We perform a grain-size trend analysis (GSTA) to investigate sediment-transport mechanisms in the Grand Rhône microtidal mouth during various hydroclimatic conditions. The objective is to determine the efficiency of this method to explain the directions of residual transport pattern and mode of distribution of the fluvial sediment input in an environment that is very difficult to equip with instrumentation during extreme events. The known biases of this sedimentological approach are reduced by using enriched geostatistical processing and choosing periods of sampling subject to unequivocal hydrodynamic forcings. The modeling results and grain-size distributions of surface sediments clearly show three vector fields during periods of low river discharge. The first vector field corresponds to an upstream zone under fluvial influence. The second field is characteristic of a mixing zone, confined inside the river mouth and marked by heterogeneous sedimentation. This sector is interpreted as the confrontation zone between alluvial and marine dynamics. Farther offshore, the vectors are oriented towards the river and at counter slope. In deeper zones, the vectors reflect the sediment-transport generated by wave refraction at the top of the mouth bar. At the delta front (bathymetry from −5 m to −20 m), the vectors are interpreted as representing a lag deposit. The periods of medium and extreme floods are associated with storms. The marine dynamic regime generates a longshore drift current, which brings sediments onto the mouth bar. During medium flood events, only the inner part of the mouth bar is highly influenced by the river. During extreme flood events, the river influences both the eastern part of the mouth and the wave regime in the central part. Consequently, the GSTA method shows that, even during flood events, the river solid discharge is not transferred towards the adjoining upper shoreface. In the context of a wave-dominated delta, the distribution of surface sedimentation in a microtidal mouth remains controlled mainly by the swell, even in periods of flood when the intensity of river dynamics is exacerbated. Finally, all these results are similar to the known hydro-sedimentological behaviours of the Rhône River mouth. Nevertheless, GSTA results seem not self-sufficient to explain all the processes, but this method proposes an alternative approach to traditional methods used to measure bed-load sediment transfer during extreme events.