Mudslides trap drivers, bury roads and damage homes in Southern California

These photos provided by the San Bernardino County Fire Department show damage caused by mudslides after storms in Forest Falls, Calif., on Thursday, Sept. 18, 2025. (Photos courtesy of San Bernardino County Fire Department via AP)

Southern California's Fire-Mudslide Cycle: Why Burn Scars Create Deadly Debris Flow Zones Across the Region

Bottom line: Southern California can now expect to see post-wildfire landslides occurring almost every year, with major events expected roughly every ten years, as the devastating mudslides in San Bernardino County's Oak Glen and Forest Falls areas highlight the dangerous correlation between wildfire burn scars and subsequent debris flows that threatens communities across Los Angeles, San Diego, and Imperial counties.

Residents of Southern California typically breath a sigh of relief when a wildfire is contained or put out, but unfortunately the danger is not gone, it is just transformed. The recent mudslides that struck Southern California mountain communities Thursday serve as a stark reminder of a deadly pattern that has claimed hundreds of lives across the region: wildfire burn scars dramatically increase the likelihood of catastrophic debris flows for years after flames are extinguished.

The Science Behind the Cycle

In the years after a wildfire, areas that are often left charred by flames and devoid of vegetation can render the soil non-permeable to rainwater. Where intense fires occur, soils can develop a layer that repels water, like rain on pavement, due to the charred remnants of organic material.

Wildfire increases the susceptibility of steep slopes to landslides and debris flows, with the return period of debris-flow initiating rainfall increasing from about one-year immediately after the fire to more than 10 years following four seasons of vegetation growth.

The threshold for danger is alarmingly low. A general rule of thumb is that half an inch of rainfall in less than an hour is sufficient to cause Flash Flooding in a burn area, far less than would cause problems in unburned terrain.

Thursday's San Bernardino County Event

The mudslides that devastated Oak Glen, Forest Falls, and Potato Canyon exemplified this dangerous pattern. The mudslides are linked to the 2020 El Dorado wildfire burn scar, which was sparked by a pyrotechnic device at a gender reveal party. The El Dorado Fire burned 22,744 acres over 71 days in 2020, cost $42,269,660 to suppress, resulted in the death of one firefighter, and destroyed 20 buildings.

One location near Forest Falls reported 1.77 inches of rain in a single hour - more than triple the danger threshold - triggered by remnants of Tropical Storm Mario.

Los Angeles County: The Most Vulnerable

Los Angeles County has the most extensive and deadly history of post-fire debris flows in Southern California, with steep mountain terrain creating ideal conditions for catastrophic events.

Recent 2025 Threats: San Gabriel Valley areas scarred by the Eaton Fire are at "high to very high risk" of debris flows following January's devastating wildfires. The National Weather Service issued a flash flood watch for late Thursday, with the greatest risks in areas burned by the Eaton, Palisades, Franklin, and Bridge fires. Tragically, one house that survived the Palisades fire was destroyed by a mudslide.

Historical Devastation: The county's deadliest post-fire event occurred in nearby Montecito in 2018, where heavy rainfall on the Thomas fire triggered debris flows that killed 23 people and damaged or destroyed over 400 homes after intense rain above Montecito triggered a series of debris flows from steep catchments that had been burned three weeks earlier by the 1,140 km² Thomas fire.

The region's vulnerability stretches back decades. A disastrous debris flow in 1969 killed 100 people after a 20-foot wave of mud rushed through Azusa in the foothills of the San Gabriels. In 1994 a man and his 9-year-old son were killed during a flash flood in a park in Sierra Madre, where a canyon had burned a year before.

The massive 2009 Station Fire created conditions for extensive damage. The Station Fire burned over 160,000 acres in the Angeles National Forest, and during the winter that followed, rainstorms triggered debris flows in the Station Fire burn area. One of these major flows filled a debris basin at the mouth of Gould Canyon in Los Angeles County. A major debris flow on Feb. 6, 2010, inundated more than 40 homes in La Cañada Flintridge's northernmost neighborhood with mud and boulders.

San Diego County: Rising Risk

While historically less prone to post-fire debris flows than Los Angeles County, San Diego County faces increasing vulnerability as development expands into fire-prone areas and wildfire patterns intensify.

"While San Diego has experienced wildfires for decades, but many of them in the back-country and burning right up to some of the neighborhoods right on the edge," said Daniel Berlant of Cal Fire. "We're seeing more and more areas that are typical urbanized areas be impacted by wildfires."

