Forty years after the mountain’s eruption, officials struggle to balance research and risk.

by Eric Wagner

The Pumice Plain in southwest Washington’s Mount St. Helens National Volcanic Monument is one of the most closely studied patches of land in the world. Named for the type of volcanic rock that dominates it, it formed during the mountain’s 1980 eruption. Since then, ecologists have scrutinized it, surveying birds, mammals and plants, and in general cataloging the return of life to this unique and fragile landscape.

Now, the depth of that attention is threatened, but not due to the stirrings of the most active volcano in the Pacific Northwest. The problem is a large lake two miles north of the mountain: Spirit Lake. Or, more specifically, the Spirit Lake tunnel, an artificial outlet built out of necessity and completed in 1985.

After nearly four decades, the tunnel is in need of an upgrade. At issue is the road the Forest Service plans to build across the Pumice Plain despite the scientific plots dotting the plain’s expanse. In this, Spirit Lake and its tunnel have become the de facto headwaters of a struggle over how best to manage research and risk on a mountain famous for its destructive capabilities.

THE ENTANGLEMENT OF THE LAND, the lake and the tunnel began 40 years ago, when Mount St. Helens erupted on May 18, 1980. At 8:32 a.m., a strong earthquake caused the mountain’s summit and north flank to collapse in one of the largest landslides in recorded history. Some of the debris slammed into Spirit Lake, but most of it rumbled 14 miles down the North Fork Toutle River Valley. Huge mudflows rushed down the Toutle and Cowlitz rivers, destroying hundreds of bridges, homes and buildings. 

The eruption killed 57 people and caused millions of dollars in damages. Mount St. Helens shed more than 1,300 feet of elevation, hundreds of square miles of forest were buried or flattened, and Spirit Lake was left a steaming black broth full of logs, dead animals, pumice and ash. Its surface area nearly doubled to about 2,200 acres, and its sole outlet, to the North Fork Toutle River, was buried under up to 600 feet of debris.

Having no outlet, and with rain and snowmelt still flowing in, Spirit Lake began to rise. The situation was dangerous: If the basin filled, the lake could overtop the debris field and radically destabilize it, unleashing another devastating mudflow that would send millions of tons of sediment toward the towns of Toutle, Castle Rock and Longview, Washington

For the complete article …

hcn.org/issues/52.5/north-scientific-research-the-threat-below-mount-st-helen

images: fs.fed.us/outernet/pnw/mtsthelens