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Landslides: General Information
Introduction
This section provides general information about types of landslides and the impacts they've had in Oregon. Much of the information and pictures were gathered from the Oregon Department of Geology and Mineral Industries (DOGAMI), the Oregon Department of Forestry (ODF), and the Oregon Department of Transportation (ODOT).

What are Landslides?
In the broadest and most general sense, a landslide is simply the down slope movement (sliding or falling) of soil, rock, or some mixture of the two, under the influence of gravity. Landslides are natural processes, but can be triggered or accelerated by one or more of the factors listed below, especially when the factors occur in combination. See page 9 of this document(pdf).
  • Intense or prolonged rainfall, or rapid snow melt, that cause sharp changes in groundwater levels.
  • Undercutting of a slope or cliff by erosion or excavation.
  • Shocks or vibrations from earthquakes or construction.
  • Vegetation removal by fires, timber harvesting, or land clearing.
  • Placing fill (weight) on steep slopes.
   The term landslide is commonly applied to a variety of somewhat distinct types of events or processes and is also generally applied to the mass of soil or rock material that results from one of these events. Some of the different processes that are sometimes referred to as landslides include:
  • Rockfall, the relatively free fall of small or large rocks that become detached from cliffs and steep outcrops. Rockfalls are common along Oregon highways where roads are cut through bedrock.
  • Rockslide, the rapid downslope movement of rock material along a plane of separation within the bedrock, which could be a fault surface, a fracture surface, or the depositional surfaces found in some sedimentary rocks. These slides can occur on relatively gentle slopes and cause serious damage. An example of this type of landslide is the recent subdivision landslide in Kelso, Washington.
  • Slump, the downward slipping of a mass of rock and/or soil that moves more or less together as a block (or group of blocks) and usually involves some amount of backward rotation during movement.
  • Debris flows, rapidly-moving landslides that typically travel long distances, often traveling within confined channels, and often involving significant amounts of water and mud. According to the ODF, debris flows can move faster than 35 mph. The slides are most common in the Tyee geologic formation found in Coos, western Douglas, and Lane Counties. The potential for debris flow hazards is also high in much of eastern Tillamook County and the Columbia River Gorge.

Landslides in Oregon
Road with Landslide
The following excerpts are from DOGAMI Special Paper 31, Mitigating Geologic Hazards in Oregon: a Technical Reference Manual, available for purchase from the Nature of the Northwest Information Center at the following website: http://www.naturenw.org/cgi-bin/quikstore.cgi?page=store-maps.htm (In the Map Search box type in SP-31)
 
"Landslides are of many types and are generally related to various combinations of slope, rock type, and climate. In general, moderate-slope slumps and steep-slope debris torrents dominate recent discussions. Losses for Oregon generally average less than one or two lives per year and $1 million to $10 million per year.
 
"In pre-historic times, large-scale landslides have formed large landforms and have blocked numerous rivers to form lakes such as at Loon Lake, Triangle Lake, and Bonneville. During historic times, landslides have resulted in: $150 million in threatened real estate at The Dalles in 1980s; 8 deaths in Douglas County in 1974; 8 deaths in several events in Oregon in 1996; and tens of millions of dollars in damage per year. Oregon can expect greater losses in the future in urban or developed areas owing to demographic trends for growth into less stable areas and increasing general population pressures on the land."
 
The following list describes some of the other major landslides that have occurred in Oregon over the last 75 years. The list is not intended to be all-inclusive, but focuses on those slides that caused loss of life or significant damage.
 

February 1926
A landslide closed Roosevelt Highway between Coos Bay and Coquille, causing at least $25,000 in damage.
November 1928
A landslide killed two workmen working on a railroad tunnel near Baker.
August 1957
A rockslide killed two quarry workers near Westfir.
February 1961
A large section of Ecola State Park, including the parking lot, slid into the Pacific ocean near Cannon Beach.
March 1972
Three motorists were injured in a mud and rockslide on Interstate 5 near Portland.
January 1974
Nine employees working in a telephone company building were killed when the building was pushed by a mudslide into Canyon Creek near Canyonville.
October 1984
Two children were killed in a rockslide along Interstate 84 near Cascade Locks. The cost of stabilizing the slide area a few years later eventually reached $4 million.
September 1990
Four highway workers were injured in a landslide near Troutdale.
February 1996
Heavy rains and rapidly melting snow contributed to hundreds of landslides across the state, many occurring on clearcuts that damaged logging roads.
November 1996
Heavy rain triggered mudslides in Lane and Douglas Counties that resulted in several fatalities.
February 1999
Two timber workers were killed in a mud and rockslide south of Florence.
January 2000
A landslide north of Florence closed Highway 101 for three months, resulting in major social and economic disruption to nearby communities.