The causes of the flood are, briefly, as follows:—The rains in southern Abyssinia cause the Sobat to rise about the middle or end of April. The yellowish-white water caused by this flood reaches Kodok the last week in April. The equatorial rains cause the Bahr El Zeraf and White Nile, above the Sobat, to rise about the end of May. The effect of the first rise is felt at Halfa about the 20th May. The Blue Nile begins to rise about this time, and brings the red fertilizing flood down to Khartoum about the 20th June, and to Halfa about the middle of July. The muddy Atbara flood (June to August) rises very rapidly after the Blue Nile, and causes the flood to attain its maximum about the end of August; at this period the river is muddiest. The White Nile continues to rise slowly, and its effects (clear white water) are felt at Halfa till October, when it falls very slowly. The Blue Nile falls rapidly after the middle of September, and the Atbara has generally disappeared by October. The Sobat begins to fall at Nasser early in December. The Bahr El Ghazal rivers rise slightly in May. These soon subside, and the main floods take place in July and August. The above is a description of an average year. An early maximum (_i.e._, about the middle of August) generally means a low summer flood, and _vice versâ_. The flood at its height moves at about 100 miles a day (below the Atbara).[17] The above dates vary largely in accordance with the time and amount of rainfall in the upper valleys of the rivers. Of the flood water annually brought down by the rivers, about half now runs to waste into the Mediterranean sea. The Aswan dam regulates and utilizes this to a certain extent, but a large proportion remains over. This it is proposed to utilize in the future for irrigating the Sudan by means of flood and catchment basins; but the time is still distant when this surplus will be entirely devoted to this purpose. [Sidenote: Historical.] Up to comparatively recent times the sources of the Nile were involved in mystery.[18] In ancient days many guesses were hazarded at their origin, some geographers maintaining that the Nile rose in the Atlas Mountains of Marocco; others were positive that the Niger formed the upper portion of the great river. During the 17th and 18th centuries it was held that the sources were to be found in the “Mountains of the Moon.” These have since been speculatively identified with the Ruwenzori Mountains between Lakes Albert and Albert Edward, and, if this theory is correct, the ancient geographers were not so far out.