At the beginning of the delta the Nile separates into two channels, the Rosetta and the Damietta, which join the Mediterranean at its south-east angle. In ancient times the delta was watered by seven branches ; five of these branches are now canals not always navigable. The ancient branches were, beginning at the west, the Canopic, Bolbitine, Sebennytic, Phatnitic, Mendesian, Tanitic and Pelusiac, of which the modern Rosetta and Damietta branches represent the Bolbitine and Phatnitic. At the head of the river is a double barrage, by means of which the water can be dammed to the height required for forcing the water into the canals which irrigate the delta. Of the two branches the Damietta is the more easterly. Both are about the same length-146 m. Behind the coast-line, which is low and sandy, are a number of salt marshes or lagoons. Whilst the Damietta branch is gradually silting up, the Rosetta branch is scouring out a wider channel. At full flood the depth of water in either branch is about 23 ft.
Hydrography.—The fertility and prosperity of Egypt and the northern part of the Sudan being entirely dependent on the irri gation of the land by the waters of the Nile, the variation in the supply at different seasons of the year is of vital importance. (In Egypt the height of the flood has been recorded annually, as the chief event of the year, since at least 360o B.c.) Above the Sobat confluence the Nile traverses a region of heavy rainfall and the water-supply is super-abundant. It is from Victoria, Albert and Edward lakes and their feeders, and in a very small degree from the Bahr-el-Ghazal, that this river obtains its constant supply of water throughout the year. The great lakes retaining a large proportion of the water they receive, act as natural reservoirs and prevent the lower Nile from ever running dry in summer. The Abyssinian affluents are the source of the Nile flood. In the equatorial regions rainfall varies from 3o to 8o in. during the year with a mean of about 5o. It is heaviest in the months of Jan., Feb., March and April, and again in Oct. and November. The most rainy portions of the lake plateau (where alone occurs a rainfall of 6o in. and over) lie along the eastern edge of the Albertine Rift valley, and west and north of Victoria lake. These rains feed Edward and Albert lakes, and, through the Kagera, supply a great part of the water of Victoria lake. The water in Lake Victoria begins to rise in Jan., the 'rise becomes marked in June, is at its height in July, the level of the water reaching its lowest at the end of November. The Bahr-el-Jebel is at its lowest in March and April and at its highest in September. The seasonal supply of the Bahr-el-Ghazal hardly varies, the maximum levels occurring in Nov. and Dec., and it has but a slight discharge. The Sobat, from Dec. to March, is at its lowest, and is in flood from June to Oct., during which period the water (milky coloured)
which it pours into the Nile is two or three times the volume of the main stream. It is the colour of the Sobat water which gives its name to the White Nile. The Blue Nile, at its confluence at Khartoum, begins to rise in June and is in flood from July to Oct.; the Atbara is also in flood during the same months. The great difference in the supply of water from the equatorial regions and from Abyssinia arises from the fact that the first-named district is one of heavy rain practically all the year round, whereas in Abyssinia the season of heavy rain is usually limited to the months of June to September. Reduced to its simplest expression, the Nile system may be said to consist of a great steady flowing river fed by the rains of the tropics, controlled by the existence of a vast head reservoir, and annually flooded by the accession of a great body of water with which its eastern tributaries are flushed.
At Khartoum the Nile is lowest in April and May and highest. in Aug. and September. Its minimum depth is 18 ft. and its maxi mum depth 25 ft. At Assuan the Nile is at its lowest at the end of May, then rises slowly until the middle of July, and rapidly throughout Aug., reaching its maximum at the beginning of Sept.; it then falls slowly through Oct. and November. At Cairo the lowest level is reached about the middle of June, after which the rise is slow in July and fairly rapid in Aug., reaching the maximum at the beginning of October. By using the water stored by the Assuan dam in the months following high Nile, the river lower down has been, since 1902, replenished at times of low water to meet the needs of cultivators (see IRRIGATION : Egypt). At Assuan the average rise of the Nile is 26 ft., at Cairo it is 23 ft. A rise of 21 ft. only at Assuan is a "bad Nile"; on the other hand, a rise of 3o ft. causes a danger of flood, or rather it used to do so previous to the building of the dam.
When the Nile below the swamps is at its lowest, the water acquires a green colour and a putrid taste and smell due to in numerable microscopic green algae, which grow freely in the conditions of heat and nearly stagnant water which prevail in some reaches of the river at low stage. This "green water" is seen at Cairo about the end of June or beginning of July, and passes away with the first rise in the later month, the algae being unable to live in turbid water. By Aug. the river in lower Egypt is full of dark red-brown sediment brought down by the Blue Nile and the Atbara from the plateaus of Abyssinia. It is estimated to be then carrying 8 cu.yd. per sec. ; by Sept. this has been reduced to half the amount, and then diminishes rapidly. It has been calculated' that the time taken by the water to travel from Khartoum to the delta barrage varies from 14 days in Sept. to 42 in May.