LONGVIEW — For the first time, federal engineers are acknowledging that they will need to battle Mount St. Helens sediment flow into the Cowlitz River into the middle of the 21st century.
And the government estimates it must spend another $384 million through 2035 to keep Longview, Kelso, Castle Rock and Lexington safe from Cowlitz River flooding related to the silt flow, a U.S. Army Corps of Engineers official said Friday in Longview.
“There will be a need for continuous action,” said Mike Turaski, the Cowlitz-Toutle project coordinator for the Corps’ Portland District.
Turaski spoke at a meeting of the Port of Longview commissioners, who are concerned that the flow of silt down the Cowlitz is clogging its shipping berths along the Columbia River waterfront.
The Corps six weeks ago finalized its long-delayed plan for controlling volcanic sediment.
“That’s very good news,” Turaski said, because the effort had been delayed several years by the need to protect endangered salmon stocks on the Toutle River.
The agency also has funding for 2019 to resume monitoring erosion and silt movement in the Cowlitz and Toutle rivers, which it has not had money to do since 2015.
As a result, Turaski acknowledged, “some people say we’re in the dark” about river conditions and flood risks along the Cowlitz. However, when last checked three years ago, flood protection levels were well within levels that Congress authorized in 1985.
Nevertheless, monitoring will “help us figure out whether we have an urgent problem,” Turaski said.
When Mount St. Helens erupted on May 18, 1980, the volcano’s summit collapsed into the upper Toutle Valley, strewing it with 3 billion cubic yards of debris. The north fork of the Toutle has been cutting away at that deposit, which is 600 feet deep in places, and washing it downstream. Unchecked, it could clog the channel of the Cowlitz and increase flooding odds.
Over the years, the Corps has raised and fortified dikes, dredged hundreds of millions of tons of debris from the rivers and built a 125-foot-high earthen dam across the north Toutle near Kid Valley to stop the silt flow.
It raised the structure seven feet in 2012, but the problem has persisted.
Major shoals showed up all along the Cowlitz this summer when water flows were low. Concerned that the silt may be reducing the river’s capacity, county officials took Turaski on a tour of the river this summer.
In addition, Port of Longview officials say the flow of debris is likely behind the shoaling in its ship berths. It expects to have to spend $6.4 million over the next 16 years to clean it out, port officials told Turaski.
Commissioner Jeff Wilson asked if the Corps’ Mount St. Helens authority applies to protecting navigation and shipping along the Columbia. It was a way of asking, in effect, whether the Corps could dredge the boat basins.
No, Turaski said, explaining that the congressional authorization for Mount St. Helens is for flood protection only.
“I know that is disappointing for you,” Turaski said.
However, the Corps’ efforts to intercept silt in the Toutle and Cowlitz should reduce the port’s silt problem, he said.
The Corps’ long-term plan is to raise the Toutle River sediment-retaining dam another 23 feet, build other silt-retaining structures upstream of the dam and dredge the Cowlitz as necessary. It also will build a new fish trap below the dam.
Turaski said the 2019 river monitoring work will determine the next steps, but the agency this year will complete design work to raise the sediment-retaining dam so the project is ready to go, Turaski said.
The agency estimates it will take $384 million over the next 17 years to combat the silt. Put in perspective, that’s nearly twice the $178 million state and federal governments have spent protecting the Cowlitz Valley from the volcano since 1984, Turaski said.
The silt problem is not going away soon, though it will be irregular depending on the severity of storms that strike the Toutle Valley. (For example, 12 million tons washed out of the Toutle in 2007, the year of a major storm, but in more normal years the figure is more like 1 million to 2 million.)
The problem, Turaski said, certainly will persist past 2035, the year the Corps’ authorization for volcano-related flood control work expires. Part of the Corps’ task in the coming decade, he said, is figuring out what needs to be done farther into the future.
“The erosion problem is not going to stop in 2035.”
