New mapping in the western portion of the Columbia Gorge in Skamania County, Washington, shows previously unrecognized landslides beneath dense forest cover.
The lidar imagery used to make the map reveals that landslides occupy about two-thirds of the 222-square-kilometer (86-square-mile) map area, expanding the area of previously known unstable terrain by 60 percent.
The majority of these landslides show evidence of multiple movement episodes: a reminder that some old landslides can reactivate and threaten nearby communities. At least one large landslide appears to have crossed the Columbia River at high speed – a scenario that has serious hazards implications if it were to be replayed.
The western Columbia Gorge has been long recognized as an area susceptible to landslides. Abundant rainfall, steep terrain, geologic structure and erosion by the Columbia River combine to create topography capable of ground movement. Yet the dense forests growing on this terrain have hampered efforts to accurately map old and currently active landslides and to fully understand the scope of this hazard.
Newly acquired lidar imagery has allowed researchers at the U.S. Geological Survey to identify and map over 200 landslides in the western Columbia Gorge, Skamania County, Washington. Lidar is a revolutionary remote-sensing technique that provides images of terrain, from which vegetation and structures can be digitally “erased” to show the underlying bare ground. Formerly hidden by the forest, telltale landslide indicators such as scarps, cracks and ridges, slope depressions, bulges and toes can be clearly seen in lidar images of the landscape.
The landslides were mapped and evaluated using a combination of lidar; InSAR, another imaging technique that detects subtle ground movement by radar from satellites; GPS; isotopic and tree-ring dating; historical records and field work. The results show that landslide occurrence in the map area has spanned thousands of years; about a quarter of the landslides are estimated to have moved within the last 1000 years, and 12 have moved within the last 20 years or are currently moving.
Two of the largest landslides in the map area, the Bonneville and Red Bluffs landslides, are each about one cubic kilometer in volume (0.2 cubic miles). Both slid into the Columbia River within the last 600 years, and evidence suggests that the Bonneville landslide moved rapidly (tens of feet per second). A number of older landslides also reached the river. The Bonneville slide temporarily dammed the river and formed the “Bridge of the Gods” that is known from Native American oral histories. For comparison to a more modern event, the volume of the debris avalanche from the May 18, 1980, eruption of Mount St. Helens was 2.5 cubic kilometers, or 0.6 cubic miles.
Further analysis of potentially unstable terrain in the western Columbia Gorge is warranted, especially in light of the potential for a large Cascadia Subduction Zone earthquake in this region. Ground motion from earthquakes can trigger landslides in terrain that might otherwise remain stable. Depending on the size, speed, and runout distance of any future landslide, vulnerable infrastructure could include a major natural gas pipeline; high-voltage power transmission lines from Bonneville Dam; road, rail, and river barge transportation corridors along the river; and Bonneville Dam itself.
The map and accompanying pamphlet, entitled, “Landslides in the Western Columbia Gorge, Skamania County, Washington,” USGS SIM 3358, are available online.