A
drainage basin or
catchment basin is an extent or an area of land where
surface water from
rain,
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 that joins a small river is tributary of a larger river, which is thus part of a series of successively smaller area but higher elevation drainage basins. Similarly, the
Missouri and
Ohio rivers are each part of their own drainage basins and that of the
Mississippi River.