Union Falls – 15.9mi / 25.6km
The majestic Union Falls, matching unique features with sheer size, has to be the most elegant falls in all of Yellowstone. The trail starts by crossing over the outlet stream from Grassy Lake Reservoir and works its way through a mixed forest. You gradually approach Falls River, first coming into view, then right alongside it. A ford of the river is necessary, which may prohibit June and early July crossings when volumes can be high and dangerous. Later in the year, it’s a cold but doable affair. (Check with a ranger in Grant Village for current conditions.)
Once across the river, the trail quickly climbs up onto Birch Hills. It mellows out as it traverses the top, and then gradually begins its slow descent through a wonderful forest. As it approaches the Proposition Creek drainage there are some nice views, then the trail quickly drops to the creek. Along the way down to the creek, the trail travels over solid slabs of rock with boulders strewn on top, creating a unique change of scenery. The trail fords the creek, and then there is a short climb back out of the drainage.
The trail then traverses down to Mountain Ash Creek and the junction with the Mountain Ash Creek Trail. Turn right at the trail junction to head upstream and continue on to the falls. The trail remains level as it works its way upstream, crossing the stream over a log footbridge, and continues to a patrol cabin and a hitching post.
The trail forks, and to the left is a recommended bonus cascade and warm creek that adds 0.7mi/1/1km to your hike. The trail to the right works its way up to the overlook for Union Falls. This 250ft/75m waterfall is composed of two streams that come together, then fan out over rock and crash down. It is powerful, complex, and beautiful.
An alternate and popular route is from the Fish Lake trailhead. It is longer at 16.5mi/26.6km, but flatter as it travels through meadows. It is a better option for those coming from the Idaho side.
Trailhead: South of the South Entrance, on the John D. Rockefeller Jr. Memorial Parkway, turn at Flagg Ranch and head west on Grassy Lake Road. Before crossing the outlet of the reservoir, a steep and rough road descends to the trailhead parking area. Inspect the road and consider parking at the top of the reservoir and walking down. The trailhead is accessed by crossing over a covered section of the outlet stream on the west end of the parking lot.
Elevation Gain: 2,000ft / 600m – Elevation Loss: 2,000ft / 600m