The present study addresses the river water composition in São Miguel island (Azores archipelago, Portugal), sampled in a set of 10 sampling locations, selected according to the actual water use and the anthropogenic pressures along the drainage basin. The selection of the rivers was made considering as main criteria the discharge. The majority of the river waters are of the Na-HCO3 type, with temperatures in the range between 16.4ºC and 25.6ºC, and the water composition is controlled by several processes, as cold carbonated and thermal waters inflows, silicate weathering, agriculture pollution and seawater spraying in the coastal area. Generally, the dissolved solids content increases from upstream to downstream, as shown by the conductivity values (104 - 428 µS/cm), and the pH values also depict the same trend, being in the range between 5.20 and 8.97.