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A serious side effect of the Mount St. Helens 1980 eruption has been the
downstream movement of enormous amounts of sediment eroded from hillslopes and
from the debris-avalanche and pyroclastic-flow deposits in the upper reaches of
the North Fork Toutle River. The SRS was constructed to trap this sediment
before it was carried farther downstream, where it could clog the river channel
and exacerbate floods along the lower Toutle and Cowlitz Rivers. An overflow
channel was added to divert lahars around the dam.
Before the 1980 eruption, the amount of sediment transported downstream annually
in the Toutle River would fill a football field to a height fo 325 feet. The
average amount of sediment transported during each of the first 5 years after
the 1980 eruption would fill a football field to a height of 360 miles, making
the Toutle River one of the most sediment-laden rivers in the world. The amount
of this sediment moving downstream, chiefly very fine material in suspension,
has been reduced significantly by the SRS, and yet it is still several times
the pre-1980 annual average.
The enormous amount of sediment transported after the eruption drastically
degraded water quality and aquatic habitat. It also increased flood potential
downstream and jeopardized homes and roads built on the floodplain and terraces
adjacent to the Toutle River. As the lower Toutle and Cowlitz Rivers filled
with sediment, their capacity to contain water during flood events was
drastically reduced. Early mitigation efforts that preceded construction of the
SRS included dredging the Cowlitz River and constructing temporary sediment dams
in the upper North Fork Toutle River.
Excerpt from:
Pringle, 1993, Roadside Geology of Mount St. Helens National Volcanic Monument
and Vicinity: Washington Department of Natural Resources, Division of Geology
and Earth Resources Information Circular 88
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Mount St. Helens
Coal Banks Bridge (west)
Link to: U. S. Forest Service - Mount St. Helens National Volcanic
Monument Website