Due the large extent (about 600 000 km2) of the Amazonian flood plain it is expected to greatly influence the Amazon River dynamics, in terms of suspended solids and chemical fluxes. However, up to now, fluxes and exchanges between the main stream and its flood plain are poorly known and it is still difficult to precisely quantify to what extent the flood plain may play a significant role. For several years, in the framework of the HyBAm (Hydrology and Geochemistry of the Amazonian Basin, IRD-CNPq) research programme, in situ hydrological and geochemical conditions have been investigated in a flood plain located in the lowest part of the river course. This data set enabled development of a model describing fluxes exchanged between the flooded area and the Amazon River. The model - in a relatively simple form - was in good agreement with the data. The results show that the flood plain is a source of water and of redox sensitive elements, such as arsenic, for the Amazon River.