Volcanic sands from the 1980 blast and landslide keep giving. The massive mound that resulted has fed the small town’s development of parks, trails, baseball diamonds and more.

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CASTLE ROCK — In this southwest Washington community, a 60-acre pile of sediment dredged out of the Cowlitz River is a reminder of the titanic power of the Mount St. Helens eruption.

This is a tiny sliver of the largest landslide in recorded history, which cut loose on May 18, 1980, to trigger the blast, and flushed downstream a roiling mass of earth and debris.

Thirty-five years later, these mountain remnants are once again on the move.

A timeline of the explosion: What happened when Mount St. Helens erupted

At 8:32 a.m. May 18, a 5.1 earthquake rumbled 1 mile beneath the volcano.

One second later, the mountain’s north flank collapsed, creating the largest landslide in recorded history. In less than 10 minutes, debris filled nearly 25 square miles of the North Fork Toutle River valley to an average depth of 150 feet.

Rock and hot gases exploded sideways at 220 to 670 mph. The blast felled trees up to 12 miles away in a 180-degree arc north of the volcano. Some 57 people were killed, most by suffocation.

A slurry of melted snow and ice, boulders and sediments swept down mountain streams around the volcano. The largest mudflow, or lahar, raced down the North Fork Toutle River valley at up to 27 mph. It deposited more than 45 million cubic yards of sediment in the Columbia River, blocking oceangoing shipping for 13 days.

A vertical column of ash and stream rose 15 miles above the mountain. The cloud drifted to the northeast, depositing more than 3 feet of ash near the volcano. By 11:45 a.m., ash fell in Spokane. The ash cloud eventually circled the globe.

Hours after the avalanche and blast, super-heated pumice and gas flowed from the crater and into the valley north of the mountain. Temperatures of these pyroclastic flows reached 1,300 degrees Fahrenheit.

The eruption subsided about 5:30 p.m. after ejecting 540 million tons of ash.

Sources: U.S. Geological Survey, Seattle Times archives

All spring long, trucks have been hauling off loads of volcanic sands sifted from the pile and transporting them to the Puget Sound region. Some is worked into the greens of manicured golf courses, and other loads are mixed with compost to spread on homeowner yards or help grow the turf for playing fields.

“It’s a beautiful clean product that comes right out of the belly of the earth,” said Greg Miller of Walrath Trucking. “Safeco Field, the Seahawks training facility, they use it. There have been some very famous shoes that have trod upon this material.”

The sediment pile is managed by the city of Castle Rock, which earns thousands — and in some years, tens of thousands — of dollars a year selling the volcanic sands. This money helps fund a broader effort to turn dredge sites around the town from liabilities to surprisingly versatile assets that have helped this aging logging town carve out a new niche in outdoor recreation.

Over the years, Castle Rock has reformed dredge piles to create a skateboard park, a boat launch and a trail system. Volunteers donated some 3,000 hours of labor to turn dredge sediments into a mountain-bike park. And, in the most ambitious transformation, the city planted sod on a large dredge pile and created five baseball diamonds and two soccer fields.

“We’ve kind of defined our economic development as quality of place. You can be fishing, playing ball or just enjoying the view,” said Dave Vorse, director of Castle Rock’s public works department.

Castle Rock was at risk

After the 1980 eruption, Castle Rock’s future was far less secure.

The community was spared the severe flooding from the landslide’s initial downstream rush of mud and debris. But north of the mountain, a transformed Spirit Lake posed more long-term risks.

The lake received enough sediment to raise the surface level by 210 feet and block the outflow. There was concern that the dam formed by this debris might fail, sending enough water downstream to put Castle Rock under dozens of feet of water.

Even if the lake stayed put, there was a greater likelihood of severe flooding from winter rains and snow melt. That’s because debris that settled in the Cowlitz River drastically reduced the flow capacity by more than 90 percent.

“There was a lot of fear of what was going to happen. A lot of anxiety,” recalls Vorse.

In the year after the eruption, Castle Rock’s population declined from 2,100 to 1,800 people, he said.

Over time, the fears eased.

By 1985, a 1.6-mile outflow tunnel had been drilled through Spirit Lake so the water level could be maintained at a safe level. The Army Corps of Engineers built a retention dam on the North Fork of the Toutle River that helped slow the downstream flow of sediments.

The corps also carried out a marathon dredging effort that began right after the eruption and continued through the early 1980s. The dredging removed enough material from the Toutle, Cowlitz and Columbia rivers to build a 12-lane highway from New York City to San Francisco, according to the U.S. Geological Survey.

Vorse says some of the first dredge piles had higher contents of ash and set up almost like concrete as they piled on the banks. So it was difficult to get grass to grow.

But later dredging produced much lighter, almost fluffy materials.

“If you weren’t careful you could sink into them,” Vorse said.

Old threat re-emerges

In the first few years after the volcano, the 60-acre dredge pile sat stark and bare.

Then, scotch broom, a yellow-bloming invasive shrub from Europe, took hold on top, along with mosses, grasses and eventually alders and willows. The cover lured rabbits. And that attracted beagle, basset hound and dachshund clubs for seasonal field trials.

Others used the area as a dump site, leaving the landscape strewn with trash.

The state owned the land. But it was viewed as such a liability that part of the acreage was transferred to Castle Rock, which eventually assumed management responsibilities for the entire property and worked to clean up the trash.

“They required us to lease the property for 40 years, and be responsible for whatever happens out there,” Vorse said.

Early on, the market for dredge sediment was weak.

By the late 1980s, across the river, one of the city’s dredged piles began to be used as fill for southwest Washington construction projects. It didn’t pay to transport the material very far.

But as sand supplies in the Puget Sound area played out, prices rose high enough that Walrath Trucking could make money hauling the St. Helen’s material north.

Last year, with Puget Sound growth surging, Walrath began excavating sand out of the 60-acre mound and separating out multicolored volcanic rock for use in landscaping.

No one can say how long it will be until the mound disappears.

“It’s all market dependent. It could be six years. It could be 60,” Vorse said.

If Castle Rock should run low on dredge materials, more keeps washing downstream.

The Army Corps has proposed raising the retention dam to capture more of the sediments.

Meanwhile, an old threat has re-emerged that again could put Castle Rock at risk.

Army Corps inspections have found significant narrowing of a midsection of the tunnel. The agency’s engineers are concerned that during high water the tunnel could back up, creating pressures that could destroy it and — in a worst-case scenario — lead to the failure of the dam and catastrophic flooding in the area.

Officials say there is no imminent danger. But the Corps is pushing to undertake a short-term repair this fall and also working to come up with a long-term fix.

“That tunnel is very important to Castle Rock, and everyone else who lives downstream,” Vorse said. “We’re encouraged that the Corps has identified the problem and come up with a plan. But if they don’t get funding, things won’t happen.”