A watershed is the land area that drains runoff – snowmelt and rain – into a connected system of lakes, streams, rivers, and other waterways. It typically is identified by the largest draining watercourse within the system. In California, for example, the Sierra Nevada is one of the state’s major watersheds.
Watersheds may be as small as a patch of land draining into a tiny pond or as large as the Sacramento River Basin, which drains an area about 27,000 square miles. Watersheds follow natural boundaries and are usually separated from one another by ridges or mountains.
A watershed has many important natural functions. It collects water from precipitation, stores groundwater in aquifers, releases water as runoff and provides habitat for plants and animals.
The flow of water from upstream to downstream can also impact a watershed’s water quality. Pollutants—both natural and man-made—entering a watershed in one place can later end up in an entirely different place.
These pollutants include natural contaminant such as sediment from stream bank erosion, bacteria and nutrients from wildlife, and industrial pollutants such as motor oil, fertilizers, and pesticides.
With this in mind, water managers are increasingly using an integrated regional management approach to better address threats to habitat, wildlife and water quality.