Page:EB1911 - Volume 19.djvu/723

 in a length of 3 m., the river fell 16 ft. Since the completion of the great dam and locks at the head of these rapids (Dec. 1902) they have to a certain extent disappeared, and a navigable channel has been formed. The dam, pierced by 180 sluices, stretches across the river—a wall 2000 yds. long and 26 ft. wide at the top. Below the water rushes between rocks in many channels (this being the relics of the cataract). Upstream from the dam a lake some 100 m. in length has been formed. The Assuan Dam was opened on the 10th of December 1902 (see under ). A ladder of four locks on the western side of the dam permits navigation between the upper and lower reaches. At Assuan the banks of the river are bordered by high granite hills. From this point to the apex of the delta the length of the Nile is 605 m. with a slope slighter than that above Assuan. The valley is comparatively narrow, being an almost level depression in a limestone plateau—the area of fertility ends where the land ceases to be irrigated by the river. At Edfu, 68 m. below Assuan, a barrage, known as the Esna barrage, regulates the flow of the water, and at Assiut, 274 m. below Edfu, is another barrage fulfilling the same purpose. Cairo, the capital of Egypt, is built on the eastern bank of the Nile 12 m. north of the apex of the delta.

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. At the bifurcation is a double barrage, by means of which the water can be dammed to the height required for forcing the river 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. In time of full flood the depth of the 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 irrigation 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 3600 ) Above the Sobat confluence the Nile traverses a region of heavy rainfall and the water-supply is superabundant. It is from Victoria, Albert and Albert Edward Nyanzas and their feeders, and in a lesser degree from the Bahr-el-Ghazal, that this river obtains its constant supply of water throughout the year. The great lakes and the region of swamps, 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 chief cause of the Nile flood. In the equatorial regions rainfall varies from 30 to 80 in. during the year with a mean of about 50. It is heaviest in the months of January, February, March and April, and again in October and November. The most rainy portions of the lake plateau (where alone occurs a rainfall of 60 in. and over) lie along the eastern edge of the Albertine Rift valley, and west and north of Victoria Nyanza. These rains feed Albert Edward and Albert Nyanzas, and, through the Kagera, supply a great part of the water of Victoria Nyanza. The water in the Victoria Nyanza begins to rise in January, 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 does not vary very greatly, the maximum levels occurring in November and December. The Ghazal has but a slight discharge. The Sobat, from December to March, is at its lowest, and is in flood from June to October, during which period the water (milky coloured) which it pours into the Nile equals in volume that of the main stream. It is the colour of the Sobat water which gives its name to the White Nile. The Blue Nile Nile, at its confluence at Khartum, begins to rise in June and is in flood from July to October; 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 several areas of repose, and annually flooded by the accession of a great body of water with which its eastern tributaries are flushed.

At Khartum the Nile is lowest in April and May and highest in August and September. Its minimum depth is 18 ft. and its maximum 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 August, reaching its maximum at the beginning of September; it then falls slowly through October 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 August, 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 : 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 30 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. This is caused by innumerable microscopic green algae, which are brought into the White Nile from the marshes of the Bahr-el-Jebel and Bahr-el-Ghazal, and descend the river when it is clear of all suspended matter. 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 August 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 cub. yds. per second; by September 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 Khartum to the delta barrage varies from 14 days in September to 43 in May. On the island of Elephantine at Assuan is the famous Nilometer, dating from ancient Egyptian times, altered and extended in Roman times and repaired in 1870 by the Khedive Ismail. It is a well built of hewn stones, marked with scales to record the level of the water, which rises and falls with that of the river. The remains of other ancient Nilometers exist at Philae, Edfu and Esna, together with inscriptions recording about forty high Niles in the XXVth Dynasty, discovered on a quay wall of the temple of Karnak. The data furnished by these give about 4 in. per century as the rate at which the Nile is silting up its bed north of the 1st cataract. The level of high Nile at the Semna rapids, between the 2nd and 3rd cataracts, is 24 ft. lower than that indicated, by the marks sculptured c. 2500 This fall is attributed to the erosive action of the water as it passes over the hard gneiss which at Semna forms a barrier across the stream. The vertical extent of such erosion is equal to about two millimetres a year.

It is noteworthy that from the mouth of the Sobat to the Mediterranean the current of the Nile is generally deepest and strongest on its right (eastern) bank; the Nile in this respect resembling other great rivers of the northern hemisphere. The pressure of the water on the right bank is attributed to the prevailing N.W. winds.

There are now gauges for registering the rise of the water at Cairo, Assuan, Berber and Khartum on the main river; at Wad Medani, Sennar and Roseires on the Blue Nile; El Duem and Taufikia on the White Nile; Nasser on the Sobat; Gondokoro on the Bahr-el-Jebel; and Ugowe, Jinja and Entebbe on Victoria Nyanza.

Navigation.— At high Nile there is uninterrupted water-communication from the sea to Fort Berkeley in 4° 40′ N.—a distance of 2900 m. Owing to the cataracts, navigation between Assuan and Khartum is impossible during low Nile, and from the 1st of March to the 1st of August the upper courses of the Damietta and Rosetta branches are closed to navigation; the water being utilized for summer irrigation in the delta. As far as Mansura (60 m.) on the Damietta branch and Kafr-el-Zayat (70 m.) on the Rosetta branch, and between Khartum and Fort Berkeley (1090 m.) the river is navigable all the year round, though between the Sobat confluence and Bor, navigation is dependent on the channel being kept clear of sudd. Above Fort Berkeley navigation is interrupted by the rapids and cataracts which extend to Dufile, but from the last-named town to Fajao at the foot of the Murchison Falls (a distance of 150 m.) the river is navigable throughout the year. There is a further navigable stretch between Foweira (just above the Karuma rapids) and the southern end of Lake Kioga. The Blue Nile is navigable for steamers during flood time from its confluence at Khartum to Roseires at the foot of the Abyssinian hills, a distance of 426 m. At low water small boats only can go up stream. The Atbara is never navigable, the current during flood time being too swift for boats. Including the Sobat and the Bahr-el-Ghazal the navigable waters of the Nile and its affluents exceed 4000 m.

Owing to the cataracts and the partial closing of the Damietta and Rosetta branches for irrigation purposes, the Nile below Khartum is subsidiary, as a means of communication, to the railways and highroads. Above Khartum the river is