Impacts of Inherited Morphology and Offshore Suspended-Sediment Load in an Amazon Estuary

The geomorphological aspects of coastal environments are influenced by the sea level variation, sediment supply, waves, tides, currents, river discharge, and tectonic evolution. In some cases, tectonic events that occurred millions of years ago still control the morpho- and hydrodynamics of modern depositional environments. The Mocajuba estuary is in the northeastern sector of the Brazilian Amazon coast, where the Amazon River Plume (ARP) provides offshore suspended sediment concentrations (SSC) substantially higher than those provided by the Mocajuba River itself. The aim of this study was to analyze the morphology and SSC dynamics of a tropical tide-dominated estuary, formed by small-scale tectonic faults. Morphology, hydrodynamics, and SSC were measured during spring tides of rainy and dry seasons of two different years. The morphology was assessed via bathymetric surveys. Vertical profiles of depth, salinity, and SSC were collected using a CTD + turbidity sensor. Current velocity and discharge were evaluated on an ADCP transect occupied for 13 h at 20 km from the mouth. The water level variation was measured with five pressure sensor/tidal gauges along the estuary. The Mocajuba estuary is a tide-dominated estuary, with peculiar morphology aspects, such as rectilinear areas instead of the funnel-shaped morphology. Furthermore, due to the structural evolution and the faults caused by the tectonic events, the Mocajuba is a deep estuary, presenting an estuarine circulation similar to fjord systems. The combination between the inherited morphology and hydrodynamics aspects allows the tidal wave propagates without substantial deformation characterizing the synchronous behavior. The seasonal conditions influenced the salinity and SSC data along the estuary. While the salinity levels were higher during the dry season, the SSC was higher during the rainy season. In both seasons, we noticed the influence of the ARP providing fine sediments to the estuary and mangroves adjacent areas. In contrast to other Amazonian estuaries, the Mocajuba estuary does not present an estuarine turbidity maxima zone due to the high deep areas, low current velocities, and low SSC. Nevertheless, a “turbid wedge” was formed near the estuary mouth.