Rivers often join together. They rarely split apart. If you’ve studied geography, or even just stared at a map for a decent amount of time, you’ll know that this is the rule. Little rivers join together to become big rivers, which join together to become even bigger rivers, which flow out into the sea.

There are some small exceptions to this rule. Winding plains rivers commonly split and rejoin around islands. Some rivers split into many small branches as they reach the coast, forming a “river delta”. River deltas are often home to large cities, like Alexandria in the Nile Delta, Shanghai in the Yangtze Delta, and New Orleans in the Mississippi Delta. A select few lakes (like Wollaston Lake in Saskatchewan) drain in two different directions. And a few smaller rivers actually do split farther upstream, mainly small mountain creeks near continental divides.

Still, rivers rarely split apart, almost never split far from their mouths, and definitely don’t, say, divert thousands of tons of water per second from one major river into another. Except for one: the Casiquiare Canal in Venezuela – which, despite the name, is a fully natural and very unusual river.

The first written record of this geographic anomaly comes from the 1639 journal of Cristóbal Diatristán de Acuña, a Spanish Jesuit missionary. A map based on Acuña’s travels was drawn up by French cartographer Nicolas Sanson, showing an interior waterway linking the Orinoco and Amazon rivers.¹

Over a century later, in 1744, another Spanish Jesuit priest named Manuel Román encountered a group of Portuguese slave traders in the upper reaches of the Orinoco. Román informed the Portuguese that they were in Spanish territory, as Spain had claimed all the land that drained out through the Orinoco as part of its colony of New Granada. The slave traders, however, insisted that they were on a tributary of the Amazon, which would be part of Portuguese Brazil.

To confirm their claim, they brought Román along the river to the Rio Negro, the Amazon’s largest tributary, after which Román returned back to the Orinoco the same way. It was Román who dubbed the waterway the “Casiquiare Canal” – since it acted as a canal despite being entirely natural. “Casiquiare” probably comes from the local Ye’kuana language name for the river, “Kashishiwadi”.

The Spanish followed up by sending a boundary commission to investigate, who verified the claimed “canal”. The dispute seems to have been settled in favor of Spain: the drainage divide remains the official border of Venezuela and Brazil today,² and the Casiquiare lies entirely within Venezuelan territory.

The Casiquiare isn’t hydrologically stable. Usually, when a river splits into two outflows, called a “bifurcation”, one eventually dominates and captures the entire flow.³ The reasons why aren’t entirely understood, but it seems to involve the smaller outflow channel getting choked off with sediment more quickly. Left alone for a few millennia, the Casiquiare will probably either dry up or capture the uppermost regions of the Orinico and redirect them fully into the Amazon. But the Amazon and Orinoco basins have so much water flowing around that another bifurcation could very easily happen before then. And maybe then this strange river won’t be quite so unique.