Process regime, salinity, morphological, and sedimentary trends along the fluvial to marine transition zone of the mixed-energy Mekong River delta, Vietnam

Sediment distribution and depositional processes along the fluvial to marine transition zone of the Mekong River delta, Vietnam

“…Based on sediment‐type associations, the fluvial to marine transition zone (FMTZ) and the delta itself could be subdivided into an upstream part and a downstream part; the boundary between these two parts is identified 80 to 100 km upstream of the river mouth (Figs , and ). This sedimentological boundary coincides with a change in geomorphological configuration of the distributary channels, because downstream of this area, they tend to become straighter, wider and shallower (Figs , and ; Oketani & Haruyama, ; Brunier et al ., ; Gugliotta et al ., ).…”

“…The grain‐size range suggests that sediment was mostly supplied in this area as suspension load (Bridge, ; Wilkerson & Parker, ), but it was locally transported and deposited as mixed bed‐load and suspended‐load by tidal and fluvial–tidal currents (Allison et al ., ). Coarser bed‐load sediments could rarely reach the downstream tract because of the seaward decrease in fluvial energy and probably also because of the geomorphological configuration of the distributary channels, with the shallowing profile along their downstream tract (Figs , and ; Oketani & Haruyama, ; Brunier et al ., ; Gugliotta et al ., ) possibly inhibiting this type of transport. Nonetheless, the presence of occasional gravelly sand (S1) in this area (Figs , , and ) might indicate the occurrence of episodic bed‐load sediment supply during stronger river floods, although part of this coarse‐grained sediment could also be derived from local erosion of the bedrock and banks or sand‐mining activities.…”

“…A). Tide‐induced water level changes are observed only up to Chau Doc and Tan Chau, ca 200 km upstream of the river mouth (Gugliotta et al ., ). Salinity of 0·5 PSU was measured ca 15 km upstream of the river mouth (Gugliotta et al ., ), whereas the freshwater input affects salinity in the South China Sea for more than 200 km outward of the river mouths (Hordoir et al ., ).…”

“…() and Gugliotta et al . (). The figure also shows seasonal variations in fluvial and wave processes and the general sediment dynamic in the delta.…”

“…The area of coastal rivers, either deltas or estuaries, affected by a combination of fluvial and marine (i.e. tidal and wave) processes is known as the fluvial to marine transition zone (FMTZ) and can extend up to several hundreds of kilometres along the river depending on the relative strength of fluvial and tidal processes and the coastal plain gradient (Dalrymple & Choi, 2007;Dalrymple et al, 2015;La Croix & Dashtgard, 2015;Gugliotta et al, 2017). The FMTZ extends from the landward limit of any tidal effect (upstream of which the system is purely fluvial) to the river-mouth area (Dalrymple & Choi, 2007;Jablonski & Dalrymple, 2016); however, in systems with relatively high river discharge, the interaction of fluvial and marine processes extends up to hundreds of kilometres outward of the river mouth (Ayranci et al, 2012;Xue et al, 2012;Loisel et al, 2014).…”