Union Falls ht. 250 feet (76.2 m) is a waterfall

Waterfall

A waterfall is a place where flowing water rapidly drops in elevation as it flows over a steep region or a cliff.-Formation:Waterfalls are commonly formed when a river is young. At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens...

 on Mountain Ash Creek, a tributary of the Fall River

Fall River (Wyoming, Idaho)

Fall River rises on the Madison and Pitchstone plateaus in the southwest corner of Yellowstone National Park, Wyoming and flows approximately to its confluence with the Henrys Fork of the Snake River near Ashton, Idaho...

 in the Cascade Corner (southwest) of Yellowstone National Park

Yellowstone National Park

Yellowstone National Park, established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872, is a national park located primarily in the U.S. state of Wyoming, although it also extends into Montana and Idaho...

. It is the second highest waterfall in Yellowstone exceeded in height only by the lower Yellowstone Falls

Yellowstone Falls

Yellowstone Falls consist of two major waterfalls on the Yellowstone River, within Yellowstone National Park, Wyoming, United States. As the Yellowstone river flows north from Yellowstone Lake, it leaves the Hayden Valley and plunges first over Upper Yellowstone Falls and then a quarter mile ...

. The falls got its name from members of the Arnold Hague

Arnold Hague

Arnold Hague was a United States geologist who did many geological surveys in the U.S., of which the best known was that for Yellowstone National Park. He also had assignments in China and Guatemala. He became a member of the U. S...

Geological Surveys sometime between 1884-86. Geologist J.P. Iddings claims the name derives from the fact that a tributary of Mountain Ash Creek joins at the very brink of the falls, thus Union Falls. Access to the falls is via the Mountain Ash Creek trail and the Grassy Lake Road.