Concern is mounting over widespread mortalities of the conspicuous F. albida trees along the ephemeral rivers in Namibia. Causes underlying the mortalities of F. albida are the lowering of the water table, natural senescence and a lack of recruitment of young trees or a combination of all three. In this study seasonal water-use efficiency determined by stable carbon isotopes and water-stress estimates provides a measure of both water used by the plant and of the water-stress impact on plant performance. Size class structure and mortality of F. albida along the Kuiseb river was also determined. The results suggest that pumping and a reduced number and magnitude of floods are the main cause of F. albida mortality on the Kuiseb river. Plants are unable to cope with the lowering of the water table and smaller trees appear to be more vulnerable than older trees. Human interference in the water cycle of the Kuiseb river and natural senescence appears to be driving the population dynamics of F. albida. In the absence ofremedial measures the recruitment of this economically important species will remain constrained.