Brahmaputra River: The Brahmaputra River originates in a great glacier mass in Kailas range of the Himalayas (elevation 5300 m) and flows through China, India and Bangladesh for a total distance of 2880 km before emptying into the Bay of Bengal jointly with the Ganges (Figure 1). It drains a combined international area of approximately 580,000 km2. It is the fourth largest river in the world in terms of average discharge at the mouth and second only to the Yellow River in China in the amount of sediment transported per unit drainage area.
In India, the Brahmaputra River flows southerly and westerly through the states of Arunachal Pradesh and Assam over a distance of approximately 800 km. In the Himalayan range, before entering India, the river is known as the Tsangpo River flowing west to east, then south through the eastern Himalayas as the Dihang River. In Assam, the Dihang River is joined by other tributaries to form the Brahmaputra River. Near the western boundary of Assam, the river turns south to enter Bangladesh changing its name to Jamuna till its confluence with the Ganges from where both the Jamuna and Ganges form the Padma flowing into the Bay of Bengal.
The total length of the river in Bangladesh is approximately 240 km. The river has a gradient of 0.09 to 0.17 m/km near Dibrugarh, Assam at the head of the Valley and it is reduces to about 0.1 m/km near Guwahati. Through Assam, the long-term average discharge increases from 8,500 to 17,000 cubic meters per second as flows are augmented by major tributaries. The width of the river varies from one km on an individual channel to as much as 10 km in some reaches with multiple braided channels.
Almost through its entire length in Assam, the river has three to six channels separated by islands and mid-channel bars under low flow conditions. These bars and islands become submerged during major floods. The pattern of channels changes frequently under flood conditions accompanied by extensive erosion of banks and disposition of sediment forming sand bars.
The geology of the Brahmaputra system in Assam Plain comprises of the Higher and Lesser Himalayas. The Higher Himalayan rocks consist of schists and marbles with amphiboles at some locations. The lesser Himalaya in the Brahmaputra system drainage is composed of quartzite and schist. The Brahmaputra valley in Assam is underlain by recent alluvium (approximate 200-300 m thick) consisting of clay, silt, sand, and gravels. The present configuration of the Brahmaputra valley in Assam is known to have evolved during 2 million years of Pleistocene and recent era.
The valley and its adjoining highlands constitute an extremely unstable seismic region. Two geological factors are ruling in determining the morphological character of the river in Assam. Firstly, the Himalayan ranges to the north are uplifting at a rate of the order of one meter per century. Secondly, the whole region is subject to frequent seismic movements and periodic major earthquakes. The recent earthquakes of 1897 and 1950 in Assam, both of magnitude 8.7 on the Richter scale, were among the largest within historical experience anywhere in the world. The 1950 earthquake seriously affected the territory in Assam causing floods and erosions.
Below is an example of the typical braided channels of the Brahmaputra near Dibrugarh, Assam: