Spatial variability in erosion in the Brahmaputra basin: causes and impacts

Abstract

The rivers from the Himalaya supply large quantities of particulate and dissolved materials to the oceans. Among the various rivers, the Brahmaputra ranks highest in contributing to the sediment budget of the Bay of Bengal. The erosion rates among the sub-basins of the Brahmaputra vary over 1-2 orders of magnitude, the highest being in the Eastern Syntaxis basin which is eroding at an enormously high rate of ∼14 mm yr^-1 , caused by the high stream power of the Siang river. These contemporary erosion rates are consistent with the time-averaged erosion and exhumation rates derived for ∼100 ka based on cosmogenic isotopes and geophysical methods. Both the Eastern and the Western syntaxes experience rapid erosion suggesting that the syntaxes, a characteristic feature of the collision belt, undergo rapid erosion under favourable conditions. The rapid erosion of the Eastern Syntaxis has caused important tectonic and geomorphological changes, such as the rapid uplift of this region resulting in the great peaks of Namche Barwa and Gyala Peri, and the nickpoint in the Tsangpo river bed prior to its entrance to the gorge. Further, the rapid erosion makes up about half of the particulate material transported by the Brahmaputra to the Bay of Bengal, an order of magnitude higher than its areal coverage. The high sedimentation rate in the Bay of Bengal over the past ∼1 Ma can be due to the high erosion rate of the Eastern Syntaxis. The mineralogical and isotopic composition of the sediments seems to suggest this inference.