Sediment dynamics of a tropical tide-dominated estuary: Turbidity maximum, mangroves and the role of the Amazon River sediment load

We investigate tidal propagation and suspended-sediment dynamics in the Caeté river estuary, Southeastern Amazonian Coastal Zone, addressing gaps in knowledge about sources and mechanisms of sediment entrapment in well mixed, tropical estuaries. Bathymetric and sedimentologic surveys were undertaken during high- and low-discharge periods, and longitudinal and cross-sectional hydrodynamic measurements were made for a range of tide and fluvial discharge conditions. Results show that, from geomorphologic and hydrodynamic perspectives, the lower Caeté River is a typical tide-dominated estuary. There is a sediment-transport convergence in the middle estuary, forming an Estuarine Turbidity Maximum (ETM), and there are substantial displacements of the ETM on lunar and seasonal time scales. Superimposed variations of suspended-sediment concentrations (SSC) related to rainfall and local fluvial discharge (landward end), and the seasonal dynamics of the shelf (seaward end), result in complex suspended-sediment dynamics in the estuary. The source of mud to the estuary is likely the Amazon River plume and its shelf deposits. During the dry season and transitional periods, the estuary imports mud from the shelf and tidal processes accrete it on the mangrove flats. Our results indicate that sediment entrapped within the ETM can accumulate in adjacent mangrove forests, and that fluvial sediment might be of secondary relevance to development of coastal zone morphology on mangrove coasts.