25, 2006 the Steelhead Haven landslide began what would become
the largest since 1967. Residents reported seeing some large sediment
slumps early in the week, but it wasn't until Wednesday that the
entire slope gave way and pushed thousands of cubic yards of earth
across the river over 200 feet, covering the sand bar in the aerial
photo below. One fisherman wasn't aware of the slide until the
water began rising around his waders.
January 28th, the river recut through an abandonned channel on
the adjacent floodplain. Residents and agency staff reported the
eerie sound of trees constantly snapping as the river pushed them
over. In an unprecedented move, Snohomish County worked with the
Army Corps of Engineers to help remove trees and excavate this
channel to prevent flooding the many homes built on the Channel
Migration Zone. By Monday the river had risen from 2000 cfs (cubic
feet per second) to 15,000 cfs. The Corps and the County placed
rock and wood to protect homes from the rising water.
As of Feb.
13, the new channel and the slide appear stable for the time being.
However, the community is still at risk from erosion and continued
headcutting and widening of the new channel. A public meeting
will be held later in February. Meanwhile the County and the Corp
will continue to monitor the slide for further changes and fulfill
permitting obligations for the emergency measures that were taken.
The top photos
is courtesy of Snohomish County, as is the following link to a
full length presentation (8MB file): Geology