The time series of 97 annual values, observed or reconstituted, of runoff of the River Amazon at Óbidos shows a slight increase of 9% in the mean annual discharges from 1903 to 1999, and of 10% for the floods. All the rupture tests indicate a break in 1970, for the mean annual discharges as well as for the floods. The evolution of the coefficients of variation indicates a reduction in the interannual variability of the mean discharges and floods, from 1927 to 1970. The comparison between the evolution of the mean annual discharges and the average annual rainfall of the basin, from 1945 to 1998, exhibits a good correlation between precipitation and runoff. Finally, the analysis of the evolution of the mean annual discharge and the average annual rainfall, expressed as a percentage of the deviation from the 1945-1998 averages, demonstrates an increase in the flow in relation to the rainfall, which could be the consequence of the Amazonian deforestation.