A fall line is the imaginary line between two parallel rivers, at the point where rivers plunge, or fall, at roughly the same elevation.

Fall lines are often located where different elevation regions, such as coastal and piedmont, meet. They are important to people and businesses. The fall line is the point at which boats traveling upriver usually cannot continue any farther. It is also the point at which hydroelectric power generation may be possible, taking advantage of the energy of the waterfalls.

In the eastern United States, there is a major fall line between the hard rock of the Appalachian Piedmont and the soft sediment of the Atlantic Coastal Plain. This line was important to early European explorers because it marked the limits of river travel for ships. Many cities developed along this fall line, including Trenton, New Jersey; Richmond, Virginia; and Philadelphia, Pennsylvania.