The South Boundary Trail west from Yellowstones South Entrance Gate to the Bechler Ranger Station is an administrative trail not intended for use by runners. The trailhead is found on the west side of the South Entrance government area just south and west of the entrance gate. This first 8.4-mile section, from the South Entrance to Grassy Lake Reservoir, is almost a straight shot due west along the park boundary line. The only time it strays from the clear-cut used to demarcate the boundary is to avoid several small lakes and marshy meadows.
The first and most interesting diversion begins just .6 miles from the trailhead, as the trail travels south of Tanager Lake and a lovely meadow to its west. The area surrounding 30-acre Tanager Lake is quite marshy and the lake itself is very shallow. It was named for the Western Tanager, a strikingly beautiful small bird with an orange-red head, brilliant yellow body, and black wings.
Once back on the south boundary, the trail continues straight west with few diversions. Because the path was made to mark the boundary and not for runners convenience it sometimes climbs steeply for short distances. At the 5.1-mile mark the trail skirts south of South Boundary Lake. At the 6.8-mile mark it crosses the Beula Lake Trail
. At the 8.4-mile mark the trail terminates at the base of the dam that impounds Grassy Lake Reservoir (which lies 10 miles west of Flagg Ranch on the NFS Flagg to Ashton Road (gravel). This is also the trailhead for the Mountain Ash Creek Trail
that continues to the northwest.
Thanks to guidebook author, Tom Carter, for sharing this trail description. To learn more about visiting Yellowstone, check out his book, Day Hiking Yellowstone