Casiquiare

Casiquiare (käsēkyä`rā), river, c.100 mi (160 km) long, S Venezuela. Also called the Canal Casiquiare, it is a branch of the Orinoco and flows SW to the Río Negro, thus linking the Orinoco and Amazon basins. The Casiquiare's flow was naturally diverted by the headward erosion of the Río Negro.

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Casiquiare 

a river in southern Venezuela. It measures 410 km long and drains an area of about 45, 000 sq km. In its upper course the Casiquiare accepts about one-third of the runoff of the Orinoco River and near the city of San Carlos joins with the Negro River of the Amazon system. The linking of the two largest rivers of South America is a classic example of the bifurcation of rivers. The Casiquiare is joined with the Orinocothrough a “canal,” a natural channel about 30 km long and witha gradient of up to 5 m, which serves to divert the water fromthe Orinoco to the Negro River basin.