A river on a diet
The Mekong River, known as the Lancang in its upper reaches, carries water and sediment. For millennia, it has transported an immense load of nutrient-rich sediment from the Tibetan Plateau, building and replenishing land along its 4,350-kilometer path to the sea. Before recent development, the river delivered about 160 million tonnes of suspended sediment to the South China Sea annually. The upper basin in China, despite making up only about 20% of the river's total drainage area, historically produced half of this sediment.
A cascade of large hydroelectric dams in China's Yunnan province has fundamentally altered this process. Massive structures, such as the 292-meter-tall Xiaowan Dam and the Nuozhadu Dam, act as enormous sediment traps. The complete cascade of dams on the Lancang is estimated to have a collective trap efficiency of 83% to 86.5%, preventing the vast majority of the upper basin's sediment from ever reaching the lower Mekong. This reduction has effectively cut the total sediment load of the entire Mekong River in half. The water released from the dams, stripped of its solids, becomes "hungry water"—a term used to describe its increased erosive power as it scours the riverbed and banks to pick up a new sediment load.
The shrinking delta
The consequences of this sediment starvation are most acute 2,000 kilometers downstream in Vietnam's Mekong Delta. This vast, low-lying plain, where millions live of people and a global hub for rice and seafood production, was built by millennia of sediment deposition. Now, deprived of its primary building material, the delta is shrinking. With the sediment supply drastically reduced, the natural processes of land replenishment cannot keep pace with coastal erosion and land subsidence.
Satellite monitoring over several decades has documented the dramatic retreat of the delta's coastline. Some areas are losing dozens of meters of land to the sea each year. The problem is compounded by local factors like unsustainable groundwater extraction, which causes the land to sink at rates of up to 2.5 centimeters per year, far outpacing global sea-level rise.
The The loss of sediment means a loss of important nutrients for agriculture and fisheries. The fertility of the delta's floodplains declines, impacting rice yields. In the coastal waters, the reduction in nutrients disrupts the marine food web, leading to a decrease in fish catches that are critical for the regional economy and food security. The entire ecosystem, from the smallest microorganisms to the critically endangered Mekong giant catfish (Pangasianodon gigas), is affected by the river's altered chemistry and flow.