environmental monitoring studies in the hydrographical basin of pardo river, rs, brazil, in the time series from 2007 to 2009, showed no agreement between the water quality evaluation obtained through the water quality index (wqi - nsf), which ranged from "regular" to "good", and the environment national council (conama) resolution 357/2005, which was bad ("class 4"), highlighting as critical variables for this classification the thermotolerant coliforms and the phosphate concentration, indicating process of water eutrophication, related to the large amount of nutrients and organic load from domestic sewage and excess fertilizers used in agriculture. aim: this research aimed to calibrate the wqi - nsf for the pardo river basin, based on the conama resolution 357/2005. methods: using the database (2007 a 2009) from the laboratory of limnology of the university of santa cruz do sul (unisc), rs, corresponding to the environmental monitoring studies realized in the hydrographical basin of pardo river new weights of the relative importance of the variables used to calculate the wqi were attributed, by means of principal component analysis. results: there was a significant agreement between the results of the assessment of water quality using the conama resolution 357/2005 and the new wqi calibrated. conclusion: the results certify the use of the calibrated wqi in environmental monitoring programs in lotic systems belonging to guaíba hydrographical basin, rs.