Estuarine outflow plumes are important coastal processes whose variable nature can make it difficult to monitor using traditional ship-based surveys. Satellite ocean color observations of the optical properties of coastal surface waters can be used to distinguish plume water from ambient water masses, particularly based on increased concentrations of Total Suspended Matter (TSM) in the plumes. In this work, 71 MERIS scenes were considered, covering the region surrounding the Douro river mouth. An automatic procedure for the identification of the Douro river plume has been proposed based on the thresholding of the MERIS data through an automatic search for the optimum value of the threshold parameter TSMcutoff. A manual identification of the river plumes was manually performed by two operators, in order to evaluate the performance of the automatic method. A set of attributes for the characterization of the river plume in terms of shape and TSM concentration was also proposed. A high and significant correlation of 0.719 was achieved between the river plume size and river discharges, which increases up to 0.838 excluding two of the images. Moreover, it was found a significant correlation between the river discharges and the orientation of the river plume, which is associated with important practical considerations in terms of sedimentary balance. Significant advances on the spatial-temporal analysis of the Douro river plume from MERIS data has been achieved in this work. Moreover, the proposed methodology may also be applied to other study areas and sources of remote sensing data.