Two separate landslides occurred at this location: a slide that failed on Dec. 3rd, along State Route 6; and a later event along the Chehalis River, probably around Dec. 7th. The landslide along State Route 6 is located about a half-mile west of Spooner Road and intersects a forest road. The landslide moved translationally and rotationally over bedrock and over the road, but did not flow onto the railroad tracks below.
Map 1: Location map of the landslides located on SR-6 and along the Chehalis River. Click on the map to view a larger image.
State Route 6 Landslide
Figure 1: A view of the main body of the debris slide. The landslide partially moved translationally and rotationally. As the landslide rotated it tilted trees, typical in rotational slides. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Figure 2: A view of the scarp of the debris slide. The landslide mass was composed of shallow soils above impermeable and unaffected substrate (bedrock). Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Chehalis River Landslides
When the Chehalis River flooded, it saturated soils and eroded into the bank, removing the stabilizing slope. As the floodwaters receded, the saturated soils began to dewater, creating increased pressure on the banks. Coupled with the high erosion, large sections of the banks along the Chehalis River flowed into the river, adding to an already sediment rich waters.
Figure 3: A view of the Chehalis River upstream from the landslide on State Route 6. The banks are highly unstable and numerous cracks along the banks were discovered. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Figure 4: A view of the Chehalis River downstream. Numerous slumps, like the one located in the center of the photo, were viewed along the riverbanks. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)