Towards identification of sediment sources, and processes of sediment production, in the Yarlung-Tsangpo-Brahmaputra River catchment for reduction of fluvial sediment loads

Sedimentation in the Brahmaputra River has led to the widening and shallowing of its channel, resulting in land loss and deposition on agricultural land, exacerbating floods, threatening the viability of flood mitigation embankments, and could even lead to the riverbed becoming higher than the floodplain over much of its length with potentially disastrous consequences when embankments breach. To reduce channel sedimentation, by soil conservation it is first necessary to identify sediment sources. The aim of this paper is to review current knowledge of sources and the processes of sediment production, with a subsidiary benefit of providing a summary of the major geomorphic processes at work in the catchment. From the existing literature and a small amount of new analysis, the quantity of sediment being delivered to the River from major source regions has been identified showing that about 45% comes from the Yarlung-Tsangpo Gorge (in the northeast syntaxis), 40% from the Himalaya and the rest from the Mishmi hills, Indo-Burman Ranges and the Shillong Plateau. While the major sediment producing processes have been identified, insufficient information exists to quantify their relative importance. Similarly, sufficient information from regions within catchments does not exist to design a targeted soil conservation program. If further progress is to be made in identification of sediment sources, a spatially comprehensive strategy is needed. Such a strategy is described which would rely upon geochemical tracers rather than measurement of sediment fluxes. This approach has the advantage of gaining results relatively quickly and can be applied to the entire catchment once a pilot study has been completed and sufficient resources made available. If about half of the current rate of sedimentation in the river could be reduced, given that the sediment coming from the Gorge probably cannot be managed, this could help alleviate part of the land loss and flood problem suffered by the people of the lower catchment.