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Excess Sediment Accumulation Causes Flooding in the Cascades

Dredging of the Toutle River after the Mount St. Helens 1980 erupti...
U.S. Army Corps of Engineers preformed dredging projects on the Toutle, Cowlitz, and Columbia Rivers and by 1987 enough material had been removed to build a 12 lane highway, one-foot thick from New York, NY to San Francisco, CA.

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.

North Fork Toutle River Above the Sediment Retention Structure, Mou...
The May 18, 1980 eruption of Mount St. Helens had a dramatic impact on the North Fork Toutle River. The debris avalanche, lateral blast and ashfall produced a substantial amount of sediment, which continues to be washed into the river and transported downstream. A Sediment Retention Structure was constructed (out of view) to minimize the amount of sediment flowing downstream. View is to the east with Mount St. Helens in the background.
Sediment retention dam on the North Fork Toutle River is designed to help stop downstream movement of sediment near where it begins on Mount St. Helens' debris avalanche deposit.