C. Drainage System

Flow of water through well-defined channels is known as drainage and the network of such channels is called a drainage system‘.

• Perennial (always with water)

1. Ephemeral (water during rainy season, and dry, otherwise)

• A river drains the water collected from a specific area, which is called its catchment area‘.
• An area drained by a river and its tributaries is called a drainage basin.
• The boundary line separating one drainage basin from the other is known as the watershed.
• The catchments of large rivers are called river basins while those of small rivulets and rills are often referred to as watersheds.
• Watersheds are small in area while the basins cover larger areas
• The drainage pattern resembling the branches of a tree is known as ?dendritic the examples of which are the rivers of northern plain.
• When the rivers originate from a hill and flow in all directions, the drainage pattern is known as radial‘.
• Example- rivers originating from the Amarkantak range
• When the primary tributaries of rivers flow parallel to each other and secondary tributaries join them at right angles, the pattern is known as trellis‘.
• When the rivers discharge their waters from all directions in a lake or depression, the pattern is known as centripetal‘.

The basis of discharge of water (orientations to the sea), it may be grouped into:

(i)the Arabian Sea drainage; (ii) the Bay of Bengal drainage.

• They are separated from each other through the Delhi ridge, the Aravalis and the Sahyadris .
• Nearly 77 per cent of the drainage area consisting of the Ganga, the Brahmaputra, the Mahanadi, the Krishna, etc. is oriented towards the Bay of Bengal
• While 23 per cent comprising the Indus, the Narmada, the Tapi, the Mahi and the Periyar systems discharge their waters in the Arabian Sea.

On the basis of the size of the watershed, the drainage basins of India are grouped into three categories-

1. Major river basins with more than 20,000 sq. km of catchment area. It includes 14 drainage basins such as the Ganga, the Brahmaputra, the Krishna, the Tapi, the Narmada, the Mahi, the Pennar, the Sabarmati, the Barak, etc.
2. Medium river basins with catchment area between 2,000-20,000 sq. km incorporating 44 river basins such as the Kalindi, the Periyar, the Meghna, etc.
3. Minor river basins with catchment area of less than 2,000 sq. km include fairly good number of rivers flowing in the area of low rainfall