Karun is the largest and the longest river in Iran which flows down the Zagros mountain range and into the Khuzestan plain. Karun enjoys a reasonably good drinking quality at the point of entry to Khuzestan plain, but after collecting agricultural, municipal and industrial effluents and etc, its quality deteriorates significantly. To investigate the effects of different pollutants in urban aquatic systems, Risk analysis of heavy metals turns significant due to their persistence, non-degradability, toxicity and accumulation. In the present study, two indices were used to determine the sediment quality of Karun. From six stations selected on Karun River where water supply points for treatments in Ahvaz and Mollasani city, sediment samples were collected in summer and winter, then, oven-dried, homogenized, and processed to assess of the metals in the sediments using flame atomic absorption spectrophotometer and the accumulations of some heavy metals (Pb, Zn, Cr, Cd) in sediment were investigated. Results indicated that the mean concentration of Pb, Zn, Cr and Cd were 26.27, 72.36, 53.47 and 3.85 mg/kg dw, respectively in summer and 13.41, 59.54, 30.28 and 0.42 mg/kg dw, in winter. Based on the results, the mean metal levels showed significant variations among study sites during the wet and dry seasons and the mean concentration of heavy metals were ranked as: Zn> Cr> Pb>Cd. Potential ecological risk analysis (R) and biological toxicity test based sediment quality guidelines (mERM-Q) applied to the results to assess the environmental risks of heavy metals in the region. On the basis of the mean values of ecological risk analysis (R), in two seasons sediments were enriched with metals in the following order: Cd> Pb> Cr>Zn, while according to sediment quality guidelines (ERM-Q), in during summer Season followed the sequence Cd>Cr> Zn> Pb and in winter season the order was Zn>Cr>Pb>Cd. So according to these results, Cd posed the highest ecological risk, although its concen
