Riverine forests were studied in the River Garonne floodplain as sites of maximum interactions between a river and its valley. A historical perspective was used as an aid to explain present processes. In the floodplain the vegetation dynamics are still largely influenced by floods in contrast with other more industrialized European rivers. Intensive agriculture in the floodplain results in high nitrate concentrations in the groundwater. Denitrification was shown to be an important process within the riparian zones. Primary productivity of these riparian zones is high owing to water and nutrient availability. In the river system, the fate of leaf litter is more linked to physical factors and processes than to the nature of leaf material or to activities of invertebrate organisms.