Orinoco, a river of Venezuela, South America, which falls into the Atlantic by numerous mouths between lat. 8° 40' and 10° N, after a course of about 1,500 m. It rises in the Sierra de Parima, near lat. 3° 40' N., lon. 64° 30'W., and flows mainly W. by S. to lat. 3° 10' N., lon. 66° 20' W., about 20 m. W. of the village of Esmeralda, where its waters are connected with those of the Rio Negro, an affluent of the Amazon, by the Cassiquiare. The Orinoco hence flows N. W. until it receives the Ventuari on the right. There it bends W. S. W., but after being joined by the Guaviare on the left at the town of San Fernando, about lat. 4°, lon. 68°, it sweeps around to the north, receiving several smaller affluents in its course, and gradually verging toward the northeast. Near lat. 6° 20', lon. 67° 45', it is joined on the west by the Meta, which forms part of the N. boundary of New Granada. It continues to flow N. E. to its confluence with the Apure, lat. 7° 30', lon. 66° 45', whence it flows nearly E. to the sea, its principal tributaries in this part being the Caura and Caroni, both on the right bank.

About 130 m. from the sea it forms a delta, by sending to the north a branch divided into several streams called the Bocas Chicas, or small mouths, some of which fall into the gulf of Paria and the rest into the Atlantic. The main stream, called the Boca de Navios, is divided for about 40 m. by a line of islands leaving a channel about 2 m. wide on each side. At the great mouth of the river the breadth is upward of 60 m., but a sand bar extends across the navigable channel in the centre, with but 15 ft. of water. Several of the other mouths are navigable, and the main stream may be ascended for about half its length. It has more than 400 navigable tributaries, and at a distance of 560 m. from the sea is more than 3 m. wide. At Angostura, or Ciudad Bolivar, the head of tide water, 240 m. from the sea, it is 4 m. wide and 390 ft. deep. The region drained by the Orinoco, comprising an area of 250,000 sq. m., is entirely occupied by immense plains, stretching from the coast chain to the Parima mountains, and from the Atlantic to the Andes, rising in some parts to the height of 1,300 ft., but in many places little above the level of the sea.

The greater part of these plains is destitute of wood, but there are some dense forests in certain regions on the N. bank, and along the course of the river. The waters of the Orinoco rise from April to October, attaining the greatest height in July and August, which in the upper part of the river is from 30 to 36 ft,, and in the lower 24 to 25 ft,; but in one confined place they are said to rise 120 ft, above the usual level. The vast plains through which the river passes are at this season to a great extent overflowed. Two remarkable rapids occur in the upper parts of the Orinoco, called the Maypures or Apures, and the Atures, the one in lat. 5° 8' N., about 80 m. below the junction of the Atabapo and Guaviare with the Orinoco, the other about 36 m. lower down. These rapids are formed by innumerable little cascades succeeding each other like steps, where numerous islands and rocks so restrict the bed of the river that, though the breadth above is upward of 8,000 ft., there only remains an open channel of 20 ft, in width. It is almost impossible to pass these rapids, and in attempting it the canoes of the natives are often dashed to pieces against the rocks.

From this place the river is navigated by steam to the Meta. Humboldt and Schomburgk are the principal explorers of the Orinoco.