Taming the Toutle River
The vast quantities of this silty debris are testament to the volcanic power of nearby Mount St. Helens, which unleashed the mighty blast of May 18, 1980. In a matter of hours, the event dramatically replumbed this Cascade drainage.
But forceful human intervention — not nature — is spreading the mud across a 2,400-acre expanse of the North Fork Toutle River Valley. The mud is backing up behind a $65 million sediment dam, which was built to keep most of the material from clogging downstream channels that wind past Castle Rock, Longview and other Southwest Washington communities.
The dam is the most-visible manifestation of a high-stakes engineering effort to try to tame the Toutle River, which over the centuries — buffeted by repeated eruptions of Mount St. Helens — has been prone to catastrophic floods and mudslides. This engineering work, the only such effort in the United States, also includes an 8,640-foot-long tunnel that drains Spirit Lake, whose natural outflow was blocked by the 1980 eruption.
As the mountain enters a new eruptive phase, this engineering is receiving renewed attention from the U.S. Geological Survey, Army Corps of Engineers and National Weather Service. Scientists expect to see a continuation of the mild period of quiet dome-building now under way but caution that the mountain still is capable of powerful explosions that might suddenly melt glacier ice or winter snows and spawn sizable mudflows.
So far, scientists say the sediment dam and Spirit Lake's manmade outfall are expected to withstand whatever the volcano unleashes and keep downstream communities safe.
"All the flow modeling that we have done indicates there wouldn't be a threat," said Jon Major, a U.S. Geological Survey scientist who specializes in volcano hydrology.
But in the decades ahead, protecting Castle Rock, Longview and other downstream communities still is expected to be an expensive, time-consuming undertaking. It may involve downstream dredging, or possibly raising the level of the sediment dam so it catches even more mountain mud, according to options now under federal review.
"We're reviewing all these options," said Christine Budai, an Army Corps of Engineers geologist who helped design these projects.
Massive transformation
When the volcano erupted in 1980, the North Fork of the Toutle River was a clear-running river that supported prize runs of salmon that migrated up into the Spirit Lake basin, which was the headwaters of the drainage.
"We used to catch huge chinook — it was a beautiful spot," said Mark Smith, whose family operated a resort at the lake before the eruption.
Within a matter of hours, the eruption transformed the river drainage. It unleashed massive mudflows that rumbled down the North Fork main stem. Though much of the mud settled in the drainage's upper reaches, an epic torrent worked its way downstream, destroying nearly 200 houses as it choked the Toutle, then coursed through the Cowlitz to fill up Columbia River navigation channels.
The eruption also remade Spirit Lake, wiping out its shoreline forests and dumping enough logs, rock and other debris to raise the surface level by some 200 feet. And it dammed the lake's outlet with a potentially unstable mix of debris topped by fine-grained ash.
Scientists feared the volcano-crafted dam would eventually crumble and unleash a second wave of mudflows that would bury portions of Toutle, Castle Rock, Longview and Kelso. That scenario appears to have played out in the past. About 2,500 years ago, a predecessor to Spirit Lake is believed to have breached and unleashed massive mudflows far bigger than those of 1980 that dammed up the lower valley to produce the 5-mile-long Silver Lake.
To stabilize the lake level and protect the new natural dam, the Corps of Engineers in 1985 built the $13.5 million tunnel to provide a drain that would keep the lake at safe levels.
But outside of the lake, in the headwaters of the Upper Toutle River, huge amounts of sediments continued downstream. This prompted a continual dredging effort to keep these materials from filling channels and increasing flood risks to Castle Rock, Longview and other communities.
So the corps built an 1,800-foot-long sediment-retention dam across the North Fork Toutle. The project site is some 20 miles downstream from Spirit Lake. Completed in 1989, it rises to a height of 184 feet.
The dam works by backing up the river, which settles out coarser sediments in a gently sloping upstream zone. Water and lighter, fine-grained sediments then flow over a spillway on its north end.
Though the dam helps protects downstream areas, it has had a harsh impact on several miles of the upstream river. Geese seem to like the mud flats, but the sediments are bad news for fish, trees and other shoreline life, according to Smith.
The dam's impact troubles Smith, who now operates a Mount St. Helens cabin and tour service known as Eco-Park Resort. Over the years, he has made many visits to the mud zone, tracking its spread, and questions whether it was the best way to deal with the sediments.
"They have made this into a sacrifice zone, " Smith said.
Both structures stable
As volcanic activity resumes, human intervention and engineering could be put to the greatest tests yet.
As in 1980, the biggest risks would be mudflows — known as lahars — that would tumble down the mountain, where the glacier-clad crater now acts as the headwaters of the North Fork Toutle.
In the months ahead, scientists say the worst-case scenario involves a mudflow equal to more than 3 million dump trucks worth of debris.
Much of that material would be expected to flow directly from the crater into the headwaters of the North Fork of the Toutle. The rest would dump into Spirit Lake. Even in this scenario, the material dumping into the lake would not cause the lake's natural dam to breach or overflow, according to Tom Pierson, a U.S. Geological Survey hydrologist.
Federal officials also are scrutinizing the Spirit Lake tunnel, which now drains into South Coldwater Creek.
In recent weeks, the tunnel underwent maintenance to shore up crumbling rock around its intake, and workers got roughly 80 percent of that job done before calling it quits earlier this month, according to Budai, the Army Corps of Engineers geologist.
But federal officials do expect the lake's tunnel to survive any new eruptions.
The other focal point of recent study is the sediment-retention dam.
Scientists expect that any major new mudflows that reach the North Fork Toutle would slow and lose their power as they approached the gently sloping muddy delta that now spreads upstream from the dam. Some of the material would move over the spillway but without the power to threaten downstream communities.
But in the years ahead, more muck piling up behind the dam could increase the rate at which downstream channels clog with sediment, forcing the corps to increase efforts to protect the downstream communities.
"If a large mudflow comes down, it becomes a whole new calculation," said Mike McAleer, a Corps of Engineers spokesman.
Hal Bernton: 206-464-2581 or hbernton@seattletimes.com
