3:30 PM - 4:45 PM
[HCG11-P10] Fluvial to tidal transition in the Mekong River delta, Vietnam
Keywords:Mekong River, fluvial-tidal transition, estuary
To better quantify the influence of river discharge and tides on river-bottom sediments, we collected >250 surface samples from distributary channels across the entire delta in Vietnam from the border to the five river mouths during the dry season (January–May 2015), and from one distributary channel from the border to the river mouth during the wet season (October 2015) with simultaneous CTD measurements. In the dry season, river- and tide-dominated areas can be spatially distinguished by the sedimentary facies (grain size and sedimentary structures) of channel bottom sediments. Tidal rhythmites (sand-mud couplets) are common as far as ~100 km upstream from the river mouths, and mud balls are often found in the middle reaches of distributaries. The spatial distribution of river- and tide-dominated areas is closely linked to channel morphology. The thalwegs of river channels show deepening trends, with large variations, downstream from the border and suddenly change to shallowing trends near their mouths. Coarse sediments are found mostly in regions with a deepening trend and sand-mud couplets are found in regions with a shallowing trend. During the wet season, most of the rivers are occupied by freshwater, and the 0.5 pss salinity line is only ~5 km upstream from the river mouth. Nevertheless, sand-mud couplets are recognized up to ~100 km upstream from the river mouth in this freshwater environment.