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 , 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 (2140 km and more than 30000 m3/s),Historial del caudal del Orinoco desde 1969, en Musinacio the extent of its basin (1 million km2) 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 proportion to the area of 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.
Geography The Orinoco river basin synthesizes ancient massifs and shields, recently raised (Tertiary) ridges, and tectonic depressions and basins or accumulation plains. The only other countries in the Americas with a similar geological disposition are Canada and the United States. To define the Orinoco basin as a natural region, it is necessary to establish the geographic characteristics that define it, such as extension, relief, climate, hydrography, vegetation, soils, and mineral resources.
Extension The Orinoco basin covers an area of almost of which or slightly more than 65% remain in Venezuelan territory, while the remaining 35% is in Colombian territory in the Colombian Llanos and The eastern slope of the Eastern Cordillera of Colombia, a stretch of the great mountain range of the Andes. This Colombian region is called Orinoquía. Of the part located in Venezuela, just over half extend from the Venezuelan Andes and the Cordillera de la Costa to the north-western bank of the Orinoco River (the left bank), forming most of the Venezuelan plains and the delta Orinoco. The southern part of the basin contains most of the waters that come from the Venezuelan Guiana.
Orinoco River Sources
The sources of the Orinoco River are located at Cerro Carlos Delgado Chalbaud, at 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.Alberto Contramaestre Torres. Expedición a las fuentes del Orinoco. Caracas, 1954. And there are other references to this expedition, for example, that of Pablo J. Anduce.Pablo J. Anduce. Shailili-Ko. Descubrimiento de las fuentes del Orinoco. Caracas: Talleres Gráficos Ilustraciones S. A., 1960 From the birth of the Orinoco at the foot of the hill Delgado Chalbaud ( at an altitude of 1047.35 meters) 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 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 40,000 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 resistance of the rocks, has been very durable (more than 1 billion of years), for which the sedimentary cover has been transformed into an inverted relief that forms the Tepuis. Moreover, were it not for the fact that the Guayanese Massif has been suffering a slow and long movement of ascent, in the present time it would have already become a penillanura in which almost all the sedimentary cover of sandstones would have disappeared. The sands coming from this erosive process have been deposited in the left bank of the river, especially in the low plains of Apure state, between the rivers Meta and Apure itself. They were not deposited in the right border because there the relief is higher. And these sands could become over millions of years in strata of sandstones that could also become converted to rise and rejuvenate the relief on plateaus similar to those that now exist in Guyana. This would be a kind of example of the theory of the geographical cycle.
In turn, these sands have come to create a unique eco-system in the world: an extensive dune field (occupies about 30,000 km2) that has the peculiarity that it is not a desert climate but a savanna climate in a landscape of natural pastures that alternate with some gallery forests, flowing rivers and dunes of more than in length and up to in height. Some of these dunes are used by the Llanero to establish in them the cheese,Fernando Calzadilla Valdés. Por los llanos de Apure. Caracas: Biblioteca Popular Venezolana nº 25, Ediciones del Ministerio de Educación, Dirección de Cultura, 1948, pp. 51-79 that, in addition to processing part of the milk, a group of cattle are being prepared to go to the front of the herd (what in the Llanos denominates the godmother of the herd). They also serve to protect the livestock from floods. Thus, this eco-system so curious and picturesque, is the result of wind modeling in a savannah climate. It is not, as stated in the Atlas of Venezuela. A spatial image (also known as the Atlas of PDVSA,PDVSA. Atlas de Venezuela. Una visión espacial. Caracas: Instituto de Ingeniería, 1992 of a paleodunas eco-system formed in an environment with a climate much drier than the present one, but of a mechanism of formation of dunes that only acts during the dry season since. When the level of the waters of the Orinoco falls due to the drought of the rivers, especially those that come from the Llanos, they remain extensive beaches of very fine sand, that the trade winds soon transfer towards the south-east forming what now constitutes the Santos Luzardo National Park, a name taken from one of the main characters in Doña Bárbara's novel by Rómulo Gallegos.
The direction of the winds during the dry season (of the summer as it is said in the Llanos) is on average very constant and with a considerable speed, from the north-east to the south-west, as can be seen in the direction of the elongated dunes in satellite images. This address may vary for a short time but in the long run it is maintained exactly in that direction. In the rainy season (or winter) the direction changes slightly and proceeds practically from the free east. But this is not the most remarkable change, but the decrease of its speed. This is due to the greater humidity that the trade winds bring and to the consequent convection: as the wet winds advance on the savanna they increase of temperature by the heat of the ground due to the solar radiation. In turn, this warming gives rise to the rise of moist air (precisely what we know as convection) and this rise gives rise, in turn, to a decrease in the speed of the winds and the increase of precipitation. So the mechanism of the winds and that of the accumulation of the sands from the Guayanese Massif are almost opposite and this opposition has been favorable for the establishment of agricultural activities in the Llanos: the dunes can become covered with vegetation and serve. The basis for the establishment of houses, herds and roads, and this process becomes more noticeable to the west, not only by the decrease of the speed of the winds as they move in that direction but also because the sands they form. These dunes come from the beaches of the Orinoco and the transport of the same diminishes when the speed of the wind descends. Fernando Calzadilla Valdés explains all this process in the central part of the Apure state, where he begins what he calls the Alto Llano, although this concept is not established with a certain level that throughout the Apure is very low until reaching the true piedmont of the Andes (established modern, this yes, in the curve of level of the 200 altitude.
Climate