Fall River rises on the Madison and Pitchstone plateaus in the southwest corner of Yellowstone National Park, Wyoming and flows approximately 64 miles (103 km) to its confluence with the Henrys Fork of the Snake River near Ashton, Idaho. Historically, the river was referred to as the Middle Fork of the Snake River or as Fall River or the Falls River by trappers and prospectors as early as the 1830s. It was officially named the Falls River by the 1872 Hayden Geological Survey, but was always called Fall River by the locals and so the U.S. Board on Geographic Names changed the official name to Fall River in 1997 at the request of Idaho authorities.