Mountain Ash Creek Trail

Mountain Ash Creek Trail is an 11.6-mile, one-way backcountry route from the Grassy Lake Reservoir dam to the Bechler River Trail junction. Treat it as a connecting leg, not a loop or a road-to-road day hike: its western end is still in the backcountry trail network. The trail meets the Union Falls Trail but does not continue to the falls. Set aside about eight hours of walking, before breaks or extra miles, and plan your onward route and exit before you start. Any overnight camping requires a Yellowstone backcountry permit for a designated campsite. Published route stats put the leg at about 545 feet up and 1,248 feet down.

The first 1.1 miles follow the southwest side of the Grassy Lake outlet and then the Falls River over mostly level ground. At the river, the trail has an unbridged ford. Route descriptions put it roughly 40 feet across, cold and swift, and knee-deep in the conditions they describe; the depth changes with the season. After the crossing, the trail climbs and rolls before dropping about 600 feet toward Proposition Creek, where views open west across the Falls River Basin.

Proposition Creek is crossed near mile 4.9, then a short climb leads to the Union Falls Trail junction around mile 5.8. Stay on Mountain Ash Creek for the through route; the Union Falls branch is a separate hike. The trail crosses Mountain Ash Creek around mile 7.5, passes Mountain Ash Cutoff near mile 8.7, and follows fairly level ground for the last three miles to the Bechler River Trail junction.

This route follows an 1880s Mormon wagon road that linked Marysville, Idaho, with Jackson Hole. The historic line and the long westward traverse make it a worthwhile leg for a prepared backcountry itinerary, but a poor match for anyone looking for a short, self-contained walk.