Union Falls Trail

Union Falls Trail from Grassy Lake is a remote, 15.4-mile out-and-back to an overlook of the waterfall. The mapped corridor is about 7.7 miles each way; walk it once to the falls and return the same way. Most of the approach is forest, with a major river ford, rolling climbs and creek crossings breaking up the long walk. Allow about nine hours and most of a day. This record uses the Grassy Lake start and ends at the falls overlook; it does not include the optional Ouzel/Scout Pool spur. Other starts, including Cascade Creek and Fish Lake, make different hikes with different distances.

The trailhead is at the west edge of Grassy Lake Reservoir, on the north side of Grassy Lake Road. The road is rough, difficult and seasonal; high clearance is strongly recommended, and the road is not open year-round. Check current road and stream conditions before setting out. The route crosses into Yellowstone within the first half mile, then continues through backcountry forest. Bring food and enough water for a full day; no food or potable-water stop was confirmed along the route.

The first major obstacle is a large, swift Falls River ford about a mile and a half from the start. Water can be dangerous at high flow, so assess it from shore and turn back if it is not safe to cross. Secure footwear for the rocky bed is useful. Beyond the river, the trail winds through forest and huckleberries, climbs over Birch Hills, then descends toward Proposition Creek. Some accounts describe log crossings there, followed by a shorter rise and descent into the Mountain Ash Creek corridor. The park estimates roughly 600 feet of total climbing, but that number does not convey the repeated ups and downs: the long downhill on the way out becomes sustained climbing on the return.

The creek-side section is comparatively level. Near the horse hitching post, an unmarked left-hand spur leads about half a mile to Ouzel Pool, also called Scout Pool, a thermally warmed pool with a small waterfall. The water temperature varies. Return to the junction and continue roughly another half mile to the Union Falls overlook. The pool is a worthwhile side trip for hikers with time, but its out-and-back distance is not included in the 15.4-mile figure.

Union Falls drops about 250 feet, making it Yellowstone’s second-tallest waterfall. Two streams join at the brink, giving the falls its name, and the overlook shows the full cascade with spray in the air. This is the standard destination and the strongest place to photograph the falls; protect a camera from the mist. A steep descent from the overlook toward the base is very wet and slippery, so do not treat it as a routine extension of the hike.

There are two backcountry campsite locations described roughly 6 to 6.5 miles from the Grassy Lake start. Overnight camping requires a current Yellowstone backcountry permit and campsite reservation; verify the current rules and site details before planning to stay. Pets are not allowed on Yellowstone hiking trails or in the backcountry. For a day hike, treat the river level and the seasonal access road as the key go-or-no-go checks, and use the overlook as the turnaround point.