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."