Blue Nile, Arab. Bahr el Azraq, river, c.1,000 mi (1,600 km) long, the chief headstream of the Nile, rising in Lake Tana, NW Ethiopia, at an altitude of c.6,000 ft (1,800 m). It flows generally S from the Lake Tana region, then W across Ethiopia, and finally NW into Sudan. At Khartoum the Blue Nile merges with the White Nile to form the Nile proper. 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 used to cause the annual Nile flood before the completion in 1970 of the Aswan High Dam (see under Aswan) in Egypt. In Ethiopia the Blue Nile, also known there as the Abbai, flows in a deep gorge and receives many tributaries. There are dams on the Blue Nile at Roseires and Sennar in Sudan; the latter is used to irrigate the Al Gezira region.