A distributary of a river is a stream that branches out of the normal flow. They are usually fairly common in river deltas, where rivers divide into smaller branches before entering the sea. However the Casiquiare river is a different story. It is a distributary of the Upper Orinoco River, many kilometers (miles) before the latter gets even close to the sea. Then the Casiquiare turns South-West (while Orinoco continues in North-Western direction) and after around 326 km (203 miles) through the lush jungle it flows into the Rio Negro, which is itself a tributary of the great Amazon river. So the Casiquiare actually connects two of the greatest river system on our planet, a unique property among all the rivers on Earth. And it is not just a small stream - in fact it is 90m (300 ft) wide when it leaves Orinoco,and almost 500m wide(1500ft) when it reaches Rio Negro.
The mysterious supposed connection of the two river systems participated in the creation of the Eldorado legend about a mighty local chieftain who bathes in a golden lake - because it was thought that if such a connection exists a big lake serving as a source of both rivers is a much more plausible explanation than a branching out river. It was long known to the local tribes, when it was discovered by Jesuit missionaries around the 17th century; however the exact geogrphy of the region was not known until in the year 1800 the famous German explorer Alexander von Humboldt and French botanist Aime Bonpland made the journey up the Orinoco and down the Casiquiare. Many of the wonders they discovered on the way will be featured in other articles.
Coordinates (where Casiquiare leaves Orinoco) : 3° 8′ 17″ N, 65° 52′ 49″ W
Google Maps at the same point:
An interesting Article about Humboldt's journey: