The highway continues into the National Volcanic Monument and turns east before reaching the border between Cowlitz and Skamania counties. SR 504 follows South Coldwater Creek upstream through a narrow valley to its headwaters near Spirit Lake, passing several trailheads, and turns west to continue its ascent. The treeless landscape is home to winter herds of Rocky Mountain elk and deer. The highway turns east once again near the Loowit Viewpoint and reaches the Johnston Ridge Observatory, the volcano's main visitor center open from spring to autumn and situated at an elevation of . SR 504 terminates downhill from the observatory at its parking lot, located north of the Mount St. Helens crater.
