A recently released USGS report finds a number of landslides that were not previously recognized due in large part to the dense forest cover.
They note that, “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.”