Land subsidence is currently considered one of the most serious threats to the Mekong Delta, as it is a silent but continuous process of surface lowering that rapidly increases the risks of flooding, riverbank and coastal erosion, and saltwater intrusion. Unlike sea level rise, which is an external forcing, land subsidence causes the delta itself to “sink,” thereby making the relative water level between the land surface and surrounding water bodies rise much faster. A study by Minderhoud and colleagues shows that over the past 25 years, the Mekong Delta has subsided by an average of about 18 cm due to excessive groundwater extraction; the current average subsidence rate is approximately 1.1 cm/year, with some locations exceeding 2.5 cm/year - many times higher than the rate of global sea level rise.

The primary cause of this phenomenon is the prolonged extraction of groundwater for domestic use, agriculture, aquaculture, and industry. As groundwater is continuously pumped out, pressure within the aquifers decreases, causing the overlying soft clay and muddy soil layers to compact and gradually lose volume, which lowers the land surface. In addition, the reduction of sediment supply from upstream due to hydropower dam systems, in-channel sand mining, and increasing loads from urbanization and transportation infrastructure has reduced the delta’s natural capacity to replenish and compensate for the compressed ground. Therefore, land subsidence in the Mekong Delta is the combined result of both natural processes and unsustainable human development activities.

The consequence of land subsidence is that the already very low-lying topography of the Mekong Delta becomes progressively lower relative to river and sea water levels, making the risk of inundation from high tides, heavy rainfall, and floods markedly greater. As the ground sinks, saline water can penetrate more easily and farther into the inland canal and river network; at the same time, riverbanks, canal embankments, and construction foundations become increasingly unstable, leading to more landslides and infrastructure damage. Many experts have warned that if subsidence rates are not controlled, the combined effects of land subsidence and sea level rise will cause many parts of the Mekong Delta to face frequent flooding even in the absence of extreme weather events.