—In the introduction to his brilliant essay on the Hydrology of the Nile[1], an essay, which, though written in 1865, foreshadowed much of what we know to day, Lombardini remarked, with much truth that, no river in the world lends itself to hydrological studies on so majestic a scale as the Nile. The most interesting river of the ancient world, it is still the most interesting river of our time; and, in spite of all that ancient and modern discoveries have unfolded, its discharges are to-day more difficult to unravel and weave together than those of any other stream in either hemisphere. These discharges are still a mystery, and it will need years and years of patient observation and study, at the hands of the Sudan Irrigation Department, to enable us to state with exactitude why its floods rise and fall with such regular and stately precision, why they are never sudden and abrupt, and why its summer supplies can never be completely cut off even in their traverse of over 3000 kilometres through the burning and parched Sahara. Though the mystery of the Nile is far from being solved to-day, still an enormous step in advance has been made by the publication of Sir William Garstin’s Report on the Basin of the Upper Nile[2]. This Report not only contains the results of three years’ observations of the Egyptian Survey Department in the Sudan, of Sir William Garstin’s own observations and studies, but also a mass of information of the Nile and its tributaries collected by Capt. H. G. Lyons. R. E., through four years of uninterrupted study. Those who know the intelligence and method with which Capt. Lyons works, will rate this information at its proper value.

Lombardini gathered together all the information available at the time that Sir Samuel Baker announced the existence of the Albert Nyanza shortly after Speke and Grant had proclaimed to the world that the Victoria Nyanza was the true source of the Nile. From the information then available he deduced the laws and operations of the great river. About twenty years later, just before the rebellion in the Sudan closed the Nile to the civilized world, a German savant, Joseph Chavanne[3], in his book on the rivers of Africa, collected and tabulated on clear and methodical lines much of the information available in 1883. Though many of his facts are erroneous, his method is clear and his ideas just. Sir William Garstin, in his Report, has developed the information at his disposal on such practical lines as are needed to study the question of insuring an abundant supply of water to the Nile in Egypt during the times of low supply.

Having myself studied the Nile for fifteen years in order to solve the problems of water storage and flood control on the Nile, and having devoted the whole of my life to this very science of Hydraulics, I have been encouraged to attempt the continuation of Lombardini’s work; and, to the utmost of my ability, to bring it to the level of the knowledge of our day.

2. Nomenclature.

—The nomenclature of the tributaries of the Nile is difficult to follow. In this book I shall call the river the Victoria Nile from Lake Victoria to Lake Albert; the Albert Nile from Lake Albert to the Sobat mouth (this reach is known generally as the Bahr el Gebel); the White Nile from the Sobat mouth to Khartoum; and the Nile from Khartoum to the sea. The Blue Nile stretches from Lake Tsana in Abyssinia to Khartoum.

3. Description of the course of the Nile.

—Chapters II and III contain detailed descriptions of the Nile and its main tributaries, and this paragraph is a short epitome of what is written there about the course of the Nile. The Nile drains nearly the whole of north-eastern Africa, an area comprising 3 million square kilometres. Its main tributary, the White Nile, has its furthest sources in south latitude 4°, near Lake Tanganyika. Known as the Kagera, it is one of the feeders of Lake Victoria, and has a course of 600 kilometres before it reaches the lake. Lake Victoria, covering 60,000 square kilometres, is the first reservoir of the Nile. The Victoria Nile leaves Lake Victoria by the Ripon Falls and after a course of 400 kilometres enters Lake Albert at its northern corner. At its southern end Lake Albert is fed by the Semliki river which has its sources in Lake Edward. Its own area is 4,500 square kilometres. The Albert Nile leaves Lake Albert at its northern end and has a course of 1280 kilometres to the mouth of the Sobat river. Of this length, the first 200 kilometres up to Dufile have scarcely any slope, the next 150 kilometres are down a series of severe cataracts. From the foot of these cataracts to its tail the Albert Nile has a gentle slope and traverses the Sudd region where the bed of the stream is often barred by blocks of living vegetation. In this latter region the stream divides into two, of which the right hand one is known as the Bahr Zeraf. After a course of 270 kilometres the Bahr Zeraf joins the Albert Nile again. In the interval the Albert Nile receives as a left-hand feeder the Bahr Gazelle. The Sobat river has its sources in Gallaland and joins the Albert Nile at the termination of the Sudd region. From the junction of the Albert Nile and the Sobat, the river is known as the White Nile, which, after a course of 840 kilometres, with an exceedingly gentle slope, joins the Blue Nile at Khartoum.

