Despite the fact that humans have dramatically altered flows in many rivers, little is known about the consequences of short-term flow changes on the movement of resident fish. We conducted a comparative study in two rivers having two different flow regimes (i.e., one regulated and one unregulated) to evaluate to what extent adult northern pike movements are influenced by river flow conditions. Data on northern pike (Esox lucius L.) sub-daily movements were collected in northeastern Ontario on a total of 40 individuals in Mississagi River, a river subjected to extreme daily flow variations associated to hydro-electric operations (i.e., hydropeaking), and Aubinadong River, an unregulated river. Using manual tracking by radio-telemetry, we found that northern pike moved at a higher rate in the river subjected to short-term changes in river flow than in the unregulated river. Results also show that the relationship between flow and movement of northern pike can differ among rivers; in Mississagi River, flow had a significant positive effect on northern pike activity, while the effect of flow in Aubinadong River depended on the water temperature. Moreover, fish in Mississagi River utilized a restricted river section and traveled more laterally, in comparison to fish inhabiting Aubinadong River where longer upstream-downstream movements were more common. This study contributes to our understanding of how fish react to short-term changes in flow, which could facilitate the management of regulated rivers by augmenting our capacity to minimize the effects of such changes on fish.