Course Tibet in Tibet The upper reaches of the Brahmaputra River, known as the
Yarlung Tsangpo from the Tibetan language, originates on the
Angsi Glacier, near Mount Kailash, located on the northern side of the
Himalayas in
Burang County of
Tibet. The source of the river was earlier thought to be on the Chemayungdung glacier, which covers the slopes of the Himalayas about southeast of
Lake Manasarovar in southwestern Tibet. From its source, the river runs for nearly in a generally easterly direction between the main range of the Himalayas to the south and the
Kailas Range to the north. 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 River joins the Tsangpo 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
Namcha Barwa in a series of rapids and cascades. Thereafter, the river turns south and southwest and flows through a deep gorge (the "
Yarlung Tsangpo Grand Canyon") across the eastern extremity of the Himalayas with canyon walls that extend upward for and more on each side. During that stretch, the river crosses the China-India line of actual control to enter northern Arunachal Pradesh, where it is known as the Dihang (or Siang) River, and turns more southerly.
Arunachal Pradesh The Yarlung Tsangpo leaves the part of Tibet to enter Indian state of
Arunachal Pradesh, where the river is called Siang. It makes a very rapid descent from its original height in Tibet and finally appears in the plains, where it is called Dihang. It flows for about southward after which, it is joined by the
Dibang River and the
Lohit River at the head of the Assam Valley. Below the Lohit, the river is called Brahmaputra and Doima (mother of water) and Burlung-Buthur by native
Bodo tribals, it then enters the state of
Assam, and becomes very wide—as wide as in parts of Assam. The reason for such an unusual course and drastic change is that the river
is antecedent to the Himalayas, meaning that it had existed before them and has
entrenched itself since they started rising.
Assam The Dihang, winding out of the mountains, turns towards 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 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 apart. As the river follows its braided course through the valley, it receives several rapidly flowing Himalayan streams, including the Subansiri, Kameng, Bharali, 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 Dikhow, and the Kopili. Between
Dibrugarh and
Lakhimpur Districts, the river divides into two channels—the northern Kherkutia channel and the southern Brahmaputra channel. The two channels join again about downstream, forming the
Majuli island, which is the largest river island in the world. At
Guwahati, near the ancient pilgrimage centre of
Hajo, the Brahmaputra cuts through the rocks of the
Shillong Plateau, and is at its narrowest at bank-to-bank. The terrain of this area made it logistically ideal for the
Battle of Saraighat, the military confrontation between the Mughal Empire and the Ahom Kingdom in March 1671. The first combined railroad/roadway bridge across the Brahmaputra was constructed at
Saraighat. It was opened to traffic in April 1962. The environment of the Brahmaputra
floodplains in Assam have been described as the
Brahmaputra Valley semi-evergreen forests ecoregion.
Bangladesh In Bangladesh, the Brahmaputra is joined by the
Teesta River (or Tista), one of its largest tributaries. Below the Tista, the Brahmaputra splits into two
distributary branches. The western branch, which contains the majority of the river's flow, continues due south as the Jamuna () to merge with the lower Ganga, called the
Padma River (). The eastern branch, formerly the larger, but now much smaller, is called the lower or
Old Brahmaputra (). It curves southeast to join the
Meghna River near
Dhaka. The Padma and Meghna converge near
Chandpur and flow out into the Bay of Bengal. This final part of the river is called Meghna. The Brahmaputra enters the plains of Bangladesh after turning south around the Garo Hills right below Dhuburi, India. After flowing past Chilmari, Bangladesh, it is joined on its right bank by the Tista River and then follows a course due south as the Jamuna River. (South of Gaibanda, the Old Brahmaputra leaves the left bank of the mainstream and flows past Jamalpur and Mymensingh to join the Meghna River at Bhairab Bazar.) Before its confluence with the Ganga, 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 Ganga"), flows past Dhaka, the capital of Bangladesh, and joins the Meghna River above Munshiganj.
