Shedding Light on a Very Dark River

Shedding Light on a Very Dark River

If the appearance of the muddy Amazon River evokes a coffee cut with cream, the Ruki River, coursing gently through the Congo Basin, is like a dark tea. On its slow path through mostly untouched lowland rainforest in the Democratic Republic of Congo (DRC), the water leaches organic material from vegetation, prompting some researchers to think it is one of the darkest blackwater rivers on Earth. The dissolved material in this distinctive water, scientists are finding, offers clues into the carbon cycle of tropical forests.

The Ruki drains an area about the size of Senegal. Most of this watershed is covered in broadleaf and lowland swamp forests. It also contains peat bogs and only a small amount of deforested land. “The Ruki is a good candidate for being one of the most pristine and homogeneous large tropical watersheds on Earth,” the authors said in a recent study about the river. The OLI (Operational Land Imager) on Landsat 8 captured this image of the Ruki River at its confluence with the Congo River, approximately 650 kilometers (400 miles) upstream (north) of DRC’s capital city Kinshasa.

For the first time, researchers have measured the chemical composition and flow of the Ruki’s dark waters. For one year, they collected water samples from a field station just upstream from the confluence and analyzed them for components such as dissolved organic carbon. Rivers are conduits of carbon to the ocean and atmosphere, especially in the tropics, so scientists are interested in knowing how much carbon they are transporting and from where.

The study reported that, as the water color suggests, the Ruki is rich in dissolved organic carbon compounds. It contains four times as much organic carbon as the Congo River and 1.5 times as much as the Rio Negro, the world’s largest blackwater river and a major tributary of the Amazon. They calculated the Ruki drains only 5 percent of the Congo Basin but contributes 20 percent of the Congo River’s total organic carbon. The Ruki watershed is very flat, such that water drains slowly and allows dead jungle vegetation plenty of time to “steep” in it, the authors said. Because of this heavy carbon load, they added, “tropical forests like those around the Ruki might not accumulate quite as much carbon as we once thought.”

The researchers also measured radiocarbon isotopes of the dissolved carbon to determine its source. The Ruki runs through areas with peat soils full of partially decomposed plant matter that could represent another source of carbon if eroded or leached into the river. Their results showed that very little carbon comes from the much older peat and that most comes from younger forest vegetation and soils. Although the peat appears stable now, they said, future drought or human disturbance in the watershed could release carbon that is now mostly locked up.

NASA Earth Observatory image by Michala Garrison, using Landsat data from the U.S. Geological Survey. Story by Lindsey Doermann.

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