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 further. 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.

These waterfalls represent a fall line, the imaginary line where at least two rivers plunge at the same elevation. The falls here are actually part of the same river.
Photograph by Jamal Harrell, MyShot
hard rock
Noun
dense rock that is difficult to crack or break. Usually igneous or metamorphic.
hydroelectric power
Noun
usable energy generated by moving water converted to electricity.
parallel
Adjective
equal distance apart, and never meeting.
upriver
Adjective
toward or near the source of a river.
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