Union Falls/Ouzel Pool hike and swim

At 250 feet, Union Falls is Yellowstone's tallest backcountry waterfall, formed by the merging of Mountain Ash Creek and an unnamed tributary. The right side of the falls (when facing it) has a small cave behind the rushing water that can be accessed by a bit of scrambling.

Nearby is a thermally heated swimming hole with a small waterfall where you can get a "waterfall massage". The temperature was slightly warmer than a heated pool when we went, but has been known to fluctuate. The pool is officially named Ouzel pool, but is sometimes referred to as Scout Pool.

Very few venture past Union Falls, but for the more adventurous 2 miles past union falls on the Northwest fork of Mountain Ash Creek is another spectacular waterfall known as Morning falls. Our group ran out of time and didn't get to make the journey, but the pictures look incredible (google image "Morning falls Yellowstone"). There is supposedly a trail upriver, but it is difficult to find and is generally unmaintained.

Can be done as an overnight backpacking trip or a long day-hike. 15-16 miles round trip, each trailhead offering similar distances. To camp overnight a permit is required, and can be bought from a backcountry office at a visitor center in Yellowstone National Park. Permits for the sites nearest to the falls go fast, so reserve one ahead of time!

Union falls can be accessed from multiple trailheads. Grassy Lake Trailhead, Fish Lake Trailhead and Cascade Creek trailhead are all options. All routes require stream crossings, which make access challenging before late summer (mid-july). Each of these trailheads are off of grassy lake road, a dirt rod connecting Ashton, ID to Flagg Ranch. 4WD and high clearance are recommended but not absolutely necessary. A map and compass will be necessary for good navigation as many trails run through the area.

The hike involves multiple stream crossings, so prepare accordingly. In peak run off season (spring snowmelt) these crossings will be dangerous and impassible. Late summer is the best time to go.

Ouzel pool can be reached by going left at the junction closest to the falls, with the horse hitching posts. This junction is about a half mile from both Ouzel pool and Union Falls.