The Blue Nile is a river originating at Lake Tana in Ethiopia. With the White Nile, the river is one of the two major tributaries of the Nile. The upper reaches of the river is called the Abbay in Ethiopia, where it is considered holy by many, and is believed to be the River Gihon mentioned as flowing out of the Garden of Eden in Genesis . According to materials published by the Ethiopian Central Statistical Agency, the Blue Nile has a total length of 1,450 kilometres, of which 800 are inside Ethiopia.
The Blue Nile flows generally south from Lake Tana and then west across Ethiopia and northwest into Sudan. Within 30 km (18.6 mi) of its source at Lake Tana, the river enters a canyon about 400 km long. This gorge is a tremendous obstacle for travel and communication from the north half of Ethiopia to the southern half. The power of the Blue Nile may best be appreciated at Tis Issat Falls, which are 45 m (148 ft) high, located about 40 km (25 mi) downstream of Lake Tana.
Although there are several feeder streams that flow into Lake Tana, the sacred source of the river is generally considered to be a small spring at Gish Abbai at an altitude of approximately 2,744 meters (9,003 ft). This stream, known as the Lesser Abay, flows north into Lake Tana. Other affluents of this lake include, in clockwise order from Gorgora, the Magech, the Northern Gumara, the Reb, the Southern Gumara and the Kilte. Lake Tana’s outflow then flows some 30 kilometres before plunging over the Tis Issat Falls. The river then loops across northwest Ethiopia through a series of deep valleys and canyons into Sudan, by which point it is only known as the Blue Nile.
There are numerous tributaries of the Abay between Lake Tana and the Sudanese border. Those on its left bank, in downstream order, include the Wanqa River, the Bashilo River, the Walaqa River, the Wanchet River, the Jamma River, the Muger River, the Guder River, the Agwel River, the Nedi River, the Didessa River and the Dabus River. Those on the right side, also in downstream order, include the Handassa, Tul, Abaya, Sade, Tammi, Cha, a, Suha, Muga, Bachat, Katlan, Jiba, Chamoga, Weter and the Beles.
After flowing past Er Roseires inside Sudan, and receiving the Dinder on its right bank at Dinder, the Blue Nile joins the White Nile at Khartoum and, as the River Nile, flows through Egypt to the Mediterranean Sea at Alexandria. The Blue Nile is so-called because during flood times the water current is so high, it changes colour to an almost black; since in the local Sudanese language the word for black is also used for the colour blue.
The distance from its source to its confluence is variously reported as 1,460 and 1,600 kilometres (907 and 1,000 miles). The uncertainty over its length might partially result from the fact that it flows through a virtually impenetrable gorges cut in the Ethiopian Highlands to a depth of some 1,500 metres (4950 feet)—a depth comparable to that of the Grand Canyon of the Colorado River in the United States.
The flow of the Blue Nile reaches maximum volume in the rainy season (from June to September), when it supplies about two thirds of the water of the Nile proper. The Blue Nile, along with that of the Atbara River to the north, which also flows out of the Ethiopian Highlands, were responsible for the annual Nile floods that contributed to the fertility of the Nile Valley and the consequent rise of ancient Egyptian civilisation and Egyptian Mythology. With the completion in 1970 of the Aswan High Dam in Egypt, the Nile floods ended.
The Blue Nile is vital to the livelihood of Egypt. Though shorter than the White Nile, 59% of the water that reaches Egypt originates from the Blue Nile branch of the great river; when combined with the Atbara River, which also has its source in the Ethiopian Highlands, the figure rises to 90% of the water and 96% of transported sediment. The river is also an important resource for Sudan, where the Roseires and Sennar dams produce 80% of the country’s power. These dams also help irrigate the Gezira Plain, which is most famous for its high quality cotton. The region also produces wheat and animal feed crops.