As the latest El Niño rainstorm moved into Southern California, there were already signs that the combination of rain and gravity was creating problems.
Mud and debris flowed onto the 101 Freeway in northern Ventura County in an area that was recently burned in a fire, shutting northbound lanes.
Here are some questions and answers about mudslides.
Why are mudslides and debris flows so common in recently burned areas?
Debris flow has long been a concern in areas where wildfires have recently burned. Vegetation, once burned, can no longer hold back loose sediment.
The burned slopes can also become "water repellent," said Jason Kean, a U.S. Geological Survey hydrologist whose research focuses on debris flow. Ash tends to clog the soil after fire, and oily substances can affect the ground's ability to absorb water. Impact from the rain can also help seal the top layer of sediment.
It's like water on a parking lot or the surface of a playground slide, Kean said: "When the rainfall hits it, it just runs right off .… The rapid runoff from these bare hill slopes can quickly pick up sediment, and that can transform into a really nasty debris flow."
How long does vegetation take to regrow?
Although vegetation grows back in approximately five years after a fire, researchers estimate that it takes up to two decades for a hillside's soil to be restored to pre-fire conditions.
Why does the drought make things worse?
Now with the drought, an unburned slope may act like a burn area. The dry conditions have caused some vegetation to die off. Officials pointed to major mudslides in northern Los Angeles County in November caused by heavy rains that hit hills where vegetation had died.
How can I protect my home?
Here are some general tips from L.A. County:
--Never underestimate the power of flows of debris.