Ethiopian highlands swell the Blue Nile until it accounts for 90 percent of the Nile’s total flow. Sudan has constructed several dams to regulate the river’s flow, including the Roseires Dam, about 100 kilometers from the Ethiopian border and the largest, the 40-meter-high Sinnar Dam constructed in 1925 at Sinnar. The Blue Nile’s two main tributaries, the Dindar and the Rahad, have headwaters in the Ethiopian highlands and discharge water into the Blue Nile only during the summer high-water season. For the remainder of the year, their flow is reduced to pools in sandy riverbeds. The White Nile flows north from central Africa, draining Lake Victoria and highland regions of Uganda, Rwanda, and Burundi. South of Khartoum, the British built the Jabal al-Awliya Dam in 1937 to store the water of the White Nile and then release it in the fall when the flow from the Blue Nile slackens. Much water from the reservoir has been diverted for irrigation projects in central Sudan, however, and much of the remainder evaporates. By now, silt deposits have curtailed the overall flow. North of Khartoum, the Nile flows through the desert in a large S-shaped pattern to empty into Lake Nasser behind the Aswan High Dam in Egypt. The river flows slowly beyond Khartoum, dropping little in elevation, although five cataracts hinder river transport at times of low water. The Atbarah River, flowing out of Ethiopia, is the only tributary north of Khartoum, and its waters reach the Nile for only the six months between July and December. During the rest of the year, the Atbarah’s bed is dry, except for a few pools and ponds. Climate Although Sudan lies within the tropics, the climate ranges from hyper-arid in the north to tropical wet-and-dry in the far southwest. Temperatures do not vary greatly with the season at any location; the most significant climatic variables are rainfall and the length of the wet and dry seasons. Variations in the length of the wet and dry seasons depend on which of two air flows predominates: dry northern winds from the Sahara and the Arabian Peninsula or moist southwesterly winds from the Congo River basin and southeasterly winds from the Indian Ocean. From January to March, the country is under the influence of dry northeasterlies. There is minimal rainfall countrywide except for a small area in northwestern Sudan where the winds have passed over the Mediterranean