Science: The Five Niles

Egypt, wrote the Greek historian Herodotus, is the gift of the Nile. He was right; Egypt—or at least its most populous and fertile area—was formed by the rich silt washed down from the East African highlands by the waters of the Nile. But which Nile? According to Egypt's leading geologist, Rushdi Said, 55, the present-day Nile is a relative newcomer to Egypt, having been around for only 30,000 years. Before that, he says, at least four different Niles had flowed through—and then disappeared from—the river basin.

Said, who heads the Geological Survey of Egypt and holds a seat in the Egyptian Parliament,...