Hydrology The
Ganges–Brahmaputra–
Meghna system has the second-greatest average discharge of the world's rivers—roughly ~, and the river Brahmaputra alone supplies about 50% of the total discharge. The rivers' combined suspended sediment load of about 1.87 billion tonnes (1.84 billion tons) per year is the world's highest. In the past, the lower course of the Brahmaputra was different and passed through the
Jamalpur and
Mymensingh districts. In an 8.8 magnitude
earthquake on 2 April 1762, however, the main channel of the Brahmaputra at Bhahadurabad point was switched southwards and opened as
Jamuna due to the result of tectonic uplift of the
Madhupur tract.
Climate Rising temperatures significantly contribute to snow melting in the upper Brahmaputra catchment. The discharge of the Brahmaputra River is significantly influenced by the melting of snow in the upper part of its catchment area. This increase in river flow, caused by the substantial retreat of snow, leads to a higher downstream discharge. Such a rise in discharge often results in severe catastrophic issues, including flooding and erosion.
Discharge at Brahmaputra River The Brahmaputra River is characterized by its significant rates of sediment discharge, the large and variable flows, along with its rapid channel aggradations and accelerated rates of basin denudation. Over time, the deepening of the Bengal Basin caused by erosion will result in the increase in hydraulic radius, and hence allowing for the huge accumulation of sediments fed from the Himalayan erosion by efficient sediment transportation. The thickness of the sediment accumulated above the Precambrian basement has increased over the years from a few hundred meters to over in the Bengal fore-deep to the south. The ongoing subsidence of the Bengal Basin and the high rate of Himalayan uplift continues to contribute to the large water and sediment discharges of fine sand and silt, with 1% clay, in the Brahmaputra River. Climatic change plays a crucial role in affecting the basin hydrology. Throughout the year, there is a significant rise in hydrograph, with a broad peak between July and September. The Brahmaputra River experiences two high-water seasons, one in early summer caused by snowmelt in the mountains, and one in late summer caused by runoff from monsoon rains. The river flow is strongly influenced by snow and ice melting of the glaciers, which are located mainly on the eastern Himalaya regions in the upstream parts of the basin. The snow and glacier melt contribution to the total annual runoff is about 27%, while the annual rainfall contributes to about and of discharge. The highest recorded daily discharge in the Brahmaputra at Pandu was August 1962 while the lowest was in February 1968. The increased rates of snow and glacial melt are likely to increase summer flows in some river systems for a few decades, followed by a reduction in flow as the glaciers disappear and snowfall diminishes. This is particularly true for the dry season when water availability is crucial for the irrigation systems.
Floodplain evolution The course of the Brahmaputra River has changed drastically in the past two and a half centuries, moving its river course westwards for a distance of about , leaving its old river course, appropriately named the old Brahmaputra river, behind. In the past, the floodplain of the old river course had soils which were more properly formed compared to graded sediments on the operating Jamuna river. This change of river course resulted in modifications to the soil-forming process, which include acidification, the breakdown of clays and buildup of organic matter, with the soils showing an increasing amount of biotic homogenization, mottling, the coating around Peds and maturing soil arrangement, shape and pattern. In the future, the consequences of local ground subsidence coupled with flood prevention propositions, for instance, localised breakwaters, that increase flood-plain water depths outside the water breakers, may alter the water levels of the floodplains. Throughout the years, bars, scroll bars, and sand dunes are formed at the edge of the flood plain by deposition. The height difference of the channel topography is often not more than . Furthermore, flooding over the history of the river has caused the formation of river levees due to deposition from the overbank flow. The height difference between the levee top and the surrounding floodplains is typically along small channels and along major channels. Crevasse splay, a sedimentary fluvial deposit which forms when a stream breaks its natural or artificial levees and deposits sediment on a floodplain, are often formed due to a breach in the levee, forming a lobe of sediments which progrades onto the adjacent floodplain. Lastly, flood basins are often formed between the levees of adjacent rivers.
