State Route 504 (SR 504, designated as the Spirit Lake Memorial Highway) is a state highway in southwestern Washington state in the United States. It travels along the North Fork Toutle River to the Mount St. Helens area, serving as the main access to the Mount St. Helens National Volcanic Monument. The highway begins at an interchange with Interstate 5 (I-5) and SR 411 in Castle Rock and terminates at the Johnston Ridge Observatory near Spirit Lake. The Cowlitz County government built the Spirit Lake Highway in 1903 and paved it in the early 1930s before it was transferred to state control, becoming Secondary State Highway 1R (SSH 1R) in 1937. SSH 1R initially ended at the boundary of Columbia National Forest (now Gifford Pinchot National Forest), but was extended in 1961 to the timberline of the mountain. It was renumbered to SR 504 in 1964 and remained popular with loggers and tourists, requiring bridges and sections to be rebuilt. A major section of SR 504 was destroyed in the May 18, 1980, eruption of Mount St. Helens and its subsequent lahar on the North Fork Toutle River. The state government rebuilt most of the highway from 1988 to 1997, relocating it further north and connecting to new interpretive centers at Coldwater Ridge and Johnston Ridge. A part of the old alignment near the Toutle River Sediment Dam is signed as a spur route. Continued volcanic activity at Mount St. Helens, particularly in the mid-2000s, has resulted in closures and evacuations along the highway at various times. Route description SR 504 begins as an extension of Huntington Avenue at an interchange with I-5 northeast of Castle Rock. The street continues southwest into downtown Castle Rock as SR 411 and I-5 Business. SR 504 travels northeasterly through a small commercial area before ascending into the hills above Salmon Creek, passing several farms and wineries along the way. The highway follows a minor stream heading southeast into the wetlands around Silver Lake, where the Mount St. Helens visitor center and Seaquest State Park lie. SR 504 continues along the north side of the lake and passes through the town of Toutle at the confluence of the Toutle River's two forks. The highway crosses over the river on the Coal Bank Bridge and continues along the north side of the North Fork Toutle River, following the narrow valley to the east of Beigle