I’m not going to mince words – the hike to Union Falls and the nearby Scout Pool in southern Yellowstone National Park is one of my all-time favorites. The waterfall, a 265-foot behemoth, is the second-largest in the park. The nearby geothermally heated Scout Pool is a refreshing place for a dip after a long hike. And the location, deep in the Yellowstone backcountry, offers plenty of solitude (although, beware of a nearby Boy Scout camp).
Union Falls is formed by the confluence of two creeks. As the waters cascade down a rock face, they intermingle, creating the unified waterfall. And not only is Union Falls an incredible 265 feet tall, it seems every bit as wide. The trip to this waterfall is worth every mile, especially since the trail ends at a perfect vantage point and you have a decent chance at seeing a rainbow in the mist from the falls. The sight and sound of the waterfall, deep in the wilderness of Yellowstone, make for one of Yellowstone’s best hikes.
The hike to Union Falls is about 7.5 miles each way. The distance makes the hike moderately difficult, but the terrain is easy to navigate and elevation gain is limited. You’ll gain 600 feet climbing a ridge about halfway through the hike back, but the trail is otherwise fairly flat. Getting to the trailhead is more challenging. There are three primary trailheads. Each is accessed from the Grassy Lake Road, which is the winding, unpaved, sometimes badly rutted and remote road that links Flagg Ranch with Ashton, Idaho. Flagg Ranch is just a small lodge and campround, and Ashton is – at best – a one-horse town, so you can imagine the condition of a road that links the two.
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This article will describe the trail from the base of Grassy Lake Dam, which is the most convenient route if you’re coming from Yellowstone or Jackson. To get to the trailhead, drive 10 miles west of Flagg Ranch along Grassy Lake Road, starting in the John D. Rockefeller Memorial Parkway, then crossing into the Targhee National Forest. Note that this road is usually passable in a car, although you would be happier in a high-clearance vehicle. Note also that the road may be closed into June. After 10 miles, you come to Grassy Lake Dam; just before the reservoir, turn right on a road that leads to a parking area below the dam. The Mountain Ash Creek trailhead is in the northwest corner near the spillway.
The trail starts just south of Yellowstone National Park; a few minutes into the hike, you will cross the park boundary, which is marked with a sign. From here, you wander through a generally forgettable forest for a mile or so until you start to parallel the Falls River, which you will soon ford. The crossing is fairly long and the river can be swift, so be careful. The water can be halfway up your thigh in mid-July, but the crossing gradually gets easier throughout the summer. I wouldn’t try the ford before July. If possible, bring a separate set of shoes, as the long crossing, chilly waters and sharp rocks are an unpleasant combination.
After the ford, you’ll quickly begin to climb above the Falls River and come to a junction with the Pitchstone Plateau trail, which eventually leads to the Phantom Fumarole and the South Entrance Road south of Lewis Lake. A backcountry campsite is also nearby. From here, it’s four miles to the next junction and six miles to the waterfall. Bear left to stay on the Mountain Ash Creek trail and you’ll continue to wander through another fairly forgettable section of forest. To the north, you’ll see the Pitchstone Plateau. Eventually, you’ll cross a ridge that offers a cool view into the south end of Yellowstone and of the Birch Hills. This area also has an interesting history. According to Tom Carter’s book “Day Hiking Yellowstone,” “You are actually following an old wagon road built in the 1880s by the Mormons to connect Marysville, Idaho, with Jackson Hole. Careful observers can still see wagon ruts in the stone at various places.”
From the divide, you descend to Proposition Creek, which is usually easy to cross on fallen logs. This section of the trail also seems to be among the best for spotting wildflowers. Less than a mile after Proposition Creek, the Mountain Ash Creek trail intersects with the Union Falls trail. Turn right onto the Union Falls trail; you’re two miles from the waterfall. The Mountain Ash Creek trail could eventually lead you to Cave Falls and the Bechler area of southwestern Yellowstone.
This section of trail parallels the serene Mountain Ash Creek. After a half mile, you come to a point where a smaller creek flows into Mountain Ash Creek. Upstream, the smaller creek flows over a small waterfall to create Scout Pool. Mountain Ash Creek, meanwhile, flows over Union Falls upstream. You can ford Mountain Ash Creek here if you’re interested, but there is a small footbridge just a minute upstream. Just beyond the footbridge is one of the two Union Falls campsites.
