LONGVIEW, Wash. (AP) - The Cowlitz River has slightly deepened for the first time since Mount St. Helens erupted in 1980. It’s good news for cities on the river, where sediment increased their flooding risk.
The Cowlitz River was naturally deeper in 2013, The Longview Daily News reported (https://bit.ly/1e5IeW7 ), the first time the river has been deeper without requiring dredging.
The finding by the U.S. Army Corps of Engineers also may hint that the sediment problem caused by the 1980 eruption may finally be easing slightly.
During the 1980 eruption, the volcano’s old north flank toppled into the upper valley. As the river tries to establish a path through the debris, it carries huge volumes of silt downstream.
Corps engineers attribute the last year’s improved river conditions mostly to raising the spillway on its Toutle River dam in 2012.
For the foreseeable future, erosion will continue and the sediment level isn’t expected to change much.
“We don’t see a drastic decline in sediment loads” in the future, said Paul Sclafani, a corps hydraulic engineer.
The finding is based on the agency’s annual survey of 95 cross sections of the river, which record the shape and depth of the channel. The “net scour,” as it’s called, amounted to about 500,000 tons of debris.
It represents the first time since the volcano blew that the corps can reliably say the river was, on average, deeper than the year before, not counting years the corps has dredged the river.
The 2012 spillway raise was a stopgap measure intended to give the corps time to develop a long-range plan for dealing with the silt flow. The corps in August expects to release its long-term plan for dealing with the sediment.
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