Purpose Many large inland rivers are contaminated by historic industrial and agricultural activities along their shorelines. Redistribution of contaminated river sediments via dredging or flooding can reintroduce previously stored toxic persistent organic pollutants into the aquatic environment.

Materials and methods We used multiple lines of evidence, including chemical measurements using gas chromatography-high-resolution mass spectrometry and cell-based bioassays using the fish liver cell line RTL-W1, to determine the risk to aquatic biota associated with sediment in three reservoirs and one delta lake in the Saskatchewan River, a large Canadian prairie river.

Results Concentrations of dioxins and furans (maximum ~ 30 ng kg−1 dry weight) and polycyclic aromatic hydrocarbons (maximum < 300 μg kg−1 dry weight) were generally low in sediments, though hotspots were identified in parts of reservoirs rich in clay and organic matter wherein these chemicals accumulated. Concentration measurements agreed well with bioassay results, as the samples with the highest chemical concentrations induced the greatest EROD activity (biological equivalent concentration = 4010 ng kg−1). With one exception, all fishes tested were below the tolerable intake level for dioxins, and PAH metabolites were only rarely detected in bile, suggesting limited risk to human consumers.

Discussion Based on our findings, the overall effects of dioxins and dioxin-like chemicals on biota in this system are likely limited. Nonetheless, disturbance and intentional movement of sediments in this river basin for potential sediment restoration in the future should be done cautiously, paying attention to sediment characteristics and local hotspots.