Managing Sediment in the Mekong River Basin: Tradeoffs between Hydropower and the Environment

The Mekong/Lancang River Basin is undergoing a period of rapid hydropower development. Construction of more than 20 dams is currently underway just in the lower Mekong Basin, with at least an additional 42 dams planned for construction in the next 20 years. The resulting alteration of the basin's natural flow and sediment regimes, geomorphologic makeup, and fish migration patterns could significantly affect the exceptional biodiversity and food production characteristics of the basin's ecosystems. This study is devoted to assessing the improvement in sediment passage (compared with current and expected conditions) that could be achieved at various locations in the basin by implementing reservoir sediment management practices such as sediment bypasses, drawdown flushing, seasonal drawdown routing, and sluicing. Specific sediment management options are discussed for selected reservoirs in the Sre Pok, Se San, and Se Kong sub-basins (called the "3S" basins). These sub-basins are particularly important with respect to biodiversity and ecological productivity and serve as an important source of flow, sediment and nutrients to the mainstream Mekong River. Simulations are performed with the Sediment Simulation Screening (3S) Model. Using a daily time step, this model simulates the mass-balance of flow and sediment to predict in relative terms the spatial and temporal accumulation and depletion of sediment in river reaches and in reservoirs under different sediment management policies. It also identifies the relative tradeoffs between hydropower output and reliability and sediment regime alteration for these sediment management options.