This article aims to advance in the knowledge of the relationships between the sand cycle and the behaviour of small river mouths (outlets) flowing into sandy beaches in the Río de la Plata estuary, Uruguay. The bars of these watercourses respond quickly with processes of erosion and/or migration of the channel when the “sand cycle” (coastal sedimentary balance) undergoes modifications, due to i) the effect of the fixation of dunes (sediments) by afforestation with exotic species and ii) the construction of infrastructure. Two watercourses were analysed: the Pando River and the Carrasco Creek that both have received different impacts and suffered modifications, not only in the sediment flow but also in their hydrological regime. A multitemporal analysis was based on a series of aerial photos and cartography since 1928, a background review, and the support of documents and chronicles. The data obtained were analysed statistically. Results evidence a significant loss of beach surface at the Pando River mouth, and an increase at the Carrasco Creek mouth, since the channelling of a wetland located upstream, allowing the entrance of sediments from the basin upstream of the wetland. Relevant outcomes are: i) the description and analysis of the relationships established between dune systems and the outlets of the watercourses, an aspect that has not received due attention in the scientific literature, in which the dissociation between the analysis of wind and fluvial dynamics is usual, and ii) the sedimentary balance was of both systems: erosion and accretion at the Pando and the Carrasco outlets respectively. The reconstruction of the dunes could likely prevent the escape of sand and reduce the erosion. The knowledge about human intervention and natural processes governing the interactions between sandy beaches, dunes and outlets is a crucial input to coastal management and adaptation to sea-level rise and storm surges.