From Evergreen Trail Guide
Lewis River Trail is a classic river trail: heavily forested, shady, rolling, and more of a workout than the map suggests. It's not as gnarly as Skookum Flats, not as developed as the lower part of the Mid Fork trail. This is a fabulous trail, mostly very smooth & buff, and a common second day option after doing Ape Canyon and/or Smith Creek on day 1. It's 11 miles one-way; you can return on paved Forest Road 90 which runs parallel if you don't want to do 22 miles of trail.
There are some options to ride upstream from the trailhead as well--consult the Forest Service website and/or your trusty Green Trails map (365, Lone Butte).
How to find it
I-5 South to exit 21 (Woodland) to SR 503. Continue past the town of Cougar. At the junction with FR-25, turn onto FR-90 (Northwoods). If you're not camping, you may want to start your ride at the lower trailhead by Curly Creek, about 4 miles up FR-90.
Most people will go about 14 miles up FR-90/Lewis River Rd (i.e. 10 miles past Curly Creek) to the Lower Falls Recreation Area (Don't go too fast, the sign is easy to miss), which is a great campground. Turn right off FR-90 and the first right is the trailhead. IF YOU HAVE LARGE RV OR LONG TRAILER check w/ FS there's one section of road that has some steep ups and downs.
Don't forget your Forest Pass.
For the full out and back experience, start riding at the Curly Creek Trailhead, which is the lowest point of the Lewis River Trail. Proceed upriver to Upper Falls, which is 2.5 miles upriver from the Lower Falls and Lower Falls campground, or to the upper road crossing, which is about 3 miles upriver from Lower Falls. Upper falls is 13.5 miles from Curly creek, for a 27 mile out and back round trip.
The trail is interesting in both directions, but don't be fooled by the elevation loss on the trip back. It's almost as hard as the trip up.
Bailout points are:
Upper Falls - continue up past Upper Falls to the road and ride the road back
Middle Falls - climb the steep switchbacks up from Middle Falls. At the top of the switchback look for another steep trail away from the river. This will take you up to the Middle Falls parking lot. From there you can take the road back.
Lower Falls - pass through the campground and to the road and take the road back.
Road crossing - approx 1 mile downriver from Lower Falls you will cross FR 90. This is the last bailout point until you get to Curly Creek. From here it's 10 miles on the trail, or about 9 miles on the road.
Mostly buff trail with its share of short but steep climbs. Some flowy & fast. Beautiful old forest. Classic river trail.
September 2009 Great shape, needs some work here and there.
There's one road crossing where the trail picks up a few dozen yards to the side; otherwise no real directions necessary. Trail map: http://www.fs.fed.us/gpnf/recreation/trails/documents/Mount_St_Helens_Trail_Guide.pdf The map and trail description start on page 50.
Map is also in the images (below). Here is a link to just the 5 pages about the Lewis River Trail.
Local Points of Interest
Northwoods general store near the SR-503/FR-25/FR-90 junction is the closest source of food, drinks, and gas. September 2009, didn't look open. Fuel up in Cougar. MSH info center has maps and info.
The campground has water pumps that produce water said to be potable but was fairly brown and metallic.
Check w/ FS for campground status.
This is for information about the trail's history with respect to mountain biking, the land owner, etc.