From the Public Policy Institute of California (PPIC) – While flood flows are dominated by water, debris flows are a slurry of a lot of things, including boulders. They are typically half water and half sediment, and they are very energetic. They move rapidly, they’re more destructive than floods, and they can jump out of a previous waterway and carve a new path. It’s a lot harder to predict where they’ll go. We are prone to debris flows here in California because many of our mountain ranges are young and really steep. Soils on these slopes are already close to being unstable, so when you really wet that soil, it falls apart. Then, under the right conditions, it can liquify and run downhill as a debris flow. (more)