Manaus, Brazil, emerged as an important port city in the nineteenth century during the rubber boom. Today, with more than 1.7 million inhabitants, it is the largest city in the Amazon Basin. It is located on the banks of the Rio Negro (darker, to the left) at that river's confluence with the Amazon River (lighter, to the right). Although it is 1450 kilometers upstream from the Atlantic Ocean, Manaus remains an important port city for oceangoing vessels. It currently exports rubber, Brazil nuts, lumber, and various fruits. Recent discoveries of oil and manganese (used in steel production) may further invigorate its economy. Surrounding this city for hundreds of kilometers in all directions is dense rainforest. Because there are no bridges spanning the rivers in the vicinity of Manaus, ships are used to transport goods across these rivers.
In this scene, the silt-laden water of the Amazon River meets the black water of the aptly named Rio Negro, and these rivers flow eastward (toward the top right). The Amazon, which originates in the Andes Mountains, carries vast quantities of light-colored eroded sediment. The Negro by contrast originates in the lowland rainforest so that it causes almost no erosion, and thus carries no visible sediment. Its load is mainly chemical, in the form of rotting vegetation that gives it a dark color. The waters do not mix completely for many kilometers downstream, and the mixing zone continues all the way to the top of the view. The Amazon and Negro are volumetrically the two largest rivers in the world before they join, so that from their confluence they make a truly gargantuan river.