Bioaccumulation of mercury in freshwater fish is a complex process driven by environmental and biological factors. In this study, we assessed mercury in fish from four tributaries to the Red Deer River, Alberta, Canada, which are characterized by high surface water mercury concentrations. We used carbon (δ13C) and nitrogen (δ15N) stable isotopes to examine relationships between fish total mercury (THg) concentrations, food web dynamics and patterns in unfiltered THg and methylmercury (MeHg) concentrations. We found that THg concentrations exceeded the tissue residue quality guideline for the protection of wildlife consumers in 99.7% of fish sampled. However, while the surface water THg concentration was highest in Michichi Creek and the MeHg concentration was consistent across streams, patterns of fish THg concentrations varied depending on species. Furthermore, body size and trophic level were only correlated with THg concentrations in white sucker (Catostomus commersoni) and Prussian carp (Carrasius gibelio). The results of this study suggest that mercury poses a risk to the health of piscivorous wildlife in the Red Deer River watershed. Despite high THg concentrations in these streams, mercury bioaccumulation is not driven by environmental inorganic mercury concentrations. Additionally, commonly cited factors associated with mercury concentrations in fish, such as body size and trophic level, may not strongly influence bioaccumulation in these stream ecosystems.