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"line separating waters flowing into different rivers," 1803, from water (n.1) + shed "ridge of high ground between two valleys or lower ground, a divide" in the topographical sense, perhaps from shed (v.) in its extended noun sense of "the part of the hair of the head" (14c.). A loan-translation of German Wasser-scheide. Figurative sense is attested from 1878. Meaning "ground of a river system" is from 1878.
watershed wa·ter·shed (wô'tər-shěd')
A ridge between two areas that directs drainage to either side.
The area of marginal blood flow at the extreme periphery of a vascular bed.
Ridges of the lumbar vetebrae and the pelvic brim formed in the abdominal cavity, which determine the direction in which a free effusion will gravitate when the body is supine.
A ridge of high land dividing two areas that are drained by different river systems. On one side of a watershed, rivers and streams flow in one direction; on the other side they flow in another direction. Also, the area drained by a water system.
Note: By extension, a “watershed” is a critical point that serves as a dividing line: “The parties reached a watershed in the contract negotiations.”
area from which all precipitation flows to a single stream or set of streams. For example, the total area drained by the Mississippi River constitutes its drainage basin, whereas that part of the Mississippi River drained by the Ohio River is the Ohio's drainage basin. The boundary between drainage basins is a drainage divide: all the precipitation on opposite sides of a drainage divide will flow into different drainage basins.