When heavy rain hits burned hillsides, it can set off dangerous floods and debris flows that can move with the speed of a freight train, picking up or destroying anything in their path. These debris flows, as parts of Colorado experienced in late July 2026, can move tons of sediment during a single storm.

What causes debris flows, sometimes called mudflows, and why are they so common and dangerous after a fire? I am a geologist whose research focuses on pyrogeomorphology, which is how fire affects the land. Here’s what we know.

How debris flows begin

When severe fires burn hillsides, the high heat from the fires, sometimes exceeding 1,000 degrees Fahrenheit (538 degrees Celsius), completely destroys trees, shrubs, grass and structures, leaving behind a moonscape of gray ash. Not only that, the heat of the fire actually burns and damages the soil, creating a water-repellent, or hydrophobic, layer.

What once was a vegetated hillside, with leaves and trees to intercept rain and spongy soils to absorb water, is transformed into a barren landscape covered with ash, and burned soil where water cannot soak in.

When rain does fall on a burned area like this, water mixes with the ash, rocks and sediment to form a slurry. This slurry of debris then pours downhill in small gullies called rills, which then converge to form bigger and bigger rills, creating a torrent of sediment, water and debris rushing downhill. All this debris and water can transform small streams and usually dry gullies into a danger zone.

Because the concentration of sediment is so high, especially when there is a large amount of ash and clay, debris flows behave more like a slurry of wet cement than a normal stream. This fluid can pick up and move large boulders, cars, trees and other debris rapidly downhill.

Dangerous flooding and debris flows on recently burned areas damaged homes and closed roads in Colorado, near Ouray and in the town of Beulah, in July 2026. In 2018, a few weeks after the Thomas Fire burned through the hills above Montecito, California, a storm triggered debris flows that killed 23 people and damaged at least 400 homes.

What controls size and timing of debris flows

The geography of the land, burn severity, storm intensity and soil characteristics all play important roles in if, when and where debris flows occur.

Fire and debris flow scientists with the U.S. Geological Survey use these variables to create models to predict the likelihood and possible hazards from postfire debris flows. After the Los Angeles fires in 2025, they developed maps to help residents, emergency managers and city officials prepare and predict postfire debris flows.

Some of the triggers of debris flows are literally part of the landscape.

For example, the slope angle in a watershed and the amount of clay in the soil are important. Watersheds with gentle slopes – generally less than about 23 degrees – and a lack of clay and silt-sized particles are unlikely to produce debris flows.

Other key factors that contribute to postfire debris flows relate to the proportion of the watershed that is severely burned and the intensity and duration of the rainstorm event.

Early important research in the field of pyrogeomorphology demonstrated that while large, intense storms are more likely to cause large, intense debris flows, even small rainstorms can produce debris flows in burned areas.

Debris flows are becoming more common

A whopping 21.8 million Americans live within 3 miles of where a fire burned during the past two decades, and that population more than doubled from 2000 to 2019. A recent study from central and northern California indicates that nearly all the observed increases in area burned by wildfires in recent decades are due to human-caused climate change.

The warming climate is also increasing the likelihood of more extreme downpours. The amount of moisture the atmosphere can hold increases by about 7% per degree Celsius of warming, leading to more intense downpours, particularly from ocean storms. In California, scientists project increases in rainfall intensity of 18% will result in an overall 110% increase in the probability of major debris flows.

Studies using models of fire, climate and erosion rates estimate that the amount of sediment flowing downhill after fires will increase by more than 10% in nine out of every 10 watersheds in the western U.S.

Even without rain, debris on fire-damaged slopes can be unstable. A small slide in Pacific Palisades, California, shortly after a fire burned through the area in 2025 split a home in two. A phenomenon called “dry ravel” is a dominant form of hillside erosion after wildfires in chaparral environments in Southern California

Preparing for debris flow risks

Research on charcoal pieces from ancient debris flows has shown fires and erosion have shaped Earth’s landscape for at least thousands of years. However, the rising risk of wildfires near populated areas and the potential for increasingly intense downpours mean a greater risk of damaging and potentially deadly debris flows.

As their populations expand, community planners need to be aware of those risks and prepare.

Jen Pierce is a Boise State University professor of geosciences. This article was originally published on The Conversation and is republished with permission. Visit theconversation.com for more articles.