Seyab River, which crosses Southwest of Tehran to Islam Shahr (10km), is an extremely important river that is in contact with a population of about one million. Freshly deposited seasonal sediments were collected from depths of 0–20 cm at 4 locations (1 to 4) along the 10-km long channel length, in the spring season of the year 2008. There is no water during the summer to the autumn seasons, with the lowest content in winter and the highest content in spring. Hence, the sampling was carried out during spring. In this research, 12 sediment samples, 9 plant samples and 8 water samples were collected from Seyab River and its periphery. The samples were analyzed by AAS for Co, Ni, Cr, Cu, Pb, Zn, Mg, Na, K, Ca, Cd, Se and HCO3-, CO3-2, Cl-, SO4-2. The concentrations of Ag, Hg, As, were less than the detection limit for all samples. Data processing shows the average concentrations of Cu, Cr, Zn, Mg and Ca were higher than the upper continental crust background levels. Se was at 174 times and Cd at 220 times the said level. Islam Shahr has been newly developed during the past 15 years, thus such high contaminations during such a short period are a cause for concern. Anthropogenic impacts, industrialization, farming activities, transport, urbanization, animal and human excrement and domestic wastes have affected the river, contaminating it and its sediments with heavy metals especially Se and Cd.