MORPHODYNAMICS OF THE AMAZON TIDAL-RIVER FLOODPLAIN ALONG THE FLUVIAL–TIDAL TRANSITION

The Amazon River is the world’s largest source of freshwater and sediment. These fluxes are estimated based on data from the lowermost non-tidal gauging station at Óbidos, ∼800 km upstream of the Atlantic Ocean. Depositional environments along the lengthy tidal river downstream of Óbidos have been proposed as important sinks for up to a third of the reported sediment discharge from the Amazon River. However, the morphology and dynamics of the intertidal floodplain have yet to be described. River-bank surveys in five areas along the Amazon tidal river reveal a distinct evolution in bank morphology between the upper, central, and lower reaches of the tidal river. The upper tidal-river floodplain is defined by prominent natural levees that control the transfer of water and sediment between the mainstem Amazon River and its floodplain. Increased tidal influence in the central tidal river suppresses levee development, and tidal currents increase sediment transport into the distal parts of the floodplain. The floodplain morphology in the lower tidal river closely resembles marine intertidal environments (e.g., mud flats, salt marshes), with dendritic tidal channels incising elevated vegetated flats. Theory, morphology, and geochronology suggest that the dynamics of sediment delivery to the intertidal floodplain of the Amazon tidal river vary along its length due to the relative dominance of fluvial and tidal influence.