Brahmaputra River one of the largest rivers in the world, with its basin covering areas in Tibet, China, India and Bangladesh. It originates in the Chemayung-Dung glacier, approximately at 31'30'N and 82'0'E, some 145 km from Parkha, an important trade centre between lake Manassarowar and Mount Kailas. It has a long course through the dry and flat region of southern Tibet before it breaks through the himalayas near the Namcha Barwa peak at about 7,755m. Its chief tributaries in India are the Amochu, Raidak, Sankosh, Mans, Bhareli, Dibang and Luhit. The several tributaries in Tibet are derived partly from a low range between the main Himalayas and the Tsang-po. The total length of the river from its source in southwestern Tibet to the mouth in the bay of bengal is about 2,850 km (including Padma and Meghna up to the mouth). Within Bangladesh territory, Brahmaputra-Jamuna is 276 km long, of which Brahmaputra is only 69 km.
The Brahmaputra is known as the Dihang in Assam Himalayas before it comes into the Great Plains of Bengal. The Dibang and the Luhit meet it from the east near Sadiya. The Dibang drains the Himalayas east of the Dihang while the Luhit drains an area between assam and Myanmar.
The course of the Brahmaputra in Tibet, known as the Tsang-po, is through a plain south of Lhasa; the flow here is sluggish. From the source to where the river enters the central Himalayan range near Namcha Barwa it is 1,600 km long. There it has three tributaries. The elevation of its bed is about 4,523m at Tradom, 2,440m at Gaela Sindong near Namcha Barwa, but only 135m at Sadiya in northeast Assam. In Assam, it flows roughly southwest to a point north of the shillong massif before entering Bangladesh.
In the plains of Assam the Brahmaputra is a mighty river and spreads into a vast expanse of water. It has numerous islands and changes its course very often. After traversing the Assam valley for 720 km, it sweeps round the Garo Hills, enters Bangladesh and flows southward for nearly 240 km before joining the ganges and the sea in the south. In Bangladesh the reach of this river is very small-up to the off-take of the old brahmaputra-and its downstream portion after the off-take is known as the jamuna.
Originally, the Brahmaputra flowed southeast across mymensingh district where it received the surma river and united with the meghna, as shown in rennell’s atlas (1785). By the beginning of the 19th century its bed had risen due to tectonic movement of the madhupur tract and it found an outlet farther west along its present course. The entire lower Brahmaputra consists of a vast network of channels, which are dry in the cold season but are inundated during the monsoon. It has numerous islands that are locally known as chars. This is the widest river system in the country flowing north-south. It meets the Ganges at goalandaghat .
The catchment of this mighty river is about 5,83,000 sq km of which about 47,000 sq km lies in Bangladesh. The river is braided within Bangladesh. It has four major tributaries: the dudhkumar, dharla, tista and the Karatoya-Atrai system. The first three rivers are flashy in nature, rising from the steep catchment on the southern side of the Himalayas between darjeeling in India, and Bhutan. Of all the distributaries, the Old Brahmaputra is the longest and was actually the course of the present Brahmaputra some 200 years ago. The switch to the present course took place after the earthquake and catastrophic flood in 1787.
During the monsoon the Brahmaputra discharges a large volume of water and at the same time brings in huge amounts of sediments. The width of the river varies from 3 km to 18 km but the average width is about 10 km. The width/depth ratios for individual channels of the Brahmaputra vary from 50:1 to 500:1. The gradient of the river in Bangladesh is 0.000077, decreasing to 0.00005 near the confluence with the Ganges. The river has a total suspended load discharge of about 725 million tons per annum.
The river is in fact a multi-channel one. Channels of many different sizes, from hundreds of meters to kilometres wide, and of different patterns including braiding, meandering and anastomosing pattern in the country. The main factor controlling the channel pattern in the Brahmaputra is its discharge. [Kazi Matinuddin Ahmed]
See map in brahmaputra-jamuna river system.