Geographic factors increase inland risk. "The risk of homes in San Diego County increases as you go inland," said Tom Larsen, AVP of Product Marketing at Cotality. "That lines up with what we expect as you get closer to hotter, drier areas near the desert."

Current conditions remain concerning. San Diego County is still under wildfire threat, as the recent rains have done little to lift the region out of an extreme drought. Parts of Imperial County are in severe and extreme drought with fire officials noting "It's not a fire season anymore. It's a fire year for us down here in Southern California."

Imperial County: Lower but Present Risk

Imperial County faces lower documented risk for post-fire debris flows primarily due to its flatter desert terrain, though drought conditions create ongoing wildfire vulnerability. The county's geography lacks the steep mountain slopes that amplify debris flow risks in coastal counties, but areas near the Peninsular Ranges could face similar threats under extreme conditions.

The Infrastructure Challenge

Southern California has developed extensive debris management systems, but coverage remains incomplete. In some parts of Los Angeles, there are large debris basins that capture sediment, boulders and other debris flowing out of the mountains. These structures, along with dams, help protect communities. But they're not in every drainage basin.

The 2009 Station Fire aftermath highlighted infrastructure limitations. A 10-ton boulder had plugged up the Mullally Canyon debris basin, which was designed to catch ashen mud as it poured from the slopes. Officials later built a $1.5-million drain pipe to divert rainwater away from the basin and worked to increase storage capacity of the area's catch basins.

Scientific Response and Warnings

The correlation is so well-established that the U.S.G.S. conducts post-fire debris-flow hazard assessments for select fires in the Western U.S. Emergency management has adapted accordingly, with debris flows and mudslides occurring many years after wildfires requiring long-term vigilance.

Climate change is amplifying the threat. "We're going to have a longer season to burn and then when it does rain, it's going to come down harder. And that's a bad recipe for these post-fire debris flows," said USGS researcher Jason Kean. "The reason you can expect one just about every year is because it doesn't take very much rain to cause one."

Cascade of Disasters

When multiple hazards such as droughts, heat waves, wildfires and extreme rainfall interact, human disasters often result, creating what researchers call cascading climate disasters. Several research studies have shown that compound events that include both drought and heat waves have become more severe and frequent in recent years.

Current Preparedness

Federal and state agencies maintain active monitoring systems. Wildfires can significantly alter the way water interacts with the landscape to the extent that even modest rainstorms can produce dangerous flash floods and debris flows, requiring specialized assessment tools and warning systems.

"This isn't like flooding; you don't put up sandbags to prevent things from happening," said USGS geologist Lisa Kostelnik. "You can't stop them." The only effective protection is evacuation when warnings are issued.

As Southern California continues to experience larger, more frequent wildfires combined with intense storm systems, the fire-mudslide cycle threatens to become an increasingly deadly feature of the region's climate reality, demanding improved evacuation systems, infrastructure investment, and land-use planning that accounts for these cascading natural disasters.

SIDEBAR: Racing Against Rain - How Scientists and Communities Fight to Restore Burned Slopes

The Revegetation Challenge

The return period of debris-flow initiating rainfall increases from about one-year immediately after the fire to more than 10 years following four seasons of vegetation growth, making rapid ground cover restoration critical for community safety. However, "The takeaway from this is the only thing that will heal the hillsides is time," said Cal Fire Chief Grant Malinowski. "We can't go plant 35,000 acres of grass and debris and brush that the fire burned."

Natural Recovery Timeline

It could take many years for vegetation to become re-established and this is the main factor in slowing the precipitation runoff that creates Flash Flooding and Debris Flows. Most burn areas will be prone to this activity for at least two years. The challenge is that communities can't wait for natural recovery - they need immediate protection.

Emergency Stabilization Methods

Hydromulching and Seeding: Effectiveness: Moderate success on slopes under 30% grade Timeline: Germination in 2-4 weeks under ideal conditions; initial soil binding in 6-12 weeks Applied immediately after fire containment, helicopter and ground crews spread seed mixtures of fast-growing grasses like annual ryegrass and barley across burn scars. These provide rapid but temporary soil stabilization while longer-term vegetation establishes.

Straw Mulching: Effectiveness: 60-80% reduction in soil erosion on moderate slopes Timeline: Immediate protection upon installation; effectiveness decreases after 12-18 months Spread by aircraft or hand crews, straw mulch provides immediate protection by cushioning raindrop impact and slowing surface water flow. However, it can be displaced by high winds or intense rainfall.

