Landslide from May 18, 1980, eruption of Mount St. Helens
The May 18, 1980 eruption of Mount St. Helens began with an enormous landslide that slammed into Spirit Lake, blocking its natural outlet and raising the lake level by 197 feet.
To mitigate downstream flood hazards, in 1984-1985, the U.S. Army Corps of Engineers constructed an 8,500 foot long, 11 foot diameter tunnel through a bedrock ridge on the west side of Spirit Lake. The lake drains through the tunnel and into the North Fork Toutle River. The tunnel has successfully controlled the lake level since 1985.
The landslide and hummocks from the May 19, 1980 eruption are pictured in the foreground, Spirit Lake in the middle ground, and Mount Adams in the background.