Flooding During the monsoon season (June–October), floods are a very common occurrence. Deforestation in the Brahmaputra watershed has resulted in increased siltation levels, flash floods, and soil erosion in critical downstream habitat, such as the
Kaziranga National Park in middle Assam. Occasionally, massive flooding causes huge losses to crops, life, and property. Periodic flooding is a natural phenomenon which is ecologically important because it helps maintain the lowland grasslands and associated wildlife. Periodic floods also deposit fresh alluvium, replenishing the fertile soil of the Brahmaputra River Valley. Thus flooding, agriculture, and agricultural practices are closely connected. The effects of flooding can be devastating and cause significant damage to crops and houses, serious bank erosive with consequent loss of homesteads, school and land, and loss of many lives, livestock, and fisheries. During the 1998 flood, over 70% of the land area of Bangladesh was inundated, affecting 31 million people and 1 million homesteads. In the 1998 flood which had an unusually long duration from July to September, claimed 918 human lives and was responsible for damaging of roads and embankments, and affecting of standing crops. The 2004 floods, over 25% of the population of Bangladesh or 36 million people, were affected by the floods; 800 people died; 952 000 houses were destroyed and 1.4 million were badly damaged; 24 000 educational institutions were affected including the destruction of 1200 primary schools, 2 million governments and private tube wells were affected, over 3 million latrines were damaged or washed away, this increases the risks of waterborne diseases including diarrhea and cholera. Also, of the rice crop was submerged and lost before it could be harvested, with 7% of the yearly
aus (early season) rice crop lost; of grazing land was affected, 5600 livestock perished together with 254 00 poultry and of lost fish production. Flood-control measures are taken by the water resource department and the Brahmaputra Board, but until now the flood problem remains unsolved. At least a third of the land of
Majuli Island has been eroded by the river. Recently, it is suggested that a highway protected by concrete mat along the river bank and excavation of the river bed can curb this menace. This project, named the Brahmaputra River Restoration Project, is yet to be implemented by the government. Recently the Central Government approved the construction of Brahmaputra Express Highways.
Channel morphology The course of the Brahmaputra River has changed dramatically over the past 250 years, with evidence of large-scale avulsion, in the period 1776–1850, of from east of the Madhupur tract to the west of it. Prior to 1843, the Brahmaputra flowed within the channel now termed the
"Old Brahmaputra". The banks of the river are mostly weakly cohesive sand and silts, which usually erodes through large scale slab failure, where previously deposited materials undergo scour and bank erosion during flood periods. Presently, the river's erosion rate has decreased to per year as compared to per year from 1973 to 1992. This erosion has, however, destroyed so much land that it has caused 0.7 million people to become homeless due to loss of land. Several studies have discussed the reasons for the avulsion of the river into its present course, and have suggested a number of reasons including tectonic activity, switches in the upstream course of the Teesta River, the influence of increased discharge, catastrophic floods and river capture into an old river course. From an analysis of maps of the river between 1776 and 1843, it was concluded in a study that the river avulsion was more likely gradual than catastrophic and sudden, and may have been generated by bank erosion, perhaps around a large mid-channel bar, causing a diversion of the channel into the existing floodplain channel. The Brahmaputra channel is governed by the peak and low flow periods during which its bed undergoes tremendous modification. The Brahmaputra's bank line migration is inconsistent with time. The Brahmaputra river bed has widened significantly since 1916 and appears to be shifting more towards the south than towards the north. Together with the contemporary slow migration of the river, the left bank is being eroded away faster than the right bank.
River engineering The Brahmaputra River experiences high levels of
bank erosion (usually via slab failure) and
channel migration caused by its strong current, lack of riverbank vegetation, and loose sand and silt which compose its banks. It is thus difficult to build permanent structures on the river, and protective structures designed to limit the river's erosional effects often face numerous issues during and after construction. In fact, a 2004 report by the Bangladesh Disaster and Emergency Sub-Group (BDER) has stated that several of such protective systems have 'just failed'. However, some progress has been made in the form of construction works which stabilize sections of the river, albeit with the need for heavy maintenance. The
Bangabandhu Bridge, the only bridge to span the river's major distributary, the
Jamuna, was thus opened in June 1998. Constructed at a narrow braid belt of the river, it is long with a platform wide, and it is used to carry railroad traffic as well as gas, power and telecommunication lines. Due to the variable nature of the river, the prediction of the river's future course is crucial in planning upstream engineering to prevent flooding on the bridge. China built the
Zangmu Dam in the upper course of the Brahmaputra River in the Tibet region and it was operationalised on 13 October 2015. ==Tributaries==