This section of the South Boundary Trail, running west to the Bechler Ranger Station near the southwest corner of the park, is an administrative trail not often used by visitors. The trailhead is found north of Fish Lake, which itself is reached by a 1.5 mile high-clearance vehicle spur road on the north side of the Flagg-Ashton NFS Road (also called the Grassy Lake Road) about 18 gravel-road miles west of Flagg Ranch. From Fish Lake, take the 1.2-mile Fish Lake-Mountain Ash Spur Trail
which leads to the trailhead for the South Boundary Trail: Grassy Lake Road-Bechler RS.
This trail has little to offer most visitors. The trailhead is one of the most difficult to reach in the Yellowstone trail system, and those wishing to reach Cave Falls or the Bechler Ranger Station can drive there from Ashton, Idaho on the mostly-paved 25 mile Cave Falls Road. As a further deterrent, the ford of the Falls River (at the 4.1-mile mark) is extremely wide and dangerous to crossing well into August.
From the southwest side of Fish Lake, follow the Fish Lake-Mountain Ash Spur Trail
1.2 miles north past Fish Lake to reach the official trailhead for this trail. At the junction, a trail to campsite 9C6 lies to the right and our trail starts to the left. After a short distance, the Fish Lake-Mountain Ash Cutoff Trail
splits off to the right. At .6 miles the trail crosses the outlet creek from Junco Lake (on the left) and at 1.2 miles it crosses the outlet creek from Winegar Lake (also on left). At 2.8 miles the trail reaches a ridge 300 feet above the mighty Falls River. The trail descends the ridge and fords the 200-foot wide Falls River at the 4.1-mile mark. Once across the river the trail turns left then right following the Cave Falls Road before bending further to the right, continuing through trees and intermittent meadows, crossing Wyoming Creek, and ending at the Bechler Ranger Station at the 7.1-mile mark.
Thanks to guidebook author, Tom Carter, for sharing this trail description. To learn more about visiting Yellowstone, check out his book, Day Hiking Yellowstone