Watershed
An area of land where all the water drains to a common river, lake, or ocean outlet.
Definition
A watershed -- also called a drainage basin -- is the area of land where all surface water, including rainfall and snowmelt, drains into a shared body of water such as a river, lake, or ocean. Watersheds can range from a few acres around a small stream to millions of square miles draining into a major river system.
Because everything that happens within a watershed -- development, agriculture, pollution -- eventually affects the water flowing out of it, watershed protection is a core strategy in both water quality management and habitat conservation.
In Practice
Pesticide runoff from a farm far upstream can affect water quality and fish populations many miles downstream, because both areas sit within the same watershed.