In 1903 the minimum discharge of the White Nile at its head was 380 cubic metres per second in April and the maximum discharge was 1470 cubic metres per second in December. Table 24 gives the behaviour of the river. The minimum discharge of the White Nile at Khartoum may be taken as 300 cubic metres per second. The preceding paragraph will explain how difficult it will be to know its maximum discharge until a gauge and discharge table are established for the Blue Nile 100 kilometres above Khartoum well above back water, and a gauge and discharge table in the Main Nile north of Omdurman. The difference between these two discharges will be the true discharge of the White Nile which, with its slope of ¹⁄₁₀₀₀₀₀ in flood, is not a river but a flood reservoir. The discharges taken at Duem on the White Nile and at Khartoum on the Blue Nile in 1902 and 1903 are interesting, but of little value for anything except the very date on which they were taken. They were all in backwaters.

19. The Blue Nile.

—Compared to any river we have yet described, the Blue Nile is a true mountain stream. Draining the southern and more rainy half of Abyssinia, it is the principal source of the Nile in flood. Whatever waters it receives, it carries to the Nile and it is the true parent of the land of Egypt, for the deposit from its muddy waters is that Nile mud which has made Egypt. The Atbara carries waters which are probably more muddy than those of the Blue Nile, but compared to the Blue Nile the Atbara is a small river, and its quota is insignificant by the side of that of the larger stream. The principal tributary of the Blue Nile, the Abai, rises at a height of about 2,700 metres above sea level and after a course of 110 kilometres falls into Lake Tsana. Lake Tsana lies at a level of about 1,760 metres above sea level, and has an area of about 3,000 square kilometres and catchment basin of about 14,000 square kilometres irrespective of the lake area. On the 31st of January 1903, after a very poor rainfall, Mr. C. Dupuis found the discharge 42 cubic metres per second. Calculating from his cross section, it seems that the maximum discharge may be 200 cubic metres per second. As at Lake Victoria, possibly not more than ¹⁄₁₂th the rainfall finds its way into the lake; and, once there, the greater part is evaporated. Little seems to leave the lake, which would consequently make a very poor reservoir. The land rises from the lake in gently undulating downs as a rule. Wherever observed by Mr. Dupuis, the lake was shallow. There are many islands and some of them considerable ones. The rivers feeding the lake are the following:—the Abai discharging 9 cubic metres per second in January 1903, the Reb 2 cubic metres, the Gumara 2, the Magetch 0·3, the Arno Garno 0·3, the Gelda 0·5, the Unfraz 1·2, and many smaller streams say 1·7, or 17 cubic metres per second in all. This of course was in the dry season. Mr. Dupuis considers the evaporation as 4 millimetres per day.

Between Lake Tsana and Rosaires, on a length of about 750 kilometres the Blue Nile falls some 1260 metres; between Rosaires and Sennar, on a length of 270 kilometres, it falls about 60 metres, and between Sennar and Khartoum on a length of 345 kilometres it falls about 50 metres. The cross sections of the river at Wad Medani, 200 kilometres above Khartoum, and at Khartoum are given on Plate IX.

In flood and early winter the river is navigable up to the Rosaires cataract. The width of channel may be considered as varying between 350 and 700 metres with an average width of 500 metres. The river rises from 9 to 12 metres in flood and has a velocity in high floods of 3 metres per second. Rivers with such velocities scour out their beds very severely in high floods and deposit silt in low floods, and for the discharges below 5 metres, cross sections should be annually established and discharge tables made depending on the sections. The summer discharge varies from 100 to 300 cubic metres per second and the flood from 7,500 to 12,500 cubic metres per second. The behaviour of the river is shown in Tables 24 and 25. The beginning of May is generally low water and the beginning of September high water. The winter discharge may be taken as 500 cubic metres per second.

The tributaries south of Rosaires are the following:—on the right bank, the Folassa, the Durra, the Fatsam, the Bir and the Temsha, veritable torrents; and on the left bank, numerous streams from north and north-west of Addis Ababa, the Anjur, and the Didessa, the latter from 100 to 150 metres wide. North of the last is the Tumat. The Didessa is about 350 kilometres long and the Tumat 200. It is not at all improbable that in the valley of the Didessa far better reservoir sites could be found than at Lake Tsana. Capt. Lyons tells me that there are important reaches here with very little slope. North of Rosaires there are two important tributaries on the right bank, the Dinder, north of Sennar, and the Rahad, just north of Wad Medani. Both these streams run only in flood and are dry in winter and summer. The Dinder has a bed width of about 120 metres, depth 4 metres in good flood and a velocity of 2 metres per second, which gives a discharge of about 1000 cubic metres per second in a high flood. The Rahad has a bed width of 60 metres, depth of 3 metres in a good flood and a velocity of 2 metres per second, which gives a discharge of 400 cubic metres per second in a good flood. The deltas of the Blue Nile, the Dinder and the Rahad are formed of the richest Nile mud. Such soil is rich in lime, potash and phosphates, but is poor in nitrates.

