While firefighters continue to battle wildfires in the Los Angeles area, another danger is fast approaching. Rain expected over the weekend of January 25–26 may help extinguish the flames, but it also carries the risk of triggering deadly debris flows in the burned hillsides. These fast-moving torrents of water, ash, and debris can wreak havoc, destroying homes, infrastructure, and lives in minutes.
The phenomenon is not new to California. In 2018, the hills above Montecito, scorched by the Thomas Fire, gave way during a storm, unleashing a devastating debris flow that killed 23 people and damaged over 400 homes. Similar events now threaten wildfire-affected areas near Los Angeles, where experts warn that the conditions for another disaster are already in place.
When wildfires scorch hillsides, they leave behind barren landscapes of ash and damaged soil. The intense heat, sometimes exceeding 1,000°F, can create a water-repellent layer in the soil, preventing it from absorbing rain. With vegetation destroyed, there’s nothing left to stabilize the ground or intercept water. When rain falls on these vulnerable slopes, it mixes with loose ash and sediment, forming a thick, cement-like slurry that races downhill, picking up rocks, trees, and debris in its path.
Experts say the danger lies in the speed and unpredictability of these flows. A storm delivering just an inch of rain per hour can be enough to unleash destruction. The U.S. Geological Survey (USGS) has already mapped areas at risk, highlighting zones where debris flows are most likely if rain intensifies.
“Debris flows can move as fast as a freight train,” explained a geologist familiar with the region. “Once they start, there’s almost no time to react.”
Adding to the urgency is the role of climate change, which scientists say is making wildfires more frequent and intense while also driving heavier rainfall events. As global temperatures rise, the atmosphere holds more moisture, leading to downpours that can overwhelm fragile, fire-damaged landscapes.
A recent study estimates that sediment flowing from burned slopes after rain could increase by more than 10% in most western U.S. watersheds, amplifying the threat to communities near wildfire zones. With more than 21 million Americans now living within three miles of a wildfire burn area, the potential for devastation is growing.
Los Angeles city officials and emergency planners are preparing for the worst, urging residents in vulnerable areas to stay alert and heed evacuation orders if necessary. Meanwhile, researchers emphasize the need for long-term planning, including better land-use policies and community education, to reduce the risks associated with postfire debris flows.
For now, as rain clouds gather over the scorched hillsides, the residents of Los Angeles can only hope that the rains bring relief rather than another round of destruction.