What are landslides?
Landslides are the sudden or steady movement of soil or rock down a slope. They occur every year in Idaho and can be devastating to property, services, highways, and wildlife habitat. They also can threaten lives. Understanding landslides is essential for reducing risks and avoid costly consequences. Both natural and man-made factors cause or contribute to these slope failures.
Types of Landslides
To explain cause-and-effect differences in slope failures, geologists have classified landslides by the type of movement and the size of the material involved.
Slides consist of blocks of material moving on well-defined shear planes. They are divided into rotational slides that move along a concave surface and translational slides that move parallel to the ground surface. Falls are the sudden release of rocks or soils dropping freely through the air with little contact with other surfaces until impact. Topples are similar to falls except that the initial movement involves a forward rotation of the mass. Flows move entirely by shearing within the transported mass and act like viscous fluids. Creep is the almost imperceptible movement of material down a slope. Lateral spreads occur when liquefaction in underlying materials causes surface rocks or soils to move down gentle slopes.
Landslides and Geology
Certain combinations of earth materials and steep topography increase the likelihood of slope failure. In Idaho, examples include basalt with sedimentary interbeds, altered volcanic rocks, fractured metamorphic rocks, glacial and lake deposits, and weathered granite.
Landslides and Floods
Many landslides in Idaho occur during or shortly after intense storms that cause flooding. These include thunderstorms in the summer or autumn, and heavy winter rains falling over prolonged periods. Intense rainfall falling on a thick snow pack and on lands recently burnt by wildfires are particularly dangerous. The connection between flooding and landslides is illustrated by the events in the Boise foothills between 1959 and 1997. On August 19, 1959, a cloudburst in the Boise foothills created mud-rich floods that ran down streets and into storm sewers, basements, and an Idaho National Guard armory. Nearly 500 houses were damaged by mud up to 10 feet deep, and an area of over 160 acres was covered by silt. The source of the mud was easily erodible material on hillsides denuded of vegetation by a wildfire only three weeks before. In January 1969, a storm struck the same area and caused extensive damage, mostly from deposits of sediment. On September 11, 1997, a cloudburst dropped 0.40 inch of rain in nine minutes in the same foothills burned by a 1996 wildfire. The resulting flood of mud and debris damaged several homes and an elementary school. Presently, Boise operates sediment control structures on the drainages that were involved in these events. The structures are designed to channel flood water and mud away from residential and business districts into holding ponds and onto playing fields.
Causes of Landslides
Landslides are caused by gravity acting on slopes. When gravitational stresses exceed the strength of rock or soil, slope failure occurs. Many landslides are initiated by triggering factors that increase stress and weaken slope materials. These triggers include:
- Heavy rains, rapid snowmelt, or irrigation that load slopes with water
- Natural erosion or human activities that increase slope angles or undercut the toes of slopes
- Shaking by earthquakes
- Removal of vegetation by wildfire, logging, agriculture, or overgrazing
- Loading of slopes with piles of rock, ore, or mining waste
Scientific studies can identify landslide-prone areas and evaluate their risk. Large landslide complexes near Lewiston were identified by geological mapping. The slides formed along steep cliffs composed of basalt interbedded with sedimentary deposits. During the last ice age, catastrophic floods deposited gravel and sand on the tops of landslides and eroded the toes of slopes. These long-ago events may have triggered initial movements in the landslide complex. Between 1994-1998, a part of the slide directly below the Elks Lodge began to fail, threatening the building and temporarily blocking a major local road.
Reducing Landslide Losses
- Learn to recognize existing landslides and landslide-prone areas.
- Avoid building on or near steep slopes.
- Many landslides occur during high intensity rainfall or prolonged periods of winter rain. Monitor news media for current conditions and check with the Idaho Transportation Department for road closures from landslides.
- Be especially alert when traveling. Bridges may be washed out or damaged, and culverts filled with debris. Never drive over flooded roads or across streams.
- Be aware that debris flow hazards may increase following wildfires because of the loss of vegetation.
For further information on Idaho landslides, see the following:
- W. Phillips, D. Garwood, and R. Stewart, 2008, Landslide hazards of Idaho: Idaho Geological Survey GeoNote G-44. Note: This is the brochure version of this web page. Free printed copies can be ordered from IGS or download the PDF file here: [Low Resolution] [High Resolution]
- W. Adams, R. Breckenridge, and K. Othberg, 1991, Landslides of Idaho: Idaho Geological Survey Surficial Geologic Map SGM-1, scale 1:500,000. [SGM-1]
Other landslide information:
- L. Highland and P. Bobrowsky, 2008, The landslide handbook—A guide to understanding landslides: Reston, Virginia, U.S. Geological Survey Circular 1325, 129 p. [Web site]