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drainage area[′drān·ij ‚er·ē·ə]
(also hydrographical basin), a land surface, bounded by a divide, whose discharge goes into a basin. The structure of the drainage area (terrain, the presence of lakes and swamps, and the type of vegetation) considerably influences the discharge of water. The nature of the terrain of a drainage area determines the hydraulic gradient and the density of the river and valley network—that is, the speed of runoff on slopes and the rate of water flow in channels. The presence of various closed depressions causes some surface water to be held in these depressions, to be lost to evaporation and seepage, and to begin to flow into a channel only after it has filled the depressions.