Landslide Reconnaissance Following the December 3, 2007 Storm - Chehalis Headwaters, Chehalis River Area
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Landslide Reconnaissance Following the December 3, 2007 Storm - Chehalis Headwaters, Chehalis River Area 
 

Stowe Creek Area

On December 3rd, an intense rainstorm blew in from the south, stalling over the Willapa Hills. Rain gages operated by Weyerhaeuser in the Willapa Hills recorded 14 to 20 inches of rain in a 48 hour period. This intense rainfall quickly saturated soils and flooded creeks and rivers, sending a surge of water downstream. In areas with shallow soils, water penetrating into the subsurface accumulated on bedrock or impenetrable substrates. At some critical point, the accumulated water reduced the pore pressure and resistive forces of the soil enough for it to initiate downhill, leaving the bedrock or impenetrable substrate relatively unscathed. The landslides moved everything above the bedrock or impenetrable substrate, carrying trees, rock, and mud downhill, often into creeks or rivers. These types of landslides we define as debris slides and debris flows, which made up the majority of the landslides recorded during this event.

Figure 1: A view of Stowe Creek and the northern flanks of Joy Mountain. A large debris slide in the center of the photo formed into a debris flow, moving into the main channel of Stowe Creek. Two small debris slides to the left (on the photo) of the large debris slide also formed into debris flows, moving into the main channel of Stowe Creek. Stowe Creek, where the debris flows entered, contains woody debris and some amount of sediment. It is possible that when these debris flows entered Stowe Creek, it temporarily blocked the creek. Another debris flow is visible at the lower right of the photo. Stowe Creek also contains a debris flow, although the origin of that landslide is unknown. (Click on photo for large image)

 

Figure 2: A view of the Stowe Creek drainage and the eastern flanks of Joy Mountain. At least four debris slides are visible in this photo and all of them flow into a channel. At least two of the landslides formed into debris flows. (Click on photo for large image)

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 Contacts

Stephen Slaughter
Geology & Earth Resources Division, Hazards Geologist
Washington State Department of Natural Resources
360-902-1498
Fax 360-902-1785
stephen.slaughter@dnr.wa.gov

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