headstream of the Nile River (q.v.) and source of almost 70 percent of its floodwater at Khartoum. It reputedly rises as the Abāy from a spring 6,000 ft (1,800 m) above sea level, near Lake Tana in northwestern Ethiopia. The river flows into and out of the lake, runs through a series of rapids, and then drops into a gorge. It flows through a deep canyon southeast and west around the Choke Mountains and then turns northwest through The Sudan to join the White Nile at Khartoum. Its length is about 907 mi (1,460 km). By far the greater part of the Blue Nile's waters come from such tributaries as the Dinder and the Rahad rivers, which rise in the Ethiopian highlands. Dams at er Roseires (ar-Ruṣayriṣ) and Sannār (Sannār Dam), in The Sudan, irrigate 1,000,000 ac (400,000 ha) in the plain of al-Jazīrah (Gezira) between the Blue and White Nile rivers; the Sannār Dam also produces hydroelectric power.