One of the driest places on Earth, the Sahara Desert, once ran with water

Traci Watson | Special to USA TODAY

A mighty river once coursed through what is now the Sahara Desert, one of the driest and most inhospitable places in the world, scientists said.

A satellite that can peer beneath the desert sands spotted a 320-mile-long stretch of ancient riverbed in the arid wastes of northwest Africa, according to a study in this week’s Nature Communications. Those 320 miles were part of a waterway so immense that if it were flowing today, its tentacles would drain more land than all but 11 existing rivers. The Colorado and Rio Grande rivers combined don’t sprawl over such a vast area.

The river network was “monumental,” said marine geologist Peter deMenocal of Columbia University’s Lamont-Doherty Earth Observatory, who was not involved in the research. “We have the smoking gun that this whole region – that today is without rainfall at any time during the year – supported a large, permanent system.”

Much of the Sahara’s vast expanse of sand is so dry and hot that it supports virtually no plant life. But it wasn’t always so. Roughly 10,000 years ago, animals swarmed across the region’s grasslands, pursued by humans who left behind rock art showing beasts such as giraffes.

Scientists have suspected for more than a decade that an ancient river drained all that land and the mega-lakes, at least one bigger than the Caspian Sea, once dotting northern Africa.

It was almost certainly a river, for example, that dumped unusual sediments found at the bottom of the sea off the coast of western Africa. It was almost certainly a river that cut the deep canyon in the seafloor west of the coast of Mauritania, a nation in northwestern Africa. Researchers christened the lost river the Tamanrasett, though there was no direct evidence for it.

To find hard proof for the lost river, study author Charlotte Skonieczny of France’s University of Lille and IFRMER, a French research institute, and her colleagues turned to radar pictures snapped by a Japanese satellite. Below the dry sands, the radar detected the traces of a large waterway that exactly lines up with the seafloor canyon off the coast. The river also aligns with the supposed path of the Tamanrasett, which stretched from the coast of what is now Mauritania east all the way to Algeria.

“Our contribution was mainly to connect the dots,” Skonieczny said via email.

The scientists aren’t sure when the river was flowing, but it’s a good bet that it was roiling with water from roughly 12,000 to 5,000 years ago. So much rain fell on now-arid northern Africa back then that the region is known as “the green Sahara.”

That gentle climate created a verdant landscape “full of life, full of humans, full of animals, full of lakes,” deMenocal says. With the discover of the river, “we now have a complete fossil ecosystem that’s frozen in time."