Often called "mudflows," debris flows are a kind of landslide: a fast-moving mix of dirt, rocks, trees and water — and sometimes ash — that starts on a hillside and rushes down the valley. They can easily overflow stream channels and badly damage houses, vehicles and other structures.
Land burned by wildfire is especially prone to them. Storms can trigger debris flows days to years after a fire, and on burned ground a flow can start on a dry slope after only a few minutes of heavy rain — about half an inch an hour or more. Debris flows can also travel many miles, threatening places downstream that were neither burned nor rained on.
How a postfire debris flow starts, travels down a stream channel and spreads. Credit: U.S. Geological Survey.
| 35 miles per hour | the speed at which a debris flow can travel |
| 25–50 people | killed by landslides in the United States each year |
Tip 1: Leave before the storm arrives — if you wait, you are too late
Get out of a debris flow danger area before the storm is forecast to arrive. Debris flows are faster than you are. If you wait until you are sure one is coming, it will be too late to leave safely.
Tip 2: If you cannot leave, get to high ground
If you are caught in a danger area, get to the highest point you can safely reach before the flow arrives. At home, go upstairs or onto the roof. Outdoors, move uphill or climb a tree.
Tip 3: Listen for warning signs, and look for anything unusual
Know the signs that a debris flow is about to arrive. Watch for rushing water or mud, and listen for unusual noises: survivors describe cracking, breaking, roaring, or the sound of a fast-moving freight train.
The fact sheet, from the USGS with the National Oceanic and Atmospheric Administration and the National Weather Service, is published in English and Spanish. More on the USGS's emergency assessments of postfire debris flow hazards is on the USGS Landslide Hazards Program website.
Sources
Based on "Postfire Debris Flow Hazards: Tips To Keep You Safe," by Steven Sobieszczyk and Jason W. Kean, U.S. Geological Survey Fact Sheet 2022–3078 (October 2022), published by the U.S. Geological Survey; rewritten in hubnx's own words.
- The diagram and the speed and death-toll figures come from the fact sheet's PDF: https://pubs.usgs.gov/fs/2022/3078/fs20223078.pdf
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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