Army Corps to raise Mount St. Helens sediment dam

The U.S. Army Corps of Engineers is set to start raising a dam used to catch millions of tons of Mount St. Helens sediment that flows each year from the mountain into the Toutle, Cowlitz and Columbia rivers.

The nearly 37-year-old sediment-retention structure was last raised in 2013. But it has become less and less effective as it fills, shifting from catching about 80% of passing sediment to allowing 80% to pass, a Corps spokesperson said in March.

That has saddled towns along the river with millions of dollars of yearly maintenance and started to fill in the Corps’ Columbia River deep-water shipping channel.

It also has worsened recent flooding along the Cowlitz, according to leaders along the river.

“It’s one of the things that we have known that we needed,” John Morgan, a spokesman for the Corps’ local operations, said last year. “It just takes a long time to get through the process, and now we’re at that point.”

Valley floor raised 100 feet in three decades

The Corps plans to raise the part of the dam that the North Fork Toutle River flows over before continuing onto the Toutle, Cowlitz and Columbia rivers.

The higher crest will slow down the sediment-packed water in the valley behind the structure, allowing heavier sediment to fall to the valley floor before passing over the structure’s spillway.

Since the dam was constructed in 1989, the sediment it caught raised the mileslong valley floor by about 100 feet, Corps local spokesperson Kerry Solan said last May, meaning its capacity shrinks every day.

The contract to raise the dam’s crest is currently out for bids, Solan said this week, and the contract will likely be awarded at the end of February.

A June listing for the project on the federal government’s contracting website said the project is valued between $25 million and $100 million.

Solan said the Corps expects to start construction by summer 2026 and added that it must be done by October 2027.

A waning solution

The sediment problem became clear the morning after Mount St. Helens’ 1980 eruption sent 1 million Olympic swimming pools’ worth of debris into the upper Toutle Valley, Col. Terry Connell told the Columbian in May. He was the commander of the Portland District back then.

In an 8 a.m. meeting the morning after the blast, a colleague told Connell that the sediment had made it to the Columbia River’s shipping channel, filling in the then-32-foot-deep channel to a depth of 12 feet by about 2 a.m. – and trapping a ship “right smack in the middle of the channel.”

The agency jumped into dredging operations in the Toutle, Cowlitz and Columbia rivers.

Before the eruption, Connell said, the Corps had been dredging about 10 million cubic yards of material at the mouth of the Columbia every year, as well as about 10 million yards from the river’s shipping channel. Then, in an instant, it had about 250 million cubic yards to deal with, Connell said.

In the months and years after the eruption, the long-term impacts started to come into focus, and in 1986 the Corps started construction on the sediment-retention structure to stop the practically unending flow of sediment near its source.

The resulting 2,300-foot-long, 180-foot-tall dam across the North Fork Toutle River valley was designed to catch about 260 million cubic yards of sediment over a 50-year period.

It certainly hasn’t failed in that mission, though the dam required a raise in 2013 similar to the one expected to begin next year.

Still, a report by the U.S. Geological Survey shows millions of tons of sediment has continued to make it past the structure every year – and the issue is getting worse.

Cowlitz River flooding worries

The growing problem has cost cities along the river millions in clogged drinking water intake systems, blocked wastewater outputs and filled-in port berths.

According to public works leaders around the county, it also has worsened flooding along the Cowlitz River.

“I can’t dump a 5-gallon bucket of sand in my bathtub and expect to have the same amount of water capacity in that bathtub,” Scott Neves, director of the county Department of Emergency Management, said last week.

Kelso’s public works manager, Castle Rock’s public works director and Longview’s public works director/assistant city manager echoed that.

In October, more than a dozen leaders around the county sent a letter to the Corps asking the agency to dredge the river for flood risk. The Corps, in a November response letter obtained by the Daily News, did not address that request.

The agency is authorized to dredge the river but has not received money from Congress to carry out that work. It has been hit by at least $1.5 billion in cuts by the Trump administration this year.

The sediment issue, meanwhile, has become especially urgent during recent flooding, which worsens the problem.

The U.S. Geological Survey report found that an average of 25% of the sediment that makes it over the dam each year does so in just a single flood event.

“We’re in the infancy of all this sediment settling out,” Solan said. “Over the next millennia, this sediment will continue to move. We know we’re going to have to keep doing these crest raises.”