Due to the great possibility of water contamination of many rivers by human activities in Iran, the study of water quality is crucial for water resource protection and human health. High level of phosphorous is the main reason for eutrophication of freshwater systems. The main aim of this study was to investigate the concentration of phosphorus in the rivers feeding into Taham dam in Zanjan, using GIS software. 40 sampling stations were selected along Taham and Ghalharod Rivers with respect to sewage discharge points and feeding characteristics of water entering to Taham dam. In total, 160 water samples were taken from rivers with regard to precipitation season in two different periods from winter 2014 to spring 2015. The obtained data were analyzed using SPSS and ArcView GIS. The findings showed that 15% of the studied stations had phosphorous levels higher than acceptable levels set by EPA. The highest levels of phosphorous contamination were observed in stations No. 145, 154, 155, 161, 166 and 168. The elevated concentrations of phosphorous in the rivers can be responsible for the eutrophication of Taham dam reservoir.