Abstract. Human interventions have drastically altered the hydrologic regime of the Razelm - Sinoe Lagoon System. Over the past four decades, following the closure of the main inlet (Gura Portiței) and the engineering of the other main communication inlets and channels, significant changes have occurred in the hydrological regime. This major disruption of the water and sediment circulation generated, at its turn, significant changes of the water chemistry, biology and lagoon ecosystems, transforming the system in eutrophic environment with all related consequences. Due to the significant decrease in sediment load transported along the coastal zone, the barrier beach separating the Lagoon System from the Black Sea is almost entirely subject to coastal erosion. Environmental improvement and rehabilitation is already a major demand of the local communities. Nevertheless, environmental recovery plans must be based on the detailed understanding of the water circulation in the lagoon system. This can be achieved only by the involvement of numerical modelling. Simulations have been performed using the hydrodynamic model SHYFEM, in order to obtain a response of the Razelm - Sinoe Lagoon System to the main forcings. The results show that the salinity distribution in the Lagoon System is, mainly, controlled by the freshwater discharge and, secondly, by wind, which can intensify the freshwater input. The calculated renewal times of the Razelm - Sinoe Lagoon System are very high, over one year, and classify the lagoon as a chocked system. This work represents a first step towards the detailed quantitative study of this sensitive part of the Danube Delta Biosphere Reserve.