Drainage System

BASIC TERMINOLOGY

• The flow of water through well-defined channels is known as ‘drainage’ and the network of such channels is called a ‘drainage system’.
• 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. There is, however, a slight difference between a river basin and a watershed. Watersheds are small in area while the basins cover larger areas.
• The drainage pattern of an area is the outcome of the geological time period, nature and structure of rocks, topography, slope, amount of water flowing and the periodicity of the flow.

Drainage Pattern: A geometric arrangement streams in a region; determined by slope, differing rock resistance to weathering erosion, climate, hydrological variability and structural controls of the landscape known as drainage pattern.

Following are the major drainage patterns −

• • Dendritic
  • Radial
  • Centripetal
  • Trellis

• A drainage pattern which looks like tree branches with lots of twigs is known as Dendritic drainage pattern. For example, the rivers of the northern plain.
• Radial drainage patterns form when rivers originate from a hill and flow in all directions. For example, the rivers originating from the Amarkantak.
• Centripetal drainage pattern is formed when rivers discharge their waters from all directions into a lake or a depression. For example, Loktak lake in Manipur.
• Trellis drainage pattern is formed when the primary tributaries of main rivers flow parallel to each other and secondary tributaries join them at right angles. For example, rivers in the upper part of the Himalayan region.

The Indian drainage system may be divided on various bases.

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

• The Arabian Sea drainage; and
•  The Bay of Bengal drainage.
  • They are separated from each other through the Delhi ridge, the Aravallis 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:

• (i) 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.
• (ii) 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.
• (iii) Minor river basins with catchment areas of less than 2,000 sq. km include a fairly good number of rivers flowing in the area of low rainfall.

Drainage System of India

The drainage system of India, that is, the rivers, is divided into two major groups:

• The Himalayan Rivers
• The Peninsular Rivers