According to the EU Water Framework Directive (WFD), the ecological status of rivers, as also that of the other watercourses, is assessed mainly on the basis of biological quality elements. The chemical and physico-chemical quality elements, as well as the hydrological quality elements, are mentioned in the WFD as supporting elements for the final evaluation of the Ecological Quality Status. It has already long been recognised that on the basis of water quality alone no reliable ecological river status assessment is possible.