While the Amazon is Earth's largest river (by volume), the Nile, of central and northeast Africa, is the longest. Rising near an elevation of 8800 feet in the mountains of southern Rwanda, the White Nile flows northward, passes through Lake Victoria and joins the Blue Nile near Khartoum, the capital of Sudan; these combined tributaries continue north to the Mediterranean Sea, producing a vast delta at Cairo and yielding an official length of 4184 miles (2700 of which are provided by the White Nile). In contrast, the Blue Nile, which originates at Lake Tana, Ethiopia, is only 870 miles long but contributes more than 80% of the Nile's flow during the wet season of May to August.
The junction of the White and Blue Niles occurs at an elevation of 1200 feet and the River's last major tributary, the Atbara, enters just 200 miles north of this point. Beyond this tributary, the Nile makes a broad curve to the south (the Great Bend) before flowing across the vast deserts of northern Africa. Exacerbated by the completion of the Aswan High Dam (Lake Nassar), in 1970, evaporation claims much of the Nile's water as it negotiates this arid landscape.
Rich in human history, today's Nile is but the latest representative of regional hydrology throughout geologic history. Climate change has led to alternating patterns of rainfall and vegetation across the Continent and the processes of orogeny and erosion have produced an ever changing topography. Indeed, volcanism along the East African Rift, which began in the Miocene, cut off a major tributary of the Nile's watershed; Lake Tanganyika, now in the Rift Valley, used to drain northward via the Albert Nile before volcanic mountains rose in its path.
