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river basin[′riv·ər ‚bās·ən]
the part of the earth’s surface that includes a layer of water-bearing rock from which waters flow into a separate river or river system. The basin of each river includes surface and underground drainage. A surface drainage area is a part of the earth’s surface from which waters flow into a given river system or river. Underground drainage areas are formed by masses of loose deposits from which the water flows into the river network. In general, surface and underground drainage areas do not coincide; however, since it is practically very complicated to determine the boundaries of an underground drainage area, the size of the river basin is taken to mean only the surface drainage area. Errors arising as a result of the arbitrary identification of the dimensions of a river basin with the surface drainage area may be essential only for small rivers and larger rivers flowing under geological conditions that favor good water exchange among neighboring basins (for example, a karst). The boundary between individual river basins runs along watershed divides.
Basins are divided into drained and undrained types. Undrained basins are intracontinental drainage regions that are not linked to the ocean by river basins. The forms and dimensions of basins are most varied and depend upon geographical position, topography, and the geological structure of the locality. The tributaries have their own small basins whose general confluence composes the area of the basin of the main river.
A. I. CHEBOTAREV