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At the beginning of the world and of Man,
There was but one fresh water river on the Earth.
Kashishiwari was the only river.
Wanadi, the Creator, had created it thus,
when he put the Earth in order.
There was no other river, nor fresh water
in all the Earth's roundness.
The sources of Kashishiwari
are in the divine mountain of Marawaca.
In Marawaca everything was born:
the water, the plants and the animals.
Then Wanadi created the men,
He created Yekwana in the High Padamo.
There were no other men on Earth...

-Yekwana legend2

A spectacular example of stream capture in action, Brazo Casiquiare is a South American river that has extended its drainage basin to intersect the upper Orinoco River in southern Venezuela.   (Yekwana legend does suggest, however, a dim memory of the Orinoco pirating the Casiquiare!) The river then flows 326 km to flow into Río Negro near the town of Solano, from which it flows into the Amazon.

With most of its water coming from the upper Orinoco, the river is actually navigable by small boats its entire length, making it possible to sail up the Amazon from its mouth, up the Rio Negro from Manaus, then up the Casiquiare and down the Orinoco to the Caribbean Sea.

The local Yekwana tribes, fabled as river navigators, gave the name "Kashishiwari" to the river, which Manuel Roman later Hispanicized into "Casiquiare".

The history of the Casiquiare and the upper Orinoco is strongly entangled with the legend of El Dorado.  Explorers looking for the fabled city would always naively ask the local natives the way to the fabled city.  The natives, perhaps weary of answering the same question over and over again, but also perhaps wisely not wanting their families killed and their homes destroyed by thwarted ambition, would point upriver, saying: "El Dorado is that way."  Somehow, currents would always force the explorers to turn back before they reached their goal.

As the maps of South America were filled in, this difficult-to-reach region became the last possible location for El Dorado to be ruled out.

Although some believe that the infamous Lope de Aguirre1 led his mutineers up the Rio Negro and down the Orinoco, rather than down the Amazon, there isn't really a lot of evidence supporting the theory. Aguirre's amazing 1561 letter mentions the hundreds of islands at the mouth of the river, which leads me to believe it was really the Amazon.

Sir Walter Raleigh's 1596 expedition up the Orinoco describes the dual drainage of the upper Orinoco, but it is clear he never actually visited the area.  The map from his expedition shows both rivers emanating from a fictitious lake, on whose shores sat El Dorado, and running parallel for a distance.

In 1744, Jesuit priest Manuel Roman apparently visited the spot where near the headwaters of the Orinoco, but he induced no-one to redraw any maps.   The 1755 Royal Orinoco expedition led by José Solano (see above) only reinforced Raleigh's erroneous description, giving the fantastic lake a name: Lake Manoa, or Lago de Parimé.

Finally, in 1800, the scientific expedition of Alexander von Humboldt encountered the Orinoco's strong currents.  Rather than turning back as the others did, he traveled overland to the Rio Negro, then downriver and up the Casiquiare, and back to the Orinoco, shattering the last illusion of finding El Dorado in the process.

Integrated from memory plus
www.museo-de-ciencias.org/SLNOrinoco/el_brazo_casiquiare.htm  (translated)
members.nbci.com/tropicland/yekwana.html (translated)

1The story of the Urroa expedition and how Agiurre came to command it really deserves its own node; it is however portrayed in the movie Aguirre: Wrath of God.
2Google translated this out of Spanish from http://www.mundolatino.org/7estrellas/orinoco.htm; the translation was very odd and I had to help it some.  I apologize if I missed an idiom.