At the top left, the capital city of Sudan, Khartoum, is located at the convergence of the Blue Nile and the White Nile. Although the Blue Nile is much shorter than the White Nile, it contributes about 80% of the flow of the river. The Dahlak Archipelago is seen off the Red Sea coast of Eritrea. Because of their isolation, the numerous coral reefs of the Dahlak Archipelago are some of the most pristine in the world. North of the Rift Valley, in central Ethiopia, are the Simien Mountains and Lake Tana. Lake Tana is the source of the Blue Nile. Photo courtesy of NASA.