The lower part of the Bahr-al-Ghazal is often lost in marshes or is widened into lake-like expanses such as Lake No, at the confluence of the White Nile. About a hundred miles to the east of this confluence with the Kir or main White Nile (also called, south of this point, the Bahr-al-Jabl or Mountain Nile), there enters a very important affluent from the east, the Sobat (Baro), which is formed by a number of streams flowing from the southwestern part of the Abyssinian Empire and the vicinity of the Lake Rudolf basin.[126] After its confluence with the Sobat the White Nile flows without any important tributary for something like three hundred and fifty miles nearly due north through a country which passes from a tropical luxuriance of vegetation to the acacias and thin grass of the steppe region. The influence of the Sahara Desert, in fact, begins to make itself felt,--that desert which extends right across from the Atlantic to the Persian Gulf, interrupted only by the exceptional mountain regions of Tibesti, Darfur, Abyssinia, Yaman, and Jabl Akhdar. At Khartum, in about 15° 40′ north latitude, the Nile receives its most important affluent, the Abai or Blue Nile (Bahr-al-Azrak). This river rises on Sagada Mountain in the Abyssinian province of Gojam, passes through the south end of Lake Tsana (a piece of water about the area of Gloucestershire) in the western part of Central Abyssinia, and, after curving to the east and south, turns west and north, and brings to the Nile (it is said) that great increase of volume in the summer time which causes the annual flooding of Lower Egypt. Lengthy as is the course of the White Nile from the Victoria and Albert Nyanzas to Khartum, and infinitely greater though the mass of its waters should be than the volume of the Blue Nile, the stream of the White River has nevertheless been much attenuated before it reaches Khartum by the waste of its volume in the region of vast swamps lying between Fashoda on the north and Lado on the south, to say nothing also of a similar waste and evaporation of water from the deflection of the Victoria Nile into the backwaters and swamps of Kioga and Kwania. Another contribution--though a much feebler one in volume of water--comes from Abyssinia in the shape of the Atbara, which, in its upper waters, is known as the Takaze.[127] This river, during the dry season, almost ceases to flow in its lower portion, though it is flooded during the summer months from the melting of the snows and the heavy tropical rains on the northern Abyssinian mountains. The Atbara is considered by Sir Samuel Baker to contribute the principal share of the black mud which fertilises Egypt.