The Blue Nile is the true parent of the land of Egypt. The deposits of its muddy waters have made Egypt. The Atbara has added its quota, but the Blue Nile is incomparably the chief contributor; fed by the timely and plentiful rains of southern and south-eastern Abyssinia, it contributes 65 per cent of the waters which pass AssuÂn. The furthest sources are those of the Abai, which, after a course of 110 kilometres falls into Lake Tsana. This lake has an area of 3,000 square kilometres and lies about 1,760 metres above sea level. The Blue Nile leaves it at its south-eastern corner and hurries down to the Sudan, fed by numerous Abyssinian rivers. At Rosaires, after a course of 750 kilometres, it has fallen 1,260 metres; and below the Rosaires cataract enters the plain country south of Khartoum. For its remaining 615 kilometres on to Khartoum, where it meets the White Nile, it is navigable for the greater part of the year. North of Sennaar it is fed by the Dinder and Rahad rivers.

Between Khartoum and El Damer, on a length of 320 kilometres, the Nile has its even passage broken by the 6th cataract at Shabluka. At El Damer the Nile receives the Atbara as a right hand tributary.

The Atbara is a very muddy torrent fed by the rains of north-eastern Abyssinia. It runs for 4 months per annum and is dry for 8 months. Rising within a few kilometres of Lake Tsana, it falls 1500 metres in its first 300 kilometres, and is then joined by the Salaama, and, 100 kilometres lower down, by the Settit river. After the junction with the Settit, the Atbara flows for 480 kilometres and joins the Nile at El Damer, contributing a fair quantity of water and a very considerable quantity of Nile mud to the river.

From the Atbara junction to the sea, the Nile has a course of 2,700 kilometres. In its first length of 1480 kilometres to AssuÂn it traverses the 5th and 4th cataracts between Berber and Dongola, the 3rd and 2nd cataracts between Dongola and Wady Halfa, and the 1st cataract at AssuÂn. All these cataracts are navigable in flood, but not so in summer. From AssuÂn to the Barrage at the head of the Delta north of Cairo, the Nile has a length of 970 kilometres and traverses Egypt without a cataract or interruption of any kind. At the Barrage, the Nile divides into the Rosetta and Damietta branches, and after a further course of about 240 kilometres in either branch, flows into the Mediterranean sea. Its greatest length from the sources of the Kagera river to the sea is 6350 kilometres, constituting it one of the longest rivers in the world.

4. The Slopes and velocities of the Nile in its different reaches.

—Table 2 of Appendix B and Plate II comprise all the information available under this head which I have been able to collect. For the slopes I have adopted the following data:

From Khartoum to Wady Halfa I have adopted the generally accepted levels of the original Soudan railway survey. From Wady Halfa to the sea I have levelled myself. Upstream and downstream from the adopted levels I have carried the levels by the aid of slopes calculated from velocity and hydraulic mean depth data. It seems to me absurd to adopt a level for Lake Choga 50 metres above that for Fowera, and then to add, that in the 140 kilometres between the two places the Victoria Nile has a gentle slope, wide bed and gentle velocity. By a strange fatality, this very error has crept into the figures under Lake Choga on Plate II. The error is noted in the corrigenda attached to the Plate. The Section is drawn correctly but these wrong figures have been interpolated by an oversight.

The Victoria Nile Falls 450 metres in 400 kilometres, but has four reaches; the first ¹/₁₂₀₀, the second ¹/₂₀₀₀₀, the third ¹/₁₈₀ past the Murchison Falls, and the fourth ¹/₁₀₀₀₀.

