The river Douro debouches into the Atlantic Ocean near Oporto (Portugal) with a funnel-shaped mouth, partly closed from the ocean with a sand spit. Both the wave-induced longshore sediment transport as the sediment yield of the river are limited by the available quantity of sediment (non-alluvial conditions). The results of some sediment transport measurements are presented. The morphology of the river mouth is discussed in relation to the geology of the area, the river discharge and the wave climate, based upon data from the field survey as well as upon some additional calculations