The construction of a water retention structure across a river in a catchment inevitably leads to changes in its natural functioning downstream. This study investigates the hydrological changes observed on the Benue at the town of Garoua before and after the construction of the Lagdo dam in the Upper Benue catchment using statistical and frequency analysis of flows upstream and downstream the dam. The results indicate that the shape of the hydrograph has changed, with a reversal of the hydrological regime characterised by a drop in flood flows and an increase in low-water flows. For the pre-dam period, the flood hydrograph shows a maximum in September and a minimum in March, which may be zero in years with dry water hydraulicity. The impoundment of the Lagdo dam in 1983 led to a reduction in peak flows of around 40% and a 90% increase in low flows on the Benue at Garoua. The maximum flows observed at Lagdo are retained in the dam for the production of hydroelectric power and part is released downstream towards Garoua with a reduction of about 34% and 35% of 100-year and 50-year floods respectively. These changes can have a significant impact by reducing the amplitude of floods in the Benue valley and consequently, the flooding phenomenon. The entire hydrological dynamic of the Benue valley has thus been modified, moving from a natural tropical rainfall regime to an altered regime characterised by the release of water to support low-water flows.
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