When Mount St. Helens erupted on May 18, 1980, it did not just flatten a forest. It dumped the forest into a lake, and more than 40 years later thousands of those trees are still out there, drifting across the water.

A Lake That Swallowed Its Own Forest

The north flank of the volcano collapsed in the largest known landslide in recorded history. The lake in its path, Spirit Lake, was thrown out of its basin and then flowed back, floating the blasted, uprooted trees as it came.

NASA satellite imagery from 2021 shows the remains of thousands of trees still on the surface. NASA says logs covered more than 70 percent of the lake right after the eruption. In 2021 they covered about 20 percent.

The Spirit Lake outlet tunnel intake, a concrete structure at the foot of the cliff with floating logs gathered in front of it (USGS photo by Lyn Topinka) — The intake where Spirit Lake drains into its tunnel

The Lake With No Way Out

The landslide debris also blocked Spirit Lake's natural outlet and pushed the water about 200 feet higher than before. With no way out, the lake kept filling behind a huge pile of loose debris that could give way.

If that debris dam failed, the water would rush down the Toutle River valley toward towns on the Cowlitz River. So the US Army Corps of Engineers bored a drain through Harry's Ridge. The tunnel, roughly 8,500 feet (about 1.6 miles) long, went into service in May 1985.

A Tunnel That Keeps Squeezing Shut

Forty years on, that tunnel is still the only way to control the level of Spirit Lake. A National Academies review calls it "the sole means of controlling water levels."

It crosses a vertical rock seam where two kinds of rock slide against each other. The tunnel buckles at that spot, and it has been closed for repairs more than once, including in 1995 and 1996. Ryan Cole, who manages Spirit Lake for the Gifford Pinchot National Forest, called past fixes "band-aid repairs," and OPB reports the rock seam will keep moving and the tunnel will keep buckling.

What Happens If It Fails

Larry Hembree, director of Cowlitz County Emergency Management, told OPB that a tunnel failure could send "a 40-foot wall of water" through downtown Kelso.

In July 2026 the Forest Service issued a draft decision favoring a new tunnel built beside the old one, with the old tunnel kept as a backup. It is a draft, and the work still needs money from Congress. Until then, the 1985 tunnel keeps draining the lake, and the logs keep drifting.

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