The Brahmaputra River has the second highest sediment yield per square kilometre in the world, exceeded only by that of the Yellow River in China. India’s central government and the state government of Assam have planned to dredge the Brahmaputra, with an initial amount of Rs 4 billion ($57 million).
One purpose of this dredging is to reduce flooding by allowing more water to stay in the river. The second is make the Brahmaputra navigable for large vessels – the river has been designated National Waterway 2 by the Inland Waterways Authority of India. It is planned as a vital component of trans-boundary inland waterways transport between India and Bangladesh.
Before the dredging starts, it is important to understand the nature of sediment transport in the Brahmaputra, because the dredged-out river is likely to be filled up again, partly or completely.
Sediment load of the Brahmaputra
The sediment deposited in the Brahmaputra varies across its length. At Tsela Dzong in Tibet it is about 150 tonnes per square km. But as the river crosses the Himalayas and reaches Pasighat at the foothills of Arunachal Pradesh in India, the deposit increases tenfold to 1,495 tonnes per square km. This shows that the river gathers sediments from soft rocks and landslide-affected areas of the Himalayas. The Higher Himalaya Range contributes about 70% of the sediments of the Brahmaputra, as explained by S. Krishnaswami and K. Singh in the September 10, 2005 issue of the journal Current Science.
The Brahmaputra then flows through Assam – forming the Assam valley, with the high Himalayas in the north and the Meghalaya plateau in the south – before entering Bangladesh. As measured at a station in Majuli – the largest river island in Assam – the suspended sediment load is slightly higher at 1,513 tons per square km, higher than at Pasighat, due to the contributions from the rivers Dibang and Lohit, which also flow down from the Himalayas on the north bank of the Brahmaputra.
Since the Brahmaputra is a trans-boundary river, data on its water discharge and sediment load are classified and thus the public has no access. Researchers can get the data after a lengthy process and only if they pledge not to share the data in public.
Sediment loads are measured as both suspended load and bed load. Suspended load is the sediment being carried by the water at the time of measurement, while bed load is the sediment that has settled down on the riverbed. Old data – published despite the ban – reveal that on an average the Brahmaputra transported 402 million tonnes of suspended sediment annually between 1955 and 1979 at the Pandu measuring station in Guwahati, the largest city of Assam.
According to Water and Power Consultancy Services, the average annual sediment yield between 1978 and 1991 was 527 million tonnes at Pancharatna near Goalpara, just a little upstream of the place where the river enters Bangladesh from India.
But this average means little. During the monsoon from May to October, the Brahmaputra transports 95% of the annual suspended load at Pandu at an average daily rate of 2.12 million metric tonnes. It would need over 141,300 trucks – of 15 tonnes each – to carry this away.
Although suspended sediment is measured at a few stations on the Brahmaputra, no convincing attempt has been made to measure its bed load. Dulal Goswami wrote in the 1989 (15.1) edition of the Indian Journal of Earth Sciences that he had estimated the bed load using several empirical equations and concluded that the bed load at Pandu was of the order of 5-15% of the total sediment load of the river.
Dredging feasible? Desirable?
Since suspended sediments form the majority of the load, will it be possible to maintain the dredged channel suitable for large vessels at the desired width and depth during the monsoon, when there is daily input of 2.12 million metric tonnes of sediments into the river?
And to what extent is this desirable? The authorities have stated that the main purpose of dredging the Brahmaputra is to prevent high flows from inundating its banks, which result in floods. But the valley of Assam has been created from the sediments deposited by floods of the Brahmaputra and its tributaries. During last two million years it has deposited 200-1,000 metre thick sediments by flooding and lateral channel migration.
Natural floods have several benefits besides increasing soil fertility. But artificial heavy floods have been created in Assam either due to breaching of embankments or sudden release of impounded water to keep dams safe. Such floods deposit enormous quantities of sterile sands rather than fertile sediments. For flood control, the solution should lie with improving or disbanding embankments and dams rather than with dredging.
Dredging seems to be a superficial answer to the challenge of drainage congestion and managing floods. Proper planning and a detailed study of the basic aspects of this government project are needed. A critical analysis of the data on both sediment input and dredging depth, together with a far better idea of the bed load, is necessary before starting this mega project.
Jogendra Nath Sarma is a retired professor of applied geology at Dibrugarh University, Assam, and the author of two books on the Brahmaputra.
