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(Bahr el Azraq; Arabic bahr—river, and azraq—blue), a river in Ethiopia and the Sudan; a right tributary of the Nile and the one providing it with the most water. Length, 1,600 km. The Blue Nile originates as the Little Abbai on the southwestern slopes of the Choke Mountains south of Lake Tana. The Little Abbai flows into Lake Tana and flows out of it under the name of Abbai. The Tis-Asat waterfall (45 m high) is located 20 km farther downstream.
The Blue Nile flows at first between low banks, but then it flows 500 km through a canyon 900–1,200 m deep and 100–200 m wide. When it flows into the territory of the Sudan, it is given the name “Blue Nile,” and it changes strikingly in character. Below Roseires it descends very gradually and meanders a great deal. Its main tributaries are the Rahad and the Dinder. At the city of Khartoum it merges with the White Nile forming the Nile.
The Blue Nile is fed by rainwater. Its discharge varies sharply. (At Khartoum, it ranges from 127 cu m per sec in February to 5,822 cu m per sec in August.) On the average, it carries 52 cu km of water per year (1,650 cu m per sec). At the city of Sennar there is a dam and a 19-megawatt (19,000 kilowatts) hydroelectric power plant. In 1971 a reservoir was under construction 8 km above the city of Roseires. The Blue Nile is navigable to a point 580 km above its mouth. During the flood season ships can travel 250 km up the Rahad and 600 km up the Dinder.