All of Ethiopia's rivers originate in the highlands and flow outward in many directions through deep gorges. Most notable of these is the Blue Nile, the country's largest river. It and its tributaries account for two-thirds of the Nile River flow below Khartoum in Sudan. Because of the general westward slope of the highlands, many large rivers are tributaries of the Nile system, which drains an extensive area of the central portion of the plateau. The Blue Nile, the Tekezé, and the Baro are among them and account for about half of the country's water outflow. In the northern half of the Great Rift Valley flows the Awash River, on which the government has built several dams to generate power and irrigate major commercial plantations. The Awash flows east and disappears in the saline lakes near the boundary with Djibouti. The southeast is drained by the Genale and Shebele rivers and their tributaries, and the southwest is drained by the Omo.