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Clearing Toutle River Sediment Issue
A July 21, 2003, editorial, “Volcano fallout,” was about the sediment retention structure on the
North Fork Toutle River at Mount St. Helens. The editorial quoted us commenting on the
structure, and in doing so presented an incomplete and inaccurate view of discussions with U.S.
Geological Survey scientists to support an assertion that the U.S. Army Corps of Engineers erred
in the structure’s design and construction.
The editorial also confused predictive uncertainty with misunderstanding natural
processes. We would like to clarify the context of our comments and discuss the real vs.
perceived effectiveness of the sediment retention structure.
The volcano’s cataclysmic eruption on May 18, 1980, deposited vast amounts of
sediment on hillslopes and in river valleys that drain the volcano. Subsequent erosion in the
Toutle River basin has led to sediment yields – the amount of sediment removal per area of basin
– as much as 500 times greater than pre-eruption yields. Sediment eroded from near the volcano
accumulated in the lower Toutle, Cowlitz, and Columbia Rivers, raised channel beds, increased
potential flood hazards, and disrupted shipping lanes. The corps dredged those channels initially,
then decided that trapping sediment close to the mountain would provide more effective
mitigation. Specifically, they sought to limit the amount of sediment entering the Cowlitz River.
When the sediment retention structure was designed, very little was known about long-term
erosion following cataclysmic volcanic eruptions, and at Mount St. Helens the USGS had
measured sediment yields for only a few years. Corps engineers and USGS scientists therefore
had to project sediment yields decades into the future on the basis of very little data. In that
sense, scientists and engineers were “groping in the dark,” as the editorial quoted.
But corps engineers used high estimates of sediment yield in their design of the retention
structure, and estimated that storage capacity would fill in 50 years. To date, the actual amount of
sediment trapped is less than projected. So why did the structure “fill” too early? Here,
perception conflicts with reality.
The retention structure was designed to trap about 250 million cubic yards of sediment.
So far, it has trapped somewhat more than 100 million cubic yards. Thus, storage capacity
behind it is not full. However, sediment behind the structure has filled to the level of the
spillway, apparently 40 years early, and now about 1 million cubic yards of silt and fine sand
bypasses the structure annually.
What happened? Perhaps the answer lies not in misprojected sediment yields, but in how
sediment has been deposited.
Sediment entering pooled water settles rapidly and commonly deposits progressively in a
downstream direction. The SRS pooled some – but not a large body of – water; thus, sediment
accumulated closer to the SRS more rapidly than was anticipated. Now it is progressively
building a wedge in an upstream direction.
Upstream migration of the sediment wedge has led to a sharp reduction in the slope of the
river valley where the natural channel meets the wedge. Continued sediment delivery from the
North Fork Toutle valley will fill that break and smooth the slope; that is how future sediment
will be stored. Once the channel slope upstream of the retention structure becomes uniform,
more and coarser sediment may bypass the structure, depending upon regrowth of vegetation
upstream.
Structure still effective
The sediment retention structure is still effective; it traps most of the sediment delivered.
It’s not as effective as it used to be, but the bypassed sediment is not changing downstream
channel shapes yet. Corps engineers project that within 20 years enough sand may bypass the
structure to raise the Cowlitz River bed. Presently, sediment bypassing the structure poses no
flood hazard to downstream communities.
Channels will be monitored closely over the coming decades to detect onset of unwanted
changes in channel conditions. Concurrently, discussions should begin to consider long-term
mitigation strategies.
Jon J. Major and Kurt R. Spicer, Hydrologists,
U.S. Geological Survey, Vancouver, WA
August 9, 2003
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