Nile River
© George Chan / Nature PL
The Nile had become a major river by around 31 million years ago.
The source of the Nile river remained a mystery to Europeans for thousands of years. Now another puzzle has finally been solved: the source of the river in deep time.

The Nile had become a major river by around 31 million years ago, reports the first team of geologists to put a firm date on its origin. "The Nile's the longest river in the world, and being able to figure out when it started is, for me, really exciting," says Yani Najman at Lancaster University, UK, who led the team.

Rivers carry sediment from their source down to the sea. So comparing the minerals in a river's sediment deposits with the rocks found upstream reveals where its waters started out from in the past.

The Nile's story has remained elusive because its most ancient deposits are buried beneath thousands of metres of Nile delta sediment, says Najman. Only oil companies have drilled to such depths in the area and they don't like to share their findings.

But after years of negotiation, BP Egypt provided samples from delta sediments dated to about 31 million years ago. These contain minerals matching those in rocks in the Ethiopian Highlands - the place where one major branch of the river, the Blue Nile, gets going (Earth and Planetary Science Letters, doi.org/cm7r).

That means the Nile was already flowing all the way from Ethiopia to the Mediterranean at least 31 million years ago. That is much older than some previous estimates.

Studying the Nile's origin is also revealing the geological history of the entire region. The findings mean the Ethiopian Highlands must have been uplifted around this time, too.

"If you're going to study rivers, you should look at them in their entirety," says geomorphologist Martin Williams at the University of Adelaide in Australia. And that, he adds, is what Najman and her team did.