Union Falls is a fan-shaped waterfall on Mountain Ash Creek in the Cascade Corner of southwestern Yellowstone National Park, Wyoming. Rising 250 feet, it is the park’s second tallest major waterfall after Lower Yellowstone Falls. Its name comes from a tributary joining Mountain Ash Creek at the brink just before the water drops, a “union” noted by geologist J. P. Iddings. Members of the Arnold Hague Geological Surveys applied the name sometime between 1884 and 1886, tying the falls to Yellowstone’s early scientific exploration. Today it remains a remote backcountry destination reached by the Mountain Ash Creek Trail and Grassy Lake Road.