Erosion Control Barriers: Effectiveness: Variable - 40-70% reduction in sediment flow depending on placement and terrain Timeline: Immediate protection upon installation; typical lifespan 1-3 years

• Straw wattles: Tube-shaped barriers made of straw wrapped in netting, placed along contour lines to slow water flow
• Silt fences: Temporary barriers that trap sediment while allowing water to filter through
• Check dams: Small temporary structures placed in channels to slow debris flows

Hydrogels and Soil Amendments: Effectiveness: Mixed results - 20-40% improvement in seed establishment Timeline: Immediate application; benefits last 6-12 months Water-absorbing polymers help burned soil retain moisture and support seed germination, though effectiveness varies significantly by soil type and climate conditions.

Long-term Restoration Strategies

Native Shrub Establishment: Effectiveness: High long-term success (80-90%) but slow initial establishment Timeline: 2-5 years for significant soil stabilization; 5-10 years for full mature root systems Restoration focuses on chaparral species like manzanita, ceanothus, and scrub oak that develop deep, extensive root systems providing superior soil binding compared to grasses.

Mycorrhizal Fungi Restoration: Effectiveness: 30-50% improvement in plant survival and soil stability when successfully established Timeline: 6 months to 2 years for fungal networks to establish; benefits increase over 3-5 years Advanced restoration programs reintroduce beneficial soil fungi that form partnerships with plant roots, though this technique is still experimental in post-fire applications.

Seeding with Native Grasses: Effectiveness: 50-70% ground cover establishment within first year on suitable terrain Timeline: Germination in 4-8 weeks; soil stabilization benefits within 3-6 months Native bunch grasses like purple needlegrass provide faster establishment than shrubs while offering better long-term stability than non-native annual grasses.

Engineering Solutions for Immediate Protection

Debris Basins: Effectiveness: 90-95% capture rate for debris flows when properly sized and maintained Timeline: Immediate protection upon construction; requires ongoing maintenance In some parts of Los Angeles, there are large debris basins that capture sediment, boulders and other debris flowing out of the mountains, though "they're not in every drainage basin."

Concrete Barriers and K-Rails: Effectiveness: 70-85% effective at redirecting flows away from structures Timeline: Immediate protection upon installation Approximately 15,000 K-rails have been set up, and catch basins and drainage systems are being cleared to reduce flooding risks in recent Los Angeles area burn scars.

Channel Modifications: Effectiveness: Variable (60-90%) depending on design and flow magnitude Timeline: Immediate protection; may require reinforcement after major events Concrete-lined channels, grade control structures, and energy dissipators help manage water flow and reduce erosion potential.

Effectiveness Limitations

Terrain Constraints: Most biological restoration techniques show significantly reduced effectiveness on slopes exceeding 40% grade. The El Dorado Fire burned steep mountain terrain already prone to landslide, rockfall, and debris flow hazards, where even successful revegetation cannot eliminate risk entirely.

Climate Windows: Seeding efforts must occur during optimal weather windows - typically late fall through early spring in Southern California. Seeds planted outside these windows often fail due to heat stress or insufficient moisture.

Scale Challenges: Large burn areas (over 10,000 acres) present logistical challenges that often mean only the highest-priority areas - those directly above communities - receive intensive treatment.

Success Rates by Restoration Type

• Emergency seeding: 30-60% effective depending on rainfall timing and intensity
• Mulch applications: 60-80% effective for first-year erosion control
• Native shrub planting: 40-70% survival rate in first year; 80-90% long-term success for survivors
• Combined approaches: 70-85% effectiveness when multiple techniques are used together

The Reality Check

"The danger is very much there," fire officials warn. "If you evacuate, you will be safe, but it's very hard to get through 6 to 8 feet of mud … to try to rescue you." While restoration techniques can significantly reduce debris flow risks over time, none eliminate the hazard completely during high-intensity storms in the first 2-3 years after fire.

Current Research and Innovation

Scientists are testing drone-deployed seed pods designed to germinate only after specific rainfall thresholds, potentially improving timing and success rates. Other experimental approaches include treated biochar applications and specialized erosion control blankets embedded with native seeds, though long-term effectiveness data remains limited.

The most successful restoration efforts combine immediate emergency stabilization (providing 40-60% risk reduction within months) with long-term native ecosystem restoration (achieving 70-85% risk reduction within 3-5 years), while maintaining evacuation infrastructure for the inevitable storms that exceed all protective measures.

 

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