Was the 2009 flood the most hazardous or the largest ever recorded in the Amazon?

Floods are fundamental components of Amazon nature and culture. The large flood of 2009, however, opened a new perspective on hazards and disasters in the Amazon basin. More than 238,000 residents from 38 municipalities were affected by floods along the Amazon River and lower reaches of its tributaries. Never before has a flood in the Amazon produced such a dramatic effect on the local population. The magnitude of the disaster suggested it was the largest recorded Amazon flood since the beginning of measurements in 1928 at Óbidos and that it could represent the largest recorded flood on Earth. A complex combination of atmospheric and hydrologic factors made the 2009 Amazon flood the most hazardous. It was the result of large scale and regional climatic events, non-typical mechanisms of flood transmission generating complex inter-relations in time and space between the main system and the tributaries, and recent urban growth of riverine cities without adequate planning. Our measurements at Óbidos, however, indicate that the 2009 flood was the highest recorded Amazon stage, but most likely not the largest water discharge. We propose as well that the magnitude of the Amazon floods at Óbidos has been overestimated for decades and that the available values of flood discharge have been a source of error for a multidisciplinary set of scientists developing climate and environmental modeling.