The Albert Nile falls 277 metres in 1290 kilometres. The first reach past Wadelai has a slope of ¹/₂₅₀₀₀, the second over the Fola and following cataracts has a slope of ¹/₇₀₀, the third ¹/₁₂₀₀₀, the fourth ¹/₂₀₀₀₀, the fifth ¹/₂₅₀₀₀, and the last below lake No of ¹/₇₅₀₀₀ in flood.

The White Nile falls 14 metres in 840 kilometres and has two slopes in flood; ¹/₅₀₀₀₀ in its upper reach, and then ¹/₁₀₀₀₀₀.

The Blue Nile falls 1370 metres in 1370 kilometres, which may roughly be divided into three reaches. The first from Lake Tsana to Rosaires on a length of 750 kilometres ¹/₆₀₀, the second Rosaires to Sennaar ¹/₄₅₀₀, and the third ¹/₇₀₀₀. These are very approximate indeed.

The Atbara falls 1640 metres in 880 kilometres. In the first 300 kilometres the slope is ¹/₂₀₀; in the next 300 kilometres the slope is ¹/₂₅₀₀, and in the last reach of 280 kilometres it is ¹/₆₀₀₀. These are approximate.

The Main Nile from Khartoum to Assuan falls 295 metres in 1810 kilometres; the so-called six cataracts occupy 565 kilometres with a slope of ¹/₃₀₀₀; and the ordinary channel occupies 1245 kilometres and has a slope of ¹/₁₂₀₀₀. From AssuÂn to the Barrage, on a length of 970 kilometres, the Nile falls 76 metres with a mean slope of ¹/₁₃₀₀₀. The Rosetta and Damietta branches are each about 240 kilometres long and have a slope in flood of ¹/₁₃₀₀₀, and of ¹/₁₂₅₀₀ in extraordinarily high floods.

From the sources of the Kagera river to the sea, on a length of 6350 kilometres, the Nile falls 2000 metres, or has a slope of ¹/₃₂₀₀. From Lake Victoria to the sea the length is 5535 kilometres and the fall 1129 metres, or the slope is ¹/₅₀₀₀.

Table III of Appendix C. gives the velocities of the river in flood and low supply, in metres per second and kilometres per day, and also the time occupied in traversing the different reaches. There are two breaks. The first is at Lake Choga and the second is at Lake Albert. As the Victoria Nile traverses the eastern arm of the many-armed and peculiar Lake Choga with a perceptible current, and as, moreover, the lake is very shallow, we may give some figure to the velocity and make it half that of the Bahr Gazelle which is ·20 metres per second and is considered perceptible. With a velocity of ·10 metres per second or 8 kilometres per day, the 80 kilometres of the lake would be traversed in 10 days. The time of traverse from Lake Victoria to Lake Albert would be 15 days. With Lake Albert it is very different. A reference to Plate V will show that it takes the Victoria Nile 5 months to fill up Lake Albert before the Albert Nile can carry off the waters of the Victoria Nile, gauge for gauge. Under these conditions it will be wise to stop at Lake Albert and begin a new calculation from this lake.

The Albert Nile takes 22 days in flood and 25 days in low supply to traverse the distance from Lake Albert to the Sobat mouth. The White Nile takes 21 days in flood and 28 days in low supply to reach Khartoum. Consequently from Lake Albert to Khartoum we have 43 days in flood and 53 days in low supply.

From Khartoum to AssuÂn the Nile takes 11 days in flood and 22 days in low supply, and consequently from Lake Albert to AssuÂn we have 54 days in flood and 75 days in low supply.

From AssuÂn to Cairo we have 6 days in flood and 12 days in low supply.

Table III is very interesting and well worth study. Through the Sudd region we have a velocity of ·6 metres per second, but only of ·35 metres per second in the White Nile. In Egypt the Nile in flood has a velocity of 1·75 metres per second, and in low supply of ·85 metres per second.

