Drainage basin

A drainage basin is an area of land in which 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, 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.

Other terms for a drainage basin are catchment area, catchment basin, drainage area, river basin, water catchment, water basin, and impluvium. 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 the drainage divide line.^{[citation needed]}

A drainage basin's boundaries are determined by watershed delineation, a common task in environmental engineering and science.

In a closed drainage basin, or endorheic basin, rather than flowing to the ocean, water converges toward the interior of the basin, known as a sink, which may be a permanent lake, a dry lake, or a point where surface water is lost underground.

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.

About 48.71% of the world's land drains to the Atlantic Ocean.^{[citation needed]} In North America, surface water drains to the Atlantic via the Saint Lawrence River and Great Lakes basins, the Eastern Seaboard of the United States, the Canadian Maritimes, and most of Newfoundland and Labrador. Nearly all of South America east of the Andes also drains to the Atlantic, as does most of Western and Central Europe and the greatest portion of western Sub-Saharan Africa, as well as Western Sahara and part of Morocco.

The two major Mediterranean Seas of the world also flow to the Atlantic. The Caribbean Sea and Gulf of Mexico basin includes most of the U.S. interior between the Appalachian and Rocky Mountains, a small part of the Canadian provinces of Alberta and Saskatchewan, eastern Central America, the islands of the Caribbean and the Gulf, and a small part of northern South America. The Mediterranean Sea basin, with the Black Sea, includes much of North Africa, east-central Africa (through the Nile River), Southern, Central, and Eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and Syria.

The Arctic Ocean drains most of Western Canada and Northern Canada east of the Continental Divide, northern Alaska and parts of North Dakota, South Dakota, Minnesota, and Montana in the United States, the north shore of the Scandinavian Peninsula in Europe, central and northern Russia, and parts of Kazakhstan and Mongolia in Asia, which totals to about 17% of the world's land.