The diatom and pollen assemblages of the upper 4.5 m of a core recovered from the bank of the Senegal River upstream of St Louis (16.03 N, 16.48 W) reveal an exceptional diversity (39 diatom taxa from which 5 mentioned for the first time in Senegambia and 147 pollen taxa), allowing reconstruction of the environmental history of the northern Sahel over the last millennium. Our study confirms that the Little Ice Age was a period of increased dryness. This resulted in the disappearance of tropical trees that were developed in gallery forests during the Holocene humid period. However, freshwater inputs from the Senegal River remained constant during the whole millennium while being strongly impacted by sea level variations: An episode of sea level rise, already recorded in the Ferlo River valley, led to a supply of salt waters up to 1250 CE. It was only after 1720 CE that a dramatic event occurred with the massive development of marine to brackish diatoms at the expense of freshwater forms as a result of the weakening of the Senegal River inputs. Increased dry conditions during the second half of the 18th and the 19th centuries culminating 1826 CE led to the salinization of the core site with the development of Avicennia populations and halophilic plant species from the rear mangrove.