Orinoco, one of the great rivers of South America, has its origin on the slopes of the Sierra Parima, in the extreme south-east of Venezuela; its exact sources were only discovered in 1886 by M. Chaffanjon. It flows at first west by north, a mountain-stream, as far as 67° W. long. A little below Esmeralda (65° 50' W. long.) it divides and sends off to the south an arm, the Cassiquiare, which, after a course of 180 miles, enters the Rio Negro, a tributary of the Amazon. The other branch on reaching San Fernando (68° 10' long, and 4° 2' N. lat.) is met by the strong current of the Guaviare; the united stream then turns due north, and, after passing over the magnificent cataracts of Maypures and Atures (glowingly described by Humboldt), and picking up the Meta on the left, meets the Apure, which likewise strikes it from the left. Below the confluence with the Apure the Orinoco turns east and traverses the llanos of Venezuela, its waters, with an average breadth of 4 miles, being augmented from the right by the Caura and the Caroni. About 120 miles from the Atlantic, into which it rolls its milk-white flood, its delta (8500 sq. m.) begins. Of the numerous mouths which reach the ocean over 165 miles of coast-line only seven are navigable. The waterway principally used by ocean-going vessels, which penetrate up to Ciudad Bolívar (Angostura), a distance of 240 miles, is the Boca de Navios, varying in width from 3½ to 23 miles. The total length of the river is some 1550 miles, of which 900, up to the cataracts of Atures, are navigable, besides a farther stretch of 500 miles above the cataracts of Maypures; area of drainage basin, 368,600 sq. m. Most of the larger affluents are also navigable for considerable distances, the
Meta, for instance, to within 60 miles of Bogotá, the capital of Colombia. As a rule the river floods the districts adjoining its banks from May to January, the country under water sometimes measuring 100 miles across.
See A. von Humboldt and Bonpland, Voyage au Nouveau Continent, vol. ii.; Sir Robert Schomburgk, Travels in Guiana (1840); Michelina y Rojas, Exploracion Oficial (Brussels, 1867); and Chaffanjon, Comptes Rendus of Paris Geog. Soc. (1887).