Orinoco ōrēnōˈkō [key], river of Venezuela, estimated to be from 1,500 to 1,700 mi (2,410–2,735 km) long. Rising near Mt. Delgado Chalbaud in the Guiana Highlands, S Venezuela, the Orinoco flows in a wide arc through tropical rain forests and savannas (llanos), forming part of the Venezuela–Colombia border, and enters the Atlantic Ocean through a large marshy delta (c.7,800 sq mi/20,200 sq km) in NE Venezuela. One of South America's longest rivers, it and its branches drain an extensive basin; the Apure River is its chief tributary. The Orinoco is joined to the Amazon system by the Casiquiare, a natural canal. The huge flow of the Orinoco varies markedly with the season.

Divided into upper and lower courses by the Ature and Maipures cataracts, the river is navigable for most of its length. Dredging permits oceangoing vessels to reach Ciudad Bolívar, c.270 mi (435 km) upstream. The major cities on the river are Ciudad Bolívar and Ciudad Guayana, which developed in an industrial zone in the late 1960s; the river is now crossed by large bridges at both cities.

Christopher Columbus probably discovered the mouth of the Orinoco in 1498, and Lope de Aguirre, the Spanish adventurer, seems to have traveled most of its length in 1560. In 1799, Alexander von Humboldt, the German naturalist, explored the upper reaches, but it was not until 1944 that an aerial expedition sighted the source area in the remote highlands. Further explorations in 1951 and 1956 located two rivulets now considered the headwaters.

See H. Acebes, Orinoco Adventure (1954).

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