Physical barriers (e.g., dams and weirs) are known to be detrimental to fish movements and this has resulted in wide spread investigation and installation of fishways or removal of barriers. However, non physical barriers such as hydraulic (high velocity flows), chemical e.g., pollution, low dissolved oxygen), noise and thermal barriers (warm or cold water) are also known to influence fish movement and habitat choice. In this study we investigated warm-water riverine fish assemblages downstream of an impoundment before and after thermal stratification and the associated downstream cold-water pollution was prevented using an aeration system that was installed in the impoundment. Temperatures below the dam significantly increased after installation of the aeration system and this correlated with an increased abundance and greater number of species downstream. This included attracting a number of diadromous species to the base of the dam in the vicinity of a now constructed fishway. Overall, aeration appeared to be beneficial for both the lake (upstream) and the downstream riverine environments.