5. Catchment basins of the Nile and its tributaries.

—Table I, of Appendix A gives the areas of the catchment basins of the Nile. The total area according to the table is 3,007,000 square kilometres. The limits of the basin are depicted on Plate I, and, with rare exceptions, they are now fairly well known everywhere. North of the 20th parallel of latitude the watershed on the west of the Nile is not far removed from the edge of the plateau skirting the Nile valley. The plateau falls away to the west, and occasional ravines find their way to the Nile down the reverse slope. On the east of the Nile the crest of the hills skirting the Red sea is the watershed. South of the 20th parallel of latitude the eastern watershed follows the crest of the hills on the west of the Red sea as far as Suakin. South of Suakin the watershed leaves the Red sea, to allow the Khor Barraka to flow into this sea. From the south east of Kassala, round by Addis Ababa, the watershed follows the crest of the high hills forming the eastern backbone of Abyssinia, and dividing the waters of the Nile from those flowing into the Indian Ocean. South west of Abyssinia the watershed travels in a south-westerly direction to the east of Gondokoro, and divides the Sobat from the rivers draining into Lake Rudolf. The watershed then moves due south to the western escarpment east of Lake Victoria. Mounts Kenia and Kilmanjaro are not within the basin of the Nile. Sweeping in a rough curve round Lake Victoria and nearly touching Lake Tangangyika in 4° south latitude, the watershed keeps close to the western shores of Lakes Edward and Albert to nearly opposite Wadelai. All the slopes of the Ruenzori mountains drain into the Nile.

From near Wadelai the watershed moves in a north-westerly direction along the hills dividing the waters of the tributaries of the Gazelle river from the Welle. Due west of the Sudd region the watershed has reached its most westerly position and from there turns northwards along the Marrah hills in Darfur, dividing the scanty waters of the Bahr-el-Arab and its tributaries from the rivers draining into Lake Chad. From the Marrah hills the watershed travels in a north-easterly direction to a point close to the Nile on the 20th parallel of latitude near Hannek.

Of the lands enclosed within this watershed, all that are drained directly into the Main Nile are desert. There are occasional showers, and some of the valleys and ravines carry water for a few hours every year, others every second, third or fourth year, but they contribute practically nothing to the volume of the Nile. The rains generally come in the winter when the Nile is falling every day, and the steady fall of the Nile is never arrested by the waters of any or all of these watercourses. The country west of the White Nile past Kordofan and Darfur to the Marrah hills is steppe land producing scanty grasses and forests of low accacias in the south, and rising to a general height of about 600 metres at the Marrah hills. The lands drained by the Gazelle river and the Albert Nile north of Gondokoro are flat plains or swamps in the north and east, and wooded and broken ground in the west and south-west, where the tributaries of the Gazelle river rise in the Blue mountains at a general height of 1500 metres. The upper waters of the Sobat and its tributaries drain the well wooded and cultivated mountain masses of Gallaland and then traverse the marshes and flat lands which lie east of the Sudd region. The Blue Nile and its upper tributaries drain the choicest portions of the high Abyssinian mountain plateau lying over 2000 metres above sea level, and rising in places to 2500 metres and upwards. The lower courses of the Blue Nile, the Rahad and the Dinder are through the black cotton soil plains of the eastern Sudan, which are either wooded or covered with dense grass in the south. The Atbara and its tributaries in their upper courses drain the northern slopes of the Abyssinian plateau, and traverse the level plains of the eastern Sudan in a direction parallel to the Blue Nile.

The Albert Nile and its tributaries between Gondokoro and Lake Albert traverse the broken and hilly country which is cut through by the Albert Nile at the Fola and succeeding rapids. The catchment basins of Lakes Victoria and Albert are the undulating hills, the flat marshy valleys, the great lakes and, in parts, high hills which constitute the highlands of Central Africa. The general level of the area may be taken as 1400 metres above sea level.

