The Brahmaputra Basin

The landscape of upper Brahmaputra basin (Tibet, Bhutan and Arunachal Pradesh) is formed by the collision of Indian and Eurasian Plate. The colliding plates have built the highest mountain range in the world while, at the same time, shaping the land downstream (Assam valley and Bangladesh). The waste products of this collision, sediments eroded from permanently rising and highly unstable mountain slopes, were transported downhill through a multitude of rivers and started accumulating along the foothills 45 million years ago. The impact of the collision is so great that both plates still converge at a rate of around cm per year and still continue building the landscape. As the Himalayas condition the monsoon patterns, extreme rainfall takes place creating some of the biggest rivers in the world, the Brahmaputra being one of them. Assam is situated in front of the eastern foothills formed by this plate collision in an area of main disturbances and fault lines where from the Brahmaputra starts. The mountain reach here, also called Eastern Syntaxis, is an area of highest uplift, steepest slopes and with world-wide highest observed surface erosion rates of upto 14 mm per year.

The Brahmaputra basin is unevenly distributed between four countries. While China covers half of the basin area of around 580,000 km² it only provides little more than one quarter of the total annual volume of water of around 600 Bm³ (or Km³). The reason being the high mountain range of the Himalayas, which block the annual monsoon movement to the north, resulting in only a few 100mm of rainfall over Tibet. In contrast India and Bhutan cover about 40% of the basin area but provides around two third of the annual volume of water.