Orinoco River

Physiography of the Orinoco

The western slopes of the Sierra Parima, which form part of the boundary between Venezuela and Brazil, are drained by spring-fed streams that give rise to the Orinoco River. The source is placed in Venezuela, at the southern end of the Parima Mountains, near Mount Delgado Chalbaud, at an elevation of some 3,300 feet (1,000 meters). From its headwaters the river flows west-northwest, leaving the mountains to meander through the level plains of the Llanos. The volume of the river increases as it receives numerous mountain tributaries, including the Mavaca River on the left bank and the Manaviche, Ocamo, Padamo, and Cunucunuma rivers on the right.

Below the town of Esmeralda, some of the waters of the Orinoco flow south into the Casiquiare River (Brazo Casiquiare; sometimes called the Casiquiare Channel). This channel, a feature peculiar to the Orinoco River system, is a natural passage that flows generally south until it combines with the Guainía River to form the Negro River, thus linking the Orinoco and Amazon river systems.

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After its bifurcation in the Casiquiare, the Orinoco bends to the northwest and flows in great meandering curves to its confluence with the Ventuari River. There the river turns to the west to run between high alluvial banks, its course marked by extensive sandbars. Near San Fernando de Atabapo, the Atabapo and Guaviare rivers join the Orinoco, marking the end of the upper Orinoco.

Downstream from San Fernando de Atabapo, the river flows northward and forms part of the border between Venezuela and Colombia. It passes through a transitional zone, the Region of the Rapids (Región de los Raudales), where the Orinoco forces its way through a series of narrow passages among enormous granite boulders. The waters fall in a succession of rapids, ending with the Atures Rapids. In this region, the main tributaries are the Vichada and Tomo rivers from the Colombian Llanos, and the Guayapo, Sipapo, Autana, and Cuao rivers from the Guiana Highlands.

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The Atures Rapids mark the beginning of the lower Orinoco basin, in which the river makes its great bend to the east. In this section, the river flows slowly through the lowest level of the plains and increases to about five miles in width. Along the bend, it receives the largest number of tributaries of its entire course, including the Meta, Arauca, and Capanaparo rivers. The Apure River contributes waters from numerous Andean streams, which form a swampy maze in their lower courses.

From its junction with the Apure, the Orinoco meanders eastward over gently sloping plains. Shoals and alluvial islands are abundant; some of the islands are large enough to divide the channel into narrow passages. Tributaries include the Guárico, Manapire, Suatá (Zuata), Pao, and Caris rivers, which enter on the left bank, and the Cuchivero and Caura rivers, which join the main stream on the right. So much sediment is carried by these rivers that islands often form at the mouths. The Caroní River, one of the Orinoco’s largest tributaries, joins the river on its right bank after passing through the Guri Reservoir formed by Guri (Raúl Leoni) Dam, above Ciudad Guayana (also called Santo Tomé de Guayana). Farther upstream, on the Churún River (a tributary of the Caroní), are Angel Falls, the highest waterfall in the world (3,212 feet [979 meters]). Many lagoons, including the Mamo, Amana, and Colorada, are located on the banks of the Orinoco west of its confluence with the Caroní and east of Ciudad Bolívar.

About 30 miles downstream of Ciudad Guayana, at the town of Barrancas, the Orinoco begins to form its great delta. The delta extends for about 275 miles along the Atlantic coast, from Pedernales on the Gulf of Paria in the northwest to Barima Point in the southeast on the Boca Grande (literally, the “Great Mouth”). Scores of islands are connected by innumerable canals (caños), which form an intricate network. The main channel of the Orinoco, known in the delta as the Río Grande, flows eastward from Barrancas to discharge into the Boca Grande.

Physiography of the Orinoco Llanos

The Llanos encompasses nearly all of the western lower Orinoco basin, occupying some 220,000 square miles (570,800 square km); most of the land is less than 1,000 feet (305 meters) above sea level. The High Plains (Llanos Altos) are most conspicuous near the Andes, where they form extensive platforms between rivers and are some 100 to 200 feet above the valley floors. Away from the mountains they are increasingly fragmented, as in the dissected tableland of the central and eastern Llanos (the Sabana de Mesas) and the hill country (serranía) south of the Meta River in Colombia. The Low Plains (Llanos Bajos) are defined by two rivers, the Apure in the north and the Meta in the south. The lowest portion of the Llanos is an area that lies to the west of the lower Orinoco valley; this area is converted annually into an inland lake by flooding.

In addition to the Apure and the Meta, the principal streams draining the Llanos include the Guaviare and Arauca rivers. Seasonal changes between saturation and dehydration have led to advanced laterization of the soil, the process in which the base minerals have been leached away or incorporated into insoluble iron and aluminum silicates. Fine-grained soils form hardpans (cemented layers of soils), and in gravel regions, iron-cemented quartz conglomerates underlie the surface. Excessive acidity and the lack of nutrient bases, organic matter, and nitrogen make virtually all mature soils infertile.