Large tropical river mouth sediment-transport dynamics and linkages to coastal sediment transport and deposition
Abstract:Rivers are the largest suppliers of particulate material to the world ocean, and we must understand the dynamics of tidal river and shallow coastal environments in order to contrast and constrain the sediment budgets of major rivers and their adjacent marine sinks. Studies in large tropical tidal rivers elucidate the processes in these regions that modify the source signal of sediment particles to nearshore environments and vegetated shorelines. Here we focus on processes within the river mouth region of the Amazon and Mekong tidal rivers in order to understand the modulation of sediment-transport signals (magnitude, grain size, pathways) between the river mouth regions and coastal environments.
The immense Amazon River mouth experiences no salinity effects (e.g., buoyancy driven flows, physiochemical aggregation) as estuarine processes occur offshore. Mouth channel-bed morphology is variable, and although near surface suspended-sediment concentration (SSC) is relatively constant across the 15-km distributary width, near-bed SSC ranges widely over the tidal cycle depending on sub-channel sediment routing. Sediment pathways differ between ebb and flood conditions inducing a spatially and temporally heterogeneous supply of sediment to the nearshore. In the Mekong River, the mouth region changes from salt-wedge conditions to a partially mixed estuary with seasonal discharge, and there has been an anthropogenic reduction in connections between river and intertidal floodplains. SSC varies throughout the water column and is seasonally modified by estuarine processes. Differences between ebb and flood flows and sediment pathways within and between distributary channels are important to the overall sediment budget, and water-column processes seasonally modify the characteristics of the river bed. These two river-mouth systems differ in channel morphology and sediment-transport mechanisms. Both systems, however, deliver sediment to the nearshore in a spatially and temporally variable manner that is modulated by tidal-river processes. Climate change, sea-level rise, and increased anthropogenic modification of the shoreline necessitate increased understanding of the linkages between processes in the river and those on the adjacent coast.