Recent sedimentation and sediment accumulation rates of the Ba Lat prodelta (Red River, Vietnam)

The Ba Lat River is the major distributary of the Red River system in North Vietnam. To assess the recent to subrecent depositional processes in the Ba Lat prodelta, a detailed sediment analysis was conducted. Bottom samples were collected during two field surveys, one in the dry season (winter) and one in the wet season (summer). A steep frontal prodelta slope is characterized by very rapid sedimentation (tens of cm per year) of muddy sediments under influence of the turbid river plume. Beyond direct influence of the river plume the bottom slope decreases and bottom transport by the prevailing southward directed currents becomes important. Coarse-grained tempestites alternate with the dominating muddy sediments. Downcore changes in the 234Th activities indicate that the subaqueous delta progrades to the southwest, with erosion and reworking of older sediments occurring north of the present outlet. The southwestward progradation is also encountered in the trend of 210Pb activities indicating that this process has continued for at least 100 years. Avulsion of the Ba Lat outlet in 1973 has led to a decrease in sedimentation rates north of the Ba Lat outlet.