Sedimentary processes building a tropical delta yesterday, today, and tomorrow: the Mekong System

River deltas are important for human habitation, commerce, food, and natural resources. Most terrestrial freshwater, dissolved substances, and suspended sediment supplied to the ocean pass through delta distributary channels. Transitions are complex as this river discharge moves through the serial environments of delta systems: tidal river, estuary, shoreline, continental shelf. The bulk of Mekong sediment accumulates as a muddy clinoform deposit on the shallow continental shelf (<20–25 m water depth), which forms the foundation over which the subaerial delta surface has grown for the past ~8,000 years. The Song Hau distributary channel, the target of this investigation, receives ~40% of the Mekong discharge and transfers the majority of it to the adjacent shelf during high flow of the river (July–November). Some of the sediment is returned to the channel and to the mangrove shoreline during low flow of the river (December–April). The sediment reentering the channel is mostly mud, deposited by estuarine processes, that temporarily buries the channel bed and interrupts sand transfer to the coastal ocean. Trapping of sediment supplied to the shoreline of the island, Cu Lao Dung, at the Song Hau mouth, is enhanced by the roughness from dense mangrove roots extending above the bed. Shoreline progradation is asymmetric with the most rapid sediment accumulation (~5 cm yr–1) in the southwestern portion of the island, and the distribution of mangroves is linked to this sedimentation pattern. About one-third of Mekong sediment discharge accumulates in the shelf clinoform near the mouths of the distributary channels, with the greatest accumulation rates (>10 cm yr–1) in the relatively steep foreset region. Intense landward and southwestward currents transport the other two-thirds of Mekong discharge during energetic shelf conditions (December–April). These sediments create a relatively shallow clinoform structure, cause the delta to grow asymmetrically toward the southwest, and form the Ca Mau Peninsula. In the future, these same natural processes will operate under different conditions. Construction of many dams within the drainage basin (>200 constructed or approved) and the impacts of climate change (i.e., alterations in monsoonal conditions) will significantly decrease Mekong River discharge. In addition, the delta land surface will be flooded due to acceleration of eustatic sea level rise and local land subsidence. Together, loss of river discharge and rise of local sea level will cause many secondary impacts, including erosion of distributary channels, ocean shorelines, and the shelf seabed; saltwater intrusion farther into the channels, along with transfer of associated estuarine processes; and decreased supply of freshwater and solutes, including nutrients, to the coastal ocean. The collaborative research among international and Vietnamese scientists described in this special issue of Oceanography provides an integrated understanding of the Mekong Delta system, and could help formulate strategies to enhance the resiliency of the system and its ability to cope with future impacts