Orinoco River

• Category: Inland Aquatic Biomes.
• Geographic Location: South America.
• Summary: The third-longest river in South America, the Orinoco, has been damaged by industrialization and exploitation of natural resources.

The Orinoco River is the third-longest river in South America, extending about 1,370 miles (2,200 kilometers) from its headwaters at the Cerro Delgado Chalbaud at the Venezuela border with Brazil to its mouth on the North Atlantic Ocean. Throughout most of its course, the river flows through Venezuela and through a wide variety of landscapes, including the impenetrable rainforest, the vast savanna also known as the llanos, marshlands, and floodplains. The navigable capacity of the Orinoco River makes it a transportation pathway that connects the different regions of the basin. In fact, the name Orinoco comes from the Guarani dialect and means “a place to paddle,” referring to its navigable features.

The Orinoco River basin, also known as Orinoquia, is a diverse ecosystem with a total area of 416,990 square miles (1.1 million square kilometers), covering about one-fourth of Colombia and four-fifths of Venezuela. The Orinoquia boasts rich ecological and cultural diversity, being one of the most vital reservoirs of South America and home to 26 different Amerindian groups. The basin can be divided into four regions: the Andean mountains, the plains and savannas also known as the llanos, the highlands of Guayana, and the Delta region. The Orinoco’s tributaries flow mainly from the south and west—from the Guyana highlands in the southeast and the Andes Mountains in the northwest. These tributaries include Rios Caura, Paraqua, Caroni, and Guaviare from the south, Rios Arauca, Apure, and Meta from the west, and Rio Cojedes from the northwest.

Climate and Vegetation

The climate of the Orinoco River Basin is tropical, with slight variations depending on the geographical features of each region. According to the precipitation range, there are two distinct seasons: a dry season from November to April and a wet season from April to November. Rainfall, air temperature, and altitudinal zonation vary throughout the basin, thus defining diverse forms of landscape and vegetation.

The Andean Orinoquia is formed by irregular and uneven slopes, with moraines and terraces in the mountainfront and canyons in the Piedmont. The annual temperature varies from 35 to 54 degrees F (1.5 to 12 degrees C), and the annual precipitation ranges from 59 to 79 inches (150 to 200 centimeters). Three vegetational layers characterize the region: the paramo community, with herbaceous vegetation thriving above 13,100 feet (4,000 meters); the Andean bush, found at 9,840–13,100 feet (3,000–4,000 meters), dominated by shrubs; and Andean forest—below 9,840 feet (3,000 meters), with arborescent vegetation.

The Orinoco plains comprise a heavy, clay sediment layer from the Quaternary. This region suffers from poor drainage and, therefore, is susceptible to flooding during the wet season. The climate is continental, with a mean temperature of 75–82 degrees F (24–28 degrees C). Every four to five years, this region experiences major floods.

According to the altitudinal zonation, this part of the biome is characterized by two well-defined landscapes: the high llanos, extending from the piedmont to 330 feet (100 meters), and the low llanos, below 330 feet (100 meters). There are four vegetation types: dry savannas, wet savannas, forests, and swamps. The boundary between forests and savannas is dynamic due to the effect of fires, fluctuations in the hydrological regime, and human activities.

The Guayana region lies on the Precambrian Guayana Shield, and most of the soils are acidic. The two well-defined physiographic units are the plains and peneplains, found below 1,640 feet (500 meters), with an annual mean temperature above 77 degrees F (25 degrees C) and a peak during the dry season of 93–104 degrees F (34–40 degrees C); and the hills, piedmont, and highlands, set at 1,640–9,840 feet (500–3,000 meters), with a mean temperature of 64–75 degrees F (18–24 degrees C) and mean annual precipitation of 59–79 inches (150–200 centimeters). The vegetation varies from dense evergreen and deciduous forests to savannas with gramineous and wide-leaved herbaceous plants.

The Delta region is a vast area formed by Quaternary sediments. Marshlands, both marine and fluvial, dominate the landscape, with a slope less than 2 percent. The mean annual precipitation is 75 inches (190 centimeters), and the annual mean relative humidity is 80 percent. The prelittoral, intermediate, and littoral zones are the three geographical zones of the Delta region. The prelittoral zone lies inland and is the highest portion of the delta. At this point, the Orinoco River merges with the Delta. The river diversifies in multiple channels, with brown water bordered by sand banks 10–13 feet (3–4 meters) high.

Vegetation was once gallery forests and palm trees, but it has been completely altered by humans. The intermediate zone is subject to seasonal flooding and twice-daily tides. The littoral zone is characterized by mangrove trees and swamp forests and is permanently flooded due to heavy rainfall and tidal fluctuation. The intermediate and littoral zones comprise the Delta’s wetlands.

Biodiversity

The Orinoco River basin has very high biodiversity and is one of the most prolific regions in the world in terms of birds. The basin boasts more than 1,000 avian species. In the Andes area surrounding the river are golden-headed and crested quetzal, emerald toucanet, the endemic (not found elsewhere) rose-headed parakeet, Andean condor, and more than 35 species of hummingbirds. There are several dozen varieties of tanagers, several species of hawks, fruiteaters, woodcreepers, and woodpeckers.

