The worst landslide in U.S. history was in 1928, when as many as 500 people were killed after the collapse of the St. Francis Dam near Los Angeles, according to geologist Lynn Highland of the U.S.Geological Survey. National Landslide Information Center in Golden, Colo.
There are many types of landslides. The one in Washington on Saturday was a "debris flow," also commonly referred to as a "mudslide" or "mud flow," the U.S.G.S. says. A debris flow is a mixture of water-saturated debris that moves downslope under the force of gravity.
Rockslides and volcanic debris flows are other types of landslides.
Landslides occur in all 50 states, according to the U.S.G.S., and cause $1 billion to $2 billion in damage and more than 25 deaths each year on average. Many of the deaths, Highland says, occur one at a time after a rockfall and are not widespread events that kill many people at a time.
For example, she said the majority of landslide deaths in Colorado are due to rockfalls.
The Appalachian Mountains, the Rocky Mountains, the Pacific Coastal Ranges and some parts of Alaska and Hawaii have severe landslide problems, the U.S.G.S. says.
The worldwide death toll per year from landslides is in the thousands, Highland reports. Most landslide fatalities are from rock fall, debris-flows, or volcanic debris flows.
Deadliest landslides in U.S. history:
1. 500 killed, St. Francis Dam, Calif., in 1928
2. 150, Nelson County, Va., in 1969
3. 129, Mamyes, Puerto Rico in 1985
4. 30, San Francisco Bay Area, in 1982
5. 26, Madison County, Mont., in 1959
Source: U.S. Geological Survey