Union Falls

Union Falls is a remote Yellowstone backcountry waterfall where Mountain Ash Creek meets an unnamed branch at the brink, then plunges about 250 feet over a sheer rock face. The shortest common approach from Grassy Lake is roughly 15 miles round trip. Save it for a serious hike or an overnight, not a quick look between drives.

The Grassy Lake approach is mostly through forest and gains about 600 feet overall, but a long descent on the way out becomes a climb on the return. A swift river ford is part of the route. The main overlook shows the whole waterfall; a steep trail continues to its wet, slippery base. The base adds a closer view, but the overlook is the easier place to take in the full drop.

Mountain Ash Creek and an unnamed branch join at the lip, forming the twin flow that gives Union Falls its name. The U.S. Geological Survey's Hague parties named it in the 1880s. The water spreads wide across the cliff before gathering again in the rocky creek below. From the overlook, keep the narrower side stream in frame with the broad main curtain; that shows why this is Union Falls rather than a single straight drop.

Ouzel Pool, also called Scout Pool, is a separate thermal swimming hole with a small waterfall on a side route near the falls. Its temperature fluctuates, so expect warm water rather than a guaranteed hot soak. It makes a worthwhile extra stop if you still have time and energy after the hike.