LAST UPDATED: Feb 19, 2019 See Article HistoryAlternative Titles: Jamuna, Tsangpo, Ya-lu-tsang-pu Chiang, Yarlung Zangbo Jiang
Brahmaputra River, Bengali Jamuna, Tibetan Tsangpo, Chinese (Pinyin) Yarlung Zangbo Jiang or (Wade-Giles romanization) Ya-lu-tsang-pu Chiang, major river of Central and South Asia. It flows some 1,800 miles (2,900 km) from its source in the Himalayas to its confluence with the Ganges (Ganga) River, after which the mingled waters of the two rivers empty into the Bay of Bengal.
Brahmaputra RiverBrahmaputra River.Encyclopædia Britannica, Inc.
Along its course the Brahmaputra passes through the TibetAutonomous Region of China, the Indian states of Arunachal Pradesh and Assam, and Bangladesh. For most of its length, the river serves as an important inland waterway. It is not, however, navigable between the mountains of Tibet and the plains of India. In its lower course the river is both a creator and a destroyer—depositing huge quantities of fertile alluvial soil but also causing disastrous and frequent floods.
The Brahmaputra’s source is the Chemayungdung Glacier, which covers the slopes of the Himalayas about 60 miles (100 km) southeast of Lake Mapam in southwestern Tibet. The three headstreams that arise there are the Kubi, the Angsi, and the Chemayungdung. From its source the river runs for nearly 700 miles (1,100 km) in a generally easterly direction between the Great Himalayas range to the south and the Kailas Range to the north. Throughout its upper course the river is generally known as the Tsangpo (“Purifier”); it is also known by its Chinese name (Yarlung Zangbo) and by other local Tibetan names.
The Brahmaputra and Ganges river basins and their drainage network.Encyclopædia Britannica, Inc.
In Tibet the Tsangpo receives a number of tributaries. The most important left-bank tributaries are the Raka Zangbo (Raka Tsangpo), which joins the river west of Xigazê (Shigatse), and the Lhasa (Kyi), which flows past the Tibetan capital of Lhasa and joins the Tsangpo at Qüxü. The Nyang Qu (Gyamda) River joins the river from the north at Zela (Tsela Dzong). On the right bank a second river called the Nyang Qu (Nyang Chu) meets the Tsangpo at Xigazê.
After passing Pi (Pe) in Tibet, the river turns suddenly to the north and northeast and cuts a course through a succession of great narrow gorges between the mountainous massifs of Gyala Peri and Namjagbarwa (Namcha Barwa) in a series of rapids and cascades. Thereafter, the river turns south and southwest and flows through a deep gorge (the “Grand Canyon” of the Tsangpo) across the eastern extremity of the Himalayas with canyon walls that extend upward for 16,500 feet (5,000 metres) and more on each side. During that stretch the river enters northern Arunachal Pradesh state in northeastern India, where it is known as the Dihang (or Siang) River, and turns more southerly.
The Dihang, winding out of the mountains, turns toward the southeast and descends into a low-lying basin as it enters northeastern Assam state. Just west of the town of Sadiya, the river again turns to the southwest and is joined by two mountain streams, the Lohit and the Dibang. Below that confluence, about 900 miles (1,450 km) from the Bay of Bengal, the river becomes known conventionally as the Brahmaputra (“Son of Brahma”). In Assam the river is mighty, even in the dry season, and during the rains its banks are more than 5 miles (8 km) apart. As the river follows its braided 450-mile (700-km) course through the valley, it receives several rapidly rushing Himalayan streams, including the Subansiri, Kameng, Bhareli, Dhansiri, Manas, Champamati, Saralbhanga, and Sankosh rivers. The main tributaries from the hills and from the plateau to the south are the Burhi Dihing, the Disang, the Dikhu, and the Kopili.
Sibsagar, India: templeShaiva temple at Sibsagar near the Brahmaputra River, Assam, India.Foto Features
The Brahmaputra enters the plains of Bangladesh after turning south around the Garo Hills below Dhuburi, India. After flowing past Chilmari, Bangladesh, it is joined on its right bank by the Tista Riverand then follows a 150-mile (240-km) course due south as the Jamuna River. (South of Gaibanda, the Old Brahmaputra leaves the left bank of the main stream and flows past Jamalpur and Mymensingh to join the Meghna River at Bhairab Bazar.) Before its confluence with the Ganges, the Jamuna receives the combined waters of the Baral, Atrai, and Hurasagar rivers on its right bank and becomes the point of departure of the large Dhaleswari River on its left bank. A tributary of the Dhaleswari, the Buriganga (“Old Ganges”), flows past Dhaka, the capital of Bangladesh, and joins the Meghna River above Munshiganj.
