In four consecutive seasons along 9 stations, water parameters such as TDs, pH, temperature, DO, BOD, COD, TOC, TP, NH₄⁺, TN and NO₃^- were determined on the siahroud River southwest of the Caspian sea in northern Iran. The results indicated higher TDS values in some parts of the river due to the agriculture and residential activities. The addition of ammonia fertilizers in the paddy fields is one of the major causes for the higher NH₄⁺ in the downstream sites. Total phosphorous (TP) and total nitrogen (TN) levels in the river were mainly in the organic forms. Factor analysis showed that agriculture and urban activities were the major pollutant sources. Four zones were identified by cluster analysis, suggesting local pollution sources or the accumulation of pollution effects downstream.