SUSPENDED SEDIMENT CHARACTER IN THE TIDAL MEKONG RIVER: OBSERVATIONS FROM LISST PROFILING

In 2015, researchers from the US and Vietnam collected hydrological and sedimentological data during a low flow season on the lower ~100 km of the Song Hau distributary of the Mekong River. The objective of this study is to describe the forcing controls (e.g., tidal and riverine flow, water column stratification, resuspension) on suspended sediment grain size (e.g. mass, volume, degree of flocculation) as measured by a Sequoia Scientific LISST 100X mounted on a profiling CTD that also carried an optical backscatterance sensor for measuring mass sediment concentration. LISST casts were collected at five cross-channel transects in the Song Hau distributary over 12.4 hour and 24.8 hour tidal periods. Stationary ADCP data was collected during each 5-15 minute cast period. Suspended sediment water samples were collected as an independent measure of suspended sediment character and used to determine primary particle grain size. In-situ suspended sediment largely falls within the silt and very fine sand range while the primary particles show a bimodal distribution with peaks in the clay and silt ranges. Sand-sized particles are found at every transect and at every depth sampled; however little to no measurable primary sand particles were found in water samples. Flocculation of the fine-grained particles was observed at every location in both fresh and brackish regions, but is enhanced by salinity. Sediment concentration and grain size generally increase toward the bed, but are influenced by fortnightly tidal phase as well as depth. Salinity stratification at neap tides results in higher concentrations of flocs in the upper water column. Neap tides produce coarser floc sizes and more homogenous sediment concentrations with depth. Over the study area, flocs make up an estimated 50% of the total sediment volume concentration. These results suggest that flocculation is an important component of the sediment dynamics in the Mekong River channel and in sediment delivery to the ocean.