What is the natural border between the coastal plain and Piedmont where waterfalls prevent further travel on the river.?

The natural border between the coastal plain and the Piedmont region is typically marked by the fall line. This geographic feature is where rivers descend from the higher elevation of the Piedmont to the lower coastal plain, creating waterfalls and rapids that historically hindered navigation. The fall line often serves as a significant boundary for various ecosystems and human settlements. Notable cities, such as Richmond and Columbia, are located along this line, taking advantage of the waterpower provided by the waterfalls.