Watershed

Note: Wikipedia's entry on watershed seems to be weak. Hopefully this community can produce a much better entry here in Waterpedia

Definition

A watershed is the total land area that contributes water to a river, stream, lake or other body of water. It is synonymous with drainage area, drainage basin and catchment.
from: http://www.city.newport-beach.ca.us/watershed/watershed defined page.htm

Linguistically, a watershed is a divide, as in a watershed event that demarks a new era in history. The reference is therefore to ridge lines, as in the Great Divide being the ridge line of the Rocky Mountains. In geographic terms, a "watershed" is a basin, with reference to the streams and rivers. If you go upstream, streams end at ridge lines, and if you go downstream, they join other streams in a tree-like structure until they reach the Ocean. This is also true for land-locked basins such as surround Great Salt Lake or the Sea of Galilee, although the "sea" may be an evaporation plain. A map of ridge lines has the same tree-like structure as a map of streams. The drainage basins defined by ridge lines are nested, smaller in larger, creating a hierarchy of drainage basins. The smallest streams, and the ridge lines defining their drainage basin, define a first order watershed. First order streams and ridge lines join to create a second order watershed, second orders form a third order, and so on. Watersheds are then hierarchical structures, with any watershed containing smaller watersheds.

If you draw a watershed map of with only ridgelines and streams (without oceans or lakes), in theory you cannot tell which lines are ridges and which are the streams. If you look closely, streams originate at the junction of ridge lines, and ridge lines originate at the junction of streams. If you view a computer-generated 3D surface (a digital elevation model or DEM) representing a region of watersheds, in theory you cannot discern if it is upside down or not. The ridges and streams would look very similar, they are a topological "dual." At Triple Divide Peak a raindrop falling onto the intersection of three ridges has, famously, a chance to end up in either of the Arctic, Pacific, or Atlantic oceans by eventually contributing to one of three major rivers (Columbia, Mississipi, Saskatchewan). Starting similarly, but confining yourself to traveling along ridge lines, you would end up at an ocean shoreline, at the boundary (in imagination, not oceanography) between two of those three oceans.