Brazo Casiquiare
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Río Casiquiare Brazo Casiquiare |
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 Brazo Casiquiare in the Amazon river system |
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| Data | ||
| location | Venezuela | |
| River system | Amazon | |
| Drain over | Rio Negro → Amazon → Atlantic | |
| source | Junction from the Upper Orinoco 3 ° 8 ′ 18 ″ N , 65 ° 52 ′ 48 ″ W. |
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| Source height | 123 m | |
| Confluence with the Río Guainía | at San Carlos de Río Negro to the Rio Negro Coordinates: 2 ° 0 '5 " N , 67 ° 6' 54" W 2 ° 0 '5 " N , 67 ° 6' 54" W |
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| Mouth height | 91 m | |
| Height difference | 32 m | |
| Bottom slope | 0.09 ‰ | |
| length | 354 km | |
| Catchment area | 42,300 km² | |
| Discharge at the Solano gauge. Location: 23 km above the mouth |
MNQ MQ MHQ |
675 m³ / s 2387 m³ / s 3977 m³ / s |
| Left tributaries | Río Pamoni, Río Pasiba, Río Siapá, Río Pasimoni, Rio Yatua | |
| Most important river bifurcation worldwide | ||
The Río Casiquiare is the left source river of the Río Negro , which in turn is the largest left tributary of the Amazon . At the same time, it connects the river systems of the rivers with the most water in South America , the Orinoco and the Amazon. Its catchment area, which is almost entirely in southern Venezuela , extends in the north into the Orinoco river bed: around a fifth of its water branches off from the Orinoco as Brazo Casiquiare ( Casiquiare arm ) to the south. This is the largest river bifurcation on earth; their existence was considered impossible until around 1800, and their origin is only partially understandable today.
Course and tributaries
In the broad, sedimented valley level of the Orinoco near Tamatama, the slow current divides: on the one hand into the approximately 90 m wide Brazo Casiquiare, which turns south and on the other hand into the remaining main stream of the Orinoco, which initially lies at the western foot of the Guayana mountainous region flows further west, later north. The Casiquiare arm, which soon winds almost in the opposite direction through the dense tropical rainforest , reaches rocky subsoil at points and gains significantly in volume and flow speed through several large, almost exclusively left tributaries. Sometimes it forms small rapids. The course of the Casiquiare often changes; especially at the confluence of the larger tributaries, it briefly swings into their valley lines.
The largest tributary is the approximately 440 kilometers long Río Siapá, whose clear water more than doubles the volume of the Rio Casiquiare. Its hardly explored upper course drains a large high plateau from which it descends via a cataract stretch. It is known for its great wealth of fish species, but also of black flies . The second largest tributary, the Río Pasimoni, drains the Serra-do-Imeri massif with the Pico da Neblina , the highest mountain peak in Brazil (2994 m) near the border . In an alluvial plain at the western mountain foot there is another bifurcation that drains partly into the Río Pasimoni, partly directly to the Río Negro. Another major tributary is the Rio Yatua.
After the Casiquiare has taken the northwest direction of the Río Pasimoni, it deepens slightly into the surrounding flat undulating hill country and meets the Río Guainía at San Carlos de Río Negro , which also forms the border to Colombia . The Casiquiare changes in its course from a cloudy white water river to a black water river with increasingly acidic water. The equally large Río Guainía, on the other hand, is a typical black water river without sedimentation.
Water data
Because of its remote location, there are hardly any reliable data about the water flow, length, catchment area and altitude of the river, which are also partly contradicting and partly inconclusive.
Depending on the estimate, the Casiquiare branches off between a tenth and a fifth of the water flow of the Upper Orinoco at low tide and between a quarter and a third at high tide. Information on the mean water flow at the junction fluctuates between 200 and 350 m³ / s (lowest monthly mean: 127 m³ / s, highest: 680 m³ / s). Up to Solano, near the confluence with the Río Guainía, the mean water flow has increased almost tenfold since the branching off from the Orinoco. It is specified either as 2,101 m³ / s or 2,387 m³ / s. The Casiquiare is roughly the size of the Moselle at the beginning and the size of the Lower Rhine at the end . The Río Guainía, the other source river of the Río Negro, has almost the same size as the Río Casiquiare at the confluence. At their confluence, the two rivers are about 500 meters wide.
The length specifications for the Casiquiare range from 300 km to 410 km; the measurement of satellite images (2010) gives 354 km. The catchment area (for the part of the runoff that does not come from the Orinoco) is usually given as 42,300 km².
The height information for the start and end point of the Casiquiare also differ. The height of the junction is given as 112 or 123 meters, and the confluence is usually 91 meters. An occasionally postulated reversal of the direction of flow depending on the water level is fundamentally ruled out with the overall gradient; this is at most conceivable in the branch area on a small scale.
Development of the river basin and the bifurcation
The Rio Casiquiare flows through a hilly land made up of migmatites and quartzites of the ancient world , which is leveled to form the hull, and lies to the west of the at least 1.8 billion year old Guiana Shield. This represents the northern part of a craton , which forms the oldest core of the South American continent. Since the beginning of the Tertiary, this shield has arched up towards the lowlands of the Amazon and Orinoco and is overlaid by very old and resistant sandstones, the island-like remains of which tower over the hill country as striking table mountains . The hill country drained mainly to the northwest until the Andes unfolded in the Tertiary and since then their alluvial fans have grown ever higher towards the east towards the Guiana shield. The water on both sides now gathers in the north-flowing Orinoco.
