Sourdough Mountain Trail

Sourdough Mountain Trail is a 10.4-mile out-and-back from Diablo to the rocky summit lookout, with 4,870 feet of climbing. Allow about nine hours. The route starts climbing almost at once, then trades steep forest switchbacks for subalpine meadow and a high ridge. The payoff is a wide view over Diablo Lake, other lake arms, glaciers and North Cascades peaks. This is a serious all-day effort; the summit panorama is the reason to take it on.

From State Route 20 near mile 126, turn toward the town of Diablo. Park beside the river across from the domed swimming pool; the trail starts behind the pool. The first switchbacks begin quickly, so there is no long approach to warm up on. Day hikers do not need a backcountry permit. Overnight stays do, and camping is limited to the two designated sites along Sourdough Ridge.

The first 3.5 miles climb steeply through forest, with little to no water. The grade is relentless: hikers gain roughly 3,000 feet in the first two miles. The forest thins in places, opening brief looks back toward Diablo, but much of the lower route has little shade. Parts of the slope were affected by fire, and falling trees or limbs are a real concern in this landscape. Take the descent slowly; the long drop back down is hard on knees.

Around 4.2 miles from the trailhead, the grade eases near Sourdough Creek and the small backcountry camp. The creek is the first likely water source, but seasonal flow is unreliable, so do not count on refilling there. Crossing can be hazardous when snow bridges weaken during the melt. Beyond the creek, the route enters subalpine meadows and follows the ridge toward the summit. Wildflowers, birds, small mammals, deer and black bears use this higher terrain. Stay on the trail, rock or snow; the vegetation is fragile.

The rocky summit opens onto a broad panorama, with Diablo Lake far below and peaks and glaciers in every direction. Mount Baker’s volcanic dome is one of the distant landmarks. For a photo, frame the lake low beneath the steep forested slopes; that angle shows how high the trail has climbed. You can reach the historic lookout site, but the building itself is closed to visitors.

Sourdough was among the early Forest Service lookout points established in 1915. Glee Davis built the original lookout in 1917; the present building dates to 1933 and was rehabilitated in 1998–99. The site was chosen to test the Osborne Firefinder, a device that became standard equipment for locating fires. Poets Gary Snyder and Philip Whalen also worked as lookouts here in the 1950s and found inspiration in the mountain.

A backcountry permit is required for any overnight stay. Sourdough Camp and Pierce Mountain Camp each hold no more than four people; fires are prohibited, and hard-sided food canisters are required. Pets are allowed only within Ross Lake National Recreation Area, not at the camps or the lookout summit, so leave them behind for this full route.