Chapter 3 — Drainage System

The flow of water through well-defined channels is called drainage. The network of such channels is called a drainage system. The drainage pattern of an area is the outcome of geological time period, nature and structure of rocks, topography, slope, amount of water flowing, and the periodicity of the flow.

Dendritic: resembles branches of a tree, seen in rivers of the northern plain. Radial: rivers originate from a hill and flow in all directions, e.g., rivers from the Amarkantak range. Trellis: primary tributaries flow parallel to each other and secondary tributaries join at right angles. Centripetal: rivers discharge from all directions into a lake or depression.

Nearly 77 per cent of India's drainage area — including the Ganga, Brahmaputra, Mahanadi, and Krishna — is oriented towards the Bay of Bengal. The remaining 23 per cent — comprising the Indus, Narmada, Tapi, Mahi, and Periyar systems — discharges into the Arabian Sea. The two zones are separated by the Delhi ridge, the Aravalis, and the Sahyadris.

A drainage basin is the area drained by a river and its tributaries, while the boundary line separating one basin from another is called a watershed. Watersheds are small in area while basins cover larger areas. The catchments of large rivers are called river basins; those of small rivulets and rills are referred to as watersheds.

Himalayan rivers such as the Indus, Ganga, and Brahmaputra are fed both by melting of snow and by precipitation, so they carry water throughout the year. Most Peninsular rivers depend mainly on rainfall, leaving them with very little water in the dry season. An exception is the Kaveri, whose upper catchment receives southwest monsoon rainfall and lower part receives northeast monsoon rainfall, giving it comparatively stable year-round flow.

The Kosi is an antecedent river with its source north of Mount Everest in Tibet. It brings huge quantities of sediment from its upper reaches and deposits them in the plains. These deposits block the river's course, forcing it to shift frequently. River Kosi is notorious for this behaviour, which causes recurring floods in Bihar, earning it the name 'sorrow of Bihar'.

The Ganga originates from the Gangotri glacier near Gaumukh (3,900 m) in Uttarkashi district of Uttarakhand, where it is called the Bhagirathi. It becomes the Ganga after the Bhagirathi meets the Alaknanda at Devprayag. It is shared by Uttarakhand (110 km), Uttar Pradesh (1,450 km), Bihar (445 km), and West Bengal (520 km), with a total length of 2,525 km. It discharges into the Bay of Bengal near Sagar Island.

First, subsidence of the western flank of the Peninsula below the sea during the early Tertiary period disturbed the symmetrical plan of rivers on either side of the original watershed. Second, the upheaval of the Himalayas caused trough faulting in the north; the Narmada and Tapi flow in these trough faults, which explains the lack of alluvial and deltaic deposits in them. Third, a slight tilting of the Peninsular block from northwest to southeast gave the entire drainage system its orientation towards the Bay of Bengal.

The Godavari is the largest Peninsular river system. It is 1,465 km long with a catchment area of 3.13 lakh sq km and rises in the Nasik district of Maharashtra before discharging into the Bay of Bengal. It is called Dakshin Ganga (Ganga of the South) because of its large size and cultural significance. Its major tributaries are the Penganga, Indravati, Pranhita, and Manjra.

Most Peninsular rivers flow west to east into the Bay of Bengal. The Narmada and Tapi are exceptions because they flow through rift valleys (trough faults). The Narmada flows between the Satpura range in the south and the Vindhyan range in the north. This structural control directs them westward — the Narmada meets the Arabian Sea south of Bharuch forming a 27 km long estuary, and the Tapi originates from Multai in Betul district of Madhya Pradesh and drains westward into the sea.

In Tibet, the Brahmaputra is called the Tsangpo, meaning 'the purifier.' It flows eastward for about 1,200 km in southern Tibet before carving a deep gorge near Namcha Barwa (7,755 m). It emerges as the Siang or Dihang and enters India west of Sadiya town in Arunachal Pradesh. After receiving the Dibang (Sikang) and Lohit tributaries, it becomes the Brahmaputra. The Brahmaputra originates from the Chemayungdung glacier of the Kailash range near Mansarovar lake.

Panjnad is the name given to the combined flow of the five rivers of Punjab that join the Indus a little above Mithankot. The five rivers are the Satluj, the Beas, the Ravi, the Chenab, and the Jhelum. The Indus receives the Panjnad and finally discharges into the Arabian Sea east of Karachi.

The Namami Gange Programme is an Integrated Conservation Mission approved as a Flagship Programme by the Union Government in June 2014. Its twin objectives are effective abatement of pollution and conservation and rejuvenation of the National River Ganga. Its pillars include sewerage treatment infrastructure, river-front development, river-surface cleaning, biodiversity, afforestation, public awareness, industrial effluent monitoring, and Ganga Gram.

The Indus is one of the largest river basins in the world, covering 11,65,000 sq km in total (3,21,289 sq km in India) with a total length of 2,880 km (1,114 km in India). It originates near Bokhar Chu at 4,164 m in the Kailash range in Tibet, where it is known as Singi Khamban (Lion's mouth). In India it flows through Ladakh and Jammu and Kashmir. Its five major Punjab tributaries — Satluj, Beas, Ravi, Chenab, and Jhelum — join it as the Panjnad. It finally discharges into the Arabian Sea east of Karachi.

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