canyon, San Bernardino County, 

California. Photograph by Sue Cannon.