The Cuito and Cubango Rivers converge at the southern Angolan border, forming a bigger river, the Okavango, which flows across the Caprivi Strip, a narrow band of Namibia, and into Botswana.

But it's really more like a miracle. Rains that fall in Angola during the previous wet season, from November through May, propel the Okavango River along its 1,000-mile downhill course in June ...