Changes to long-term discharge and sediment loads in the Vietnamese Mekong Delta caused by upstream dams

The discharge and sediment loads of the Mekong River (MR) have been significantly altered because of natural processes and anthropogenic activities. We examined the impacts of dams in the MR basin on changes to the long-term discharge and sediment loads from upstream of dams in China (e.g., Jiuzhou) to the Vietnamese Mekong Delta (VMD) at daily, monthly, and annual scales over a 55-yr period (1961-2015) using various statistical methods. Results revealed sediment trapping in the Lancang cascade dams. To clarify the consequences of the reduced MR sediment load on morphological changes in the VMD, bathymetric data measured in 2014 and 2017 were examined along 100 km of the Tien river. The results show significantly reduced suspended sediment loads at all stations in the lower MR, but the sediment loads at Jiuzhou was increased. Thus, the reduced sediment load is caused not by a reduced sediment supply from the upper MR basin but by sediment trapping in the Lancang cascade dams. The sediment was reduced by 74.1% in the VMD; 166.7 +/- 33.3 Mt/yr occurred in the predam period and 43.1 Mt/yr occurred in 2012-2015, with 40.2% caused by six mainstream dams in the Lancang cascade. Therefore, the Tien river in the VMD was severely incised, with an incision rate of -0.5 m/yr in 2014-2017. Upstream development has caused large-scale morphological changes in the VMD. Sand mining was responsible for a maximum of 14.8% of the annual riverbed incision in the VMD, while the remainder was caused by upstream hydropower dams. (C) 2019 Elsevier B.V. All rights reserved.