Video shows waves of sludge powering down a hill and into a steakhouse parking lot, covering it with feet of mud.
Evacuations had been ordered in parts of Riverside and San Bernardino counties, as mud overpowered cars and damaged homes.
One person has been reported missing in the wake of the devastation, AP reports, and officials say clean-up efforts could take days.
The mudslides were spurred by heavy rainfall, brought on by remnants of Hurricane Kay — which dissipated after hitting Mexico’s Baja California, sending storms across southern California and Arizona.
A meteorologist with the National Weather Service (NWS) told the San Bernardino Sun that nearly 2.5 inches (6.4 centimetres) of rain had fallen near two areas that have burned from wildfire in recent years.
Wildfires can leave the land primed for floods and mudslides by stripping away vegetation and hardening the soil surface, pushing tons of water into runoff down the mountain.
One of the fires that created a burn scar in the area, the El Dorado Fire, killed one firefighter after allegedly being started by a “gender-reveal party” in 2020. The couple in question were later charged with involuntary manslaughter.
After the rains, mud rushed down around Oak Glen, Forest Falls and Yucaipa, California — covering roads in thick layers of dirt and burying vehicles. In addition to the mud, boulders the size of cars were washed onto the road, the Sun reports.
The mud also crashed into some homes in the area, causing damage. One local resident reported finding “feet” of mud surrounding her home, AP reports, while another building reportedly had mud so high that the roof collapsed.
The owner of the steakhouse that was seen being covered in waves of mud told ABC 7 News that their building had sustained serious damage and is closed indefinitely.
Additional mudslides hit parts of northern Los Angeles county on Sunday night, in the wake of other remnants from Hurricane Kay, requiring rescues as people got trapped with their cars.
These kinds of disasters could become more common in the coming decades as the climate crisis grows, creating both more wildfires and more intense storms.
Heatwaves and drought, two conditions perfect for sparking and spreading wildfires, are projected to get a lot more intense and common as the planet heats up, according to a UN climate science panel.
In addition, hurricanes like Hurricane Kay are forecast to become much stronger on average — potentially dropping tons of rain when they reach land.
Major wildfires are burning in California right now, including the Fairview Fire, which is nearby these recent mudslides. That blaze has spread to more than 28,000 acres outside the town of Hemet and left two people dead.
The past few years of intense drought in California — part of a decades-long and climate crisis-powered “megadrought” in the southwest US — has left the landscape prone to massive fires that can quickly get out of control.