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.