Firehole Falls is a roughly 40-foot waterfall on the Firehole River in Firehole Canyon in southwestern Yellowstone National Park, about half a mile above the confluence with the Gibbon River at Madison Junction. Early explorers recorded the falls during Yellowstone’s exploration era, including Lt. Gustavus C. Doane in 1871, but the origin of the name remains unknown even though it was in common use by the mid-1880s. The falls also shaped the river’s ecology: they long blocked fish from moving upstream, leaving the upper Firehole fishless until brook, brown, and rainbow trout were introduced above the barrier in the late 19th and early 20th centuries.