Great Falls of the Potomac

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 falls, falling a total of 76 feet in elevation over a distance of less than 1 mile. 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). -from Wikipedia