Drainage basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, the drainage divide,[1] made up of a succession of elevated features, such as ridges and hills. A basin may consist of smaller basins that merge at river confluences, forming a hierarchical pattern.[2]



Other terms for a drainage basin are catchment area, catchment basin, drainage area, river basin, water basin,[3][4] and impluvium.[5][6][7] In North America, they are commonly called a watershed, though in other English-speaking places, "watershed" is used only in its original sense, that of a drainage divide.
In a closed drainage basin, or endorheic basin, the water converges to a single point inside the basin, known as a sink, which may be a permanent lake, a dry lake, or a point where surface water is lost underground.[8]
Drainage basins are similar but not identical to hydrologic units, which are drainage areas delineated so as to nest into a multi-level hierarchical drainage system. Hydrologic units are defined to allow multiple inlets, outlets, or sinks. In a strict sense, all drainage basins are hydrologic units but not all hydrologic units are drainage basins.[8]