The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined (not including Madeira and Rio Negro, which are tributuaries of the Amazon). The Amazon, which has the largest drainage basin in the world, about 7,050,000 square kilometres (2,720,000 sq mi), accounts for approximately one-fifth of the world's total river flow.
In its upper stretches, above the confluence of the Negro river, the Amazon is called Solimões in Brazil; however, in Peru, Colombia and Ecuador, as well as the rest of the Spanish-speaking world, the river is generally called the Amazon downstream from the confluence of the Marañón and Ucayali rivers in Peru. The Ucayali-Apurímac river system is considered the main source of the Amazon.
The width of the Amazon varies between 1.6 and 10 kilometres (1.0 and 6.2 mi) at low stage, but expands during the wet season to 48 kilometres (30 mi) or more. The river enters the Atlantic Ocean in a broad estuary about 240 kilometres (150 mi) wide. The mouth of the main stem is 80 kilometres (50 mi). Because of its vast dimensions, it is sometimes called The River Sea. At no point is the Amazon crossed by bridges. This is not because of its huge dimensions; in fact, for most of its length, the Amazon's width is well within the capability of modern engineers to bridge. However, the bulk of the river flows through tropical rainforest, where there are few roads or cities, limiting the need for crossings.