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.