The Mt St Helens climbing route trail-head is located at the Climbers Bivouac parking lot. It is open now for the 2015 season. To get to the Climbers Bivouac, drive east on Lewis River Highway through the town of Cougar. Cross the canal, and continue straight on Road #90. Turn left on Road #83. After about 3 miles, turn left on Road #81. After about 1.5 miles, turn right on Road 830 (there is a Climbers Bivouac sign there).
The zoomable map at the bottom of the page shows the location of the Climbers Bivouac. You can also use it for directions.
Rest rooms at Climbers Bivouac
Most people who are climbing Mt St Helens, head out on the trail at sunrise, or even earlier. Many of them camp overnight at the camp sites at the parking lot. There are multiple primitive, but level, camp sites with fire pits. No water or power though. There are no designated RV pads, but there is plenty of room for the RV at the parking lot. There are good bathrooms at the parking lot.
The Climbers Bivouac is a Forest Service camp-ground, and a Forest Service parking pass is required. You can buy a $5 day-pass at the parking lot.
You can also stay overnight in Cougar, at Lone Fir Resort, or any other accommodations (of course if you live close enough, you can just get up early and head up there).
You will need to self-register your climb at the Climbers Bivouac before you head on the trail.