Drainage Patterns and River Basins of India: GK Notes

Drainage means the network of channels through which the water of an area flows out. The whole area drained by a single river together with its tributaries is its drainage basin, a smaller area drained by one stream is a watershed, and the upland that separates two basins is a water divide. The shape that the channels take on a map is the drainage pattern, and it depends on the slope of the land, the hardness of the rocks and the lines of weakness in them. Exams ask the pattern and its example, the rank and size of the basins, and the peculiarities of the rivers that behave differently from the rest.

The drainage patterns

Two further terms are asked often. Antecedent or pre-existing drainage means that the river is older than the mountain it crosses; the Indus, the Satluj and the Brahmaputra were flowing before the Himalaya rose to its present height, and they kept their courses by cutting deep gorges across the range. Superimposed drainage means that a pattern formed on a cover of rocks that has since been stripped away, leaving the streams flowing across the structure beneath; the Damodar, the Subarnarekha and the Chambal show this on the old Chotanagpur and Vindhyan surfaces.

Basin, watershed and water divide

The Central Water Commission treats a catchment of more than twenty thousand square kilometres as a major basin, and India has a little over a dozen of them. A water divide is usually a ridge or a plateau edge: the Delhi ridge and the plain near Ambala separate the Indus system from the Ganga system, the Western Ghats separate the short Arabian Sea rivers from the long Bay of Bengal rivers, and the Amarkantak plateau sends water in three directions at once. Where the water of a basin never reaches the sea, the drainage is called inland or endoreic, as in the Sambhar lake catchment of Rajasthan and in the lower course of the Ghaggar, which dies out in the desert.

Himalayan and Peninsular drainage compared

Himalayan rivers are perennial, fed by both the monsoon and the melting snow, and they are young and energetic: they cut gorges and V-shaped valleys in the mountains, then meander wildly over the plains, shift their beds and build great deltas. Peninsular rivers are seasonal and rain fed, they flow in shallow, broad and almost graded valleys, their courses are fixed by hard rock and their catchments are small by comparison. The east flowing peninsular rivers, the Mahanadi, the Godavari, the Krishna, the Cauvery and the Pennar, reach the Bay of Bengal and build deltas; the west flowing Narmada and Tapi run through rift valleys between the Satpura and the Vindhya ranges and empty into the Gulf of Khambhat through estuaries, with no delta at all.

The major river basins

The rivers that break the rules

A few rivers are asked precisely because they do not behave like their neighbours. The Narmada rises at Amarkantak in Madhya Pradesh, flows west through a rift, makes the Dhuandhar falls and the marble gorge at Jabalpur, and ends in an estuary. The Tapi rises at Multai in Betul district, also flows west, and is the only large river parallel to the Narmada. The Chambal flows north into the Yamuna and is famous for its ravines. The Luni is the only sizeable river of the Thar and ends in the Rann of Kachchh instead of the open sea. The Brahmaputra enters India as the Dihang in Arunachal Pradesh, carries the largest silt load of any Indian river, and braids so widely that it holds Majuli, one of the largest river islands in the world. The Sharavati of Karnataka, a short west flowing stream, drops as the Jog falls.

Why the pattern matters

Drainage shapes everything a geography paper asks next. The soils of the northern plain are the alluvium the Himalayan rivers brought down, the deltas are the richest farmland of the east coast, the west flowing rivers in rift valleys have little delta land but good sites for dams, and the interlocking of basins is the reason for interstate water disputes and for the river linking proposals. A student who can draw the divides and name the basins can answer most map questions on India without learning them one by one.