Impoundment of hydroelectric reservoirs deeply modifies habitats available for waterfowl because it involves transforming a fast-flowing river, its tributaries and nearby ponds and wetlands into a large body of water. Using a Before-After-Control-Impact design, we evaluated whether the creation of the Péribonka reservoir, a steep-sloped hydroelectric reservoir with low water level fluctuations, affected the abundance and species composition of waterfowl breeding pairs and broods in the area. We used helicopter-based waterfowl survey data covering a period of 2 years before and a period of 10 years after the creation of the reservoir. We also used 9 5x5 km plots and 72 km of river as control sites. Our results show that breeding pair density slightly increased after impoundment, while brood density increased significantly (sixfold), especially for Common Goldeneye (Bucephala clangula). This suggests that there were favorable habitat gains for waterfowl after impoundment, probably due to low water level fluctuations and localized areas of shallow water, and that mitigation measures likely helped to reduce the impact of the project. Because this BACI study ended 10 years after impoundment, it remains difficult to ascertain whether conditions in the Péribonka reservoir have stabilized or are still evolving.