The Chattahoochee River flows through Columbus, Georgia, one of the cities located along the fall line marking the boundary between the Piedmont and Coastal Plain geologic provinces. The hard rocks of the Piedmont form outcrops that create rapids and waterfalls along the fall line. Image source: Georgia Department of Economic Development.
The fall line is a line along which a number of nearby rivers have waterfalls, marking the progress of the rivers from hard to softer rock. Since this marks the farthest inland point navigable from the sea, and because the falls can supply hydroelectric power, many important industrial centers have sprung up along fall lines.