Hints of lakeshore on hills around the White Nile River Valley have long led scientists to suspect the region was once home to a mega lake. The large freshwater lake almost certainly existed, but no one knows for sure how large it was – as scientists have found, it’s hard to measure something that doesn’t exist.
Estimates have gotten more accurate with time and new tech. A study published in the journal Geology is the most recent to propose a size. It came at the question a different way. The answer, it seems, was in the old lakeshore.
Beryllium 10 is a naturally occurring element created by cosmic rays. It takes millennia to build up, but accretes in soil at a known rate. With samples measured around the lakebed and a little figuring, scientists estimated the lake probably existed about 109,000 years ago.
Drainage ditches along the Blue Nile River during that time period were dated. The ditches showed extremely high flow during the same period – enough, the scientists say, for the river to overflow, block the White Nile’s path and flood the valley.
The records were used alongside topography data collected by the Shuttle Radar Topography Mission to show how far the lake could have stretched. They place the massive lake at 45,000 square kilometers (17,000 square miles) in size, larger than Lake Erie and Lake Ontario put together.