Managing the Impacts of Reservoirs in the Mekong River Basin

Abstract

The Mekong/Lancang River Basin in China and Southeast Asia is undergoing a period of rapid hydropower development. Despite the basin’s yield of 160 million tons of sediment per year, and the importance of that sediment load to geomorphology, and ecosystem habitat and productivity, very little attention has been paid to reservoir sedimentation and sediment management. This study is devoted to assessing the potential for sedimentation occurring over time in existing and planned dams in the Se San, Sre Pok and Se Kong sub-basins. These three basins (collectively called the “3S” basins) are a particularly important set of adjacent, transboundary watersheds with respect to biodiversity and ecological productivity, and serve as an important source of flow and sediment to the mainstream Mekong River. Daily massbalance simulations of flow and sediment are performed to predict in relative terms the spatial and temporal accumulation and depletion of sediment in river reaches and in reservoirs under different reservoir operation (for hydropower and sediment management) policies. Modeling sediment transport and management requires invoking many assumptions and establishing values for many parameters. This study addresses the sensitivity of the tradeoffs among hydropower production and sediment regime alteration to various assumptions and parameter values. This study finds that 50%-80% (depending on what assumptions are made) of the 3S basins’ sediment load may be trapped in reservoirs, thereby significantly affecting natural sediment regimes. However, even after 100 years of simulated sedimentation, reservoir storage capacities at most reservoir sites were not significantly reduced.