The changing sediment loads of the Hindu Kush-Himalayan rivers: an overview

The rivers originating from the Hindu Kush-Himalayas (HKH) are an important water resource for billions of people in Asia. These rivers used to contribute a large proportion of the global land-ocean suspended sediment flux and the Huanghe and the Ganges/Brahmaputra were characterized by the highest sediment loads of all world rivers. This paper identifies the key sediment source areas and identifies the main causes behind the recent dramatic changes in sediment load. The large river basins of this region can be broadly divided into several critical zones based on their elevations. The high plateau, with an elevation of >3500 m, has or will be affected by melting glaciers and snow as a result of global warming, and this in turn could generate increased sediment loads. The high mountainous areas with elevations ranging from 1000 to 3500 m on the northern side of the HKH and >1000 m on the southern side are the main sediment source areas, due to frequent slope failures and severe surface erosion. The main sediment source areas for some of the large rivers also include areas of lower elevation (around 500 m above sea level) with intensive human activity. Such areas include, for example, the Loess Plateau for the Huanghe, the hilly areas of the Sichuan Basin for the Changjiang, and the dry area in Bagan for the Irrawaddy. Most of these large river systems have been subject to a dramatic decline in sediment loads in recent years, due to both climate change and human impacts. The total sediment load transported from the HKH and neighbouring regions to the oceans has decreased by about half, from about 4.3 Gt year(-1) prior to the 1980s, to similar to 2.1 Gt year(-1) currently.. The ranking of the large Asian rivers in terms of the annual sediment fluxes to the oceans has changed from the Brahmaputra, Huanghe, Ganges, Changjiang, and Irrawaddy in the pre-1980s to the Brahmaputra, Irrawaddy, Ganges, Changjiang, and Mekong in the post-1990s. The major Chinese rivers have become less important than those in the South, especially the Southeast Asian rivers, in terms of land-ocean sediment flux. It is anticipated that the sediment loads of the HKH rivers will continue to change due to intensive economic activity and the rapid pace of climate change in the region and throughout all the main river basins. It is possible that it will take a long time for the rivers to achieve equilibrium or quasi equilibrium. These dramatic changes can also give rise to important problems, including river channel/bank instability, loss of habitats, coastal instability, and sea water intrusion.