Changes in the sediment load of the Lancang-Mekong River over the period 1965–2003

Abstract

Changes in the sediment load of a river can have important impacts on river channel evolution, nutrient fluxes, aquatic ecology and delta erosion and sedimentation, and the possibility of changes in the sediment load of the Lancang-Mekong River has attracted increasing concern in recent years. Existing studies present conflicting findings regarding the nature and magnitude of recent changes in the sediment load of the Lancang-Meking River and the authors have attempted to assemble the most reliable data on annual sediment loads for the period 1965–2003, to assess recent trends in the sediment load of the river. The changes in annual sediment load at 7 stations on the river are analyzed. Important sediment contributing areas are found in the reaches between Gajiu and Yunjinghong, Chiang Saen and Luang Prabang and downstream of Nong Khai. The sediment load increased at Gajiu, Yunjinghong and Chiang Saen over the period 1985–1992 because of serious soil erosion caused by the expansion of cultivation, the replacement of natural forest by plantations and land disturbance associated with hydropower dam construction. A marked reduction in sediment load occurred at Gajiu after the impoundment of the Manwan Hydropower dam on the Lancang River, but this reduction was not evident downstream at Yunjinghong and the stations further downstream. Significant increases in sediment load appeared at Mukdahan and Khong Chiam. These contrasting patterns of change reflect the influence of sediment contributions from the intervening catchment areas and channel systems as well as storage and remobilization of sediment from the channel system and the impact of hydraulic works such as irrigation systems. The long term mean annual sediment load of the Mekong River at its mouth is estimated to be ca. 145×106 t a−1, which is lower than previously reported values and it seems likely that this will be reduced in the foreseeable future.