In the south, meanwhile, a trench-like depression opened the way to the Atlantic for the Amazon and also caused the area of the Río Negro to sink. At its northern edge, and thus also in the area of the Casiquiare, the rivers flowing west or north were gradually diverted to the south to the Río Negro, which leads to unusual mouth angles of the left (northern) tributaries, especially in the neighboring Río Guainía.
The upper Orinoco hardly cuts into the subsoil; its sediments are deposited and fill the flat valley floor further and further. The subsidence towards the Amazon also helped that part of the Orinoco water could eventually overflow into neighboring valley hollows and now also fill these with sediments. As a result, most of the tributaries are dammed back like a lake in front of their mouths by the deposits of the Río Casiquiare, which is rich in suspended matter in the upper part, similar to what is typical in the Amazon.
The Río Casiquiare itself has another, temporary bifurcation that allows water to drain into the Río Conorichite when the water level is high. This continues the valley line of the Río Siapá to the right of the Casiquiare, also backs up its tributaries by sedimentation and later flows into the Río Guianía, the other source river of the Río Negro.
The southern catchment area of the Upper Orinoco is gradually "tipped out" towards the Río Negro. The sequence in which the river diversions took place, which ultimately led to the course of the Casiquiare, is still the subject of research.
Origin of name
The name of the Casiquiare goes back to the Ye'kuana (also Makiritare , roughly: boat people , called) living on the upper Orinoco . The first from European culture to come into contact with their culture was the Jesuit missionary Manuel Román , who discovered the connecting waters to the Amazon while on a tour of the Orinoco in 1744. In his notes, he changed the name Kashishiwari from the Ye'kuana for a mythical primal river into the more easily pronounced form Casiquiare .
exploration
The first news of the river connection seems to have come to Francisco de Orellana in 1535 and Walter Raleigh in 1596 . In 1639 the Spanish explorer Cristóbal Diatristán de Acuña reported about it. The travel report by Father Roman, presented by La Condamine , a good 100 years later , also contradicted the postulate that was common at the time that rivers above their mouths only had a pure tree structure, and thus met with passionate rejection. The branching of the river was confirmed by a Spanish Orinoco expedition to establish the border in 1755. However, it was not until 1801 that Alexander von Humboldt cleared up the last doubts , who mapped this division during his trip to South America and in a natural history context. In 1924/25, Alexander H. Rice ( Harvard University ) carried out more precise mapping of the waterways for the first time using aerial photographs .
Shipping route
Contrary to what Alexander von Humboldt had suspected, the Casiquiare did not develop any importance as a shipping connection between the river basins of the Amazon and the Orinoco. Changing sandbanks and rock bars make the journey difficult for larger ships. Since the construction of two large dam projects on the Rio Madeira , the idea of an inland waterway from the Río de la Plata via the river bifurcations in the north and south of the Amazon basin to the Orinoco has become more topical again, but the isthmus of the Pimichín is considered more suitable for this, a low valley watershed in the valley of the former upper course of the Atabapo, which now flows to the Guaviare and Orinoco, diverted to the Río Guainía . Today the Casiquiare is increasingly used for tourists, with contacts with the indigenous people of the Yanomamo in the foreground.
Individual evidence
- ↑ a b Overview of discharge data according to MARN ( Memento from November 20, 2011 in the Internet Archive )
- ↑ Data according to GRDC ( Memento from January 18, 2012 in the Internet Archive )
- ↑ Note: Taking into account the catchment areas and information on the Rio Negro a good 100 kilometers below (near Cucui), the point of confluence results in the Río Guainía being the somewhat larger river in one case and the Casiquiare River in the other.
- ↑ google earth
- ↑ Note: This value stands up to close scrutiny, but not the information of 80,960 km² for the GRDC measuring station Solana. It is apparently based on an incorrect assignment to the Orinoco system with the reverse modeled flow direction.
- ↑ The epicontinental Amazon basin (Institute for Geography of the University of Innsbruck)
- ↑ Natural Areas of Latin America (Institute for Geography at the University of Innsbruck)
- ↑ Alexander v. Humboldt indicated ( on the connection between the Orinoco and the Amazon River ): "The interior of Guiana [...] is so flat that the smallest inequalities of the soil determine the course of the rivers there". In: Monthly correspondence for the transport of the earth and sky customer 26, Sept. 1812, pp. 230-235.
- ↑ Ludwig Wagner: “Favored by the less pronounced slope of the floor, bifurcations still occur repeatedly; so on the Casiquiare itself in the form of the approx. 100 km long Conorichite or Itinivini, which branches off approximately in the middle of the south-western stretch of the Casiquiare and flows under Maroa at Guzman Blanco 284 m wide (H) into the Rio Negro; however, this connection seems to exist only in the rainy season ... ”In: The Orinoco electricity system . Thirty-first report of the Upper Hessian Society for Nature and Medicine, Gießen 1897, p. 20
- ↑ Alfredo Cedeño: La fascinación por los Yekwana ( Memento of the original from November 30, 2015 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Alexander von Humboldt, Through the tropical South America. Page 147 ff. (PDF; 112 MB)
