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Landscape Characterization / Impervious Surfaces / What are watersheds?
What are watersheds?
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A watershed is the area drained by a stream and its tributaries.
Watersheds range in size from under a square mile to hundreds of thousands of
square miles (the Chesapeake Bay watershed, for example, has an area of 66,388
square miles). Any rain that falls within a watershed will eventually drain
from the bottom of the basin through the main stream channel. Because watersheds
may incorporate broad or diverse land areas, they are better units for analyzing
environmental problems than are political jurisdictions. As a case in point,
a study of water pollution in the Maryland portion of the Chesapeake Bay watershed
would have to take into account pollution sources in the upper Chesapeake Bay
watershed of New York and Pennsylvania.
But a watershed is more than a collection of streams and adjacent
land areas--it is a natural resource system in which humans and other organisms
interact with the land and its associated resources for sustenance, shelter,
and security. The physical condition of a watershed, therefore, directly affects
the health and well being of natural and social systems within its divides and
indirectly affects those systems beyond its divides.
What is a watershed?
A watershed is the area drained by a stream and all of
its tributaries. Any rain that falls within the watershed will pass
through the main stream channel.
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A divide separates each basin from the surrounding drainage basins.
Divides follow ridges and hill tops. If a raindrop falls on one side of
a divide, it will flow down one side of the hill, and into one drainage
basin. If the raindrop falls on the other side of a divide, it will flow
into a different drainage. |
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Watersheds are composed of many smaller drainage
basins. In the diagram, a sub-basin has been drawn for every
tributary. |
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