River water quality mostly depends on the quantity of dropped pollutants and on the purifying capacity of the river. Often the results of water analyses are not easily readable, especially when referred to the density of close urban agglomerations. This paper focuses on how GIS can be used in explaining different pollution levels in some northern Italy rivers (monitoring realized in 1990 by Trento Province). This target was carried out through thematic maps of demographic, industrial, and agricultural pressing (1991 census data) on the districts intersected by the rivers.