FINDING NEW AND COMPLEX LANDSLIDES IN FORESTED TERRAIN: MAPPING USING LIDAR IN THE WESTERN COLUMBIA RIVER GORGE

Recent LiDAR imagery has allowed us to identify and map a large number of previously unrecognized landslides in heavily forested terrain in the western Columbia River Gorge, Skamania County, Washington, and it has revealed that the few previously mapped large slides are actually composites of multiple smaller landslides. The high resolution of the imagery further reveals that slides in the study area have complex movement histories and span a wide range of relative ages. Movement histories are inferred from cross-cutting relationships of surface features. Estimated order-of-magnitude ages are based on (1) limited absolute dating; (2) fineness of surface textures of currently active or recently active dated landslides (InSAR, GPS, historical records); (3) the sharpness and steepness of surface morphologic features, calibrated by dating in other regions; (4) the degree of surface erosion; and (5) evidence of erosion or deposition by late Pleistocene (13–15.5 ka) Missoula floods within 200 m elevation of current river level. The relative age categories are recent (0 to ~1,000 yBP), intermediate (~1,000 to ~10,000 yBP), and old (>~10,000 yBP). Within the 221.5 km² study area, we identified 183 discrete landslides, covering 144.9 km² (65 percent of the study area). At least six of the mapped landslides are currently active. Mapping for this project expanded the area of mapped unstable terrain from about 87.3 to 144.9 km². Landslide geometries suggest that most of these slope failures are translational slides or complex slides with translational elements, with failure occurring along gently sloping bedding planes in zones of deeply weathered, clay-rich volcaniclastic sedimentary units. Approximately 64 percent of the mapped landslide area comprises landslides that have remobilized parts of older slides, and many of these reactivated slides have involved two or more older slides. The largest two recent landslides each have volumes >1 km³and runouts >6 km. One of these, the Bonneville landslide, temporarily dammed the Columbia River around AD 1440-1450, and subsequent dam-break flooding inundated downstream areas. A similar landslide occurring today could have a catastrophic impact on downstream cities and the transportation and energy-distribution infrastructure of the Pacific Northwest.