Geologically, the Amazon basin is a sediment-filled structural depression between crystalline highlands of Brazil and Guiana. The riverbed (1–8 mi/1.6–12.9 km wide) is in a wide floodplain that is up to 30 mi (48 km) wide. For much of its course, the Amazon wanders in a maze of brownish channels amid countless islands, but is unobstructed by falls.

Its headstreams, however, arise cold and clear in the heights of the Andes. They descend northward before turning east to join and form the Amazon (which is, however, occasionally called the Solimões from the Brazilian border to the junction with the Rio Negro). Of the Amazon's more than 500 tributaries, the chief ones are the Negro, Japurá (Caquetá), Putumayo (Içá), and Napo, which enter from the north; and the Javari, Juruá, Purús, Madeira, Tapajós, and Xingú rivers, which enter from the south. The Casiquiare River, a natural canal, links the Amazon basin (through the Rio Negro) with the Orinoco basin.

Below the Xingú the river reaches its delta, with many islands formed by alluvial deposit and submergence of the land. Around the largest of these, Marajó, the river splits into two large streams. The northern stream is the principal outlet and threads its way around many islands. The southern channel, called the Pará River, receives the Tocantins River and has the important port of Belém. The awesome tidal bore (up to 12 ft/3.7 m high) of the Amazon is called pororoca; it travels c.500 mi (800 km) upstream. The river's immense silt-laden discharge is visible far out to sea.

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