The result of this analysis of sediment load from this study aligned with the global context of the sediment trend of the major rivers in the region and the world (Vörösmarty, Meybeck, Fekete, Sharma, Green, and Syvitski (2003) More than 40% of global river discharge is intercepted locally by the large reservoirs and maintains a theoretical sediment trapping efficiency in excess of 50%. D. Walling and Fang (2003) assessed trends in the sediment loads for 145 of the world’s rivers, and concluded that most showed decreasing sediment loads. Based on annual runoff and sediment data to 2005, C Liu et al. (2007) found that major rivers in Southern China (e.g., Yangtze, Qiantang, and Pearl rivers) were transporting only about 60%–80% of their prior loads, while their annual runoff remained relatively stable. The most important influence on land-ocean sediment fluxes is probably the reservoir and dam construction, but the influence of other controls resulting in increased sediment loads could also be detected, H. Gupta, Kao, and Dai (2012) found that the combined annual sediment flux of the large Chinese rivers has been reduced from 1800 Mt to about 370 Mt in the last 50 years. From the mid-1990s to early 2000s dam development accelerated in China and Vietnam with the construction of mainstream dams on the lower Mekong and tributary dams (G. M. Kondolf et al., 2018).
A decline in sediment flux along the Lower Mekong River carries many implications downstream (Kite, 2001). In the Mekong River basin, a recent study on the effects of rapid development dams on sediment by Kummu, Lu, Wang, and Varis (2010) demonstrated that more than 50% of the total Mekong River sediment load (∼140 Mt) will be trapped annually if the entire cascade of eight dams is constructed on the upper Mekong (Lancang), such as Manwan reservoir, which trapped around 60.5 % of the total sediment load transported between 1993 and 2003, causing significant effects on sediment variation downstream (Fu, He, & Lu, 2008). The Xayaburi, Don Sahong, and Pak Beng dams are the first three of 11-12 dams planned for the mainstream in the lower basin (Fox & Sneddon, 2019). Another proposed hydroelectric dam (Sambor dam) on the main Mekong River located at Kratie Province, Cambodia is expected to prevent significant sediment quantities from reaching the Tonle Sap Lake and the Mekong Delta (Wild & Loucks, 2015). Under a scenario of 38 dams built and under construction (main river and tributary), about 77 Mt/yr of sediment would be delivered to Sambor from upstream of which it would trap about 38 Mt/yr, significantly affecting sediment delivery to downstream reaches (G. Kondolf, Rubin, & Minear, 2014). The concern stems not only from dam development on Mekong mainstem but also on its main tributaries. For example, The Srepok, Sesan, and Sekong basin (the 3S basin), the largest tributary to the Mekong, represent a significant portion of the sediment load reaching critical Mekong ecosystems such as the Vietnam Delta and Cambodia’s Tonle Sap Lake (Wild & Loucks, 2014), but this has been cut off by the recently completed Lower Sesan 2 dam, which blocks the Srepok and Sesan. The changes in natural sediment dynamics would threaten the long-term stability of the Mekong Delta (Ian C Campbell, 2007; Saito, Chaimanee, Jarupongsakul, & Syvitski, 2007). Beside the dam development in Mekong River Basin, climate change and land use change are also the crucial factors to the sediment transport in the basin. Deforestation accelerates erosion, increasing river sediments heading to reservoirs and decreasing hydropower production in a watershed of the Mekong River (Kaura, Arias, Benjamin, Oeurng, & Cochrane, 2019). Shrestha et al. (2013) indicate high uncertainties in the direction and magnitude of changes of discharge as well as sediment yields due to climate change in a watershed of the Mekong River.