The spectacular flood of the Blue Nile is what sets the Nile apart from many other great river systems of the world. For just a few weeks each year the Blue Nile swells to over 50 times its dry season size and carries with it a staggering 140 million tonnes of rich, fertile silt as it rages, thick and brown, towards the sea.

One of Africa's great crossroads, Khartoum in Northern Sudan is a unique melting pot, where Arabia meets Africa, and east meets west. The Nile dominates Khartoum and here the Blue Nile and the White Nile meet and merge in what Arab poets call 'the longest kiss in history'. The clearly different colours of the Blue and White Nile suggest their different characters and origins.

The Ethiopian highlands contain 80% of Africa's mountains - a legacy of the region's volcanic past. The moorlands here support one of the highest densities of rodents on earth - up to three tonnes of them in every square kilometre. Ethiopian wolves and augur buzzards are master rat-catchers. Gelada baboons also make a living here - no monkey lives at higher altitude. Once a year the slopes are lashed by violent rains during which a metre of rain can fall in just a few weeks. Loosened by the activities of rodents, farmers and ice, the black volcanic soil is eroded and carried away by the river's headwaters.

At nearly 2,000m, Lake Tana is Africa's highest lake. The hippos here live higher than any others. Dissolved volcanic soil has enriched the lake to make good fishing for both people and animals. More than 60 rivers flow into Lake Tana, but only one flows out - the Blue Nile. A few kilometres downstream the river plunges off a sheer rocky step more than 45m (150ft) high transforming the sedate river into a boiling cauldron. This waterfall, the second largest in Africa, is called Tis Isat, which means 'smoking fire'. Virtually dry in June, by September it has swollen tenfold into a wall of water and mud over 400m (1,310ft) across.

For a million years the Blue Nile has been carving this huge gash through the Ethiopian Highlands. Well over 1,000m deep, nearly 20km wide and over 600km long, this is Africa's own Grand Canyon. From all over the highlands, huge rivers pour into the Blue Nile Gorge. By the time it leaves Ethiopia the Blue Nile will be 50 times the size it was in the dry season.