10 GEOGRAPHY (No. 98 a > into a solid block , often having an under -water thick ness of 15 or 20 ft. These blocks, which maybe up wards of a mile in length, dam the main channel, cause the flooding of surrounding marshes, and prevent navi gation. They constitute a great danger to shipping, as a steamer caught amongst them is exposed to the risk of being crushed, and is certain to be imprisoned for an indefinite period . Of recent years , however, supervision during thestormy season has kept the river open to traffic .

At a point about 7° 12 ' north latitude the river throws off from the right bank a branch called the Bahr el-Zeraf (river of giraffes), really a relief channel for the floods when the main stream is blocked by sudd. The Zeraf runs through little -known and inaccessible swamps for over 200 miles, and rejoins the White Nile fifty miles below its confluence with the Bahr el-Ghazal.

The Bahr el-Ghazal joins the Bahr el-Jebel at the west end of Lake No ( 90 29 ' N. ) , and on issuing from this lake the river , henceforth called the White Nile (Bahr el-Abiad ) flows east for ninety -six miles in a sluggish stream , broken by many islands and back waters, to the confluence of the Sobat ( 9° 20' N. ) . Thence to Omdurman ( 516 miles ) it receives no more perennial affluents. The only obstacle to navigation on this long reach is the Abu Zeid ford ( 13° N.),where the water sometimes falls as low as 2 ft. At Khartum the combined stream of the White and Blue Niles is about three miles wide; and between this point and Halfa , after receiving the waters of the Atbara below El Damer ( 17° 32' N.), it attains its greatest volume . The Island of Argo , between Dongola and Kerma, is perhaps the most fertile spot in the Sudan .

The whole stretch of 946 miles between Khartum and the northern frontier is full of reefs and rapids, and contains five of the six great cataracts, which are caused by the volume of flood -water having eroded the bed of the river and brought to the surface numerous dykes and ridges of volcanic rock . Navigation here