THE BASIN OF THE NILE a great part of its course, the Nile plays an important part in the economic development of the countries through which it passes, and hence it is necessary to know some thing of the regime of that river and its tributaries. It rises on the Lake Plateau of Equatorial Africa, where the Kagera and various other streams flow into Victoria Nyanza. The Victoria Nile from Victoria Nyanza, and the Semliki from Lake Edward, both descend rapidly from the plateau to Lake Albert in the western branch of the rift valley, and the river which leaves Lake Albert— the Bahr-el-Jebel—also has a swift descent into the plains of the Sudan. From the Nile-Congo watershed, on the south-west of these plains, comes a number of streams, the majority of which eventually unite to form the Bahr-el-Ghazal, which flows into the Bahr-el-Jebel, the combined river taking the name of the White Nile. A short distance below the confluence of these rivers, the Sobat flows into the White Nile, which is also joined at Khartum by the Blue Nile, and near Berber by the Atbara, all three rivers flowing from the Abyssinian plateau.
From the point at which it enters the Sudan plains to its con fluence with the Blue Nile at Khartum, a distance of over 1,000 miles, the fall of the Nile is very gentle, and only averages about two inches to the mile. Below Khartum, however, the river passes through a series of cataracts before it enters the Nile Valley at Assuan, where its course again becomes gentle. In this valley, which owes its origin to a fracture in the earth's crust, the Nile has, by the deposition of silt, built up a plain which has a length of about 600 miles and a breadth varying from 5 to 10 miles. The delta at the mouth of the river has likewise been built up of silt.
On the equatorial plateau there is a double rainy season, and the discharge from Victoria Nyanza is fairly constant through out the year. Further north, on the slope of the plateau and in the basin of the Bahr-el-Ghazal, there is a heavy summer rainfall, which leads to much flooding in the southern plains of the Sudan, but does not affect the height of the river below Lake No. The basin of the Sobat has also a summer rainfall, but its effect is not felt in the valley of the White Nile until December. The Blue
Nile and the Atbara are, therefore, the factors of most importance in determining the rise and fall of the waters of the Nile as far, at least, as the lower part of its course is concerned. On the Abyssinian plateau, whence these two rivers flow, there is a heavy monsoonal rainfall between the middle of May and the middle of September. Both rivers come down in flood, the Blue Nile reaching its maximum about the beginning of September, and the Atbara about the third week in August, with the result that the maximum height of the Nile at Wady Haifa is also reached about the beginning of September. But to this, it ought to be noted, the White Nile contributes but little, as, when the Blue Nile is in high flood, the waters of the White Nile are ponded back and do not begin to run off until the later months of the year, when the former river has fallen very considerably. Below its confluence with the Atbara, the Nile receives no regular stream, but only the results of occasional showers on the hills between it and the Red Sea. Consequently, in its progress through Egypt it loses in volume as a result of evaporation, seepage, and the withdrawal of water for irrigation purposes. At Wady Haifa, where the Nile enters Egypt, the regime of the river is somewhat as follows : it is at its lowest about the middle of June, when it begins to rise rapidly and continues to do so until the beginning of September, throughout which it remains very high. During October and November it quickly loses volume as a result of the fall of the Blue Nile and Atbara ; and, after the waters of the White Nile have been drained off in November and December, the river falls steadily until the following June. At Cairo the maximum occurs about a month later than at Wady Haifa. During the flood period the Blue Nile and the Atbara bring down large quantities of volcanic matter from the Abyssinian plateau, and it is this silt that has built up the plain and delta of the Nile which constitute modern Egypt, the rate of deposition being •10 of a metre per century. Concerning the fertilising qualities of the silt there are some differences of opinion.