Researchers have uncovered the remains of a vast ancient river system that crossed what is now known as the Sahara Desert. The river system was so vast that if it still flowed today, it would be ranked as the 12th largest watershed on Earth, the researchers say in their report published Tuesday in Nature Communications.

Radar images reveal ancient rivers that once flowed through the Sahara Desert. Photo credit: Philippe Paillou

Using radar images taken from a Japanese Earth observation satellite, researchers have discovered ancient riverbeds stretching from the middle of the Sahara to the Mauritanian coast in West Africa, which appear to originate from the Atlas Mountains to the north and the Hoggar Mountains to the east.

This is not the first time that someone has suggested that the Sahara was once “wet and humid” and, in fact, teeming with life. In 1957, an expedition led by French ethnologist Henri Lhote discovered cave paintings of giraffes and elephants. Since then, scientists have postulated that the Sahara has alternated wet and dry periods over the past 300,000 years. Until 7,000 years ago, “cattle, sheep and goats roamed the green savannah,” explains Livescience.

This map shows the current river systems of northern and central Africa, as well as the ancient Tamanrasett River. Photo credit: Skonieczny et al.

IFLScience explains how we now know that water once flowed through this extremely arid climate:

The possibility that a river system once existed in the region was first raised about ten years ago, following the discovery of fine river sediments and a deep submarine canyon carved into the continental shelf off the Mauritanian coast. However, the direct evidence needed to confirm this was lacking. This time around, the scientists used orbital radar satellite images, which allowed them to take images of the geology of the Sahara a few meters below the sandy surface using microwaves. From this data, scientists were able to see the ancient riverbeds of the waterway, which corresponded incredibly to the canyon off the coast.

The river is estimated to have flowed periodically during what are known as African Wet Times (AHP), the last of which ended around 5,000 years ago when the lush, damp and humid Sahara teeming with animals and of life, turned into the dry, dusty place we know today. These shifts between wet and dry periods are estimated to occur every 20,000 years or so as the Earth wobbles on its axis. Whether the old river under the desert will flow again during the next AHP is difficult, however, as climate change is currently disrupting weather patterns and making things harder to predict.

Russell Wynn of the National Oceanography Center in the UK was among the researchers who found evidence of an ancient river system more than a decade ago. He was not involved in this study, but he told The Guardian: “This is an excellent geological detective story and it more directly confirms what we expected. This is more convincing evidence that in the past a very large river system flowed into this canyon, ”said Wynn. “This tells us that still 5,000 to 6,000 years ago, the Sahara Desert was a very dynamic and active river system.”

Today, the Sahelian zone south of the Sahara Desert is threatened by desertification. Almost 75 percent of Africa’s drylands are degraded “with rapidly growing populations trying to make a living by farming or grazing herds on less and less productive land,” says Janet Larsen of the Earth Policy Institute. “Desertification is particularly acute in Burkina Faso, Chad and Niger, as well as Nigeria, Africa’s most populous country, where around 868,000 acres are lost to desertification each year.”

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