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(ōrēnō`kō), 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 BolívarCiudad Bolívar
, city (1990 pop. 225,340), capital of Bolívar state, E Venezuela, an inland port on the Orinoco River. It is the commercial center of the eastern llanos, the Orinoco basin, and the Guiana Highlands.
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 and Ciudad GuayanaCiudad Guayana
, formerly Santo Tomé de Guayana,
city (1990 pop. 453,047), Bolívar state, Venezuela, at the confluence of the Caroní and Orinoco rivers in the Guiana Highlands.
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, 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).



a South American river in Venezuela and Colombia. It is 2,500 to 2,730 km long (sources differ) and drains an area of 1,086,000 sq km.

Rising on the western slopes of the Parima Mountains in the southwestern Guiana Highlands, the Orinoco flows through the Guiana Lowlands and empties into the Atlantic Ocean, forming a delta. It receives the Ventuari, Caura, and Caroni from the right and the Guaviare, Vichada, Meta, Arauca, and Apure from the left. In its upper course the Casiquiare River branches off to the left, carrying about one-third of the Orinoco’s flow into the Amazon basin. Until the mouth of the Meta River the Orinoco flows through rugged terrain, forming rapids, particularly in the section between the mouths of the Vichada and Meta rivers.

In its middle course the Orinoco becomes a deep river 1–1.5 km wide (sometimes increasing to 3 km) and 10–20 m or more deep. Here, its broad valley (3–10 km) forms narrows, called angosturas, in places. The last such narrow stretch is in the lower course, near Ciudad Bolívar, after which the river flows through a broad valley, dividing into numerous branches and channels. Near Barrancas, 200 km from the sea, the river begins to form a swampy delta covering 20,000 sq km and extending for about 300 km along the coast. In the delta the Orinoco divides into 36 branches and numerous channels. The main branches are the Mánamo (far left), Macareo (navigable), Araguao, and Boca Grande (right), the largest branch, with widths of 15–20 km.

The Orinoco is fed mainly by rain. Its water level and discharge vary greatly during the year. In the lower course, near Ciudad Bolívar, high water begins in the second half of April or early May. The water reaches its highest level in September, after which it gradually recedes, reaching its lowest point in March or April. At the mouth of the Meta River the water level rises 8–10 m, and near Ciudad Bolívar, 10–15 m. Marine tides affect the river as far as Ciudad Bolívar. During syzygy tides the water level rises by about 2 m. At the head of the delta the average annual flow rate is about 29,000 cu m per sec and the annual discharge totals about 915 cu km. During high water, the rate of flow may reach 50,000–55,000 cu m per sec or more. During the dry season that lasts from November to April and in years of little rainfall the rate of flow decreases to 5,000–7,000 cu m per sec. The river carries 45 million tons of sediment annually.

There are about 12,000 km of navigable waterways in the Orinoco basin. Oceangoing vessels with a draught of 8 m travel as far as Ciudad Bolívar, about 400 km from the mouth. During the rainy season river boats travel upstream as far as the Guaviare River, halting at the rapids. Whereas the right tributaries of the Orinoco are navigable only along their lower courses, the left tributaries are navigable throughout their courses for the greater part of the year. The Orinoco’s hydroelectric resources are as yet poorly developed. In 1974 a system of hydroelectric power plants was under construction on the Caroní River. The principal cities along the river are Santa Bárbara, Puerto Ayacucho, Ciudad Bolívar, and Puerto Ordaz in Venezuela and Puerto Carreño in Colombia.

In 1498, Columbus reached one of the estuary branches of the Orinoco, and it is believed that members of the Spanish expedition of A. de Ojeda and A. Vespucci saw one of the Orinoco’s branches. In 1531 the Spanish conquistador Diego de Ordaz first navigated the Orinoco to its confluence with the Meta River. In 1800 the German scientist A. Humboldt and the French botanist A. Bonpland traveled up the Orinoco and established the linkage between the Orinoco and Amazon systems. The sources of the Orinoco were discovered by a Franco-Venezuelan expedition in 1951.


Grelier, J. Aux Sources de I’Orénoque. Paris, 1954.
Gómez, P. R. “La hoya hidrográfica del Orinoco y la Orinoquia Colombiana.” Boletin de la Sociedad Geografica de Colombia, 1960, vol. 18, no. 65.
Perrin, P. “Caractéristiques générales des rivières vénézuéliennes.” Revue de géographie Alpine, 1969, vol. 57, fasc. 2.



a river in N South America, rising in S Venezuela and flowing west, then north as part of the border between Colombia and Venezuela, then east to the Atlantic by a great delta: the third largest river system in South America, draining an area of 945 000 sq. km (365 000 sq. miles); reaches a width of 22 km (14 miles) during the rainy season. Length: about 2575 km (1600 miles)