Depletion of freshwater quality due to increase in nutrient loads has become an important problem that administrations have to face off. This problem is especially critical in Mediterranean countries because of the scarcity of water. This project aims to evaluate the self-purifying capacity in polluted streams and to examine the mechanisms that control it (nutrient sources from catchments and in-stream physical, chemical and biological processes). With this empirical information and from stream manager experts we propose to develop an Expert System as the final product of this project. This Expert System, a management tool, will propose strategies to ameliorate stream water quality problems based on the type The Expert System will be an user-friendly software to infer the percentage of stream self-purification capacity and to provide solutions to maximise this capacity.