Great Falls - Mather Gorge
Roughly 35,000 years ago, the Potomac River began carving out the Great Falls of the Potomac. The river cascades over a series of 20-foot (6.1 m) falls, falling a total of 76 feet (23 m) in elevation over a distance of less than 1 mile (1.6 km), making the Great Falls the steepest fall line rapids of any river in the eastern United States.
The Potomac narrows significantly as it passes over the falls and through Mather Gorge. Heavy rain or snow in the watershed upstream causes white-water floods which entirely submerge the rocks and even threaten the adjacent park visitor center (built on stilts for this reason).