River sediment supply

Sediment supply from the Mekong River is a fundamental factor governing the geomorphological stability and long-term evolution of the Mekong Delta. Under natural conditions in the past, the river transported large volumes of suspended sediments and bed material—estimated at approximately 160–170 million tons per year—thereby sustaining accretion processes, nourishing coastal zones, and supporting the development of floodplains as well as mangrove ecosystems. This continuous sediment flux enabled the delta to maintain a dynamic equilibrium between deposition and erosion.

However, in recent decades, sediment supply has declined significantly, with an estimated reduction of approximately 70–80% compared to historical conditions. The primary causes are sediment trapping in upstream hydropower reservoirs and the intensification of in-channel sand mining. Hydropower dams retain a substantial portion of sediments, from coarse to fine fractions, thereby greatly reducing downstream sediment flux; meanwhile, sand mining directly removes bed material, further disrupting sediment balance within the system.

The reduction in sediment supply has led to profound changes in sediment transport dynamics across the delta. With decreasing sediment loads, the capacity to replenish material at river mouths and along the coast is severely constrained. As a result, depositional processes have weakened, while erosional forces driven by waves, tides, and currents have become dominant. This shift has contributed to a clear transition from accretion to erosion in many coastal areas of the Mekong Delta.