Not far from the mouth of the Amazon, dead animals, including manatees -- mammals up to 3m long with flat, paddle-shaped fins -- and distinctive pink dolphins, line the banks of some tributaries. Normally, you would have to take a boat to cross these rivers but today, because of the Amazon basin's worst drought in memory, they are little more than mudflats with a trickle of water in the middle.
So far, the drought has had its most serious impact in the upper reaches of the river and its hundreds of tributaries in Brazil, Colombia and Peru. There, along many stretches, the water has fallen to the lowest levels ever recorded and has become impassable even for canoes. Some 600 Brazilian schools in Amazonas state have had to be closed and many hamlets, whose only contact with the outside world is by river, are running short of food and medicines. Several districts have been declared disaster areas and the army is having to bring emergency supplies to 900 towns and villages.
The problems are expected to get worse before the drought eventually breaks, perhaps in the next month when the Amazon's rainy season usually comes.
"Most little towns don't have sewage treatment," says Dan Nepstad, a US scientist based in Brazil. "Their sewerage is to put a pipe into the river. When you reduce the flow of these rivers, you have less water to dilute the sewage and cesspools build up. That has got all the makings for intestinal diseases and cholera. Water that's not moving is also a breeding ground for insects that carry malaria, dengue and other diseases."
But what is worrying some scientists even more than the growing scale of the humanitarian crisis is a suspicion that this year's drought may be the harbinger of a much greater disaster that could push the whole Amazon forest to a critical flip-over point and into an unstoppable process of self-destruction.
This is how the theory goes: the Amazon river contains a fifth of the planet's fresh water. Over 300km wide at its estuary, it carries more water than the world's next nine largest rivers combined. In a remarkable process, much of this water is recycled within the forest.
"The watering of the Amazon basin is a cycle that starts with the trade winds that fly over the surface of the Atlantic Ocean from Africa," explains Peter Bunyard, science editor of The Ecologist and an expert on the Amazon.
"The winds flow over warm tropical water so they become utterly laden with moisture. When the winds reach the Amazon forest, some of the vapor comes down in rainfall, but three-quarters of this rainfall is then recycled back into the atmosphere through evaporation and transpiration -- which occurs when water is sucked up from the soil into the trunks and then out through the leaves," Bunyard says.
"This process of convection, as it is called, occurs seven times as the winds cross the continent. It leads to the absorption of huge amounts of solar energy," he says.
When the winds eventually hit the Andes in the west, a huge air mass is pushed high up into the atmosphere. It moves out of the influence of the spinning of the Earth and drifts back to Africa.
"By the time this air mass reaches Africa, it is high, cold, dry and dense," Bunyard says. "It forms a high-pressure zone and, in sinking, causes the dry winds that blow across the Kalahari and Sahara deserts towards the Atlantic ocean. Africa's deserts are the other side of the coin to the tropical forest in the Amazon."
This process occurring around the very middle of the Earth is the driving force in the world's climate. It is directly responsible for much of the rainfall in South America and even, scientists have discovered, in faraway places such as the corn belt in the US mid-west. If there is poor rainfall in the Amazon's rainy season (November to February), then the US is likely to have a drought four months later during its all-important growing season.
What happens to the Amazon affects the rest of the planet, and scientists have long been aware that if too much forest is felled by loggers, cattle rearers and soya farmers, the convection process will be disrupted, with disastrous global consequences.
"If the Amazon loses more than 40 percent of its forest cover, we will reach a turning point where the world's largest forest will begin an irreversible process of savannization," says Carlos Nobre, a senior scientist at INPE, the Brazilian institute of space research, as well as a leading climatologist.
The consensus is that a smaller forest would be unable to sustain the convection process and will start drying out. A vicious spiral will begin in which the drier forest becomes more vulnerable to forest fires and the fires, by destroying vegetation, will make the Amazon more vulnerable to drought.
Until last week it was widely assumed that only 17 percent of the forest -- an area larger than the size of France -- had been felled.
This is well short of the critical 40 percent postulated by Nobre, but an article printed in Science journal last month shows that the satellite images on which this figure was based were not telling the full story.
The images, the scientists said, were detecting only clear-cut swaths of land, where all the trees had been removed. They were failing to detect so-called "selective logging," where timber companies go into a forest under the canopy and take out valuable hard timber. The scientists reckoned that this kind of activity was destroying on average an additional 15,500km2 of forest each year. This means that the destruction, previously put at about 20,000km2 a year, is almost twice as serious as had been thought.
Dorothy Stang, an American nun who was killed in February by gunmen sent in by local landowners, was a passionate environmentalist. Living in the Amazon basin for over 20 years, she complained in one of her last filmed interviews of the devastation caused by loggers and cattle farmers, which, she believed, was changing the climate.
"It used to rain steadily for nine months," she said. "Now it rains for six or seven months at best. The destruction is killing the forest. We've got to stop it."
Sue Branford was the BBC's Brazil correspondent.