Water is a limited, irreplaceable and indispensable natural resource. Statistical methods are important tools for controlling and forecasting changes in the management process of water quality. The main objective of this study is to classify the water quality monitoring sites into homogeneous groups and to measure the risk for water pollution influenced by the monthly dissolved oxygen concentration, which has been selected and considered relevant to characterize the water quality. The methodologies are illustrated using a data set of the Douro River basin (in Portugal), measured monthly in 18 water quality sampling stations and recorded in the period from January 2002 to December 2013. A clustering analysis is performed to group similar sampling stations considering the environmental variable, and by taking into account the seasonal variations. Several risk measures, such as loss probability, entropy or value at risk, are determined for the considered variable in order to assess the risk of water pollution taking into account the monthly nature of the data. The risk measures are used to classify the months in order to analyse and to confirm the distinction of the dissolved oxygen concentration according to a wet and a dry period.