Washington State Route 504

State Route 504 (SR 504, also known as the Spirit Lake Memorial Highway) is a state highway in Cowlitz and Skamania counties, in the U.S. state of Washington. It extends from an interchange with Interstate 5 (I-5), SR 411 and Interstate 5 Business (I-5 Bus) in the city of Castle Rock, east to end at the Johnston Ridge Observatory. The route serves as a connector from Castle Rock to Mount St. Helens NVM.

The highway formerly was Secondary State Highway 1R (SSH 1R) from 1937 until 1964. SSH 1R ran from Castle Rock to the border of the Columbia National Forest (now known as the Gifford Pinchot National Forest) from 1937 until 1961. In 1961, the Washington State Legislature extended SSH 1R east to Spirit Lake. In 1964, SSH 1R became SR 504. When Mount St. Helens erupted on May 18, 1980, SR 504 was closed and was heavily damaged from SR 505 to Spirit Lake. The highway was rebuilt and partially opened in 1991 on a higher and safer route to Coldwater Ridge. The new highway was completed when the road to the Johnston Ridge Observatory was opened in 1994.

SR 504 also has a spur route, named SR 504 Spur, that runs along the former route of SR 504 in the Gifford Pinchot National Forest. The route extends along the former route of SR 504, before the Mount St. Helens eruption. The highway closes every winter because of...
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