Dam-Induced Sediment Starvation in Deltas: Implications for the Mekong

Abstract:

Dams interrupt the downstream transport of sediment in rivers, accumulating sediment in the reservoir and depriving downstream reaches of their natural sediment load, with serious consequences for many deltas where lack of sediment supply has contributed to accelerated subsidence and coastal erosion. From the Indus to the Yellow, major Asian rivers are undergoing unprecedented rates of dam construction. On the Mekong, 7 dams are under construction on the mainstem in China and 133 proposed (or already built) for the Lower Mekong River and tributaries. Given its naturally high sediment loads (owing to its active tectonism), we can ask how much sediment will these dams trap, how will sedimentation affect reservoir sustainability, and how will sediment trapping deprive downstream reaches of their natural sediment loads and thus create problems of ‘hungry water’ in downstream channels and the delta. The question has been difficult to answer before because of the lack of sediment data for critical tributaries and uncertainties even for the better-documented mainstem. To overcome the lack of data, we adopted a geomorphic approach, delineating nine distinct regions for which we estimated sediment yields based on geomorphic characteristics, tectonic history, and the limited sediment transport data available. We used the 3-W model to estimate cumulative sediment trapping by these dams, accounting for changing trap efficiency over time and multiple dams on a single river system. Under a 38-dam ‘definite future’ scenario, cumulative sediment reduction to the Delta would be 51%. Under full build-out of all planned dams, cumulative sediment trapping would be 96%, ultimately depriving the Delta of nearly all its sediment load. This scenario would have profound consequences on productivity of the river and persistence of the Delta landform itself, and suggests that strategies to pass sediment through/around dams should be urgently explored to prevent or reduce the consequences of downstream sediment starvation.