This study forms part of a French Government initiative supporting research into the function of wetlands as regulators of water regimes and supporters of a rich biodiversity. Aquifer recharge has been studied in a section of the floodplain of the River Adour, between Saubusse and Le Vimport, one of the major wetland areas in southwest France. The study has focused primarily on a major flood event observed during 6 to 19 March 1999. The March flood allows 2.1 × 10⁶ m³ of water to be stored in the sub-surface as the water-table rises to the surface of the floodplain giving complete saturation. After the passage of the flood wave, the water table declines to a level that restores the original volume of water stored in the sub-surface. The 7 km² floodplain has an estimated total sub-surface storage capacity of ∼ 6.3 × 10⁶ m³ down to a layer of impermeable clay. During summer low water periods, the sub-surface storage is estimated to be ∼ 4.2 × 10⁶ m³ (i.e., 67% of the total available sub-surface storage in the zone studied). A hydrologic balance was calculated for the flood event that considers inputs and outputs to the floodplain. The input volume from stream overflow, surface runoff, rainfall, and input from a small stream in the floodplain was estimated to be 6.9 × 10⁶ m³ (of which 5.6 × 10⁶ m³ is retained on the surface). Water outputs including drainage channels, direct drainage by the river, and evapotranspiration were estimated to be 7.5 × 10⁶ m³. These volumes contribute water representing ca. 1.4% of the total flood event flow of 5.40 × 10⁸ m³. Relatively low nitrate concentrations and associated low conductivities were observed in the water sampled at a number of piezometers placed within the study zone. A mechanism is suggested that involves the mixing of water of low nitrate content and conductivity from small streams and the sub-surface within the floodplain, with water containing elevated levels of dissolved nitrate and high conductivity from the River Adour.