Reduction of Risk
Contact Information
PI-70 (pdf)

Public Information Series 70
by William F. Case

Debris flows are often called mudslides, mudflows, or debris avalanches.

(U.S. Geological Survey Open-File Report 83-635)

They consist of debris mixed with water. Debris is mainly soil and anything else, such as trees and rock fragments, that are picked up as the flow moves down a slope or channel.

Debris flows may be generated when hillside colluvium or landslide material becomes rapidly saturated with water and flows into a channel.

Intense rainfall, rapid snowmelt, or high levels of ground water flowing through fractured bedrock triggers the movement.

Debris flows and floods also occur when heavy rains on slopes cause extensive hillside erosion and channel scour.

Debris-flow scar, Rudd Creek, Davis County.	Flow onto alluvial fan, Birch Creek, Sanpete County.

Repeated debris flows and/or floods deposit sediment at the mouth of a canyon, forming an alluvial fan. The fan shape is a result of periodic diversion of the main channel back and forth across the fan.

Rudd Creek debris flow, May 1983, Davis County, showing boulders and muddy flood waters.

Flows may travel farther down the fan from the mouth of the canyon if the channel becomes entrenched and the flow is confined. Alluvial fans are risky places for homes because it is difficult to predict where flooding or debris flows will occur.

Debris flows can be as thick as wet concrete and can transport boulders as large as a car; debris flows may eventually become muddy flood waters as they deposit their debris.

Debris flows tend to move in pulses. Early pulses or previous debris flows form levees that channel the flow until the levees are breached.

Levee of the Rudd Creek debris flow,
May 1983, Farmington, Davis County.

The presence of older levees indicates the recurrence and characteristics of debris flows in a particular canyon. This is valuable information for developing land on the alluvial fan.

How debris-flow risk can be reduced.

Contact Information

The Utah Geological Survey documents and evaluates geologic hazards such as debris
flows to protect citizens of Utah. If you wish to report a debris flow or other landslide, or
want more information, please contact the Utah Geological Survey at:

• Utah Geological Survey, PO Box 146100, Salt Lake City, UT 84114-6100, phone 801-537-3300. The office is located at 1594 West North Temple, Salt Lake City.

• The Southern Utah Regional Office, Electronic Learning Center, Room 103, Southern Utah University, Box 9053, Cedar City, UT 84720, phone 435-865-8126, cell phone 435-559-0876.