Impacts of Channel Depth and Estuarine Stratification on Particle Aggregation and Deposition

Robin Lydia McLachlan1, Andrea S Ogston1, Mead A Allison2,3 and Diana Di Leonardo4, (1)University of Washington, School of Oceanography, Seattle, WA, United States, (2)Tulane University of Louisiana, New Orleans, United States, (3)Tulane University, River-Coastal Science and Engineering, New Orleans, United States, (4)The Water Institute of the Gulf, Baton Rouge, United States
Aggregation of suspended sediment promotes settling and deposition of fine particles and is enhanced by large particle concentrations, moderate shear stresses, and salinity. Thus, vertical profiles of suspended sediment, turbulence, and salinity influence where sediment can deposit. In-situ fluxes were measured along two distributaries within a tide-dominated, large tropical delta ̶ Vietnam’s Mekong Delta ̶ to determine the combined impacts of channel depth and estuarine stratification on sediment aggregation and deposition along a fluvial-to-estuarine reach. Vertical profiles were measured over a tidal cycle with CTD and LISST 100X casts, and suspended-particle size and settling were video recorded within an onboard settling chamber. These data show that aggregates were present throughout the study region, from the fresh tidal river to the saline estuary. Most particles >40 μm in diameter were aggregates with a lower effective density than their discrete constituents. During ebb and flood tidal phases, the shallower distributary channel hosted larger, faster-settling particles than the deeper channel. When estuarine conditions were present, the shallower channel was more vertically mixed, likely due to its lesser depth and freshwater flux, and, thus, saline water catalyzed aggregation more uniformly throughout the entire water column. These results suggest that suspended particles are more likely to deposit within relatively shallow channels, where aggregation is enhanced and settling distances are shorter. Additionally, continued dam installation along the Mekong River, and other large tropical rivers, will further promote aggregation, deposition, and channel infilling by decreasing the fluvial discharge and enhancing estuarine intrusion and vertical mixing.