A watershed is typically defined as the area of land where all precipitation drains to a common water body, such as a river or lake. Watersheds are not made of water, but of land. This means that the boundaries of a watershed are determined by the shape of the land that surrounds them. Water runs downstream, so buttes and ranges often form the boundaries of watersheds.

Often called a drainage basin or hydrologic unit, a watershed can cover a large multi-state area like the Columbia River watershed or a relatively small area, like the watershed of a small stream, pond, or wetland. Larger watersheds are made of numerous smaller watersheds, often called sub-watersheds or sub-basins. No matter where you stand, a watershed surrounds you.