Background & Aims of the Study: Pollution from heavy metals in aquatic ecosystems, through entering into food chain, threats living organisms' health, especially humans, so that it can poses an ecological risk on them. Hence, the present study aims to assess concentration of heavy metals (lead, chromium, copper, zinc) and their ecological risk in the surface sediments of the Bashar River in Yasuj during the summer and winter seasons. Materials & Methods: To determine metals concentration in 8 stations along the river, three samples were taken in each station. Having samples preparation and digestion, the concentration of these metals was determined using atomic absorption spectrometry. Results: The results showed that the average total concentration of metals in the summer season was (14.45 ± 3.67 ± 3.35 ± 8.41 ± 11.88 ± 13.48 ± 13.48) and as for winter was (12.33 ± 19.30.33 ± 0.66 vs. 26.83 ± 9.236.886.83) mg/kg. In addition, evaluating geo-accumulation indexes, pollution load and pollution factor indicated the low pollution status of the area in both seasons. Also metals ecological risk and environmental hazard assessment showed that in summer and winter, pollution concentrating were (Cr> Zn> Pb> Cu) and (Cr> Zn> Cu> Pb) respectively. Conclusions: According to results, pollution status and the ecological risk level of heavy metals in the study area is low. Also, the sources and changes in elements concentration in different stations and seasons may be due to the establishment of various urban and industrial pollution sources along the river, and its physical and geo-biochemical changes such as bed type, slope and variation in water flow volume entering the river, especially in winter season.
