fall line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The steepest possible line of descent from a given point on a slope.
• noun A line connecting the waterfalls of upland rivers before they reach a common coastal plain, often marking the limit of upstream navigation and indicating a zone of potential water power for industrial purposes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geomorphology A geomorphologic unconformity marking a boundary between an upland region and a plane; sometimes marked by a series of waterfalls on rivers that cross it.
• noun snowboarding The imaginary line downhill that something falling downhill would naturally follow.