Suspended Sediment Character in the Tidal Mekong River: Observations from LISST Profiling

In two recent cooperative field campaigns, teams of researchers from the US and Vietnam collected hydrological and sedimentological data during a low flow season and a high 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, granulometry, degree of flocculation) as measured by a Sequoia Scientific LISST 100X mounted on a profiling CTD. LISST (Type C, 2.5-500 µm size range) casts were collected at five transects in the Song Hau distributary. Four transects were located in the Dinh An and the Tran De channels immediately above the ocean interface with one additional transect located above the channel bifurcation, ~40 km from the river mouth. Casts were collected at multiple stations across each channel transect for 12 hour and 24 hour continuous periods. Stationary ADCP data was collected during each 5-15 minute cast period and used to characterize shear stress. Preliminary results from the LISST suggest that the majority of suspended sediment is in the silt and very fine sand range. Increasing concentrations of all size fractions towards the bed suggests a local sediment source. Bimodal grain size distributions, with the coarser peak in the 150 µm to 250 µm range, are observed frequently, especially in the low discharge study. Grain size frequencies from the high discharge study tend to be more often unimodal. While there was effectively no salinity observed during the October 2014 high flow season, a maximum of 25.8 PSU was observed in the March 2015 low flow season. These results suggest that flocculation is an important process in the Mekong River, particularly during periods of higher salinity.