The river flow regime of the Amazon basin exhibits considerable variability at the interannual and interdecadal scales. A major source of variation is the El Niño-Southern Oscillation (ENSO) events. El Niño events cause a decrease in rainfall, river flow, and inundation patterns throughout the entire region, with the strongest reductions happening in the northern part of the basin. On the other hand, La Niña events cause increased river flow for the northern tributaries and the main stream, but apparently do not cause a discernible pattern of climate variability in the southern part of the basin. ENSO events are different one from the other. While most El Niño events cause reductions in precipitation and river flow over the entire area of northern Amazonia, some El Niño events change precipitation only over northwestern Amazonia. The strength of the ENSO events through the decades is modulated by an interdecadal signal possibly associated with the Pacific Decadal Oscillation. La Niña (rainy) events are rainier during the 1940s-1950s and 1970s, while El Niño (dry) events are drier during the 1960s and since the 1980s. It is also apparent that the interannual variability was damped during the 1930s-1960s. In addition to these modes of variability caused by varying rainfall patterns, in some regions where changes in land cover are extensive, changes in evapotranspiration may drive increases in river flow, increasing the runoff coefficient. This has been clearly documented for the Tocantins basin, and there is evidence that the Óbidos runoff coefficient is also increasing. © Copyright 2009 by the American Geophysical Union.