The water quantity and quality of a watershed are determined by the combined influences of its:
A watershed can also be described as an extent of land where water from rain and melting snow or ice drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, and is separated from adjacent basins by a drainage divide. The watershed acts as a funnel by collecting all the water within the area covered by the basin and channelling it into a waterway. Each watershed is separated topographically from adjacent basins by a geographical barrier such as a ridge, hill or mountain, which is known as a water divide.
Other terms that are used to describe a watershed are drainage basin, catchment area, catchment basin, drainage area, river basin, water basin and watershed. In the technical sense, a watershed refers to a divide that separates one drainage area from another drainage area. However, in the United States and Canada, the term is often used to mean a drainage basin or catchment area itself. Drainage basins drain into other drainage basins in a hierarchical pattern, with smaller sub-drainage basins combining into larger drainage basins. Catchments often refer to streams, rivers, wetlands, lakes and/or aquifers.