Definitions for fall line
Random House Webster's College Dictionary
the natural boundary between an upland and a lowland, as a piedmont and a coastal plain: marked, in temperate or humid areas, by waterfalls and rapids.
Category: Geography (places)
Skiing. the path of natural descent from one point on a slope to another.
Origin of fall line:
A line marking a boundary between an upland region and a plane; sometimes marked by a series of waterfalls on rivers that cross it.
The imaginary line downhill that something falling downhill would naturally follow.