This promise—not a guarantee—of prosperity embodied by the High Dam has come at a price. Instead of renewing the soil, Nile nutrients now sink to the bottom of Lake Nasser, and the chemical fertilizers necessary for crop production are creating unforeseen environmental hazards downstream. The flood no longer nourishes shrimp hatcheries nor does it flush out Mediterranean sand and salt water from the mouth of the Nile Delta. The altered ecosystem is thus threatening the traditional livelihoods of fishermen from Damietta to Rosetta and has led to a shrinking of coastal agricultural land already under pressure from a rapidly growing population. Perpetual irrigation, meanwhile, has caused the water table to rise in the Nile Valley. The water transports salt and other minerals to the surface, forming a crystal crust that further reduces the land's natural fertility and infiltrates and defaces modern and historic buildings.

There's more. Geologists believe that the weight of Lake Nasser's water makes Egypt more earthquake-prone. And the lake remains at risk from political tremors too. Last December, Kenya abruptly announced its intention to withdraw from the 1929 Nile Basin Treaty that regulates water consumption in the ten countries through which the river flows. Tanzania has followed suit, and Egypt now fears that Ethiopia and Sudan will also abandon the pact. Though negotiations have always been friendly, nobody can rule out the possibility of a future water war.

Indeed, the world seems more uncertain today than it did five thousand years ago, when people believed that a fat hermaphrodite god called Hapy might fail to make the Nile rise if he woke up in a bad mood.

Fortunately, there's nothing like a little fishing to keep angst at bay. We've motored five hours from Aswan when a sandstorm obscures the horizon with a curtain of orange. As whitecaps build, we seek shelter on a small island, once the tip of a granite mountain. It's an arachnophobe's nightmare: All of Nubia's marooned spiders seem to live here, and they race for their holes when I disembark for a walk. Joining us are a handful of Nile cruisers on the three-day run from Aswan to Abu Simbel, and the archaic wooden ice boats that pick up catches from fishermen camped for weeks at a time on tiny islands. The fishermen, who cast nets and long lines from crude wooden rowboats, form a low-tech emergency survival network. "If we get into trouble," Yousef confidently promises, "they will help us."

When the weather clears, two wild-haired boys row over from their nearby desert island. The crew greet them like long-lost friends, handing out cigarettes and bread and trading a packet of South African jelly beans for a string of tilapia—small African whitefish—which we eat for supper.

The next morning, ospreys patrol a fjord lined with black-striated cliffs, but our trolling produces only nibbles and we head for a submerged cataract where a vise of granite boulders once squeezed the Nile into a narrow rapid. At lunchtime, we stop by a sandbank where Moursi serves chili pasta and sweet potatoes. It tastes fine to me, but the South Africans grumble about having to eat "boarding school grub." Take-charge Dermott stalks off with his rod to catch something better. Expletives break the desert silence. We come running in time to see him waving the rod in one hand and the reel in the other as a man-sized Nile perch explodes out of the water. He manages to put his broken rig back together without losing the fish, which turns out to be all size and no fight.