Drainage

📚 Explained

1. Drainage, Basin and Water Divide

The term drainage describes the river system of an area. The area drained by a single river system together with its tributaries is called a drainage basin. An elevated area, such as a mountain or an upland, that separates two drainage basins is known as a water divide. For example, the Ambala town is located on the water divide between the Indus and the Ganga river systems. A large river basin contains many smaller basins of its tributaries. The world’s largest drainage basin is that of the Amazon river; in India, the Ganga has the largest drainage basin.

2. Drainage Patterns

The streams within a drainage basin form different patterns depending on the slope of land, the underlying rock structure and the climate. In a dendritic pattern the stream branches like the veins of a leaf and develops where the river follows the slope of the land. A trellis drainage pattern forms when the main streams flow parallel and tributaries join them at right angles — common where hard and soft rocks lie in alternate bands. A rectangular pattern develops on strongly jointed rocky terrain. A radial pattern develops when streams flow in all directions from a central high point like a dome or a volcanic cone.

3. The Himalayan Rivers

The Himalayan rivers are perennial, which means they flow throughout the year because they receive water from both rain and the melting of snow from the lofty mountains. The two major Himalayan rivers, the Indus and the Brahmaputra, originate on either side of Mansarovar lake. These rivers are long and are joined by many large and important tributaries. In their upper courses, while still in the mountains, they have carved out steep V-shaped valleys, gorges and waterfalls through intense erosion. When they reach the plains they form flat features such as meanders, oxbow lakes, flood plains and large deltas at the mouth.

4. The Indus River System

The Indus rises in Tibet near lake Mansarovar. Flowing west, it enters India in the Ladakh region of Jammu and Kashmir. Several tributaries — the Zaskar, the Nubra, the Shyok and the Hunza — join it here. The Indus flows through Baltistan and Gilgit and emerges from the mountains at Attock. The Satluj, the Beas, the Ravi, the Chenab and the Jhelum join together to enter the Indus near Mithankot in Pakistan. Beyond this, the Indus flows southwards and finally reaches the Arabian Sea, east of Karachi. The Indus plain has a very gentle slope. With a total length of about 2900 km, the Indus is one of the longest rivers of the world. According to the Indus Waters Treaty (1960), India can use only about 20 per cent of the total water carried by the Indus river system.

5. The Ganga River System

The headwaters of the Ganga, called the Bhagirathi, are fed by the Gangotri Glacier and are joined by the Alaknanda at Devprayag in Uttarakhand. At Haridwar the Ganga emerges from the mountains on to the plains. It is joined by many tributaries from the Himalayas, such as the Yamuna, the Ghaghara, the Gandak and the Kosi. The Yamuna rises from the Yamunotri Glacier and joins the Ganga at Allahabad (Prayagraj). The peninsular tributaries like the Chambal, the Betwa and the Son rise from the semi-arid southern plateaus. Enlarged by all these waters, the Ganga flows eastwards till Farakka in West Bengal — the northernmost point of the Ganga delta. Here the river splits: one part (the Bhagirathi-Hooghly) flows south through the deltaic plains to the Bay of Bengal, while the mainstream flows into Bangladesh where it is joined by the Brahmaputra. Together they form the Sundarban delta, the world’s largest and fastest-growing delta. The length of the Ganga is over 2500 km.

6. The Brahmaputra River System

The Brahmaputra rises in Tibet east of Mansarovar lake, very close to the sources of the Indus and the Satluj. It is slightly longer than the Indus, but most of its course lies outside India. In Tibet, where it is known as the Tsang Po, it carries a smaller volume of water and less silt because of the cold, dry climate. In India it passes through a region of high rainfall, so it carries a large volume of water and a huge amount of silt. Here it has a braided channel in most of its course and forms many riverine islands, including Majuli, the largest inhabited riverine island in the world. Every year during the rainy season the river overflows its banks, causing widespread devastation due to floods in Assam and Bangladesh. Unlike other north Indian rivers, the Brahmaputra is marked by huge deposits of silt on its bed, causing the river bed to rise.

7. The Peninsular Rivers

The major water divide in peninsular India is formed by the Western Ghats, which run from north to south close to the western coast. Most of the major rivers of the Peninsula — such as the Mahanadi, the Godavari, the Krishna and the Kaveri — flow eastwards and drain into the Bay of Bengal, forming deltas at their mouths. The Narmada and the Tapi are the only two large rivers that flow westwards and make estuaries instead of deltas. Peninsular rivers are mostly seasonal (rain-fed), so their flow depends on rainfall; during the dry season even the large rivers shrink. They have shorter and shallower courses than their Himalayan counterparts, and they flow through fixed beds with very little erosion because the Peninsular plateau is an old, hard landmass.

8. Important Peninsular Rivers

The Narmada rises in the Amarkantak hills in Madhya Pradesh and flows west in a rift valley, creating the spectacular “Marble Rocks” near Jabalpur and the Dhuadhar falls. The Tapi rises in the Satpura ranges and also flows west through a rift valley, parallel to the Narmada but shorter. The Godavari is the largest peninsular river, rising in the Western Ghats of Maharashtra; it is often called the Dakshin Ganga (Ganga of the South). The Mahanadi rises in the highlands of Chhattisgarh. The Krishna rises near Mahabaleshwar, and the Kaveri rises in the Brahmagiri range and is famous for the Shivasamudram waterfall.

9. Lakes and Their Value

India has many lakes that differ in size and origin. Some are formed by the action of glaciers and ice sheets, some by wind, river and human activity. Wular lake in Jammu and Kashmir, formed by tectonic activity, is the largest freshwater lake in India. Most freshwater lakes lie in the Himalayan region. A lagoon such as the Chilika lake (Odisha), the Pulicat lake and the Kolleru lake is formed when spits and bars build across a shallow part of the sea. Lakes are very useful: they regulate the flow of rivers, prevent flooding during heavy rains and maintain an even flow during the dry season; they are also used for generating hydel power, developing tourism and providing recreation.

10. Rivers in the Economy and River Pollution

Rivers have been of fundamental importance throughout human history. They provide water for drinking, irrigation, navigation and hydro-electricity, and the riverside plains attract dense settlement and farming. But rising demand from industries, agriculture and growing population has increased the use of river water, while untreated sewage and industrial effluents are dumped into rivers. This affects not only the quality of water but also the river’s self-cleansing capacity. To save our rivers, the government has launched action plans such as the Ganga Action Plan (GAP) and the National River Conservation Plan (NRCP).