The relationship between the climatic characteristics of a river basin and its hydrological response is very well understood. River flow is the synthesis of a broad range of phenomena such as rainfall, evapotranspiration, vegetation, soil, and geomorphological characteristics. This paper analyzes the Uruguay River in terms of its flow variability and the possible influences of the El Niño/Southern Oscillation (ENSO) phenomenon over a transition region between the subtropical and the mid-latitude regimes. A number of relationships were found between the warm, cold and neutral ENSO phases and the maximum floods of the Uruguay river. The conclusion of this work enhances knowledge of the climatic characteristics of the Uruguay river basin, and it may also be useful for flood forecasting.