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

By JOHN ANTCZAK
Associated Press

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. Larger objects such as boulders, trees and cars are turned into battering rams. In the blink of an eye, property is damaged or destroyed and lives are put at risk. These dangerous torrents are commonly called mudslides. But geologists and first responders refer to them as debris flows. The U.S. Geological Survey describes them as fast-moving landslides capable of exceeding 35 mph. In Los Angeles alone, crews have responded to 475 slides as of Tuesday evening.