Mud and debris are flowing down hillsides across California. What causes the slides?

LOS ANGELES (AP) — When rainstorms like this week’s powerful atmospheric river hit California, the earth begins to move.

Water rushing down mountains and hills picks up soil and vegetation, as well as boulders, trees and cars that can become battering rams. In the blink of an eye, property is damaged or destroyed and lives are put at risk.

Commonly called mudslides, these dangerous torrents are usually referred to by geologists and first responders as debris flows, which the U.S. Geological Survey describes as fast-moving landslides capable of exceeding 35 mph (56 kph).

In Los Angeles alone during the current storm, crews responded to more than 380 slides as of Tuesday morning. Fire Chief Kristin Crowley, who toured hard-hit areas the previous day, said 35 buildings required inspections, five were deemed uninhabitable and there were seven where people were only allowed to remove possessions.

“Yesterday I was reminded of the pure force and magnitude with which a hillside can suddenly come down,” Crowley said.

WHY IS CALIFORNIA PRONE TO MUDSLIDES?

The state has relatively young mountains from a geology standpoint, meaning much of its steep terrain is still in motion and covered in loose rocks and soil that can be sloughed off easily.

That's especially true when the ground is wet, according to geologists, and California was plenty wet from a pounding by a previous atmospheric river, a moisture plume that stretched far over the ocean to near Hawaii.

Downtown Los Angelesrecorded 7.03 inches (17.8 centimeters) of rain Sunday through Monday, the third-wettest two-day total since weather records began in 1877. Some other parts of the city got far more, including more than 12 inches (30.4 cm) in Bel Air.

WHAT ARE THE MOST...