By discharge volume, the Amazon and the Orinoco are the largest and fourth largest rivers in the world. The Casiquiare River in Venezuela connects them.
Back in the post on swimming across North America I mentioned the phenomenon where a river splits in two. Often those two branches meet up again downstream, creating a river island. Most river islands are tiny, but they get as large as Marajó, an island in the middle of the Amazon that’s as large as Switzerland.
Sometimes, though, the two branches of a bifurcated river go in completely different directions. Sometimes they drain into completely different river systems, or even completely different seas. This peculiarity of hydrography means that someone could travel up one river system and then cross over and go downstream into an entirely different river system.
One such connection is the Casiquiare River in Venezuela. It’s a natural canal, beginning life as a bifurcation from the upper Orinoco. The Orinoco, by the way, is a huge river – not by length, but by the amount of water that pours out of it each day into the Caribbean Sea. In terms of volume, it rivals the Ganges.
The Orinoco’s bifurcation is a curious one, because it turns south and connects up with another major river: the Amazon. (The Amazon is number one in terms of discharge volume; one quarter of the world’s river output comes from just that one watercourse.) Specifically, the Casiquiare flows into the Rio Negro, which joins the Solimões River to form the Amazon.
This means that you could theoretically swim all the way up the Amazon, into the Rio Negro, into the Casiquiare River, from there into the Orinoco, and then all the way down to the Caribbean. It also technically makes the land south of the Orinoco and north of the Amazon into a kind of island, because it is separated from the rest of South America by the waters of those two connected rivers (although, unfortunately, most definitions of an island conveniently ignore river bifurcations).