Debris flows, fast-moving masses of mud, rocks, and water, are becoming a growing concern in the United States, particularly in areas recently affected by wildfires. A map produced by the U.S. Geological Survey (USGS) highlights high-risk zones near Los Angeles after the Palisades Fire. The model simulates a scenario where 15 minutes of rainfall, at just under 1 inch (24 mm) per hour, could trigger debris flows in vulnerable regions.
The landscape itself can create ideal conditions for debris flows. Steep slopes, the presence of clay-rich soil, and silt-sized particles in watersheds are key contributors. Generally, slopes gentler than 23 degrees and lacking fine-grain materials are less likely to experience debris flows.
Post-wildfire conditions worsen the risk. Severely burned areas lose their vegetation, making the soil more unstable and susceptible to erosion. Even minor rainstorms can trigger debris flows in these landscapes, as highlighted by early research in pyrogeomorphology. Intense storms increase the likelihood of larger, more destructive flows, but even lighter rains can cause significant damage in burned regions.
In recent years, debris flows have become more common due to climate change and increasing wildfire activity. A staggering 21.8 million Americans now live within three miles of areas burned by wildfires in the past two decades a number that has more than doubled between 2000 and 2019.
Climate change is intensifying rainfall events, as warmer air can hold more moisture, leading to stronger downpours. In California, rainfall intensity is projected to rise by 18%, doubling the likelihood of major debris flows. Models suggest sediment flow after fires will increase by over 10% in 90% of watersheds across the western U.S., exacerbating risks for nearby communities.
Even without rain, fire-damaged slopes remain unstable. For example, a small landslide in Pacific Palisades after a wildfire split a home in two, showcasing the destructive potential of “dry ravel” a type of hillslope erosion common in Southern California chaparral environments.
Debris flows have shaped Earth’s landscapes for thousands of years, as shown by ancient charcoal deposits. However, with more people living near wildfire-prone areas and the rising frequency of intense rainstorms, the risk of destructive debris flows is higher than ever.
To mitigate these dangers, community planners must incorporate debris flow risks into their strategies. Enhanced mapping, early warning systems, and resilient infrastructure are critical to protecting lives and property as climate change continues to reshape the landscape.