From the crossing, you’re about 1.5 miles from Union Falls. A few minutes up the trail, you’ll pass the turnoff to the second campsite, which is a few minutes’ walk off the trail on the shores of the smaller creek. It’s a low-impact campsite so no fires are allowed. You’ll also pass the Union Falls “Ranger Station,” which is a small A-frame building that is presumably used for the occasional backcountry patrol, but has always been boarded up when I’ve walked by.
About a mile from the campsites, you’ll pass a horse hitching post and see an unmarked trail that heads to the left (north) behind the hitching area. This trail leads to the geothermally heated, and very refreshing, Scout Pool (sometimes called Ouzel Pool). Bear right to go to Union Falls – it’s only another half mile. As you walk the final half mile, you can sometimes see a quick glimpse of the waterfall through the forest. As you get closer to the overlook, you’ll start to hear its roar. And then you’ll come around a corner and the gorgeous waterfall will be on full display. I always plan to spend plenty of time here, taking pictures and soaking in the gorgeous sight. It’s nature at its best.
From the main viewing area, you will see a steep, unofficial trail that leads to the base of the falls. The spray from Union Falls makes this area very muddy and very slippery. Unless you’re really confident, I’d skip the trip to the base of the falls, although the view is gorgeous.
After you’ve seen Union Falls, take the side trip to the Scout Pool. A small creek that is geothermally heated upstream flows over a small waterfall, creating a very refreshing swimming hole — and warm water for the campsite downstream. The temperature seems to fluctuate a little bit, but it’s usually somewhere between a swimming pool and a hot tub. There is a spot where you can perch yourself on a rock under the waterfall and let the waters tumble over your shoulders, which probably deserve the massage after carrying your backpack. Note, of course, that the waters move quickly and the pool is fairly deep, so be careful.
From here, retrace your steps to get home. Be aware that the climb up from Proposition Creek is much more challenging on the way back, as you will quickly gain about 600 feet of elevation. Also, note that the junction after the Falls Creek ford can be confusing, so be sure to take the correct trail. If you’re following the route I have described, you need to take the trail to the left; the trail to the right leads to the Cascade Creek trailhead.
Union Falls notebook
First, this section of Yellowstone gets a lot of snow and is an incredible breeding ground for mosquitoes. They aren’t generally too bad during the day, but in the evenings they can be quite the nuisance, particularly through July. The mosquitoes naturally die out throughout the summer and aren’t a problem by September. The tradeoff, of course, is that there is less water flowing over the waterfall.
Second, bears love this area of Yellowstone, so take the normal bear precautions.
Third, the elevation for this hike generally hovers within a few hundred feet of 7,000 feet, which makes it a reasonable later-season hike. I’ve done the hike well into September without any problems. And even though there isn’t as much water cascading down Union Falls, it is still gorgeous.
Fourth, this trail is a popular horse route, so don’t be surprised to see a few of our equestrian friends. Because it is in Yellowstone, dogs and mountain bikes are not allowed.
The other trail options
The Cascade Creek trailhead is located just a couple of miles west of Grassy Lake Reservoir on Grassy Lake Road. It intersects with the trail I described above just before the Falls River ford. This trail has the advantage of being a couple miles closer for people coming from Eastern Idaho but requires an additional minor ford of Cascade Creek. If you take this trail, you should consider taking the spur trail to Terrace Falls, which is 1.4 miles northwest of the Cascade Creek ford. At Terrace Falls, the Falls River passes through a gorgeous series of cascades.
A third trailhead is located further west along Grassy Lake Road. It joins the route I have described two miles from Union Falls at the junction of the Mountain Ash Creek and Union Falls trails. A spur road leads from Grassy Lake Road to the trailhead, and a high clearance vehicle is definitely required for the spur road. This trail passes Fish Lake less than a mile from the trailhead. Note that the ford of the Falls River will be marginally tougher because you will be further downstream.
I’ve taken all three trails and really don’t have a strong preference. If I were coming from Idaho in a high-clearance vehicle, I’d probably take the Fish Lake trail.
Note: This is a complete rewrite of an article that I posted in May 2008.