California continues to take stock of its unprecedented conflagrations—between the Camp Fire in the north, which on its own is the deadliest and most destructive fire in California history, and the Woolsey Fire in the south, 91 lives have been lost so far, with hundreds more still missing.

As firefighters keep battling the blazes, a new menace is emerging: debris flows. The rains have come to California, and the fires have primed the soil for another disaster that can claim yet more lives.

The coastal town of Montecito, in Southern California, learned this lesson less than a year ago. Last December, the massive Thomas Fire burned through the hills, right up to the edges of town. Thanks to the community’s comprehensive fire-preparedness plan, only 7 homes were lost. But a month later, the scorched hills outside of town let loose a storm-driven debris flow that enveloped the town, killing 21.

The problem is a fundamental change to the soil itself. When a wildfire burns vegetation, it releases organic compounds. “These compounds are heated in the fire, volatilize, and then recondense on soil particles,” says Brian Ebel, a hydrologist with the US Geological Survey.

This soil normally absorbs water, but the organic compounds turn it hydrophobic, literally meaning “fear of water.” It is not an understatement: You can pour a bottle of water on this burned soil and it’ll bead right off, as if the dirt were polished marble. The water will pool up and sit there, and it can take more than an hour for it to penetrate fire-scorched soil, Ebel says.

[#video: https://www.youtube.com/embed/3a02Bf8977s

Problem number two: vegetation anchors the soil. When fire burns it away, the soil loosens. “You can think of vegetation as a protective blanket,” says Jason Kean, a USGS hydrologist. Without it, sediment has nothing to hold it in place. “Before there's even rain, debris can roll down the hill and accumulate in channels.”

Now hydrologists and meteorologists are keeping a close eye on communities that butt up against these latest fires, where the conditions are set for catastrophic debris flows when the rain comes. “Once it starts raining hard on a burn area, the things can get generated really fast, like five minutes,” Kean says.

The debris flow can carry boulders and trees along with mud, making it more dangerous than a regular flood or a dry landslide. “When you mobilize them—keep them mobile with this muddy matrix in between them—they can go 3 kilometers,” Kean says. The video below shows just how easily the flow tosses boulders around.

[#video: https://www.youtube.com/embed/OTuHQOHjC6Q

Boulders can also act like a moving dam, retarding the flow. “So you get this surging behavior, where these things are holding back a whole bunch of flow behind it,” Kean says. “It really makes the flow depth increase several times what you'd get in a water-only flood.” The Montecito debris flow reached 30 feet deep at points, lodging boulders in trees. Humans are powerless to stop it—sandbags just become more debris for the flow.

But it can be predicted, giving residents a chance to evacuate. The work begins as soon as a wildfire’s smoke clears enough for satellites to get a good look at how badly areas have burned. This information is relayed to scientists, who enter those burned regions as soon as it’s safe.

The greatest risk of debris flow is in the zones that burned the most intensely, and scientists spend two or three days wandering the fire perimeter, taking soil samples. “That involves testing the hydrophobicity of the soil, the water repellency,” says Eric Huff, staff chief of Cal Fire’s Forest Practice Program, “and actually getting down on hands and knees and looking at what that surface soil layer is showing, whether or not organic matter has been completely burned off.”