Abstract The Saloum river (Sénégal, West Africa) is an inverse estuary, with salinities of more than 80 g l −1 reached 100 km from the sea. Monthly salinity measurements have been done 120 km inland since 1927. Seasonal salinity increase (during dry season) proceeds at a constant rate ( ⋍ 0.3 g l −1 d −1 ). This would indicate that evaporating water masses are shallow (average depth ⋍ 0.4 m ). Since 1950, annual maximum and minimum salinities have been increasing, with decreasing rains, at a rate of about 1.3 g l −1 per year. Across the 1927–1987 period, both yearly extremes are well correlated with rainfall in the previous years, indicating a “memory” spanning three years or less. We have computed a water budget as a function of rainfall with three different hypotheses about the extent of the evaporating surfaces. Comparison with actual data indicate that about 60% of the lowlands are evaporating as shallow open waters would. We discuss the implications of these results for the possible future of the estuaries in the region, especially in the “green-house effect” hypothesis.