Debris flow damage in Jamestown, Colorado, November 2013. Photo credit: Jonathan White for the CGS.

Debris and Mud Flows

Debris flows are a common hazard in many hillside areas of Colorado. Heavy rainfalls commonly trigger flash floods on steep slopes. These torrents pick up anything in their paths and may contain more solid material than liquid. They tear into the hillsides and deposit accumulated material when they reach flatter ground, creating an alluvial fan.

A debris flow (commonly called a mud slide) is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity. To be considered a debris flow, the moving material must be loose and capable of “flow”, and more than half of the solids in the mass must be larger than sand grains, including gravel-, pebble-, cobble- and boulder-sized material. The speed of a debris flow may reach 100 miles per hour, although most commonly they are slow and move only a few feet per year downslope.

A mud slide or mud flow is a mass of water and fine-grained earth materials that flows down a stream, ravine, canyon, arroyo, or gulch. To be considered a mud flow, more than half of the particles must be sand sized or smaller that can flow very rapidly. A mud flow is the sandy, more water-saturated analog of a debris flow.

The media often use the term mud slide for events that are technically debris or mud flows.

Noting that we were getting hundreds of search hits on a previous version of an info-brochure that we originally published in 2010, we decided to issue a new, updated version: HAZ-2021-01 Post-wildfire Hazards: Mud Slides :: Debris Flows. The subject is unfortunately very relevant given the exceptional drought conditions in the US West and elsewhere in the world exacerbating the threat of major wildfires.