Watershed n. : The whole region or extent of country which contributes to the supply of a river or lake.

Watershed n. : The line of division between two adjacent rivers or lakes with respect to the flow of water by natural channels into them; the natural boundary of a basin.

Example:
How is the environmental quality of a Watershed related to surface water supply?

Taxonomy
Watershed, free encyclopedia, American usage, watershed area, Watershed stroke, algorithm, search, free dictionary, non-American, North American, image processing, blood supply, brain, Watershed Media Centre, arts centre, Bristol, England, Music, disambiguation pages.

Defination from the web:
Watershed (television), a time each evening after which certain material may be shown. Watershed Media Centre, an arts centre in Bristol, England ... List of watershed topics. Watersheddings, former British rugby stadium. This disambiguation page lists articles associated with the same title. ...