The Nile, south of Khartoum, forms a complicated stream. For 500 miles it proceeds through the desert on a broad and more or less regular course. There are trees, and low, bare, and occasional hills or mountains on both banks. But, at the point where the Sobat river comes in from the mountains of Abyssinia not far from the town of Malakal, the river turns west. Here, the air becomes more humid, and the two banks more green.
 
This is the first warning of the great obstacle or the dam which lies ahead. No swamp in the world is more frightening than this dam. The Nile loses itself in a vast area of papyrus ferns and rotting plants. In the foetid heat, there is a rich tropical life that hasn't changed much since the beginning of the world. It is primitive and hostile to man. Crocodiles and hippos lie heavily in the muddy water. Mosquitoes and other insects choke the air, and the strange water-birds guard the banks. This region is neither land nor water.

Year by year, the current continues bringing more floating plants and packing them into solid lumps, perhaps 20 feet thick, and they are so strong that an elephant can step upon them. But this debris soon breaks away in the form of islands, and re-forms in another place. This is repeated in a thousand indistinguishable patterns and goes on forever.