"Catchment basin" redirects here. For the human geography concept, see
Example of a drainage basin. The dashed line is the main water divide of the hydrographic basin.
A drainage basin or catchment basin is an extent or an area of land where all
surface water from
melting snow, or
ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another body of water, such as a
ocean. Thus if a tributary stream joins a brook that in turn joins a small river which is a tributary of a larger river, there is a series of successively larger (and lower elevation) drainage basins. For instance, the
Ohio rivers are within their own drainage basins and also within the drainage basin of the
Other terms used to describe drainage basins are catchment, catchment area, drainage area, river basin and water basin.
North America, the term watershed is commonly used to mean a drainage basin, though in other English-speaking countries, it is used only in its original sense, to mean a
 a ridge that separates adjacent drainage basins. Drainage basins drain into other drainage basins in a
hierarchical pattern, with smaller sub-drainage basins combining into larger drainage basins.
closed ("endorheic") drainage basins 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
 The drainage basin includes all the streams and rivers that convey the water towards the sink, as well as the land surfaces from which water drains into those channels.
The drainage basin acts as a
funnel by collecting all the water within the area covered by the basin and channelling it to a single point. Each drainage basin is separated topographically from adjacent basins by a perimeter, the
drainage divide, making up a succession of higher geographical features (such as a
mountains) forming a barrier.
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.