The Congo River in Africa is home to some of the largest and most extreme rapids in the world, and scientists say the turbulence is so powerful that it’s forced entirely new fish species to form.

According to research published in the journal Molecular Ecology, the rapids essentially cut off populations of fish from each other, creating tiny communities that have evolved independently.

It’s not uncommon for speciation (the formation of new species) to occur when physical barriers like mountains or large bodies of water prevent two groups of the same species from breeding with each other. So the researchers theorized that the rapids could be acting as natural barriers.

The Congo is the ninth longest river in the world. Image: Wikimedia Commons

They analyzed DNA from more than 50 different species of fish from the genus Teleogramma throughout a 200-mile stretch of the lower Congo River. What they found confirmed their theory: while there were several different species in the river, each one was found in a different area separated by rapids.

“The genetic separation between these fishes show that the rapids are working as strong barriers, keeping them apart,” said one of the study’s lead authors in a statement. “What’s particularly unique about the lower Congo is that this diversification is happening over extremely small spatial scales over distances as small as 1.5 kilometers. There is no other river like it.”

The section of the Congo River where the researchers studied is famous for its incredible biodiversity: more than 300 species of fish have been identified there. About a quarter of the fish are endemic to the region, and the study’s authors say they could be threatened by a major dam that is being proposed for development.