Geomorphologic and stratigraphic research, supported by radiocarbon datings, give us an approach to reconstruction of the evolution of Mar Chiquita lagoon. The estuarine body originated during the regressive phase that followed the transgressive "maximum". After this maximum, a short rapid fall of the sea level restricted the estuarine environments which resulted in the formation of the lagoon. Predominant landforms are related to the regressive phase. Mar Chiquita lagoon has a mean surface of 46 km super(2) and a drainage basin of about 10,000 km. It is subject to significant salinity variations. Hydrogeological investigations show the influent character of the water table in relation to the lagoon. The water table rise associated with the "rejected recharge" effect causes important variations in the lagoon body. The lagoon inlet zone exerts an important control in its hydrological behavior. Historical and present surveys indicate a northward migration pattern. This trend has been emphasized by the constrictions of man, casuing environmental disturbances such as erosion and silting. Neighboring beaches are subject to rapid erosion.