—From Assuân to the Barrage, the length of the river is 973 kilometres in summer and 923 in flood. The slope in summer is 1⁄13000 and in flood 1⁄12200 The mean fall of the valley is 1⁄10800. The slopes vary in the different mean reaches, the least being 1⁄14800 in the Kena Mudiria and the greatest 1⁄11400 in Beni Suef. In a high flood with a rise of 9 metres at Assuân, the rise in Kena will be 9.5 metres and only 8.2 in Beni Suef. [Table 42] gives the mean areas of cross sections of the Nile, while [table 44] gives the mean widths. Neglecting spill channels, we may state that in a high flood the mean area of the section of the Nile is 7,500 square metres and the mean width 900 metres. In the Kena Mudiria, the area is 7,000 square metres and the width 800 metres, while in Beni Suef the mean area is 8,000 square metres and the mean width 1,000 metres. Speaking generally it may be stated that where the Nile valley is narrow the slope of the river is small, its depth great and width contracted; while where the valley is broad the slope is great, the depth small and the width enlarged. The mean velocity in flood ranges between 2.0 metres and 1.0 metre per second, while the velocity in summer varies from 0.5 to 0.9 metre per second. We may say that the Nile in soil has a natural section whose width in flood is 110 times its depth, while its mean velocity is 1.50 metres per second.

The natural canals, which take off the river and which never silt, have a mean velocity of some 65 centimetres per second, while the proportion of width to depth is about 12 to 1. Artificial canals of this section do not silt if their velocities are 80 centimetres per second, while silting takes place as readily when the velocity is greater as when it is less than the above. In muddy streams, like the Nile in flood, certain velocities demand certain proportions of width to depth, and if these are not given to it, they will make it for themselves by eating away the sides if they can, or, if they cannot eat away the sides, by silting up and raising the bed.

To the north of Assiout is situated the Assiout weir or barrage across the Nile with 111 openings of 5 metres and 10 metres depth of water in high flood. It was regulated on for the first time in August 1902.

On Roda island, opposite Cairo, has stood a gauge from the earliest times. It has been frequently reconstructed. The present gauge is reputed to have been erected in A.D. 861 with its zero at the same level as a more ancient one whose readings have been preserved since A.D. 641. When the gauge was constructed, a reading of 16 cubits meant the lowest level at which flood irrigation could be insured everywhere. The level to-day is 201⁄2 cubits on the gauge and the difference between them is 1.22 metres. As 1,026 years have elapsed since the construction of the gauge it means a rise of 12 centimetres per 100 years. This is slightly under the rise calculated at Assuân by the French savants.

The following table gives the means of the maximum flood and low water levels per century:—

7thcentury17.5R. L. flood11.0R. L. low water6.5Difference.
8th„17.4„11.1„6.3„
9th„17.5„11.2„6.3„
10th„17.5„11.3„6.2„
11th„17.5„11.4„6.1„
12th„17.7„11.5„6.2„
13th„17.7„11.6„6.1„
14th„17.9„11.7„6.2„
15th„18.2„11.8„6.4„
16th„18.4„11.9„6.5„
17th„18.8„12.0„6.8„
18th„19.1„12.1„7.1„
19th„19.5„12.2„7.3„

It is evident from the above that the head of the Delta, or the bifurcation of the Nile, was much nearer to Cairo in early days than just now, and the last three centuries have seen great changes. The fall of watersurface is very considerable at every bifurcation, and the difference between mean high and low supply at the Barrage to-day is 6.0 metres against 7.2 metres at Cairo. Judging from the above figures, I should say that from the 7th to the 13th century the bifurcation was gradually approaching Cairo, while since the 13th it has been receding.

PLATE XII.

LONGITUDINAL SECTION of the NILE from ASSUAN to CAIRO
ALONG CENTRE LINE OF FLOOD