Located on the West Beninese coast of Grand-Popo administrative unit, "Bocca del Rio" commonly called “Bouche du Roi”, is the mouth of swell dominant microtidal estuary river (Figure 1 and Figure 2). It plays an important part of exchange of water and sediments between the ocean and a fluvio-lagoonal system constituted by the coastal lagoon, the Ahô channel and the Ahémé lake (Figure 2). The fresh water supply is primarily ensured by the Mono river with a monthly average flow of rising neighbor of 400 m / year (Figure 3). Since the bringing into service of the Nangbéto dam on Mono river in 1987, the “Bouche du Roi” morphodynamic is driven by the morphogenesis of a system of split-creek which forces the mouth to a west-east migration, with a speed reaching 700 m / year. Based on ground observations made in May, 2014 and on multi-temporal Landsat satellite images from July, 2013 till June, 2014, the present work has for objective to analyze the processes of sedimentary by-pass and “Bouche du Roi” split sandy construction on one hand, and to look for the influence and the relative importance of the swell (and the current partners) and river currents implied in the aforementioned processes, on the other hand. The methodological approach is inspired by the hydrodynamic functioning of the “Bouche du Roi” extern tidal system. Indeed, waves raised on the encounter of the terminal lobe are forced to break immediately with propagation towards the sea bottom, under the panache of estuary water. The electromagnetic signal of the breaking body of water and the generated morphosedimentaries entities allow to appreciate the “Bouche du Roi” morphodynamic evolutions and the ones of its external delta (Figure 4). The results show that the sedimentary by-passing as well as the construction of the split sandy are essentially under the command of the regime of the swell and the hydrological seasons of the Mono river, with a secondary interference of tidal currents. These two sedimentary processes (by-passing, construction of split sandy) involve a sedimentary stock in the form of sandy bars (swash bars and mouth linear bars) which form and deform on the swash platform. Bars migrate then to the shore and transfer the platform sedimentary stock to the updrift coast or to the downdrift coast, according to the relative importance of the marine and river hydrodynamic agents. This diachronic study based on the images Landsat, allowed to identify the various stages of the annual hydro-sedimentary functioning of the “Bouche du Roi” and to determine the relative importance of the forcing which are involved (Plate 1a; Plate 1b and Figure 5). In summary, four stages are distinguished: -the marine stage, from January till June, in the course of which the hydrodynamism of the swash platform is firmly controlled by the swell and the derived currents; the river flows (river forcing) being seriously paid off; -the both of fluvio-marine stages respectively of southern winter (from June till September) and summer southern (from November till December) during which the dynamics of the deltaic system is under the double control of the marine and river forcing, with a morphodynamic resultant printed by swell (marine forcing); -the river stage during which the dynamics of the external platform is firmly controlled by the river flows (river forcing); the marine forcing by the swell being seriously paid off during this stage.