[2] 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.

[5][9] Further southeast along the river, SR 504 passes the privately owned Eco Park resort, a youth camp, and a heliport near the Hoffstadt Bluffs Viewpoint.

The company also operates the Charles W. Bingham Forest Learning Center, a museum that overlooks the North Fork Toutle River and also includes a rest area.

From the Elk Rock area, the highway forms the northern boundary of the Mount St. Helens National Volcanic Monument and encounters more mountainous terrain that results in several hairpin turns between other scenic viewpoints.

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.

[21] The highway turns east once again near the Loowit Viewpoint and reaches the Johnston Ridge Observatory,[5] the volcano's main visitor center open from spring to autumn and situated at an elevation of 4,314 feet (1,315 m).

[33][34] Expanded logging and a planned ski resort on Mount St. Helens hastened the construction of a modern highway linking Spirit Lake to the timberline on the mountain's northern slopes in the 1950s.

[35][36] Despite support from the state government, the Forest Service, and Bureau of Public Roads, the highway project was put on hold for several years due to funding shortfalls.

[46] The Forest Service established a roadblock on SR 504 approximately 4 miles (6.4 km) west of Spirit Lake on March 28 because of potential avalanche risks.

[64][65] A visitor center was also opened at Silver Lake in 1987 to serve the growing number of tourists to the area, but the highway would remain closed beyond the sediment dam.

The highway would include 23 miles (37 km) of new road built at a higher elevation between Kid Valley and the newly formed Coldwater Lake, crossing over various creeks with ten major bridges.

[72] After finding that elk herds were leaving areas with improved highway access, the Forest Service established restricted zones within the national monument that prohibited off-trail hiking, pets, fires, and camping.

[79] A long-proposed extension of SR 504 to the eastern side of the Mount St. Helens area gained traction in the early 2000s, sparking outcry from scientists and environmentalists.

[89][90] An evacuation of the area was ordered in early October 2004 due to seismic activity, indicating a possible buildup of magma, forcing 2,500 visitors to leave the Coldwater and Johnston centers for several days.

[93][94] A mudslide and debris flow, known as the 2023 South Coldwater Slide, occurred on May 14, 2023, destroying the 85-foot (26 m) Spirit Lake Outlet Bridge and severing SR 504 northeast of Johnston Ridge Observatory at milepost 49.

Twelve people were airlifted from the area beyond the mudslide; the observatory's seasonal reopening was postponed and the highway east of Coldwater Lake was closed to all traffic.

[95][96] WSDOT contractors cleared debris over the highway by July and created a temporary gravel road to access Johnston Ridge for vehicle retrieval and to prepare for reopening in 2024,[97][98] but the interim roadway was washed out in November due to a collapsed culvert caused by heavy rainfall and erosion unrelated to the earlier landslide.

[99] WSDOT announced in early 2024 that because of the costs and difficulties in making repairs to the bypass, based on a limited construction window due to weather and elevation as well as the steepness of the terrain, the highway and access to the observatory was to remain closed until 2026.