The Quequén Grande river is one of the most important rivers of the Buenos Aires province. The river basin is found in a very important farming zone of the Argentine country and it flows in the Atlantic Ocean. At present very little is known about the river basin and its tributaries, in fact, few studies have been performed in the estuary. Therefore, the main objective of this research is to study the hydrography of the Quequén Grande river basin with particular detail en the river mouth. Specific objectives were to analyse the Quequén Grande basin, to establish the climate and the patterns of precipitation of the basin, to study the stream discharge, to analyse the tidal into the river, to study the hydrography of the estuary, temperature, salinity and current velocity variations, to analyse statistic of waves in Quequén and to describe the basic problems of the harbour because of the climatic factors. The surface water divide and the total area of the river basin were identified and described, including relief and drainage. The Quequén Grande river basin has an area of 9370.7 km2, the basin perimeter is 482 km and the river is 173 km in length and it has a dendritic drainage pattern. The origin of the names of main streams and tributaries ware investigated using historical information. The annual flow cycle and its relationship with the hydric balance in the Quequén Grande river basin were investigated. Potential evapotranspiration and stationary changes of the hydric efficiency have been determinated. By that, different areas were identified and a degree of dryness in East - West direction had been detected. The area has a temperate rainy climate. Mean annual temperature is 14 °C. Precipitations varied in the river basin between 800 mm in the East to 700 mm in the West. The results showed that in dry periods February had water deficit meanwhile in wet periods August and September had water surplus. Four watersheds were defined by partial river discharge (0,57 m3 seg-1 to 1,5 m3 seg-1) and geomorphic variation between them were identified. The maximum and minimum stages observed in the four watersheds have occurred in different months of the year, although all minimums occurs in February, except in the Pescado Castigado river basin. Infiltration is a very important factor that determines the main hydrological characteristics of the basin. A statistical relationship between precipitation and river discharge is presented here. Soil characteristics showed an increase of salinity in the fields where irrigation is applied. The farmers employed the groundwater or the freshwater for irrigation without control. It was determined in the basin of the Pescado Castigado river. Not only soils were damaged but trees too. The hydrography of the Quequen river estuary was analysed. The salinity gradient in the estuary is important producing highly stratified surface water in the first meter depth. Water temperature profiles were homogeneous in the water column. Velocity currents were up to 0,09 m seg-1. Historical records (1929 - 1969) of water waves in Quequen harbour showed that the significant wave height is 1,16 to 1,46 m. The sea state is good and fair only 75 days per year. The harbour has to be close when waves are 1,8 m high, therefore 76 days per year, in average, the harbour is closed. Tides are semidiurnal with a mean range of 0,93 m. Therefore, the estuary is microtidal one. Wind produces sea level fluctuations of 0,37 m and 0,42 m.