When Mount St. Helens erupted on May 18, 1980, its northern flank collapsed, triggering the largest landslide in recorded history. A good portion of this debris avalanche tore through Spirit Lake. Within moments, the lake was violently altered. The picturesque recreation site, home to youth camps and visitor lodges was gone. When the debris avalanche hit the lake, it caused a wave that scoured the northern slope of the lake basin. Hundreds of logs from the pre-eruption forest can still be seen floating in the lake. When the dust settled, the lake had roughly doubled in surface area but was much shallower; the debris deposit raised the lake bed nearly 200 feet. The debris also blocked Spirit Lake’s outlet to the North Fork Toutle River.
“What had been a lake with a babbling trout stream as an outlet suddenly became a closed basin,” explains Gordon Grant, a research hydrologist with the U.S. Forest Service Pacific Northwest Research Station.