The Ruki River in the Democratic Republic of the Congo may be the darkest river in the world due to high levels of organic matter, according to a study published in Limnology and Oceanography.
When a team of researchers from ETH Zurich, Switzerland, reached the Ruki River — a tributary to the Congo River — they were stunned. The water was so dark that they couldn’t see their hands in front of their faces. “We were struck by the colour of the river,” said Travis Drake, lead author in the study. The authors believe the Ruki is Earth’s darkest large blackwater river. It’s certainly a lot darker than the famous Rio Negro in the Amazon.
The water in the Ruki is so dark because it contains large amounts of dissolved organic matter. These compounds are mostly dragged into the river by rain, which falls on dead vegetation and leaches organic compounds from the decomposing plant material. In addition, the river floods the forest during the rainy season, and it can take weeks for the water to drop, during which time it leaches organic substances. “The Ruki is essentially jungle tea,” said Drake.
The Ruki is remarkable for more than just its dark water. The river is about one kilometre wide and empties into the Congo. Its basin is about four times the size of Switzerland and is still covered by untouched primary lowland rainforest. Along the river, peat bogs contain large amounts of undecomposed plant material, making them significant carbon sinks.
Despite all this, the Ruki has never been officially studied. The river’s water levels have been documented since the 1930s, but no data on its chemical composition is available. Surprisingly, nobody ever decided to find out how much dissolved organic carbon (DOC) is in the water or where it comes from.
To answer these questions, a team of researchers from ETH Zurich set up a measuring station near the city of Mbandaka, just a short distance upstream from where the Ruki and the Congo converge. The aim was to measure the daily water level and water discharge every two weeks to calculate the annual flow. “Our measurement methods on site were quite basic,” said Drake. Mbandaka has no permanent power supply and hardly any infrastructure. “So, we often had to improvise.”
Water samples were collected and sent to the lab at ETH Zurich for analysis to determine the content of dissolved organic carbon (DOC). “We need to collect samples from only one location to gain information about a huge area – like a doctor who takes a blood sample to determine the condition of a patient’s health,” says the study’s coauthor, Matti Barthel.
The analyses confirmed that “the Ruki is one of the most DOC-rich river systems in the world,” said Barthel. The water contains four times more organic carbon compounds than the Congo and about 1.5 times more than the Rio Negro’s in the Amazon. Under normal circumstances, DOC is present in the water as organic acids that make the water more acidic and stimulate the release of carbon dioxide (CO2) as the acids dissolve carbonates present in the water. This is not the case for the Ruki. “CO2 emissions are relatively high across the Ruki’s entire drainage basin but not dissimilar to other tropical rivers,” explained Drake. This is because the Ruki is a sluggish and placid river, which makes it more difficult for the CO2 in the water to escape into the air. “In a turbulent river, we’d see higher emissions.”
According to the authors, the analysis also shows that most of the carbon comes from the vegetation, not the peat. There’s only a short period at the end of the rainy season (between March and April) that the peat bogs release carbon into the water. “Overall, however, we see very little peat in the river,” said Drake. “And that’s good news because it also means that the peat bogs are stable.”
This means there is no danger that the peat’s organic matter will be released since they’re underwater most of the year and not exposed to oxygen. However, companies have taken an interest in using Ruki’s basin to harvest natural resources. If this is allowed to happen — leading to deforestation — it could dramatically change the river. If peat bogs are allowed to get dry, bacterial decomposition will start and release huge amounts of CO2. “The peat bogs in the Congo Basin store some 29 billion tonnes of carbon,” said Barthel. “It would be better for the climate if they stayed wet.”
After their trip to the Ruki, the scientists are now studying other tributaries of the Congo, such as the Kasaï and the Fimi River – and studying their carbon cycles.
Drake T, Barthel M, Mbongo C, et al (2023) Hydrology drives export and composition of carbon in a pristine tropical river. Limnology and oceanography, https://doi.org/10.1002/lno.12436