Why engineers drilled a 1.5-mile tunnel through Mount St. Helens just to drain one lake

In 1980, Mount St. Helens erupted and its collapsing north side buried Spirit Lake's only outlet. The landslide shoved the lake roughly 200 feet higher, and runoff kept pushing it up behind a "dam" made of loose ash, shattered rock, and trees. If that debris eroded, the lake could tear down the valley toward about 50,000 people, and a channel over the top would chew through the whole dam. The Army Corps pumped the lake from a barge, but only while the engines ran. So they flipped the problem: don't touch the dam, drill through the solid bedrock ridge beside it. In 1984 crews bored a 1.5-mile tunnel about 11 feet wide, and since May 1985 gravity has kept the lake below the danger line. Sources: • USGS OFR 2025-1046 — https://pubs.usgs.gov/publication/ofr20251046/full • Wikipedia — https://en.wikipedia.org/wiki/Spirit_Lake_(Washington) • USGS — https://www.usgs.gov/volcanoes/mount-st.-helens/science/lakes-and-drainages-around-mount-st-helens