Controls on Hydrodynamics, Suspended-Sediment Concentration and Sediment Flux in the Tidal River to Estuarine Reach of the Mekong River, Vietnam

Sediment dynamics within the Song Hau distributary of the Mekong River vary temporally due to fluctuating magnitudes of marine and fluvial forces in this freshwater to estuarine tidal river. Seasonal variations in river discharge and estuarine regime result in export of fine-grained sediment when discharge is high (~1.6 t/s) and import when discharge is low (~0.1 t/s). The movement of fine-grained sediment is influenced by fluctuations in bed stress in the tidal river and the periodic presence of a salt wedge and associated turbidity maximum in the estuary. A fluctuating salinity regime induces variations in flocculation, settling, and trapping of sediment within the river channel. Generally, the river exhibits partially mixed estuarine conditions during times of low river discharge then shifts to salt-wedge conditions during high discharge. Tidal range effects are superimposed on this pattern with neap tides producing a water column with more vertically stratified salinity and suspended sediment concentration than spring tides. Sediment dynamics also vary spatially in this large tropical river. Pre-flocculation of fine-grained particles in the freshwater reach of the tidal river, which extends more than 200 km upstream of the river mouth, aids in settling and trapping of sediment in the lower river reach. Bathymetry associated with bifurcation into two distributary channels near the river mouth influences the spatial distribution and transport pathways of salt and sediment, with one sub-channel having more vertically mixed salinity and suspended sediment. The presence of salinity throughout the water column in this channel almost doubles the percentage of flocculated particles and enhances sediment settling. Offshore currents augment this imbalance between the distributaries by forcing marine water more predominately into one channel based on prevailing seasonal currents.