The barrage is a double bridge, or weir, the eastern part spanning the Damietta branch of the Nile, the western part the Rosetta branch. The appearance of the structure is so light and graceful that the spectator finds it hard to conceive of the difficulty and the greatness of the work itself. Architecturally, the barrage is very beautiful, with a noble front and a grand effect, produced by a line of castellated turrets, which mark the site of the sluice gates. There are two lofty crenellated towers, corresponding with the towers over the gateway of a mediaeval baronial castle. The sluices are formed of double cones of hollow iron, in a semicircular form, worked on a radii of rods fixed to a central axis at each side of the sluice-gate. They are slowly raised or let down by the labour of two men, the gates being inflected as they descend in the direction of the bed of that part of the river whose waters are retained. The working of the barrage was never what it was intended to be. After the year 1867 it ceased to be of any practical utility, and was merely an impediment to navigation. Between the years 1885—90, however, during the British occupation, Sir Colon Scott-Moncrieff successfully completed the barrage at a cost of $2,500,000, and now the desired depth of eight feet of water on the lower part of the Nile can always be maintained.
It proved to be of the greatest advantage in saving labour worth hundreds of thousands of dollars a year, and in the irrigation and navigation facilities that had been contemplated as among the benefits which would naturally accrue from its successful completion.
Compared with the advance of the land seaward at the estuary of the Mississippi and the Ganges, the advance of the Nile seaward is very slow. This is accounted for by the geological theory that the Delta of the Nile is gradually sinking. If this is so, the tendency of the periodical deposit to raise the level of the Delta will be counteracted by the annual subsidence. These phenomena account for the gradual burial of Egyptian monuments under the sand, although the actual level of the sea above what it formerly was is quite unappreciable.
The periodical rise in the Nile, recurring as regularly as the revolutions of the heavenly bodies, necessarily remained an unsolved mystery to the ancients, for until the discovery of the tropical regions, with their mountainous lakes and deluging rains, it was impossible to learn the occasion of this increase. It is now known that the Blue Nile, flowing out of the mountainous parts of Abyssinia, is the sole cause of the periodic overflow of the Nile. Without the tropical rains of the Ethiopian tablelands, there would be no great rise nor any fertilising deposits. Without the White Nile, which runs steadily from the perennial reservoirs of the great Central African lakes, the Lower Nile would assume the character of an intermittent wady, such as the neighbouring Khor Baraka, periodically flushed by the discharge of the torrential downpours from Abyssinia. Though there is a periodical increase in the flow of the upper waters of the White Nile, yet the effect of this, lower down, is minimised by the dense quantities of vegetable drift, which, combining with the forest of aquatic growth, forms those vast barriers, known by the name of sudd, which not only arrest navigation but are able to dam up large bodies of water.
The sudd, it is supposed, stopped the advance of the Roman centurions who were sent up the Nile in the days of Nero. Sir Samuel Baker was the one who first pointed out the great disadvantage of allowing the vegetable matter to accumulate, both to merchants and to those who were employed to suppress the slave-trade. In the year 1863 the two branches of the White Nile were blocked above their junction at Lake No. Once blocked, the accumulation rapidly increased from the stoppage of outlet, forming the innumerable floating islands which at this part of the Nile customarily float down-stream. A marsh of vast extent had been formed, and to all appearance, as Baker narrates, the White Nile had disappeared. Baker cut through fifty miles of the sudd, and urged the khédive to reopen the Nile. The work was successfully undertaken by Ishmail Ayub Pasha, and the White Nile became clear for large vessels when Gordon reached Khartum in 1874. It is practically impossible to keep the central provinces of the Nile open to civilisation unless the course of the Nile is free. Yet in 1878 the obstruction had been renewed, and during the occupation of these provinces by the rebel dervishes under the Mahdi and the califa the Nile was completely blocked, as formerly, at Lake No. The alarming failure of the Nile flood in 1899—1900 was generally attributed to this blockade, and in 1899 fifty thousand dollars was placed at the disposal of the governor-general for reopening the White Nile by removing the vast accumulation of sudd which blocked the Bahr-el-Jebel from Lake No almost as far as Shambeh. The work was started under the direction of Sir William Garstin in 1899. In 1900 the greater part of the sudd had been removed by the strenuous labours of Major Peake, and the Nile again became navigable from Khartum to Rejaf. The sudd was found to be piled up and of almost as close a structure as peat. It was sawn out in blocks ten feet square and carried away by gunboats. In the years 1901—02 further progress was made, and twenty thousand dollars appropriated for the work; and by means of constant patrolling the sudd is now practically absent from the whole course of the White Nile.
The discharge of the flood waters from the Upper Nile begins to make itself felt in Lower Nubia and Egypt in the month of June, at first slightly, and after the middle of July much more rapidly, the river continuing to rise steadily till the first week in October, when it reaches high-water mark, nearly fifty-four or fifty-five feet at the Egyptian frontier, and twenty-five or twenty-six feet at Cairo. A subsidence then sets in, and continues till low-water level is again reached, usually about the end of May. The floods are then much higher and confined to a narrower space in the Nubian section of the Nile, while they gradually die out in the region of the Delta, where the excess seawards is discharged by the Rosetta and Damietta branches. In place of the old Nilometers, the amount of the rise of the Nile is now reported by telegraph from meteorological stations.