Ruki, a tributary of the Congo River, has recently been dubbed the darkest river in the world, with water so dark that you can’t even see your face in front of you.

In what is considered the first-ever scientific study of the African river, scientists concluded that the dark-colored water is caused by the high levels of dissolved organic matter from the surrounding rainforest. Scientists found that the color is caused by carbon-rich compounds leached out of rotting plant matter and washed into the Ruki River by rainwater and floods. Dr Travis Drake, lead author of the recently published study, said that the Ruki is “essentially jungle tea” in which carbon-rich plant matter is brewed. The resulting coloration of the water makes the Ruki darker than the Rio Negro.

Photo:  Matti Barthel / ETH Zurich

“The organic compounds leached from this vegetation absorb light, so the higher the concentration, the darker the water will appear,” Dr. Drake said. “It would be like brewing an extra concentrated tea using many tea bags.”

Using a measurement system, the team of scientists ETH Zurich, in Switzerland, found that Ruki was 1.5 times darker than the Amazon’s Rio Negro, the world’s blackwater river.And although the Ruki only makes up a twentieth of the Congo Basin, it provides a fifth of all dissolved carbon in the Congo.

“The Ruki is one of the most DOC-rich river systems in the world,” Matti Barthel, the study’s co-author, said. Its water contains four times as many organic carbon compounds as the Congo’s and 1.5 times as much as the Rio Negro’s in the Amazon.

 

“We were struck by the color of the river,” said ETH Zurich researcher Travis Drake, adding that it is so dark that you literally can’t see your hand in front of your face.