The Ghazal River contributes very little to the flow of the Nile, owing to wide lagoons through which it passes, and which cause great evaporation. The river then passes sharply to the right, and sixty miles further on is joined by the Sobat from the eastern hills, which in flood brings down a volume equal to that of the Nile, but during summer contributes little or nothing. From the white sediment brought down by the Sobat the White Nile derives its name and colour. Hence it flows in a wide bed, a mile across on the average, 540 miles to its junction with the Blue Nile at Khartoum. Khartoum is still 1,800 miles from the sea, and 390 metres above it. Two hundred miles north of Khartoum, near to Berber, the Atbara River flows in, and hence the Nile pursues its solitary way through the desert until, after a circuitous bend round Dongola, it bursts through the rocky defiles of Nubia, and emerges at Assouan into Egypt proper. Of the distance between Khartoum and Assouan about 350 miles consist of so-called cataracts, during which the total drop is 200 metres; about 750 miles are ordinary channel, with a total drop of nearly 100 metres. Of all the affluents of the Nile, the Blue Nile (assisted by its tributaries, the Rahad and the Dinder) and the Atbara have been of infinitely the greatest importance to Egypt in the past. Not only do their waters contribute the largest proportion of the annual flood, but also down them comes the rich volcanic detritus swept from the Abyssinian hills by the heavy summer rains, which composes the red-brown silt so dear to the Egyptian cultivator. To understand the system of irrigation in Egypt it is necessary to have a clear view of the amount of water derived from the different sources at different times of the year. The great lakes and swamps of Uganda, acting as reservoirs, prevent any great differences in the discharge of water above Lado. At that place the low Nile discharge is about 500 cubic metres[2] per second; but, in spite of the Ghazal River and the Sobat, so great is the loss by diffusion in the marshes and by direct evaporation, that at Khartoum the discharge is no more than 300 cubic metres per second. At this time the Blue Nile is giving no more than 160 cubic metres per second at Khartoum, and the Atbara is not running at all. The loss between Khartoum and Assouan is about 50 cubic metres, and consequently the amount of water passing Assouan in May in an ordinary year is 410 cubic metres per second. This is the summer supply of Egypt.