‘A flood on steroids’: Hydrologist explains the mechanics, dangers of debris flows in SoCal

A dangerous atmospheric river is poised to deliver “excessive rainfall” across Southern California, raising fears that the rain could unleash a threat that has been lingering in the burn scars of wildfires that ravaged Los Angeles communities in recent years.

Called debris flows, these fast-moving slurries of floodwater and sediment can hurtle down slopes carrying cars, trees and even boulders with them.

They’re like “a flood on steroids,” said Jason Kean, a research hydrologist with the U.S. Geological Survey’s landslide hazards program. “It’s really hard to stop these things. The best thing to do is get out of the way.”

Northern California is already reeling from the atmospheric river that unleashed catastrophic flash flooding in Redding over the weekend, killing a 74-year-old man who became trapped in his pickup truck on a flooded roadway.

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Now, another storm is expected to reach coastal Northern California this evening, with strong winds and especially heavy rainfall pushing into Southern California.

Forecasters warn that the rain “will cause life-threatening flash flooding, along with landslides, rockfalls, and/or mudslides,” particularly for areas along Southern California’s Transverse Ranges, including the San Bernardino, San Gabriel, and Sierra Madre mountains. “Urban flooding in the greater Los Angeles metro area is likely.”

Burn scars — slicked by fire and stripped of plants — are especially dangerous during heavy rains. A storm after the Thomas Fire in 2018 spurred debris flows in Montecito that killed 23 people. And in February, a debris flow in the Palisades Fire burn zone swept a Los Angeles Fire Department member and his SUV into the Pacific Ocean.

The Los Angeles County Department of Public Works warns that there’s a risk of moderate debris and mudflows capable of blocking roadways and endangering some structures in the burn scars of more than a dozen fires — including January’s Eaton, Hurst and Palisades fires.

The county has issued evacuation warnings in and around recent burn scars, and urged those who may take longer to evacuate to consider leaving now. Officials also announced some targeted evacuation orders for specific properties “at higher risk for mud and debris flows impacts.”

“Recent burn areas, including those impacted by the January wildfires, remain highly susceptible to mud and debris flows,” county officials warned Monday.

Gov. Gavin Newsom announced Monday that more than 225 personnel and resources including 45 fire engines, 10 swiftwater rescue teams, helicopters and more have been pre-deployed to a dozen northern and southern California counties.

Meteorologists are discussing it as a “one in every five years type of storm,” said Lisa Phillips with the National Weather Service. “There's going to be problems everywhere. We have a lot of canyon roads and mountain roads, and things will fall down.”

Phillips said she expects to see mudslides, landslides, sinkholes — even after the rains have ended, and urged people to stay off the roads if they can.

“There's going to be issues outside of the burn scars too, and flooding,” Phillips said. “We want everyone to stay at home, stay safe and don't get yourself into any trouble unnecessarily.”

We spoke with the U.S. Geological Survey’s Kean, an expert on debris flows after wildfires, last month about what to expect when storms strike burn scars. This conversation has been edited and condensed.

As this storm really takes hold in L.A. and Southern California, I'm hearing a lot of concern about it hitting areas that burned this past year, including in the Eaton and Palisades fires. Why is this such a big concern? What could happen?

Last January those fires removed much of the vegetation on really steep slopes, and that made those slopes really vulnerable to erosion during intense rainfall. That protective blanket of vegetation is gone, and heavy rain can rapidly make a flash flood. And that flood, in some cases, can pick up material and turn into what we call a debris flow — which is like a flood on steroids.

Damage along Tanoble Drive near Mendocino Street is visible after heavy rainfall triggered multiple mudslides in the Eaton Fire burn scar area in Altadena on Feb. 14, 2025. Photo by Joel Angel Juarez for CalMatters These burn areas are still vulnerable, even though it's now many months after the fire and there have been flows already. There's still plenty of material that could be mobilized. So the threat’s still there. And so we know they're bad actors, and we’re concerned they could be bad actors again.

I’m hearing a lot of different terms: mudslide, debris flows, landslide. What are the differences, and which ones are the burn scars at risk for? 

Landslide is an umbrella term that captures all kinds of mass movements, from rock falls to debris flows — these floods on steroids — to big, slow movers. The type of flow that we're most concerned about in a recent burn area is a debris flow. It's also called a mudslide. But geologists don't like to use the word mudslide as much because it sounds like there's some mud on your driveway — not a big issue, not something that could kill you. And these things, if you're in the wrong spot at the wrong time, they can cause serious damage.

You called it a flood on steroids. What happens in a debris flow? 

Flash floods are bad, and they can cause lots of problems, too. They can get even worse if they pick up enough sediment to turn into the consistency of wet concrete. But it's worse than just concrete, because it can contain boulders the size of cars. And, very close to the mountain front, it can move very quickly — faster than you can run. And when it gets all bulked up with debris, the rocks, the gravel, the mud, trees, the flow can be a lot bigger. It just turns into a different animal.

Now, debris flows pack a bigger punch than floods, but thankfully, they don't have as long of reach. So usually, the debris flows are confined really close to the mountain fronts. That's where they put those debris basins to catch them. But if there isn't one protection like that, then they can travel downslope and impact neighborhoods, and then flooding can extend even further down.

Is there something about Southern California that makes it higher risk?

Southern California's kind of the world capital for these kinds of events. It's got this combination of very steep topography, like the San Gabriel Mountains that just shoot right up, Santa Monica Mountains, Santa Ynez — very steep topography. It burns fairly frequently. And then there are a lot of people living very close to the mountain front, so that's what puts the risk up.

The thing about a burn area is it takes much less rainfall to cause a problem than it would in unburned conditions. So we've now made the slopes really vulnerable. They're extra steep. There's a lot of people there. That's why the risk is so high.

We've seen debris flows in Northern California burn areas as well. It's not just a Southern California problem, and it's not just a California problem.

Is there anything that could have been done to reduce this risk? Anything that should be done now? 

Not long after the fires, in particular the Palisades Fire, (there were) a number of fairly widespread debris flows that disrupted the roads. There were also, in the Eaton fire, floods and debris flows there. Thankfully there's a dense network of LA County debris basins, which are designed to catch the material before it enters neighborhoods, and those largely saved the day.

Planners have planned ahead and put in these debris basins — these big, giant holes in the ground — designed to catch the material. That's the best defense against these. They're not everywhere, but there is a good network of protection. Other than that, it's really hard to stop these things.

What should people who live near the recent burn scars know? What should they do now, as the rain starts? 

The best thing you could do is, if you're really close to a drainage in one of these burn areas, is to get out of the way. You're going to get a heads up from the National Weather Service, who's closely monitoring the rainstorms. They know how much rain it's going to take to cause a problem, and they'll get out warnings, and local authorities will reach out to get people out of the way. So there's a lot of eyes on the situation. And so at this point, the best thing to do is listen to the weather service, listen to local authorities.

If they ask you to get out of the way, take their advice. These things can happen really fast if there is an intense burst of rain, a flash flood, where debris flow can start within minutes.

So there is no escaping a debris flow once it starts? 

It's pretty difficult. If you have a two-story home and you happen to be there at the wrong time, get up to that second floor for sure. Fight like heck if you get trapped in one. But best to be out of the way.

This article was originally published on CalMatters and was republished under the Creative Commons Attribution-NonCommercial-NoDerivatives license.