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Excess Sediment Accumulation Causes Flooding in the Cascades

Sediment carried downstream remains as a lasting legacy of volcanic activity.

Eruptions, and subsequent erosional processes, can deliver vast quantities of sand and gravel to rivers on or near volcanoes. Mobilized material can move rapidly as voluminous slurries of rocks and debris (lahars) that can destroy structures along their path and deposit vast quantities of sediment along a valley floor. Flood-transported sediment can do similar damage but over a longer period of time, and post-eruption sediment transport can have socioeconomic consequences more severe than those caused directly by an eruption.

Release of excessive sediment from volcanically disturbed watersheds can persist for decades and greatly extend the duration of an eruption's damaging effects. In the three decades following the Mount St. Helens eruption, the U.S. Army Corps of Engineers has spent more than $0.5 billion to dredge sediment from the Toutle, Cowlitz, and Columbia rivers, build a sediment retention structure, and construct a tunnel to stabilize the level of Spirit Lake; it continues efforts to mitigate ongoing volcanic sediment release. Sediment accumulated (aggradated) to about 20 m (65 ft) thick 60 to 90 km (40 to 55 mi) downstream of Mount Hood (Oregon) during a modest dome-building eruption (A.D. 1781 to 1793), owing simply to the steady erosion and transport of sediment shed from the growing lava dome. The size or type of eruption may not determine the impact from prolonged sedimentation, which can occur for decades after an eruption.