Drainage basin
A
drainage basin or
watershed is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean. For example, a tributary stream of a brook which joins a small river, which is tributary of a larger river is thus part of a series of successively smaller area but higher elevation drainage basins. Similarly, the Missouri and American rivers are each part of their own drainage basins/watersheds and that of the Mississippi River. Other terms that are used to describe a drainage basin are
catchment,
catchment area,
catchment basin,
drainage area,
river basin and
water basin. In North America, the term
watershed is commonly used to mean a drainage basin, the one meaning an area, the other its high elevation perimeter of that area. Drainage basins drain into other drainage basins in a hierarchical pattern, with smaller
sub-drainage basins combining into larger drainage basins.