The area draining into Lake Victoria is 240,000 square kilometres. At the outlet of Lake Albert this has increased to 380,000, and at Gondokoro to 470,000. The Gazelle river drains 470,000 square kilometres, and the Sobat 160,000. The White Nile drains altogether 1,690,000 square kilometres, or more than half the total area of the catchment basin of the Nile. The Blue Nile drains 300,000 square kilometres and the Atbara 240,000. The Nile below the Atbara junction is draining 2,290,000 square kilometres. Between the Atbara mouth and the sea, the Nile drains whatever falls on a desert area of 720,000 square kilometres.

If we take 3,000 cubic metres per second as the average annual flow past AssuÂn we may say that the White Nile supplies 24% off more than half the area of the whole basin, the Blue Nile 65% off ¹/₁₀ the area, and the Atbara 11 % off ¹/₁₂ the area. The Gazelle river drains about ¹/₆ the total area and adds practically nothing to the discharge. Table 24 should be very carefully studied by any man who wants to understand the Nile. It does not pretend to exactitude, but embodies the best information I have been able to obtain.

6. The climate of the Nile valley.

—This paragraph would have been much more complete if Capt. Lyons’ monograph on the Meteorology of the Nile valley had been published. In considering the climate I shall follow the subdivisions of the catchment basin of the Nile contained in Table I.

In the catchment basins of Lakes Victoria and Albert, the mean annual rainfall may be taken as 1.25 metres, with great fluctuations between good and bad years. Neglecting here and through this paragraph, the light occasional falls of rain which are trying to travellers but which have no effect on the rivers, it may be said that in these basins there are two rainy seasons, the greater in March, April and May, and the lesser in October, November and December. The former are followed by dry southern winds, while north winds blow in the winter.

Along the whole of the Albert Nile, the mean annual rainfall may be taken as 1 metre, with severe famines in occasional years and heavy rainfall in others. The principal rains are between May and November, with the maximum between August 15 and September 15. In years of deficient rainfall, the June, July and August rains seem to fail. The catchment basin of the Gazelle river may be credited with a mean annual rainfall of 75 centimetres between May and October, while the mean annual rainfall on the Arab river cannot be more than 30 centimetres between June and September. The Sobat river in its upper reaches enjoys an annual rainfall of about 1.25 metres and of ·75 metres in its lower reaches. The time of rain is between March and September. The lands draining into the White Nile north of Tewfikieh have an annual rainfall of about 20 centimetres between June and September.

The Abyssinian part of the catchment basin of the Blue Nile enjoys a good rainfall throughout nine months of the year from February to October, with generally heavy rain between May and September, and very occasionally in October. The rainfall here may be taken as 1.25 metres per annum. In the plains of the eastern Sudan traversed by the lower reaches of the Blue Nile and the Atbara the rainfall is very much lighter and may be considered as 30 centimetres between July and September; fairly constant and heavier in the south, and very inconstant and lighter in the north. The Atbara and its tributaries in their upper reaches on the northern slopes of Abyssinia, have rain from May to the end of August and occasionally into September. There are great fluctuations in the rainfall. The mean annual rainfall may be taken as 75 centimetres.

The desert area between Khartoum and Cairo has occasional winter rains especially in the parts near the Red sea, but as these rains are nearly all soaked up by the desert, and very little, here and there, reaches the Nile, we may neglect them altogether. Railways have to be provided with culverts and bridges where they cross the terminal reaches of the khors and wadis which run considerable bodies of water for a few hours after rain; but the effect on the Nile is practically nothing. Along the sea-board of the Mediterranean there are a few inches of rain every winter, sufficient as a rule to raise poor crops of barley.

In the catchment basins of Lakes Victoria and Albert the direction of the winds may be taken as north-east in winter and south-east in summer. The maximum monthly temperature may be taken as 35° and the minimum as 12°, with a mean for the year of 21°.

Along the Albert Nile the north wind blows through the winter, and southerly winds prevail from about the 15th of April to October. The temperature may be taken as ranging from a monthly maximum of 38° to a monthly minimum of 16°, with a mean of 27°.

