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 down into the Cowlitz River each year.

And that flow is causing costly problems for towns along the way.

Updating Toutle’s roughly 36-year-old sediment retention dam has been delayed, leaving nearby cities to find their own solutions to drinking water needs, and maintaining deep draft levels at ports.

Kelso is looking to update its water system due to impeding sediment, while Castle Rock and and Longview have changed their systems in light of the blast, and the U.S. Army Corps of Engineers continues to monitor sediment levels.

The problem

The issue starts when 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 before much of it dumps into the Columbia on its way to the Pacific Ocean.

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 has long managed the issue. It constructed the sediment retention dam in 1989 on the North Fork of the Toutle River, and still spends between $400,000 and $600,000 a year monitoring the problem, said Corps’ spokesman Jeffrey Henon.

But the structure has become less and less effective as it fills, switching from catching about 80% of passing sediment to allowing 80% to pass, said Henon.

While work to refurbish the structure again is planned, it has been delayed, said David Vorse, Castle Rock Public Works director.

And — with the Corps spread thin managing an almost endless number of project around the country including most of the Columbia River hydropower system — the burden of dealing with Mount St. Helens’ sediment has increasingly fallen to cities along the rivers’ path.

Kelso plans for new water system

That management can be a costly task, said Kelso City Councilperson Mike Karnofski, who 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 Collector Well system in 1979, said Kelso Public Works Manager Devin Mackin. Ranney wells collect naturally filtered groundwater using pipes that extend out under a riverbed horizontally from a main vertical shaft.

The switch was planned to create a more consistent water supply, not 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 deal with.

“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 the issue, the city has needed to dredge above the intakes, buy water from Longview and even refurbish the system.

The last of that trio alone cost the city about $1 million last year, said Karnofski. And the continuing expense is significant enough that it has led the City Council to budget for maintenance and an eventual replacement years into the future.