page_title: Orinoco Basin
section_title: Relief
hierarchical_section_title: Orinoco Basin / Relief
caption_attribution_description: English: La Piedra del Medio, named by Alexander Humboldt the Orinocometer.
context_page_description: The Orinoco Basin is the part of South America drained by the Orinoco river and its tributaries. The Orinoco watershed covers an area of about 990000 km², making it the third largest in South America, covering most of Venezuela and eastern part of Colombia.
  The Orinoco is one of the most important rivers in the world due to its length and flow, the extent of its basin and especially its historical importance and economic and the meaning it has had for Venezuela, where most of its basin is spread, with almost two-thirds of it. It is probably the largest river in the world in relation to its basin, similar in extent to that of the Danube but five times greater in volume. Its discharge is the third largest of any river worldwide, after the Amazon and the Congo, and much greater than that of many rivers of greater length and basin area.
context_section_description: The sources of the Orinoco River are located at Cerro Carlos Delgado Chalbaud, at 1047 meters above sea level, discovered in 1951 by the Franco-Venezuelan expedition that went back and explored the Upper Orinoco course to the Sierra Parima, headed By Venezuelan army officer Frank Risquez Iribarren. The first reference to this expedition was that of Alberto Contramaestre Torres in 1954. And there are other references to this expedition, for example, that of Pablo J. Anduce. From the birth of the Orinoco at the foot of the hill Delgado Chalbaud (02º19’05”07 Of latitude north, 63º21’42”63 of longitude west and 1047.35 meters of altitude) until its opening in the Atlantic Ocean, the Orinoco describes a great arc and its basin extends like fan, reason why the north-western part Of the basin is somewhat more extensive than the south-east.
  As already indicated, the two sub-regions of the basin have quite different characteristics, due to differences in their geological constitution. The maximum height of the basin is located in the Sierra Nevada del Cocuy, in Colombia (more than 5000 meters above sea level), which forms part of the Eastern Andean Cordillera of Colombia.
  The north-western edge of the basin would consist of the Colombian-Venezuelan Andean slopes and the southern slopes of other mountainous reliefs of northern Venezuela, while the southern edge of the basin would be marked for the most part by the watershed between the Orinoco and the Amazon, which is located on the Guayanés massif. Between the two sides extends the Venezuelan Guayana on the right bank of the Orinoco and the Llanos, both Colombian and Venezuelan on the left bank. As we see, the Orinoco river itself marks the natural boundary between these two regions; it could be said that the Orinoco is one of the most remarkable natural borders in the world, although this fact has a simple explanation: the rivers have little slope and have been building for millions of years a level of accumulation with the sediments that they carry from the mountain ranges where they are born. And it is these sediments that push the channel of the Orinoco against the Guayanese shield itself, to the point that in most of its course, the channel is riding on the rocks of the guayanés shield, as can be seen in Piedra del Medium in front of Ciudad Bolívar (before Angostura). The ancient name of Bolivar City, Angostura of the Orinoco is due to the fact that the rocks of the shield are very resistant to the erosion and presented in that point, a narrowing of about 800 meters of width that gave origin to a species of imprisonment during, probably, millions of years, until the river was gradually excavating the channel on granite rocks.
  Thus, the Venezuelan Guiana constitutes, unlike the Llanos, a surface of erosion. From the combination of these two forces that modify the relief, a constructive one, the sedimentation and another destructive one, the erosion, arises the current situation in which the river marks approximately the limit between the two regions. As can be seen from the above, this limit presents exceptions since, in some sections, rounded hills of granite origin (and therefore, Guayan reliefs) can be seen on the left bank of the Orinoco, that is, on the border of the Llanos. In the stone of the middle, you can see the different levels reached by the waters of the river, expressed in the different coloration of the granite, which explains the value of this granite island as "nilometer" - according to Alejandro de Humboldt. These lines of distinct coloring should not be interpreted as a reduction of river flow over geological time, but as a descent of river level with sediment transport from the bottom to the sea: we recall that the river delta (almost 40000 km²) was built with these sediments, while granitic rocks (such as Piedra del Medio) have been much more resistant to erosion.
  On the other hand, the sandstones of the Venezuelan Guayana (of the Roraima formation) have been transformed into sand by the erosion that, although never very intense by the extra-ordinary res
caption: The Piedra del Medio, located in front of Ciudad Bolivar is a kind of  nilometer  or  orino-meter  in this case, in which the lines of different coloration indicate the successive levels reached by the water.
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