Tables 75 to 81 give the principal meteorological data for many places in the Nile valley and compare the Bombay rainfall with the AssuÂn gauges. The latter show how closely the AssuÂn gauge in flood follows the rainfall at Bombay.

At Addis Ababa in the highlands of Abyssinia, the mean monthly temperature ranges between 19° and 15°, with a mean of 17°. The winds are south-east and east through the year. In 1902 the rainfall was 980 millimetres, and in 1903 it was 1340 millimetres. (Table 75).

At Wad Medani in the Grezireh south of Khartoum, the mean monthly temperature ranges between 35° and 24°, with a mean of 30°. The winds from October to April are from the north and from May to September from the south. In 1902 there were 350 millimetres of rain and in 1903 there were 310. (Table 76).

At Khartoum the mean monthly temperature ranges between 34° and 19°, with a mean of 28°. The winds from October to April are from the north and from May to September from the south. In 1902 there were 120 millimetres of rain and in 1903 there were 70. (Table 77).

For Alexandria, Cairo and AssuÂn, representing the Nile valley in Egypt, I have prepared the following table:—

From the above table we may conclude that at Alexandria, Cairo and AssuÂn the absolute maximum thermometers may be taken as 40°, 45° and 47° Centigrade; or 104°, 113°, 116°, Farenheit. The minimum thermometers as 5·5°, -0·7°, 4·0° Centigrade; or 42°, 31°, 39° Farenheit. The rainfall at Alexandria, Cairo and AssuÂn respectively may be taken as 235, 31, and 0 millimetres; or 9, 1¹/₄, and 0 inches. The heaviest rainfall in any individual year at Alexandria and Cairo respectively has been 308 and 55 millimetres; or 12 and 2 inches. The lightest rainfall at Alexandria and Cairo respectively in any individual year has been 108 and 7 millimetres; or 4¹/₄ and ¹/₄ inches. AssuÂn is practically rainless. It does rain sometimes at AssuÂn, but there has been no rain during the last three years while meteorological observations have been taken.

7. The Geology of the Nile Valley.

—South of Gondokoro along the Victoria and Albert Niles, and at the lakes, the rocks are generally granites, crystalline schists and quartzites. The hills of Uganda are covered with red clay and marl on the higher lands, while the valleys consist of a rich black loam. All the cataracts are granites and granitic rocks or diorites. The Ruenzori range consists of lofty volcanoes. The surface of the ground is covered with a fine Kankar (nodulated limestone) in many places. North of Gondokoro the plains are formed of sandy deposits mixed with coarse peat in places. The hills of the Bahr-el-Gazelle and Arab river are all crystalline. Abyssinia is a volcanic plateau. It is the detritus of this rich volcanic soil swept down by the Blue Nile and Atbara which constitutes the richness of the soil of Egypt and of the water of the Nile. Those parts of the eastern Sudan south of Khartoum and El-Damer and at Kassala, which are the deltas of the Blue Nile, the Dinder, the Rahad, the Atbara, and the GaÄsh, are possessed of a soil in every sense similar to that of Egypt itself. At Khartoum and in the bed of the Blue Nile at Kamlin are extensive deposits of nodular limestone corresponding to the Kankars of India.

The main Nile from Khartoum to AssuÂn flows between low hills and tables of Nubian sandstone overlying crystalline rocks of gneiss, mica schists, hornblendic granite and red granite. Where the crystalline rocks come to the surface we have cataracts; where the Nubian sandstone is at the surface we have reaches of unbroken water.

From AssuÂn[4] to near Edfu the Nile flows between hills of Nubian sandstone, the best known of which is Gebel Silsila. From Edfu to near Luxor, the Nubian sandstone which overlies the crystalline rocks dips under the Nile and its place along the Nile Valley is taken by green and grey clays containing nitrate and phosphate deposits. The former are inexhaustible and have constituted the manure of this part of the valley for thousands of years. With these deposits are thick banks of soft white limestone.

[4] Condensed from a description of the geology of the Nile Valley in Egypt written by Capt. Lyons for the second edition of “Egyptian Irrigation”.