Fine-sediment Dynamics in the Mekong River Estuary, Vietnam

Field studies of fine-sediment transport were carried out in the Mekong River estuary, Vietnam, in the high-flow season of November 1993. In the freshwater region, erosion and deposition of suspended sediment occurred at tidal frequency with a strong down-river transport at a mean velocity of 1 m s−1and mean suspended-solid concentration of about 250 mg l−1. Also, the suspended sediment was mostly fine silt, and the clay fraction accounted for only 15%. The suspended sediment was transported either as individual particles or agglomerated with organic detritus. At the mouth of the estuary, a salt wedge was present but was flushed out of the estuary at low tide. Flow reversal occurred across the pycnocline. A turbidity maximum zone was present at the toe of the salt wedge at flood tide. Most of the suspended sediment was coagulated with little organic matter. The bulk of the suspended sediment was exported to coastal waters, but some sediment returned to the estuary in the salt wedge. Data for the low-flow season are sparse but suggest that partially well mixed estuarine conditions prevail, with salinity penetrating about 40 km inland, carrying fine-sediment up-river to a turbidity maximum zone. Most of this sediment would have been deposited in shallow coastal waters in the previous high-flow seasons. The construction of large hydro-electric dams further upstream on the Mekong River may exacerbate siltation patterns in the estuary