The velocity of the Blue Nile may be taken as 75 centimetres per second in low supply and 3 metres per second in high flood.

20. The Atbara.

—The Atbara river flows into the Nile at El-Damer, south of Berber. It is essentially a torrent fed by the rains of north-eastern Abyssinia. The rains here begin early and end early, so that the Atbara is in high flood in August and falls quickly through September. Its floods last from June to October and the river is dry for the remaining months of the year. By dry it is meant that there is no running water, for the bed of the river contains numerous pools of water, which are nearly always deep and often very extensive.

Mr. Dupuis has given a rough longitudinal section of the Atbara river. Rising within 16 kilometres of Lake Tsana, at a height of about 2000 metres above sea level, in its first 300 kilometres it falls 1500 metres to 530 metres above sea level, where it is met by the Salaam river. In the next hundred kilometres it falls 40 metres and is joined by the Settit river, a larger and more permanent stream than the Atbara itself. Sixty kilometres lower down is the Khasm-el-Girba gauge, just upstream of Fasher and about 420 kilometres from the Nile. Two hundred kilometres below the Settit junction and about 280 kilometres from the Nile is Gosrejeb, and 150 kilometres lower down Adarma. Finally, after a total length of about 880 kilometres, the Atbara flows into the Nile.

The Settit junction is about 490 metres above sea level, Fasher 470 metres, Gosrejeb 410, Adarma 380, and El-Damer about 365 metres above sea level. In the last 280 kilometres there is therefore a fall of 45 metres or about ¹⁄₆₀₀₀. In this reach the river has a width of about 330 metres and depth in flood of 6 metres.

Tables 24 and 25 give the behaviour of the river. In 1902 and 1904, two very low years, the maximum discharge was about 2000 cubic metres per second, and in 1903 about 3000 cubic metres. In high floods the Atbara can discharge 5000 cubic metres per second.

The principal tributaries of the Atbara are the Salaam and Settit already mentioned. On its right bank between Gosrejeb and Adarma it is joined by the Gaash river, which flows past Kassala and loses itself in the deserts. In years of extraordinary rainfall the Gaash reaches the Atbara. The Gaash at Kassala has a width of 150 metres, depth of 1 metre and approximate discharge of 300 cubic metres per second in an ordinary flood. It has a course of about 160 kilometres before it disappears in the desert.

21. The Nile from Khartoum to Assuân.

—The Nile begins its course without any gauge to record its varying height. A gauge north of Omdurman, another upstream of the 6th cataract and a third downstream of the cataract are badly needed. Until these three gauges are erected and recorded, and another erected and recorded on the Blue Nile at Kamlin, about 100 kilometres above Khartoum, the behaviour of the Nile and its tributaries at their junction will never be exactly understood. Making use of the information which is obtainable, we may say that the Blue Nile is generally at its lowest between the 15th April and 15th May with a mean low-water discharge of about 200 cubic metres per second, falling to nearly zero in certain years; it is at its highest between the 15th August and 15th September with a mean maximum discharge of some 10,000 cubic metres per second, rising to 13,000 and falling to 6,500 in maximum and minimum years. If the larger figure is correct, the Blue Nile bank at Khartoum is over a metre too low, and the town is liable to be flooded out. If reference is made to Plate VIII it will be seen that the flood of the Blue Nile in July, August and September travels up the White Nile, holds back its waters and converts the valley of the White Nile into a flood reservoir. When the Blue Nile falls rapidly in October and November, the discharge of the Nile is maintained by the stored-up waters in the White Nile and by the White Nile flood which has slowly travelled down its almost level bed. Table 24 shows this more clearly than any description could. I do not think that the maximum discharge of the Main Nile on any given day is ever equal to the maximum discharge of the Blue Nile.

The Nile between Khartoum and Berber has a channel wider and deeper than that between Wady Halfa and Assuân and a gentler current. I have not taken, or seen any discharges which have been taken in this reach, but judging from what I saw I should say the channel was 800 metres wide on the average. At a distance of 86 kilometres from Khartoum is the Shabluka or 6th cataract. Here the Nile descends 6 metres on a length of 18 kilometres. Two hundred and twenty kilometres below the cataract the Atbara flows into the Nile and repeats on a very small scale what the Blue Nile does at Khartoum. The Atbara is a flood torrent and is dry from October to May. In flood it discharges from a low maximum of 1,700 to a high maximum of 5,000 cubic metres per second, with a mean maximum of 3,500 cubic metres.

In this reach the Nile has a maximum range of 8¹⁄₂ metres and an ordinary range of 7 metres.

