The Río de la Plata drains the second largest basin of South America. It flows into the Atlantic Ocean generating an estuarine system of about 35, 000 km 2, with only 5-15 m water depth. On the basis of temperature and salinity data from the last 29 years, property seasonality at the estuary were studied. Surface salinity distribution is controlled by the balance between onshore and offshore winds, the river discharge and the Coriolis force. As a result of the combined effects of these forces, two periods were observed for the salinity distribution. Fall-winter is characterized by a balance between onshore and offshore winds and a maximum in the continental drainage, generating a main NNE drift of the estuarine waters along the Uruguayan coast. During spring-summer, onshore winds become dominant and a minimum in the runoff is observed, resulting in Ekman surface drift advecting freshwater southwards along the Argentine coast. Consequently, shelf waters penetrate the estuary up to Punta del Este (Uruguay). Bottom salinity distribution does not exhibit seasonality, because the shelf water intrusion is controlled by bathymetry. The temperature field, driven by the atmospheric cycle, presents a warm and a cold season. Homogeneity in the vertical as well as horizontal scales characterizes these periods. High-resolution CTD data were arranged into salinity sections to describe the river-ocean interaction. A salt wedge is a quasi-permanent feature for the central and southern sectors of the estuary, defining an area of strong vertical stratification. However, the salinity stratification is destroyed by moderate to strong onshore winds.