fall line

noun
1.
an imaginary line, marked by waterfalls and rapids, where rivers descend abruptly from an upland to a lowland.
2.
(initial capital letters) Eastern U.S. the imaginary line between the Piedmont and the Atlantic coastal plain.
3.
Skiing. the path of natural descent from one point on a slope to another.

Origin:
1880–85, Americanism

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
fall line
 
n
1.  skiing the natural downward course between two points on a slope
2.  the edge of a plateau

Fall Line
 
n
a natural junction, running parallel to the E coast of the US, between the hard rocks of the Appalachians and the softer coastal plain, along which rivers form falls and rapids

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
fall line   (fôl)  Pronunciation Key 
A line connecting the waterfalls of nearly parallel rivers that marks a drop in land level.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

fall line

line of numerous waterfalls, as at the edge of a plateau, where streams pass from resistant rocks to a plain of weak ones below. Such a line also marks the head of navigation, or the inland limit that ships can reach from a river's mouth; because navigation is interrupted both upstream and downstream, important cities often occur along the fall line. In the eastern United States, a fall line exists between the Appalachian piedmont and the Atlantic coastal plain; waterfalls or rapids occur on all the principal rivers (e.g., the Delaware, Schuylkill, Patapsco, Potomac, James, and Savannah rivers), and the cities of Trenton, Philadelphia, Baltimore, Washington, Richmond, and Augusta are among those built along this line. Other examples are the extensive fall lines that separate the narrow coastal plains of Africa, Western Australia, Brazil, and India from their interior continental shields.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
It took her a little while to work out how to position them at right angles to the fall line.
The historic fall line was the farthest point west a cargo ship could go before reaching the rapids.
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