Stretching across 9,400,000 square kilometers of northern Africa—a land area comparable to the entire United States—the Sahara is the largest hot desert in the world, an expanse of shifting sands where only the hardiest wildlife can eke out a living.
But new satellite imagery of the Sahara has revealed that the desert wasn't always such a bone-dry dead zone. In a study published today in Nature Communications, researchers led by Charlotte Skonieczny of IFREMER (L'Institut Français de Recherche pour l'Exploitation de la Mer), present evidence of an ancient river system deep beneath the desert sands.
According to the team, these lost paleochannels are periodically reactivated as river systems during warmer climate cycles, which are called African Humid Periods (AHPs). The most recent AHP ended only 6,500 years ago.
"In terms of geological structures, generally, paleochannels are associated with an absence of flowing water and sediment deposition," Skonieczny told me over email. "However, this large paleo-river system is likely connected to large amounts of fossil water resources in shallow aquifers located where the channels were mapped. This gives them some interesting geographical information on where to drill for finding some new water resources in desert regions."
Skonieczny and her colleagues were able to identify the paleo-river using an orbital satellite system called Phased Array type L-band Synthetic Aperture Radar (PALSAR). Using microwave sensing, PALSAR can peer into several meters into the Saharan sands and detect the fossil water still flowing there.
With this data, the team estimates that the river stretches over 520 kilometers underneath the desert landscape. It may be linked to a much larger paleowater system called the Tamanrasett river valley, which "has been described as a possible vast ancient hydrographic system that would rank twelfth at present among the top 50 largest drainage basins worldwide," according to the study.
So, when will this expansive river valley rise to flow above the Saharan sands again? Skonieczny said that human-driven climate change has made that a much trickier question for scientists to answer.
"Without anthropic impacts, we could have hypothesized a potential reactivation of the coastal part of the Tamanrasett river during the next African Humid Period," she told me. "However, considering the ongoing global perturbation of the climate system due to the increase of greenhouse gases, only the development of climate models might be able to provide some clues in the near future."
In the meantime, Skonieczny and her co-authors hope to use satellites to hunt down more rivers hidden beneath the sands. "Other large paleo-river systems are still to be discovered in Sahara, with the help of orbital radar data," she said. "[T]hat may help us to fill the gap between giant marine systems and the absence of superficial features associated on the continent."
The Sahara already evokes a sense of mystique and otherworldliness, so it seems completely in character for it to have long-lost rivers flowing beneath its surface. It may seem like an eternal wasteland from our perspective on Earth, but when viewed from space, this desert is revealed to be as dynamic as its iconic shifting sands.