Drainage basin

The Mississippi River drains the largest area of any American river, including most of its agricultural regions. Agricultural runoff and other water pollution that flows to the outlet is the cause of the hypoxic or dead zone in the Gulf of Mexico.

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.

In North America, this is commonly called a watershed, though in other English-speaking places, "watershed" is used only in its original sense, that of the drainage divide line. Other terms for a drainage basin are catchment area, catchment basin, drainage area, river basin, water catchment, water basin, and impluvium.

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.