Tista RiverTista River, a major tributary of the Brahmaputra River, flowing through the Siwalik Hills, northeastern India.Anupam Manur
The Jamuna joins with the Ganges north of Goalundo Ghat, below which, as the Padma, their combined waters flow to the southeast for a distance of about 75 miles (120 km). After several smaller channels branch off to feed the Ganges-Brahmaputra delta to the south, the main body of the Padma reaches its confluence with the Meghna River near Chandpur and then enters the Bay of Bengal through the Meghna estuary and lesser channels flowing through the delta. The Meghna forms the eastern limit of the Sundarbans, a vast tract of forest and saltwater swamp that constitutes much of the Ganges-Brahmaputra delta. The growth of the delta is dominated by tidal processes.
The Ganges-Brahmaputra system has the third greatest average discharge of the world’s rivers—roughly 1,086,500 cubic feet (30,770 cubic metres) per second; approximately 700,000 cubic feet (19,800 cubic metres) per second of the total is supplied by the Brahmaputra alone. The rivers’ combined suspended sediment load of about 1.84 billion tons per year is the world’s highest.
Climate
The climate of the Brahmaputra valley varies from the harsh, cold, and dry conditions found in Tibet to the generally hot and humid conditions prevailing in Assam state and in Bangladesh. Tibetan winters are severely cold, with average temperatures below 32 °F (0 °C), while summers are mild and sunny. The upper river valley lies in the rain shadow of the Himalayas, and precipitation there is relatively light: Lhasa receives about 16 inches (400 mm) annually.
The Indian and Bangladeshi parts of the valley are governed by the monsoon (wet, dry) climate, though it is somewhat modified there compared with other parts of the subcontinent; the hot season is shorter than usual, and the average annual temperature ranges from 79 °F (26 °C) in Dhuburi, Assam, to 85 °F (29 °C) in Dhaka. Precipitation is relatively heavy, and humidity is high throughout the year. The annual rainfall—between 70 and 150 inches (1,780 and 3,810 mm)—falls mostly between June and early October; however, light rains also fall from March to May.
Hydrology
The course of the Brahmaputra has changed continually over time. The most spectacular of these changes was the eastward diversion of the Tista River and the ensuing development of the new channel of the Jamuna, which occurred in 1787 with an exceptionally high flood in the Tista. The waters of the Tista suddenly were diverted eastward into an old abandoned course, causing the river to join the Brahmaputra opposite Bahadurabad Ghat in Mymensingh district. Until the late 18th century the Brahmaputra flowed past the town of Mymensingh and joined the Meghna River near Bhairab Bazar (the path of the present-day Old Brahmaputra channel). At that time a minor stream called the Konai-Jenai—probably a spill channel of the Old Brahmaputra—followed the course of today’s Jamuna River (now the main Brahmaputra channel). After the Tista flood of 1787 reinforced it, the Brahmaputra began to cut a new channel along the Konai-Jenai and gradually converted it after 1810 into the main stream, now known as the Jamuna.
Ganges-Brahmaputra delta cycloneSatellite image of the Ganges-Brahmaputra delta cyclone, November 12, 1970.NOAA
Along the lower courses of the Ganges and Brahmaputra and along the Meghna, the land undergoes constant erosion and deposition of silt because of the shifts and changes in these active rivers. Vast areas are subject to inundation during the wet monsoon months. The shifts in the course of the Jamuna since 1787 have been considerable, and the river is never in exactly the same place for two successive years. Islands and sizable newly deposited lands (chars) in the river appear and disappear seasonally. The chars are valuable to the economy of Bangladesh as additional cultivable areas.
In Tibet the waters of the Brahmaputra are clear because little silt is carried downstream. As soon as the river enters Assam, however, the silt load becomes heavy. Because of the speed and volume of water in the northern tributaries that flow down from the rain-soaked Himalayan slopes, their silt load is much heavier than that carried by the tributaries crossing the hard rocks of the old plateau to the south. In Assam the deep channel of the Brahmaputra follows the southern bank closer than the northern. This tendency is reinforced by the silt-laden northern tributaries pushing the channel south.
Another important feature of the river is its tendency to flood. The quantity of water carried by the Brahmaputra in India and Bangladesh is enormous. The river valley in Assam is enclosed by hill ranges on the north, east, and south and receives more than 100 inches (2,540 mm) of rainfall annually, while in the Bengal Plain heavy rainfall—averaging 70 to 100 inches—is reinforced by the huge discharge of the Tista, Torsa, and Jaldhaka rivers. Extensive flooding is virtually an annual occurrence in the Brahmaputra valley during the summer monsoon. In addition, tidal surges accompanying tropical cyclones sweeping inland from the Bay of Bengal periodically bring great destruction to the delta region. One such storm—the Ganges-Brahmaputra delta cyclone (also called the Bhola cyclone) of November 1970—caused an estimated 300,000 to 500,000 deaths and inundated a vast area. In the 21st century the delta has also been affected by rising sea levels as a result of global warming.
