Anatomy of a landslide

Erosional Phases No erosion Rapid Erosion Slow Erosion

No erosion Can last seconds to decades Volume of deposit increases as upstream sediments are trapped Debris dams are more likely to form and persist with large amounts of woody debris

Rapid erosion Incision through sediment occurs during the development of new channels Downcutting may be limited by bedrock or coarse armoring material supplied by landslide Rough estimate of material moved during this phase = length of channel affected x channel width x depth of deposit Phase ends when channel geometry is equilibrated to upstream water and sediment discharge

Slow erosion Sediment is supplied primarily by bank erosion Supply rate is a function of flood occurrence, channel shifting and the establishment of bank vegetation

Significance of landslides Sediment input affects riparian habitat Infilling of Salmon redd’s Gravel recruitment Societal impacts Damage to infrastructure Potentially lethal

The 1983 Thistle landslide at Thistle, Utah-This landslide began moving in the spring of 1983 in response to groundwater buildup from heavy rains the previous September and the melting of deep snowpack for the winter of 1982-83. Within a few weeks the landslide dammed the Spanish Fork River, obliterating U.S. Highway 6 and the main line of the Denver and Rio Grande Western Railroad. The town of Thistle was inundated under the floodwaters rising behind the landslide dam. Total costs (direct and indirect) incurred by this landslide exceeded $400 million, the most costly single landslide event in U.S. history.