A worsening drought in the Amazon basin has prompted Brazil to extend an emergency across the Amazonas state.
Lakes such as the Anama have been drying up in the drought
Brazil's military has been distributing supplies and medicine to tens of thousands of people stranded by the dramatic drop in water levels.
Witnesses say rivers and lakes have dried up completely, leaving behind kilometres of sand and mud.
Environmental campaign group Greenpeace has blamed deforestation and global warming for the drought.
It quoted scientists as saying that the burning of forests has raised temperatures in the Amazon, preventing the formation of clouds.
Brazilian government meteorologists, however, have said the drought is the result of unusually high temperatures in the Atlantic Ocean, that have also been linked to this year's devastating hurricanes.
A state of emergency has been declared in all 61 municipalities of Brazil's Amazonas state as the drought has started affecting towns and cities further downstream, reports the BBC's Tom Gibb in Sao Paolo.
Brazil's armed forces have been delivering water, food and medical supplies to communities isolated by the worst drought in the Amazon for decades.
The air force has been distributing water-purifying chemicals to counter the threat of disease from water supplies contaminated by dead fish in the Amazon.
Low river levels are preventing boats - for many the only means of transport - from using the Amazon safely, leaving communities depending on government airlifts for their survival.
Big ships have been left stranded in the world's second-largest river and millions of fish are rotting in the sun, witnesses say.