The Brahmaputra system is another important river system of India. It occupies a toal drainage area of 5, 80,000 km2 of which 2, 58,008 km2 lies in India (annual water yield of 6, 27,000 million m). The total power potential of the river is estimated at 1,248 lakh km.
The source of the B rahmaputra (called Tsangpo in Tibet) is at Tamchok Khambab Chorten in the Chemayung-dung glacier (31° 30′-N and 82° E). It flows for 1,100 km eastward parallel to the main Himalayan range before it makes a sharp southward bend and carves out a deep gorge at Namcha Barwa.
Here it is called Dihang and adopting a westward course for 725 km in Assam valley before entering Bangladesh. The Subansiri, Kameng, Dhansiri (north), Jaibhorelli, Manas and Tista join the Brahmaputra from the north; the Burhi Dihing, Dhansiri (south), Disang and Kopili from the south.
The Dibang and Luhit (Zayul) join the Brahmaputra near Sadiya. Tista became its western most tributary after the floods of 1787 (deserting from Ganga). In Tibet most of the tributaries flow in a direction opposite to the Tsangpo which proves that originally the course of the Brahmaputra was from east to west in the Tibet region (Holditch, 1905, p. 111).
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The average rainfall in the catchment area of the Brahmaputra is 212 cm per annum which enables it to carry huge quantity of water, during rankly season. The width and the depth of the river during the south-west monsoon become 8 km and 10 meters respectively and the river looks like a slowly moving lake. It carries huge quantity of dissolved material (666.7 cubic meters per sq. km.) which leads to the formation of a number of riverine islands. The maximum and minimum discharge of water at Pandu is 72,460 and 2,680 cum/ sec. respectively. The river serves as an important transport route in the Assam valley and its entire course from Sadiya to Dhuburi (891 km) has been declared as National Waterway on 26.10.1988.
Also the efforts are being made to use it as an important link between Assam and West Bengal through the co-operation of Bangladesh. All the important urban centers of Assam like Dhuburi, Goalpara, Guwahati, Tezpur, Dibrugarh and Sadiya etc. lie along its banks. It forms the largest riverain island (Majuli Island near Gosaingaon) within its bed.
The course of the Brahmaputra in Bangladesh is called Jumna which is an outcome of the river capturing. Originally the river flowed south-east across the Mymensingh district where it received the Surma river and joined the Meghna river before emptying into the Bay of Bengal (Rennell’s Map of 1785). The old channel still retains its original name while the new stream arising out of river capturing has been named as Jumna and has more westerly location.
TheTista(TrishnaorTrisrota)risesinChitamu Lake in Tibet (about 28°28 ‘N ; 88° 44 ‘E) and joins the Brahmaputra in the Rangpur district of Bangladesh. It forms a magnificent gorge called the Sivok Gola Pass in the Darjeeling district. Prior to the floods of 1787 it was a tributary of the Ganga but during the floods it turned eastwards and joined the Brahmaputra. Its Himalayan tributaries include the Ragpo, Rangit, Rangjo, Ryeng and Sivok while the Lish, Gish and Saldanga etc. are the tributaries in the plains.
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The Meghna is the name given to the course of the original Brahmaputra after its confluence with the Surma River, from Bhairab Bazar onwards. It enters the sea by four streams, enclosing several islands. During floods it expands as a vast sheet of water. Its lower course is susceptible to severe tidal bores and cyclonic havocs.
The Surma rises on the southern slopes of the mountain range to the north of Manipur. Its upper part, called Barak, is characterised by steep banks and several falls. It has 900 km long course before it joins the Brahmaputra. It offers navigation channel supporting towns like Silchar, Sylhet, Badarpur and Sunamganj etc.