Plant and animal life
Along the upper reaches of the Brahmaputra (Tsangpo) on the high Plateau of Tibet, the vegetation is mainly xerophytic (drought-resistant) shrubs and grasses. As the river descends from Tibet, increased precipitation supports the growth of forests. Forests of sal (genus Shorea)—a valuable timber tree that is also utilized to cultivate the lac insect, which produces the resin used to make shellac—are found in Assam. At even lower elevations, tall reed jungles grow in the swamps and depressed water-filled areas (jheels) of the immense floodplains. Around towns and villages in the Assam Valley, the many fruit trees yield plantains, papayas, mangoes, and jackfruit. Bamboo thickets abound throughout Assam and Bangladesh. Nipa palms (Nypa fruticans) and other halophytic (salt-tolerant) flora predominate in the delta region’s mangrove swamps.
The most-notable animal of the swamps in Assam is the one-horned rhinoceros, which has become extinct in other parts of the world; Kaziranga National Park (designated a UNESCO World Heritage sitein 1985) provides a refuge for the rhinoceros and for other wildlife in the valley, including elephants, Bengal tigers, leopards, wild buffalo, and deer. Numerous varieties of fish include the pabda (Omdok pabda), chital (Notopterus chitala), and mrigal (Cirrhinus cirrhosus).
People
The people living in the different sections of the Brahmaputra valley are of diverse origin and culture. North of the Great Himalayas, the Tibetans practice Buddhism and speak the Tibetan language. They engage in animal husbandry and cultivate the valley with irrigation water taken from the river.
The ancestry of the Assamese includes peoples speaking Tibeto-Burman languages from the surrounding highlands and peoples from the lowlands of India to the south and west. The Assamese language is akin to Bengali, which is spoken in West Bengal state in India and in Bangladesh. Since the late 19th century a vast number of immigrants from the Bengal Plain of Bangladesh have entered Assam, where they have settled to cultivate vacant lands, particularly the low floodplains. In the Bengal Plain itself the river flows through an area that is densely populated by the Bengali people, who cultivate the fertile valley. The hilly margins of the plain are inhabited by the tribal Garo, Khasi, and Hajong of Meghalayastate in India.
Economy
Irrigation and flood control
Flood-control schemes and the building of embankments were initiated after 1954. In Bangladesh the Brahmaputra embankment running west of the Jamuna River from north to south helps to control floods. The TistaBarrage Project is both an irrigation and a flood-protection scheme.
Until the 21st century, little power had been harnessed along the Brahmaputra, although the estimated potential was great—some 12,000 megawatts in India alone. An increasing number of hydroelectric stations have been completed in Assam, most notably the Kopili Hydel Project in the south of the state. Another major project, the Ranganadi plant, has been built in Arunachal Pradesh, which has considerably more generating capacity than the Kopili station. In addition, a giant hydropower installation in Tibet on the Tsangpo River became fully operational in late 2015.
Navigation and transport
Near Lhazê (Lhatse Dzong) in Tibet, the river becomes navigable for about 400 miles (640 km). Coracles (boats made of hides and bamboo) and large ferries ply its waters at 13,000 feet (4,000 metres) above sea level. The Tsangpo is spanned in several places by suspension bridges.
Because it flows through a region of heavy rainfall in Assam and Bangladesh, the Brahmaputra is more important for inland navigation than for irrigation. The river has long formed a waterway between the Indian states of West Bengal and Assam, although, on occasion, political conflicts have disrupted the movement of traffic through Bangladesh. The Brahmaputra is navigable throughout the Bengal Plain and Assam upstream to Dibrugarh, 700 miles (1,100 km) from the sea. In addition to all types of local craft, powered launches and steamers easily travel up and down the river, carrying bulky raw materials, timber, and crude oil.
The Brahmaputra remained unbridged throughout its course in the plains until the Saraighat Bridge—carrying both road and rail traffic—was opened in 1962 near Guwahati, Assam. A second crossing in Assam, the Kalia Bhomora road bridge near Tezpur, was opened in 1987. Ferries, however, have continued as the most important—and in Bangladesh the only—means of crossing the Brahmaputra. Sadiya, Dibrugarh, Jorhat, Tezpur, Guwahati, Goalpara, and Dhuburi are important towns and crossing points in Assam, while Kurigram, Rahumari, Chilmari, Bahadurabad Ghat, Phulchari, Sarishabari, Jagannathganj Ghat, Nagarbari, Sirajganj, and Goalundo Ghat are major crossing points in Bangladesh. The railheads are located at Bahadurabad Ghat, Phulchari, Jagannathganj Ghat, Sirajganj, and Goalundo Ghat.
