The river Brahmaputra is one of the largest rivers in the world. For the people living in Brahmaputra valley, the mighty river is a great gift of nature as the river has been enriching the soil of the valley from the time immemorial through deposition of silt. The enormous amount of water available in the river not only can meet the irrigation and other requirements of the people of valley but also has the potential to meet such requirement for a large part of rest of India for future generations.
There is huge hydroelectric potential in the river that can change the entire economy of the region and the country. The fish that the river produces, huge river tourism potential, famous national parks on its banks that sustains on the river etc are other gifts to the people by the river. But at the same time, the great river has the propensity to cause large destruction and inflict huge tragedy through its devastating floods and bank erosion.
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For this reason, people living on its bank has a love and hate relationship with it at the same time. For them, the river is a demigod- the son of Brahma and someone whose arms are very powerful (Mohabahu). For poets and artists of the valley, the river is an enigma and has inspired great works of art and creativity. But how much do we actually know about this great river?
The Brahmaputra river is an antecedent snow fed river which flows across the rising young Himalayan Ranges. Geologically, it is the youngest of the major rivers of the world. It is the 3rd largest river in the world in terms of its annual discharge i.e. annual volume of flow and carries highest silt load amongst rivers of its size. At Indo-Bangla border, the average annual discharge of the river is estimated to be 600 billion cubic meter (BCM) against 1770 BCM of water carried annually by entire river system of India.
Therefore, Brahmaputra alone has more than one third of India’s total surface water resources! In the course of its 2,906 km journey from source to sea, the river receives as many as 22 major tributaries in Tibet, 33 in India and 3 in Bangladesh.
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The drainage basin of the Brahmaputra extends to an area of about 580,000 sq km. Of this, 50.51% is in Tibet (China), 7.75% in Bhutan, 33.52% in India and 8.1% in Bangladesh. Its basin in India is shared by six states namely, Arunachal Pradesh (41.88%), Assam (36.33%), Nagaland (5.57%), Meghalaya (6.10%), Sikkim (3.75%) and West Bengal (6.47%).
Contrary to the popular belief that the famous Manasarowar lake is the source of Brahmaputra river, its actual origin is the Kanglung Kang glacier located about 63 km south east of the lake at an altitude of 5300m on Kailash range of Himalayas. The river is known as Yarlung Zangbo/Tsanpo in Tibet. It traverses eastward for 1625 km along southern Tibet, keeping a course roughly parallel to and about 160 km north of the Central Himalayas.
Large numbers of glaciers contribute their snow melt in the initial stage of the river from its south as well as from north. Almost entire precipitation, mostly in the form of snow, received by the north face of Central Himalayan Range, contributes to the river flow. Later, the river is joined by larger tributaries; mostly from its North where the catchment area is wider.
The total catchment of Yarlung Tsangpo constitutes more than 50 per cent of the Brahmaputra basin. This vast high altitude and barren catchment is responsible for some of the unique characteristics of Brahmaputra river. It is remarkable to note that the river is quite wide (600m and above) in many parts of Tibet and is navigable for a length of 650 km at an altitude of 12000 ft or above, making it the highest inland navigation system in the world. There are many locations where the river is highly braided at such high altitude.
Before entering India, the river turns abruptly towards North-East and North, making its way through stupendous gorges between huge mountain masses of Gyala Peri (7150m) and Namche Barwa (7755m) in the process of crossing the Central Himalayas from its north to south. Through these deep narrow gorges, the Tsangpo rushes down in a series of cascades and rapids through a hairpin bend and finally turns south-east to enter India at Geling in Arunachal Pradesh. This last stretch of Tsangpo is about 260km in length in which the river drops by about 2200m in altitude! In this reach lies the only known fall of about 24m on the main Brahmaputra river.
After entering India, it flows generally towards southern direction through hills of Arunachal Pradesh. It takes the name of Siang in Arunachal Pradesh and Dihang in Assam for a total of 278 km. It has a meandering single channel for the initial 226 km upto Pasighat. From here, the river widens, starts braiding i.e. becomes multichannel and after flowing for a distance of 52 km, meets Dibang and Lohit rivers and take the name of Brahmaputra.
From this tri-junction of the three rivers at Kobochapori (near Jonai), the 640 km stretch of the river in Assam is called Brahmaputra. Starting with a comparatively steep slope for a river for its size with 1m drop in every 3700m of its length at Kobo, it gradually becomes flatter. The slope at Dhubri is as flat as 1m drop for 14650m of its length. The river has a general direction of east to west in this portion, flows through the narrow Brahmaputra Valley (average width=80km) and is joined by large numbers of tributaries from both banks.
In the process, the river swells in size and discharge doubles during its journey from Sadiya to Dhubri and accordingly the nature of the river undergoes changes. From the coarse sand bed at Kobo, the river bed material becomes much finer at downstream. Thus, the river islands or ‘Chars’ of upper Assam with ‘Ikora’, ‘Khagori’ and ‘Birina’ gradually turns into good paddy lands at Pancharatna and downstream. From a turbulent silt laden river near Dibrugarh, it transforms into a sluggish giant at Dhubri with only few comparisons in the world. The river remains braided throughout its entire length, generally covering 10 km of the valley (average).However, the width exceeds 15 km in many locations; it is as wide as 22 km just before it enters Bangladesh.
As it crosses international border and enters Bangladesh, the river turns south and takes the name of Jamuna. From Dhubri, it flows for 258km to meet Ganges (which is known as Padma in Bangladesh) at Goalundo. It flows by the name of Padma or Ganges for another 105 km where it is met by Meghna, known as Barak at upstream in India. After this, the combined flow takes the name of Meghna before emptying into Bay of Bengal. As the confluence point with Meghna is the apex point of the delta of Meghna, the length of the river is considered only up to this point.
DJ Borgohain is former chief engineer of the Brahmaputra Board.