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Where the Mekong River discharges into the East Sea (also known as the South China Sea), it has formed the world's third largest delta plain with an area of ~50,000 km2. Numerous cores recovered from the subaerial delta reveal that it has prograded ~220 km southeastward within the past 7,500 years. Recent extensive seismic and geochemical surveys of the adjacent subaqueous delta indicate that the Mekong River forms a classic sigmoidal, cross-shelf clinoform immediately off its distributaries that is up to 15 m thick, with topset, foreset, and bottomset facies. These deposits are constrained within water depths of 20 m. Mekong-derived sediment packages extend ~300 km along shelf in the southwestward direction to the tip of the Ca Mau Peninsula, where they form a distal mud depocenter up to 22 m thick. These sediment packages can also be traced into the Gulf of Thailand to water depths of 25 m. The proximal and distal deposits cover ~11,000 km2 of the shelf. Historically, the Mekong Delta has prograded seaward at a mean rate of >30 m yr-1, or 7 km2 yr-1; however, study of the past 43 years of Landsat images indicates that the mode of sedimentation in the delta shifted starting in 2005. From 1973 to 2005, the Mekong Delta's seaward shoreline growth decreased gradually from a mean of 7.8 m yr-1 to 2.8 m yr-1, and after 2005 it became negative, with a retreat rate of -1.4 m yr-1. The net deltaic land area gain has also been slowing, with the mean rate decreasing from 4.3 km2 yr-1 (1973-1979) to 1.0 km2 yr-1 (1995-2005), and then to -0.05 km2 yr-1 (2005-2015). Thus, in about 2005, the subaerial Mekong Delta transitioned from a constructive mode to an erosional (or destructive) mode. Furthermore, not only is the subaerial Mekong Delta land area gradually diminishing, but high-resolution CHIRP sonar profiling surveys off the east-central Ca Mau Peninsula reveal that this portion of the subaqueous delta is also eroding. With the construction of more dams, sand mining, delta subsidence, increasing storms, and sea level rise, the Mekong Delta will likely face more destructive changes, with erosion both of coastlines and underwater deposits. [ABSTRACT FROM AUTHOR]

As international concern for the survival of deltas grows, the Mekong River delta, the world's third largest delta, densely populated, considered as Southeast Asia's most important food basket, and rich in biodiversity at the world scale, is also increasingly affected by human activities and exposed to subsidence and coastal erosion. Several dams have been constructed upstream of the delta and many more are now planned. We quantify from high-resolution SPOT 5 satellite images largescale shoreline erosion and land loss between 2003 and 2012 that now affect over 50% of the once strongly advancing >600 km-long delta shoreline. Erosion, with no identified change in the river's discharge and in wave and wind conditions over this recent period, is consistent with: (1) a reported significant decrease in coastal surface suspended sediment from the Mekong that may be linked to dam retention of its sediment, (2) large-scale commercial sand mining in the river and delta channels, and (3) subsidence due to groundwater extraction. Shoreline erosion is already responsible for displacement of coastal populations. It is an additional hazard to the integrity of this Asian mega delta now considered particularly vulnerable to accelerated subsidence and sea-level rise, and will be exacerbated by future hydropower dams. [ABSTRACT FROM AUTHOR]

Three radiocarbon-dated sediment cores from the northeastern Vietnamese Mekong River Delta have been analysed with a multiproxy approach (grain size, pollen and spores, macro-charcoal, carbon content) to unravel the palaeoenvironmental history of the region since the mid Holocene. During the mid-Holocene sea-level highstand a diverse, zoned and widespread mangrove belt (dominated by Rhizophora) covered the extended tidal flats. The subsequent regression and coeval delta progradation led to the rapid development of a back-mangrove community dominated by Ceriops and Bruguiera but also represented locally by e.g. Kandelia, Excoecaria and Phoenix. Along rivers this community seems to have endured even when the adjoining floodplain had already shifted to freshwater vegetation. Generally this freshwater vegetation has a strong swamp signature but locally Arecaceae, Fabaceae, Moraceae/ Urticaceae and Myrsinaceae are important and mirror the geomorphological diversity of the delta plain. The macro-charcoal record implies that natural burning of vegetation occurred throughout the records, however, the occurrence of the highest amounts of macro-charcoal particles is linked with modern human activity. [ABSTRACT FROM AUTHOR]