The large floodplains associated with the Amazon River and its main tributaries affect water transport, influence sediment and chemical budgets and support highly diverse ecosystems and productive fi sheries. Approximately 25% of the Amazon River discharge is routed through these systems along a 2,000 km reach between Sao Paulo de Olivença and Obidos (Riche y et al., 1989), with an extension of over 100,000 km2 (Melack and Hess, 2010). A major challenge in understanding the Amazon fluvial system is evaluating how and to what extent biogeochemical floodplain processes influence the hydrological, chemical and biological dynamics of the Amazon River. We focus our study on one of the largest floodplain systems of the Amazon, "Lago Grande de Curuai", which is representative of the central and lower reaches of the Amazon basin. A ten-year study of the "Varzea do Lago Grande de Curuai" floodplain allowed us to obtain a comprehensive understanding of how hydrological and geochemical processes occurring in floodplains alter the dissolved and particulate flux of the Amazon River. In terms of internal hydrological functioning, we demonstrated that floodplain water balances are influenced by direct rainfall, local runoff and seepage as weil as flooding from the river and that the relative importance of different inputs varies seasonally. Regarding the sediment and particulate carbon budget of the Curuai floodplain, the annual volume of sediment trapped in the floodplain was of the same order of magnitude as the mean annual sediment fluxes outflowing from the floodplain into the Amazon River, and the floodplains act as an important source of particulate and dissolved organic carbon. The water passing through the floodplains undergoes important biogeochemical transformations under the influence of biotic processes, sorption and redox reactions. Floodplains also play an important role in the organic carbon balance of the Amazon basin. Coupling the hydrological model with the database of elemental concentrations, we discuss the conservativity or non-conservativity of certain selected elements and present the mass balance of these elements between the floodplain and the main stream.