On August 25, 2009 at approximately 1pm, a landslide along Porcupine Bay (part of the Coulee Dam Recreational Area) failed, probably something akin to a debris avalanche. The landslide plunged into the lake, creating a seiche, approximately 5-6 feet high. The seiche struck a camp dock across the bay, damaging some boats and destroying the dock. The landslide also injured two boys, who were thrust under the dock, resulting in minor injuries.

The landslide is reported to be up to an acre in size. The material of the landslide is probably Fraser age glaciolactustrine beds (equivalent to Vashon Stade in Western Washington). It is difficult to determine a trigger mechanism at this point, but this has been a fairly dry year and the lake level might have been dropping fairly rapidly, resulting in a rapid dewatering of the material, resulting in an increase in instability. Unfortunately, such things are hard to determine in photos.

Porcupine Bay Landslide

Photo from NWCN

Porcupine Bay Landslide Photo from KREM

Porcupine Bay Landslide Photo from KREM

I haven’t been able to imbed this within the website, but here is a link to the raw video by Karen Mustard, a camper at Porcupine Bay, who caught the landslide on video:
Watch Video

This is the second landslide this year to have caused a seiche in the Lake Roosevelt area. A landslide that moved in January caused a seiche and damaged docks. No injuries were reported. Link to the January Lake Roosevelt Landslide

Media Links:
KREM
Northwest Cable News
KiroTV
Seattle Times
Spokane Spokesman
KATU (Portland)
Seattle PI Washington GeoHazards Blog

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