SR 504 starts where SR 411 ends at I-5 Exit 49 in Castle Rock and heads east via Toutle, Kid Valley, and Coldwater Ridge to Johnston Ridge inside the Mount St. Helens National Volcanic Monument. Most of SR 504 east of Kidd Valley had been destroyed by the mud flows along the Toutle River that resulted from the eruption of Mount St. Helens in May 1980. The highway was rebuilt on higher ground north of the Toutle. By the early 1990s, the new route was opened to Coldwater Ridge. It was the mid-late 90s before the highway opened to Johnston Ridge. There is some talk of extending SR 504 to Windy Ridge to tie into NF Route 99. SR 504 is 51.76 miles long.
Before 1964, SR 504 had been SSH 1R.
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