Sediment load crisis in the Mekong River Basin: Severe reductions over the decades

The boom in water infrastructure in the Mekong Basin has raised concerns over the annual supply of water and sediment among its riparian communities. By consolidating various datasets, continuous series of sediment load data were estimated for several stations located within the Lower Mekong Basin. At Chiang Saen in Thailand, the nearest station to the Chinese dams, the average sediment load was 79 +/- 32 Mt./yr during the pre-dam period of 1965-1991. However, from 2010 to 2019 - during which a series of mega-dams were built in China - the sediment load decreased drastically by 84 % to only 12.5 +/- 4.6 Mt./yr. This phenomenon of reduced annual sediment load during the mega-dam era (2010-2019) as compared to during the pre-dam era (1965-1991) can be observed at stations downstream from Luang Prabang (-53 %) to Nong Khai (-62 %) to Khong Chiam (-33 %). One of the drivers of this sediment load crisis is the rapid development of upstream dams. Especially after 2003, Chinese dams have reduced sediment supply to the downstream Mekong Basin severely. Concurrently, there was an increase in sediment contribution from the stretch of the Mekong River from Chiang Saen to Khong Chiam. A positive outcome of this increased sediment contribution was its buffering effect against the reduction in sediment load from the Chinese part of the Mekong Basin. Although sediment load at Kratie - the gateway station before the alluvial stretch comprising the Cambodian floodplains and Vietnam Mekong Delta - decreased from 78 +/- 22 Mt./yr (1995-2009) to only 60 +/- 21 Mt./yr (2010-2019), the reduction would have been higher without the increased sediment from the Chiang Saen - Khong Chiam stretch. However, with upcoming planned dams in Laos and Cambodia, this buffering effect is likely to be temporary, implying that the sediment load crisis as already experienced by the downstream communities can only become more severe.