Rio Negro

Rio Negro (English: Black River) is the largest left tributary of the Amazon and the largest blackwater river in the world. It has its sources along the watershed between the Orinoco and the Amazon basins, and also connects with the Orinoco by way of the Casiquiare canal. In Colombia, where its sources are, it is called the Guainía River. Its main affluent is Vaupés, which disputes with the headwaters of the Guaviare branch of the Orinoco the drainage of the eastern slope of the Andes of Colombia. The Rio Negro flows into the Rio Solimões to form the Amazon River near Manaus, Brazil.

Rio Negro is navigable for 450 miles (720 km) above its mouth for 4 feet (1 m) of water in the dry season, but it has many sandbanks and minor difficulties. In the wet season, it overflows the country far and wide, sometimes to a breadth of 20 miles (32 km), for long distances, and for 400 miles (640 km) up, as far as Santa Isabel. It is a succession of lagoons, full of long islands and intricate channels, and the slope of the country is so gentle that the river has almost no current. But just before reaching the Vaupés, there is a long series of reefs, over which it violently flows in cataracts, rapids and whirlpools. The Vaupés is full of similar obstacles, some fifty rapids barring its navigation, although a long stretch of its upper course is said to be free from them, and to flow gently through a forested country. Despite the impediments, canoes ascend this stream to the Andes.

While the name Rio Negro means Black River, its waters aren't exactly black -- they are similar in color to clear tea. The name arises from the fact that it looks black from afar.