On May 18 , 1980 , the north face of Mount St. Helens slid away and triggered a massive eruption that created a lahar that devastated the North Fork Toutle River . The lahar destroyed about of SR 504 , including seven of the highway 's eight major bridges , leaving only the Kid Valley bridge standing because of its sufficient clearance . The remaining section between Toutle and Kid Valley was buried in up to of sediment , but was able to re-open by September with temporary Bailey bridges and other structures . Several vehicles on the highway were swept away by the lahar , including those carrying some of the eruption 's 57 victims
massive eruption
1980 eruption of Mount St. Helens
Washington State Route 504