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The most enduring geological consequence of the eruption of Mount St. Helens, Washington,
on May 18, 1980, and the most costly single
element in the recovery effort, has been the persistent downstream
sedimentation caused by erosion of the approximately 3 cubic kilometers
(km3) of sediment deposited on the landscape surrounding the volcano.
Most of the sediment was associated with the emplacement of a 2.8 cubic kilometers
debris avalanche in the upper part of the watershed of the North Fork Toutle River,
and debris flows in the channels of the South Fork Toutle
River, Pine Creek, Swift Creek, and Muddy River. An additional 0.2-0.3 cubic kilometers
of volcanic material was emplaced by pyroclastic flows, blasts,
and ash fall. Part of this vast quantity of volcaniclastic sediment
has been subsequently eroded by runoff and streamflow. This brief report
summarizes the changes in sediment yield at five locations around
Mount St. Helens in the first 13 years following the May 18, 1980 eruption.
-- Costa, 1994
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[Map,26K,InlineGIF]
