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The biggest reservoir of fresh water in Africa, second deepest lake in the world, and major reservoir of biodiversity, Lake Tanganyika and its catchment exert a major control over Africa's resources, settlements, and economies. Shared by five countries, the watershed contributes to the Congo River. The level of the lake, which reflects the water balance between inputs (rain, runoff) and losses (discharge at the outlet, evaporation), is sensitive to regional atmospheric circulation. The fluctuations reach 70 cm at the seasonal scale, but considerable variations (more than 100 meters) are recorded at the millennial scale, thus illustrating the considerable impact of climate changes. The lakeside population (more than 12 million inhabitants) is differently distributed, with the highest concentrations concerning the North. A cross-border cooperation for the socio-economic development and for the sustainable management of the lake resources has been undertaken, especially for drinking water production, maritime transport fisheries management and environmental preservation : water quality, soil husbandry and biodiversity.

Formed on the upper basin by the annual monsoon, the flood of Senegal River inundates a vast floodplain on its lower course, called "the valley". This one, several hundred kilometres long, can be regarded as an inland delta. This annual flooding, which offers ecosystem services and promotes traditional activities, must be maintained despite the presence on the Bafing of the Manantali reservoir dam, which controls since 1987 half of the river flows through the valley. In addition to its other objectives (energy production, low water support for irrigation and navigation, mitigation of too high floods), this dam therefore achieves a flood support, designed to maintain adequate flood in the valley. Here we present a decision support system dedicated to flood support that aims to: 1) define a flood hydrograph goal for Senegal river, according to a target area of recession crops; 2) define the optimal date for flood support, in order to penalize the least possible energy production at the dam; 3) calculate the discharges to release of the dam to support the flood, according to the real-time hydrological situation known. We finally analyse the respective impacts of natural climate hazard and of Manantali dam on the river's flood observed.

Une retenue artificielle de petite dimension est un lieu privilégié pour l'observation des bilans d'érosion sur un petit bassin versant. La majorité du transport solide reste piégé dans le réservoir et celui exporté en aval du barrage, lors des rares débordements par le déversoir, peut être estimé à partir de la connaissance des débits déversés et de leur concentration. Un équipement hydro-pluviométrique du barrage, couplé à des mesures annuelles de la bathymétrie du lac, permet d'établir des bilans précis en eau et en sédiments du bassin versant drainé. Dans la zone semi-aride de la Tunisie Centrale, un réseau de 26 lacs collinaires a été équipé en 1993. Les données qu'il fournit peuvent être généralisées à l'ensemble des petits bassins versants de la dorsale tunisienne. (Résumé d'auteur)