/ Water quality levels and loads of nutrients transported by the AgL~era stream (northern Spain) were studied for a year to assess the self-purification capacity of this system. The main villages produce an increase of nutrient concentrations and a degradation of water quality. Nevertheless, the high retention capacity, especially for phosphate, allows the stream to recover its previous levels of quality after a short reach. The retention of nutrients depends on a complex combination of the flow level, hydrologic stability and the development of periphytic communities.Increasing concern about the conservation of rivers, and mainly about the quality of stream waters, led to the development of several quality criteria and quality indexes (Tuffery and Verneaux 1967, Nisbet and Verneaux 1970, Janardan and Schaeffer 1975, Hellawell 1977, as well as to the assessment of the role played by fluvial systems in the dilution and purification of the nutrient and pollutant inputs (Margalef 1983). Rivers are not just sewers that channel the materials exported out of the terrestrial ecosystems, but centers of intense biological activity, with an important capacity for processing the transported materials (Hearne and Howard-Williams 1988, Cooper 1990). The capacity of purification is often .exceeded by nutrient inputs, and rivers all around the world show strong eutrophication and pollution. Therefore, in order to manage our environment in a sound way, it is necessary to assess the ability of self-purification of any running waters and to determine the maximum permissible levels of inputs, as well as the ways to attain those levels. Similarly, it is important to take into account any changes in the self-purification capacity of river systems as a result of human-caused impacts such as canalization, modification of river margins and riverbed or clear-cut of riparian vegetation (Pinay and others 1990).