ORINOCO, a river and river system of northern South Amer ica. Its basin embraces about one-half of the eastern plains of Colombia and nearly all of Venezuela south and cast of the Andes; it ranks third in area among the South American river basins. The main river is about 1,5oo m. long from its source in the Sierra Parima on the Venezuela-Brazil boundary to the Atlantic ocean, and, except for a stretch of about 200 m. between the mouths of the Guaviare and Meta rivers, where it forms a part of the Vene zuela-Colombia boundary, it is in Venezuela. At about 1 so m. from its source it bifurcates into the Casiquiare canal to the Rio Negro of the Amazon system, affording a through water way to the Amazon basin. Three major rapids and numerous minor ones obstruct the upper river. At the mouth of the Apure the Orinoco is 2 m. wide in the dry season and often 7 m. wide in time of floods, and narrows to Boo ft. at the city of Ciudad Bolivar. Large steamers can navigate as far as the Cariben rapids, 700 m. from the ocean and only 6 m. from the mouth of the Meta. At the Cariben rapids the difference between high and low water averages 32 ft., while at the Angostura, at Ciudad Bolivar, the average rise is 5o ft. and has been known to reach 6o.
The Orinoco enters the ocean by a delta of approximately 700 sq.m. of islands and swamps covered with dense vegetation and so little above sea-level that they are periodically flooded. The Boca Grande at the mouth of the Corosimi river (the southern most channel of the delta) is the deepest outlet; but the Cano Macareo, one of the outlets of the Vagre river (the westernmost channel of the delta), is usually taken by steamers because it is the most direct navigable route to the Gulf of Paria.
Ordaz, whose expedition (1531-32) entered the Orinoco by the Boca de Navios and, with much loss of life, ascended to the mouth of the Meta, was the first to explore any part of the river, although Columbus, while exploring the Gulf of Paria in 1498, noticed the freshness of its waters without investigating their source, and Ojeda, following closely the track of Columbus in 1499, probably passed in sight of one or more of the mouths of the river. There have since been many expeditions and surveys.
Except for a few outliers which form isolated hills north of the lower river, the Orinoco is the dividing-line between the llanos and the highlands of Venezuelan Guiana. The tributaries from the Guiana highlands are little known. They all have their sources in the divide which carries the Venezuela-Brazil boundary. The largest, the Ventuari, joins the Orinoco about 90 m. above the mouth of the Guaviare. The Caura, the next large stream to the east, is much obstructed with falls and rapids, while the lower Caroni is more or less navigable for some 400 miles. The llanos are divided into a broad, well-watered western section and a nar rower and much drier eastern section. North of the upper Gua
viare and its tributaries the western section rises gently from an elevation of about 13o ft. at the mouth of the Apure to about 700 ft. at the border of the Andes. Its principal rivers have their sources in innumerable mountain torrents that rise in the eastern ranges of the Eastern Cordillera of Colombia. From the Guaviare north to 6° 3o' N. lat. these streams are fed so abundantly by the condensation of moisture from the north-east trade winds that they have headed far back toward the high savannas of the East ern Cordillera.
The most important tributaries of the Orinoco in this section are the Apure, Arauca, Meta and Guaviare. The Apure was, until recently, formed by the junction of the Uribante and Sarare, hut the latter has now dammed its lower channel and flows to the Arauca, leaving the Uribante, an important stream with 16 large affluents, and the Nula, formerly a tributary of the Sarare, as the chief sources of the Apure. The Apure is about 65o m. long from the Uribante-Nula junction. It is in large part navigable for large craft but is obstructed by rapids about i oo m. from its mouth. The northern tributaries drain the eastern and southern slopes of the Venezuelan Andes. Of these the Portuguesa, which the cattle route from San Fernando to Puerto Cabello follows, and a few others, are navigable for short distances. The Arauca parallels the Apure on the south. When in flood, the additional water now brought to the Arauca by the Sarare causes it to overflow and send part of its surplus water by several canos to the Capanaparo, a tributary which joins the Orinoco about 18 m. S. of the mouth of the Arauca.
The main stream of the Guaviare, southernmost of the great western tributaries of the Orinoco, is known as the Guayabero from its source to the mouth of the Ariari, a large tributary from the north-west. It marks in a general way the dividing line be tween the low savannas of the north and the higher and more varied topography of the south. The main stream and many of its tributaries after leaving the piedmont cut their way through four distinct zones: (I) a dissected sandstone plateau, the crests of which rise 1,5oo ft. above the llanos; (2) a zone east of the escarpment of the plateau of sandstone remnants separated by clayey depressions; (3) a crystalline peneplain where the granites and gneisses are mostly covered with lateritic clays; and (4) a low plain on which stand isolated massifs of the dome-like form characteristic of crystalline mountains in wet tropical climates. The Ariari tributary is navigable for large barges for a consider able distance. About 30o m. of the Guayabero-Guaviare are said to be navigable but narrows and rapids which mark the passage of the river from one zone to another offer serious obstacles.
(R. R. P.)