Programs and Services

Geologic Hazards—Landslides

"Landslide" is the general term for the slow or rapid movement of a soil and rock mass down a slope, or for the mass itself. It covers a variety of processes and landforms known as rockslide, rockfall, debris flow, liquefaction, slump, earthflow, and mudflow.

The Idaho Geological Survey has identified and plotted over 3,000 landslides in the state for the U.S. Geological Survey's national landslide appraisal. Landslides are a recurrent menace to waterways and highways and a threat to homes, schools, businesses, and other facilities.

Landslides may be triggered by other geologic hazards such as earthquakes and floods. Factors of weather and climate, like melting snow and rain that increase the water content of earth materials, may fuel slope instability. The activities of urban and rural living with excavations, roads, drainage ways, landscape watering, logging, and agricultural irrigation may also disturb the solidity of landforms. Late spring-early summer is slide season, particularly after days and weeks of greater than normal precipitation.

Landslides are costly. One nightmare for Idaho is maintaining highway U.S. 95, the primary and, in most places, only north-south link from the Canadian border. Redirecting local and through traffic around a landslide is not an option in many places on this heavily traveled road. Alternate routes do not exist, and detours in steep terrain are difficult or impossible to construct. The unimpeded movement over roads—whether for commerce, public utilities, school, emergencies, police, recreation, or tourism—is essential to a normally functioning society. The disruption and dislocation caused by landslides can quickly jeopardize that freedom and vital services.