In the llanos or plains area, there are scarlet macaw, yellow-crowned parrot, three species of storks including jabiru, seven species of ibis including scarlet, more than 12 species of heron and egret types, more than twelve different shorebirds, wattled jacana, gray-necked wood-rail, three species of whistling-ducks that can be seen sometimes in flocks of several thousands, plus muscovy and comb duck, Orinoco goose, horned screamer, great black and white-tailed hawks, ornate hawk-eagle, snail kite, king vulture, aplomado falcon, owls, the rare and primitive hoatzin, the near-endemic yellow-knobbed curassow, the pale-headed jacamar, nightjars, nighthawks, more than 40 flycatchers, manakins, antbirds, puffbirds and many more.

In the Guyana highlands area, there have been more than 500 bird species identified, including red-billed toucan, black-necked and green aracari, Guianan toucanet, red and green macaw, caica, red-fan parrot, tepui parrotlet, hawks, eagles, falcons, hummingbirds, the Guinean cock-of-the-rock, pompadour cotinga, spangled cotinga, the amazing capuchin bird, antbirds, woodpeckers, puffbirds, jacamars, tanagers, and flutist wrens.

Among the large diversity of fish are the piranha, the electric eel, and the laulao, a catfish that can weigh more than 200 pounds (91 kilograms). Other fish species commonly found in various parts of the river include: rubi tetra, cardinal tetra, checkerboard cichlid, twig catfish, leaf fish, peacock bass, pike cichlids, red hook, oscar, spotted leporinus, basketmouth cichlid. These are just a few of the more than 1,000 fish species in the Orinoco. Also found in the river are the Orinoco crocodile (endangered), pink and grey river dolphins (endangered), giant river otters, the giant anaconda, and the Arrau turtle.

Most mammals of the biome dwell in the gallery forests of the Llanos region. Common dwellers of the forests are deer, rabbits, tapirs (which are an endangered species), jaguars (the third-largest feline species and listed as near threatened), armadillos, capybaras (the largest living rodent species), shrews, bats, and opossums. The spectacled bear, also known as the Andean bear, is the last remaining short-faced bear species. Its status is vulnerable. The harpy eagle population in the greater region is believed to be declining due to loss of forest habitat and in 2021 was assessed as vulnerable, according to the International Union for Conservation of Nature and Natural Resources.

Human Impact

Most of the indigenous groups of Venezuela dwell within the Orinoco River basin. Some of the major aboriginal settlements include the Maquiritare in the rainforest area; the Warao in the delta region, and the Guaica, Guahibo, Yaruro, and Yanomami in the llanos. Before the 1950s, the settlements were limited to a few villages, but after oil and gas strikes at el Tigre and Barinas, slow but steadfast industrialization and urbanization of the region began. Several towns grew into sizable cities of 10,000 or more inhabitants. Intensive agricultural practices were adopted along the river valleys and Andean piedmont.

The Orinoco River basin is rich in natural resources. The vast plains of the Llanos region are used to raise livestock, in particular cattle. Plantations of cotton, rice, and sugarcane are also harvested on a large scale. The Guayana region is rich in mineral deposits, including iron, nickel, manganese, bauxite, vanadium, diamonds, chromium, and gold. Pollution from mining practices negatively affects the water quality within the river. While mining threatens the region, the largest exploitations are petroleum and natural gas in the Orinoco Llanos and Orinoco Delta regions, followed by industrial development around Guayana City, where steel, aluminum, and paper factories have been established.

Industrial success and hydrocarbon exploitation are possible due to the construction of the Macagua and Guri dams, which provide the energy needed. Industrialization also brought major road constructions to connect the newly formed cities with the rest of the country. A bridge across the Orinoco at Bolivar City connected the llanos with the Guayana region in 1967. Around that time, a bridge was built across the Caroni River to connect the industrial city of Puerto Ordaz with the San Felix port, creating the new city of Guayana. The latest one was later connected to Caracas by a major highway.

The predictions for the impact of climate change on Venezuela in general, and the Orinoco basin in particular, are increases in temperatures, decreases in precipitation and increases in dry land masses prone to desertification. Floods may also severely affect agriculture, much as they already have in the past. In Venezuela, nearly 95 percent of agricultural land is rain-fed, making this sector vulnerable to any changes in climate.

Fortunately, much of the Orinoco River basin is still forested. Misión Árbol is an initiative the government created in 2006, aimed at increasing reforestation activities in the country to help increase carbon-capturing capacity of the land.

Bibliography

Echezuria H., J., et al. "Assessment of Environmental Changes in the Orinoco River Delta." Regional Environmental Change, vol. 3, 2002, pp. 20-35.

Gasson, R. "Orinoquia: The Archaeology of the Orinoco River Basin." Journal of World Prehistory, vol. 6, no. 13, 2002, pp. 237-311.

Hilty, Stephen L. Birds of Venezuela. Princeton University Press, 2002.

Lewis, W., Jr., et al. "The Orinoco River as an Ecological System." Interciencia, vol. 15, no. 6, 1990, pp. 346-57.

Mosquera-Guerra, Federico, et al. "Home Range and Movements of Amazon River Dolphins Inia Geoffrensis in the Amazon and Orinoco River Basins." Endangered Species Research, vol. 45, 2021, pp. 269-82, doi.org/10.3354/esr01133. Accessed 31 Oct. 2024.

World Bank. "Venezuela: Country Note on Climate Change Aspects in Agriculture." World Bank, siteresources.worldbank.org/INTLAC/Resources/Climate‗VenezuelaWeb.pdf. Accessed 31 Oct. 2024.