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Mount St. Helens Sediment Retention Structure

The north fork of the Toutle River flows through a notch in the spillway at the U.S. Army Corps of Engineers sediment retaining dam in the fall of 2012. The notch is in a portion of the spillway that was raised seven feet to restore the sediment-trapping efficiency of the dam.

The Cowlitz River was slightly deeper in 2013 than it was in 2012, and cities along the river are a tad safer because of that.

The finding by the U.S. Army Corps of Engineers also may hint that the sediment problem caused by the 1980 eruption of Mount St. Helens may finally be easing slightly.

For 34 years, the Toutle River has washed tons of Mount St. Helens debris into the Cowlitz, which generally has been unable to naturally disgorge itself of the muck. As silt built up in the river bed, it increases flood risks by diminishing the channel’s capacity.

Millions of tons of silt still are washing into the river. But from August 2012 to September last year, the Cowlitz flushed more silt out into the Columbia River than the Toutle washed into it, the U.S. Army Corps of Engineers reported recently.

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 engineers call it, amounted to about 500,000 tons of debris.

That’s a small amount when spread over the lower 20 miles of the Cowlitz, from its mouth to the Toutle River. But 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), even if just by an inch or two.

Are we seeing the end of the sediment problem that has dogged the area for three decades? Yes and no, the corps says.

Most of the debris comes off the so-called debris avalanche deposit — the crumbled, sandy and boulder-strewn remains of the volcano’s old north flank. During the 1980 eruption it toppled into the upper valley, burying the north fork of the Toutle up to 600 feet deep in places. 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 sediment-retaining dam in 2012. The project, which boosted the spillway of the 125-foot-high earthen dam 7 feet, restored the efficiency of the dam, which had largely filled with debris.

Over two winters, the raised spillway project has trapped about 2 million cubic yards of sediment, said Chris Nygaard, a hydraulic engineer in the sediment and hydrology section in Portland. That’s a pile that would measure 900 feet high is spread across a football field.

Corps officials say the spillway raise is the main reason that flood protection levels at Cowlitz River cities are still well-guarded from 100-year-storms (those with a 1 in 100 chance of occurring annually).

Clearly, though, a lot of silt continues to erode out of the blast area, according to hydrologists. The U.S. Geological Survey estimates that 3 million to 6 million tons of silt a year are washing out of the volcano’s blast area, an amount that is generally unchanged over the past decade. And as recently as 2007, a major winter storm walloped the area and dropped 2.5 million tons of silt into the Cowlitz, mostly in the stretch from Lexington to its mouth. A major storm like that could do the same today, Corps engineers say.

Still, the silt flow through the Toutle Valley is a fraction as much as it was in the years immediately following the eruption, when the Toutle carried anywhere from 20 million to 100 millions tons of silt downstream annually.

A U.S. Department of Agriculture analysis for the corps last year shows that the Toutle still is meandering through the debris, and as long as the channel continues to evolve it will continue to erode lots of material.

“We don’t see a drastic decline in sediment loads” in the foreseeable future, said Paul Sclafani, another corps hydraulic engineer.

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, said Tim Kuhn, the agency’s Cowlitz-Toutle coordinator.

The plan will be subject to public review and comment before a final one is adopted in 2015. The earliest any work could be done under the plan would be 2017, he said.

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City Editor Andre Stepankowsky can be reached at 360-577-2520 or andre@tdn.com.

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