The influence of the Plata, the second largest river in South America, extends along a coastal strip of 1300 km. Historical hydrographic and wind data and numerical simulations are combined to determine the seasonal and interannual variability of the Plata plume and its relationship to the magnitude of the river discharge and the intensity and direction of the wind stress. Our results indicate that the seasonal variability of the river plume is controlled by the alongshore component of the wind stress. During El Ni¿o the effects of the wind and precipitation anomalies tend to compensate each other, preventing anomalous northeastward plume extensions associated to large outflow events. Numerical experiments confirm this finding and indicate that during El Ni¿o the discharge from the Plata River spreads offshore.