Connection between macrotidal estuaries along the southeastern Amazon coast and its role in coastal progradation

Sediment transport in the southeastern Amazon coastal zone (SACZ) is substantially affected by tidal asymmetry, seasonal variation of the wind and wave regime and especially rainfall (i.e., river discharge). The regional climate and geological configuration have resulted in numerous estuaries and large mangrove-covered coastal plains that partially divide the estuarine basins, which are connected by tidal channels. Based on the investigation of the Furo Grande tidal channel, we assess the relevance of connectivity on the sediment transport and evolution of the ~8000 km2 SACZ mangrove belt. Extensive sedimentologic and bathymetric surveys, as well as hydrodynamic measurements including water level, water and sediment flow, and physical-chemical characteristics of the water, were undertaken at different seasonal periods. Water-level variation along the channel shows that sediment-transport convergence (STC) tends to be formed within the connecting channel, as the tide propagates from both ends. Morphologic and sedimentologic results show the accumulation trend of fine sediments within the STC zone, and this is relevant for mud retention and further delivery to the mangrove plains, which may eventually lead to channel-connection closure. Despite the STC, results also show that there is an important net flux of suspended sediments between estuaries through the studied channel. However, the connected estuaries of Taperaçu and Caeté have substantial differences regarding riverine discharge, and thus estuarine processes, tidal propagation and sediment transport. Bottom sediments and morphology reflect long-term trends and show STC migration towards the Taperaçu as a result of substantially smaller fluvial discharge in that estuary, and therefore higher sediment accumulation rates, compared to the Caeté estuary. The migration of the STC prevents the closure of the connection, and its maintenance preserves these channels as sediment-flow conduits between estuaries. The connectivity is a key process to redistribute mainly muddy sediments along this coastal sector, which helps to explain the evolution and maintenance of the relatively homogeneous and widespread progradation of mangroves along the SACZ, with uneven mud supply.