The Least Bittern (Ixobrychus exilis) is a threatened species in Canada under the Species at Risk Act and breeds in manmade impoundments on Department of National Defence land in Nicolet/Baie-du-Febvre, Québec, a designated Migratory Bird Sanctuary. We conducted Least Bittern call-broadcast surveys in three adjacent basins in these impoundments from 2004 to 2006. Least Bittern abundance greatly fluctuated between these years with a maximum of 37 individuals in 2004, 16 in 2005, and increasing to 32 individuals in 2006, translating into 38, 18 and 31 estimated breeding pairs. A breach in a dyke occurred on the larger basin in the fall of 2004 where most Least Bitterns were detected, which decreased the water level of the marsh and altered the habitat structure the following spring. The dyke was repaired in the fall of 2005 followed by a significant increase in the water level. The fluctuating Least Bittern abundance observed during these years suggests a rapid response of the local population to interannual changes in water level and habitat structure. Water-level management may be necessary to ensure long-term ecological integrity of artificial wetlands but management activities should be conducted outside the breeding season to ensure that the Least Bittern's reproductive needs are fulfilled.