The Orinoco phenomenon – when rivers break hydrological rules

Most rivers flow according to the laws of nature: smaller streams within a drainage basin feed into the main river. However, there is a place where this rule is broken. That place is the Orinoco River. On the map, the Río Casiquiare appears to be a tributary, but in reality, it flows in the opposite direction. What causes this hydrological anomaly?

The rebellious Río Casiquiare – the apparent tributary of the Orinoco

When we look at satellite maps of Venezuela, everything seems normal. Río Casiquiare appears to be one of the right-hand tributaries of the Orinoco, which flows from west to east. In reality, though, things are different. The water in the Orinoco moves in the opposite direction, while the Río Casiquiare branches off from the main stream, then flows south and eventually becomes a tributary of the Río Negro. In this way, it creates a navigable route between the Orinoco and the Amazon (into which the Río Negro flows).

This unusual situation was observed thanks to images taken by the Operational Land Imager-2 (OLI-2), a sensor mounted on the American satellite Landsat 9. It is part of a research project in which scientists monitor the Earth’s surface for climate change, wildfire impacts, glacier melting, and human activity. The hydrological anomaly observed on the Orinoco River is not central to their research – it’s more of a curiosity that shows how nature likes to defy expectations.

How is it possible?

Researchers do not agree on the causes of the phenomenon observed on the Orinoco River. Some link it to the terrain and frequent flooding: when large amounts of water flow into a low-lying valley, the top layers of soil and rock erode, making it easier for the Río Casiquiare to carve a path. They call this process river capture and emphasize that the outflow of large volumes of water from the Orinoco basin to the Amazon basin may negatively affect biodiversity.

The Orinoco phenomenon is not unique. Another example also comes from Latin America. The Wayombo River in Suriname can, depending on conditions, flow either east or west and connect to different main rivers. This phenomenon was also observed thanks to the OLI-2 sensor on the Landsat-9 satellite.

The Orinoco River – endangered riches

The Orinoco is the third-longest river in South America. It stretches about 2,150 km and lies mostly within Venezuela. The Orinoco basin is an area of extraordinary biodiversity – over 1,000 bird species live there.
The river and its surroundings are also home to other remarkable animals: electric eels, catfish weighing up to 90 kg, Orinoco crocodiles (among the largest in the world), pink river dolphins, caimans, jaguars, and giant otters.

For centuries, the Orinoco Valley was inhabited by indigenous tribes that had no contact with global civilization. This began to change after oil deposits were discovered in the region. Today, the well-being of the river is threatened by deforestation, intensive extractive industries, and climate change.