The recently established large reservoirs Manantali on the river Bafing, tributary to the Senegal River and Selingue, located on the river Sankarani, tributary to the Niger River, have followed similar changes in limnology and fish populations as those established in West Africa 20 to 30 years ago. Basic water parameters, such as oxygen content, pH, electrical conductivity and temperature show seasonal variations largely determined by the changes in wet and dry seasons. Both waterbodies are monomictic, with water mixing taking place in January, the coldest period ofthe year. Pre-impoundment forest clearence ofthe future reservoir bottom in the shallow SelinguC reservoir was carried out only in the proximity of the dam site. In the deeper reservoir Manantali the savanna forest was cleared from an area of about 25 percent, and the tree gallery was cleared along the river. The initial filling of Manantali reservoir was followed by a nearly complete drainage of the reservoir. After refilling some months later, the dissolved oxygen content stayed reasonably high and therefore there was no fish mortality. Also no hydrogen sulphide or a large-scale spread of noxious aquatic plants developed. At present the fish fauna is dominated by cichlids and pelagic species, i.e. at SelinguC by clupeids, and at Manantali by cyprinids (Lepfocypris niloticus). In both reservoirs was the genus Citharinus considerably reduced. At Selinguk reservoir disappeared Proroprerus annectens. The fish yield estimate is 27 kg/ha/year for Manantali, and 40 kg/ha/year for Selingue. Both rerservoirs have recorded a slight increase in electrical conductivity. They both serve as a valuable water resource and fish supply for the Sahel region.