Orinoco, river, Venezuela, one of South America's longest rivers, extending 2,560 km (1,590 mi). Its source is in the Guiana Highlands, on the slopes of the Sierra Parima, in extreme southeastern Venezuela, on the border of Brazil. It flows northwest to a point near La Esmeralda, where it divides. One arm, the Casiquiare River, goes south and after a course of 290 km (180 mi) enters the Río Negro, a tributary of the Amazon River. The main branch continues northwest to the town of San Fernando de Atabapo, where it receives the Guaviare River and, flowing generally north, forms the border between Venezuela and Colombia. After passing over the Maipures and Atures Rapids and receiving the Meta River on the left, it meets the Apure River. The Orinoco River then turns northeast and traverses the Llanos, or plains, of Venezuela before emptying into the Atlantic Ocean.

The Orinoco, which averages 6 km (4 mi) in width, is augmented from the right by several rivers, including the Caura and the Caroní. The delta of the river, with an area of 21,000 sq km (8,000 sq mi), begins 190 km (120 mi) from the Atlantic. The total area of the drainage basin is 1,200,000 sq km (450,000 sq mi). The Orinoco is navigable for oceangoing ships for 420 km (260 mi), from the mouth to the city of Ciudad Bolívar, the major commercial and communications center for the drainage basin. It is navigable for smaller craft for a distance of 1,600 km (1,000 mi).

The Orinoco was sighted in 1498 by Christopher Columbus and was first explored by Europeans (1530-1531) to the confluence with the Meta River. The German naturalist Alexander von Humboldt explored the upper reaches in 1799. An expedition led by the American physician and explorer Herbert Spencer Dickey claimed to have reached its headwaters in 1931. Several Brazilian and Venezuelan expeditions in 1944 and in the 1950s penetrated further to the site that is now accepted as the headwaters.