Twenty-four kilometres downstream of the Atbara junction is Berber, and 45 kilometres downstream of Berber is the beginning of the 5th cataract, which has a length of 160 kilometres and a drop of 55 metres with three principal rapids, the Solimania, Baggâra and Mograt. The village of Abu Hamed is situated at the foot of this Cataract. Between Abu Hamed and Dongola is the 4th Cataract, which begins at a point 97 kilometres downstream of Abu Hamed, and has a length of 110 kilometres with a drop of 49 metres. In this series of rapids are the Um Dâras and Guerendid. Between the 4th and 3rd Cataracts is a reach of 313 kilometres on a slope ¹⁄₁₂₀₀₀. On this reach is the town of Dongola. The 3rd Cataract has a length of 72 kilometres and a drop of 11 metres with the Hannek and Kaibâr rapids, surveyed and levelled by De Gottberg in 1857. Upstream of the Hannek rapid, on the left bank of the Nile, is the termination of the long depression in the deserts which goes by the name of the Wady-el-Kab and is considered by many as lower than the Nile valley. Between the 3rd and 2nd Cataracts is an ordinary reach of 118 kilometres. West of this part of the Nile are the Selima Wells and according to some travellers an old abandoned course of the Nile slightly above the present high level of the river. This waterless river is said to terminate in the Oasis of Berys which is separated from the Khargeh Oasis by a limestone ridge.

The 2nd Cataract, known as the “Batn-el-Haggar” or “Belly of Stone,” has a length of 200 kilometres and a drop of 66 metres with the rapids of Amâra, Dal, Semna and Abka. At Semna are the rocks where Lepsius discovered the Nile gauges cut by one of the Pharaohs some 4,000 years ago. The Nile flood recorded there is 8 metres higher than any flood of to-day. As the Nile at Semna could be very easily barred by a dam, it struck me when I was there in 1892 that probably King Amenemhat (of Lake Mœris fame) had tried to bar the river with a dam in the hope of creating a reservoir. At Wady Halfa, near the foot of the 2nd Cataract, a masonry gauge divided into metres has been erected and read since 1877. Its accidental zero is R. L. 116.69 and the mean low-water level, or true zero, is R. L. 117.89. Between the 1st and 2nd Cataracts, the Nile has a length of 345 kilometres and a slope of ¹⁄₁₂₅₀₀. The mean width of the river is 500 metres, and the mean depths in flood and summer are 9 and 2 metres. The velocity in summer falls to 50 centimetres per second and rises to 2 metres per second in flood. The river in this reach is generally within sandstone, and the greater part is provided with gigantic spurs on both banks. These spurs perform the double work of collecting soil on the sides in flood and training the river in summer. They were probably put up by the great Rameses 3,000 years ago, as some of the most massive of them have evidently been constructed to turn the river on a curve out of its natural channel on to the opposite side in order to secure deep water in front of Rameses’ temple of Jerf Husain (“Jerf” means steep, scoured bank). The spurs have been constructed with care, and as the courses of roughly-dressed stone can be examined at fairly low water (I have never seen them at absolutely low water) it is evident that there has been no great degradation of the bed during the last 2,000 or 3,000 years. The first, or Assuân Cataract, has a drop of 5 metres on a length of 5 kilometres.

From Khartoum to Assuân, on a total length of 1809 kilometres, there are 565 kilometres of so-called cataracts with a total drop of 192 metres, and 1,244 kilometres of ordinary channel with a total drop of 103 metres.

At the head of the 1st Cataract is the Assuân dam, regulated on for the first time in October 1902. It has 140 openings of 2 metres × 7 metres and 40 openings of 2 metres × 3¹⁄₂ metres.

At the foot of the 1st Cataract, opposite the town of Assuân, on the Island of Elephantine, has stood a Nile gauge from very ancient times. An officer belonging to the Roman garrison in the time of the Emperor Severus marked an extraordinarily high flood on the gauge. The maximum flood-mark at the time of the visit of Napoleon’s French savants was however 2.11 metres higher than the above. As the middle of Severus’ reign was A.D. 200, and the visit of French savants A. D. 1800, they concluded that the bed and banks of the Nile had risen 2.11 metres in 1600 years or 0.132 metres per 100 years. The new gauge divided into cubits and twenty-fourths was erected in 1869 and has been recorded daily since then (a cubit = 54 centimetres). The accidental zero of the gauge is R. L. 84.16. The mean low-water level or true zero is R. L. 85.00.

22. The Nile from Assuân to the Barrage.

—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:—

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.

The following table gives the highest and lowest floods at Cairo in intervals of 25 years from A.D. 639 to A.D. 1904.

The gauges are in pics and kirats and are referred to mean low water or R. L. 12·25 metres above mean sea.