The area drained by a single river system is called a drainage basin or river basin. The network of such channels is called the drainage system. The arrangement of rivers and their tributaries is defined as a natural drainage pattern. The drainage pattern depends on the nature and structure of the rocks, topography slope, amount of water flowing and periodicity of the flow.
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When the drainage develops the shape of a tree, the drainage pattern is called dendritic; when they radiate from a hill, it is called Radial; it is Centripetal when a river blow into a depression or lake, in the trellis pattern the primary tributaries join from the sides; in the dome pattern, the drainage combines the elements of annular and radial drainages.
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The rivers in India have been more or less adjusted to its physical conditions. It is largely influenced by the evolution of three fold physiographic. India has about 113 river basins, the most conspicuous of which are the Ganga and Brahmaputra Basins. Mr. K.L. Rao, an eminent engineer has divided the river basins of India into three broad groups on the basis of their catchment areas. River basins with catchment area of 20,000 sq. km or more are classified as major or larger river basins. There are 14 large river basins.
River basins with catchment areas between 20,000 river basins. River basins with catchment areas between 20,000 sq. km. and 2000 sq. kms. Are called medium basins and there are 44 such river basins in the country. Rivers with catchment areas below 2,000 sq. km are termed as rivers. These are by and large, coastal rivers. There are 55 minor rivers which originate generally in the coastal mountains.