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

The latest headlines from our reporters across the US sent straight to your inbox each weekday Your briefing on the latest headlines from across the US Your briefing on the latest headlines from across the US SIGN UP I would like to be emailed about offers, events and updates from The Independent. 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. On Jan. 9, 2018, intense rain fell on a weeks-old wildfire burn scar in the mountains above Montecito on the Santa Barbara County coast, unleashing what residents remember as the “ 1/9 Debris Flow.” Huge torrents tore through the community, killing 23 people and destroying or damaging hundreds of homes. In 1934 a storm over the Southern California mountains unleashed runoff so intense that 30 people were killed, more than 480 homes were destroyed and a nearly 60-ton boulder was pushed out of a canyon. The California Geological Survey, a state Conservation Department division, studies the potential for post-fire debris flows and creates maps and reports that the state Office of Emergency Services can pass along to agencies for emergency response and alleviating problems in places where people and property may be in danger.