Seasonal hydro-sedimentary dynamics of a small tropical estuary under Amazone influence (the Cayenne river estuary, French Guiana)

The Cayenne River estuary is a small mesotidal tropical estuary located in the French Guiana (04° 54’ N / 52° 21’ W). It forms at the confluence between the Cayenne and Montsinery rivers, and is one of the numerous estuarine systems situated between the mouths of the Amazon and the Orinoco Rivers, likely to be controlled by coastal mud banks formed by suspended sediment discharge of the Amazon river. This estuary drains a small coastal catchment composed of mangrove forests. Dynamics of this estuary are described for the first time from oceanographic and sedimentary data collected in 2022 during dry and rainy seasons. Surveys consisted of 13h anchor stations at three locations distributed across the main channel, at 5 km from the mouth, where vertical profiles of salinity and turbidity were measured every 30 mn. Complementary vessel-towed ADCP transects were performed in order to capture the current field and residual flow in the same cross section. The data collection was performed during one semidiurnal tidal cycle at neap and spring tides. Water levels records indicate that frictional forces dominate, since the tidal range decreases from the mouth toward the upper estuary, although the tidal wave propagates up to 30 km usptream from the mouth, with increasing asymmetry between ebb and flood. Current patterns show that flood current dominates near the left bank and the ebb current is more intense in the center and the right bank. Residual flow shows inflow at west and inflow at east boundaries, except for neaps in dry season, when a two-layer estuarine circulation develops on the central channel. During the dry season, tidal currents induce strong resuspension of fine sediments. A very turbid estuary is found in the dry season, suggesting pumping of mud from the coastal area. Less loaded waters in the wet season result from the predominance of the fluvial dominance which flushes the suspended matter to the ocean. Residual fluxes of suspended sediment were calculated at each of the three anchor stations. Residual flux follows trends of residual currents, meaning that they are all directed downstream in the wet season, with the exception of the left bank of the estuary, while in dry season they are upstream oriented near the bottom and downward oriented near the surface. These first observations confirm the influence of coastal mud in the infilling of the estuary. However they need to be completed to get a reference state of the estuary, as it is soon about to experience significant anthropogenic changes related with an increasing development of port activities.