While firefighters work to extinguish the Los Angeles-area wildfires, city officials and emergency managers are also worried about what could come next.

Light rain began falling on January 25, 2025, helping firefighters who have been battling fires for nearly three weeks, but rain can also trigger dangerous floods and debris flows on burned hillslopes. The National Weather Service issued a flood watch for the burned areas through Jan. 27.

Debris flows can move with the speed of a freight train, picking up or destroying anything in their path. They can move tons of sediment during a single storm, as Montecito, just up the coast from Los Angeles, saw in 2018.

What causes these 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.

When severe fires burn hillslopes, 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 hillslope, with leaves and trees to intercept rain and spongy soils to absorb water, is........

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