The upper course of the Brahmaputra was explored as early as the 18th century, although it remained virtually unknown until the 19th century. The explorations of the Indian surveyor Kinthup (reported in 1884) and of J.F. Needham in Assam in 1886 established the Tsangpo River as the upper course of the Brahmaputra. Various British expeditions in the first quarter of the 20th century explored the Tsangpo upstream in Tibet to Xigazê, as well as the river’s mountain gorges. More-recent scientific work has concentrated on understanding the hydrology of the Brahmaputra for watershed management and flood-hazard mitigation.
Use of mechanical device to pull kelp plants from beds would destroy local ecosystem, say campaigners
A proposal to mechanically dredge kelp forests off the coast of Scotland has led to an outcry from conservationists, who say it would destroy local ecosystems.
Ayr-based company Marine Biopolymers has approached Marine Scotland to apply for a licence to use a comb-like device that pulls entire kelp plants from the bed. In order to inform the environmental appraisal required by Marine Scotland, Marine Biopolymers has published a report describing the potential environmental impacts to be researched further for a full assessment.
Rake used to harvest L. hyperborea The harvesting technique proposed by MBL is similar to that used in Norway for L. hyperborea. It comprises use of a comb-like harvesting head (3-4 m wide) that is situated within a curved frame . It is deployed from a vessel, and trawled through the kelp bed at approximately 0.5 m above the rock substrate at a speed of around 3 knots. Photograph: Steen, H., Moy, F.E., Bodvin, T./Institute of Marine Research
Public comment on this report is open until Friday 24 August, and various stakeholders have expressed concern over the proposals. “Kelp habitats are vital ecosystems that absorb the power of waves along stormy coasts, lock up millions of tonnes of carbon every year and provide shelter for hundreds of species,” said Calum Duncan of the Marine Conservation Society.
“This scoping report is only the first stage of an extensive consultation process,” said a statement issued by Marine Biopolymers. “The next stage is the full environmental survey, which will be carried out by internationally renowned scientists.”
The report describes plans to harvest up to 34,000 tonnes of kelp per year, an estimated 0.15% of the kelp in Scotland. Proposed sustainability measures include plans to avoid harvesting young kelp, and to leave harvested beds to recover for five years before returning.
However, these proposals may not be truly sustainable, according to Dan Smale, an ecologist at the Marine Biological Association. “I’m not opposed to wild kelp harvesting if it’s managed appropriately, and it’s been shown from both Norway and France that to an extent it can be done sustainably,” he said. “My problem here is that we don’t have enough baseline ecological information or understanding of how our systems work [in Scotland].” The recovery rate of five years may be insufficient not only for the kelp itself to recover, but also for associated animal communities to return, he explained.
Ailsa McLellan, who harvests kelp by hand, is concerned about the precedent set by granting a licence to Marine Biopolymers, given the lack of legislation protecting wild seaweed from unsustainable harvesting: “Even if they’re the most careful company in the world, there’s no pressure on anyone else to do it that way.” There is also a conflict with the strict rules applied to hand harvesters, she added: “I have to record every single invertebrate bycatch. It can’t be one rule for us tiny operators and they’re allowed to go at it with a dredge.”
The Marine Conservation Society supports the exploration of more sustainable alternatives to dredging. “Mechanically stripping swaths of pristine kelp forest clean from the reef at the scale proposed simply cannot be considered sustainable,” said Duncan. “We would urge a complete rethink and lower impact alternatives, such as managed hand-gathering and seaweed culture, to be explored instead.”
Kelp forests grow predominantly on the Pacific Coast, from Alaska and Canada to the waters of Baja California. Tiered like a terrestrial rainforest with a canopy and several layers below, the kelp forests of the eastern Pacific coast are dominated by two canopy-forming, brown macroalgae species, giant kelp (Macrocystis pyrifera) and bull kelp (Nereocystis leutkeana).
A host of invertebrates, fish, marine mammals, and birds exist in kelp forest environs. From the holdfasts to the surface mats of kelp fronds, the array of habitats on the kelp itself may support thousands of invertebrate individuals, including polychaetes, amphipods, decapods, and ophiuroids.
California sea lions, harbor seals, sea otters, and whales may feed in the kelp or escape storms or predators in the shelter of kelp. On rare occasions gray whales have been spotted seeking refuge in kelp forests from predatory killer whales. All larger marine life, including birds and mammals, may retreat to kelp during storms or high-energy regimes because the kelp helps to weaken currents and waves.
