The Casiquiare River, also known as the Casiquiare Canal, is an extraordinary natural waterway in southern Venezuela. As the only major river on Earth that connects two vast and distinct drainage basins, the Casiquiare serves as a rare and important geographical feature. It links the upper Orinoco River, which drains into the Atlantic Ocean, with the Rio Negro, a key tributary of the Amazon River. This unusual bifurcation is a product of natural hydrological and geological processes, creating a unique passage between the two largest river systems in South America. The Casiquiare's ecological significance, history, and role in regional hydrology make it an essential subject of study in the fields of geography, ecology, and hydrography.

The Casiquiare represents a rare case of a river bifurcation, a phenomenon in which a single river splits and drains into two separate basins. The Orinoco, one of the longest rivers in South America, loses a portion of its flow to the Casiquiare, which gradually descends toward the Rio Negro. The volume of water diverted from the Orinoco varies seasonally, with higher flows occurring during the rainy season when increased precipitation leads to flooding and stronger currents. The Casiquiare's existence has long fascinated explorers and scientists, as it provides an uninterrupted natural link between the Atlantic and Amazonian drainage systems.

Approximately 25% (one-quarter) of the upper Orinoco’s discharge is currently diverted into the Rio Casiquiare and then into the Amazon system.