The Hadejia-Nguru wetlands in north-east Nigeria are of international importance for resident and migratory Anatidae but the construction of large dams upstream seriously threatens their continued existence. An analysis of waterfowl census data collected over the period 1988 to 1998 inclusive demonstrated that the extent of flooding had no effect on the species richness of waterfowl but had a significant impact on total numbers. White-faced Whistling Duck Dendrocygna viduata and Garganey Anas querquedula in particular respond to changes in flood extent. The present practice of releasing water from upstream reservoirs should therefore be continued.