Nearly 45 years after Mount St. Helens’ eruption sent almost 90 billion cubic feet of debris into the upper Toutle Valley, millions of tons of sediment still pour into the Cowlitz River each year. And that flow is causing costly problems for towns along the way.
A sediment retention dam has been in place for the past 36 years but it needs updating. That work has been delayed, leaving nearby cities to find their own solutions to drinking water needs and to maintain deep draft levels at ports.
Kelso is looking to update its water system due to impeding sediment, while Castle Rock and Longview in Cowlitz County have changed their systems in light of the blast. The U.S. Army Corps of Engineers continues to monitor sediment levels.
The problem
Mountain runoff carries ashen remains of what was the volcano’s peak into the North Fork Toutle River and down to the Toutle. From there, the lower Cowlitz River ferries an average of nearly 3 millions tons of sediment through Castle Rock, Kelso and Longview, where it dumps into the Columbia River
That 3 million tons is 10 times greater than pre-eruption levels, and it causes damage by burying cities’ water intake systems and filling in deep water ports’ berths.
The U.S. Army Corps of Engineers constructed the sediment retention dam in 1989 on the North Fork Toutle River, and still spends between $400,000 and $600,000 a year monitoring the problem, said Corps’ spokesman Jeffrey Henon.
But the sediment retention dam has become increasingly less effective as it fills. It used to catch about 80% of passing sediment, but now allows 80% to pass, Henon said.
Planned work to refurbish the structure has been delayed, said David Vorse, Castle Rock’s Public Works Department director.
With the Corps spread thin managing the Columbia River hydropower system and other projects around the country, the burden of dealing with Mount St. Helens’ sediment has increasingly fallen to cities along the path of each river.
Kelso’s water system
That management can be costly, said Kelso City Councilor Mike Karnofski. He used to run utilities for Weyerhaeuser’s local operations and now budgets for upgrades to the city’s water system.
The city switched from drawing water directly from the Cowlitz River to a Ranney well system in 1979, said Kelso Public Works Manager Devin Mackin. Ranney wells collect naturally filtered groundwater using pipes that extend out horizontally under a riverbed from a main vertical shaft.
The switch was planned to create a more consistent water supply that wasn’t as vulnerable to seasonal changes in water level, contamination and natural disasters.
But, when the mountain blew one year later, fine sediment deposited on the riverbed began to slow the percolation of water into the intake pipes. That problem has not gone away — and it’s not cheap to correct.
“We do not have a precise cost estimate for managing the silt and sediment in the Cowlitz River because the expenses are too extensive to accurately track, and the silt buildup itself is not systematically monitored,” Mackin said.
To combat sediment, the city has needed to dredge above the intakes, buy water from Longview and refurbish the system.
That system upgrade cost Kelso about $1 million last year, Karnofski said. And the continuing expense is significant enough that it has led the Kelso City Council to budget for maintenance and an eventual replacement years into the future.
The current system has about five to eight years left, Karnofski said. After that, the city will have to build a new water system.
Longview dredging costs
Longview gets its water from groundwater wells near the Mint Farm Regional Water Treatment Plant.
That approach started in 2013 specifically to avoid costly and frequent sediment-related maintenance, said Chris Collins, Longview Public Works director and assistant city manager.
“It was to the point where the plant was needing a full rebuild, which would have cost $40 million over the course of nine years, and our new plant was $32 million,” Collins said. “So it made that decision pretty easy.”
But, as Port of Longview Director of External Affairs Dale Lewis will tell you, that doesn’t mean the city totally dodged the bullet.
Last year alone, the port spent more than half a million dollars dredging about 200,000 cubic feet of Mount St. Helens sediment from the mouth of the Cowlitz and nearby port berths.
“We dredge almost every year,” Lewis said. “The amount of dredging at the docks varies by year but is almost always needed since the eruption of the Mount St. Helens.”
The recently passed 2024 Water Resources Development Act authorized the Corps to start dredging at the mouth of the Cowlitz River to support navigation and not just mitigate flood risk, said Henon, the Corps’ spokesman. That could reduce some of the Port of Longview’s costs.
Castle Rock issues
“When the mountain blew, the log debris that came down the river literally wiped out the intake structure to the water treatment plant,” said Vorse of Castle Rock Public Works.
When Castle Rock rebuilt, it placed the water intake on the upper Cowlitz River — above where the Toutle, and all the Mount St. Helens sediment it carries, meets the river.
Vorse, who started in his job the year after the eruption, said the system built in the aftermath still provides water to many communities around the area.
Despite that combination of lucky location and ambitious engineering, even Castle Rock hasn’t come out totally unscathed.
Water cleaned by the city’s wastewater treatment plant is supposed to be released back into the river by a set of diffuser pipes, but they’re buried in 3 feet of sediment at all times.
“Again, it’s a sediment issue that has impacted all three of the communities along the river at some level,” he said.
About the project: The Murrow News Fellowship is a state-funded journalism project managed by Washington State University. For more information, visit news-fellowship.murrow.wsu.edu.
