The Nile in 1904

by Sir William Willcocks

24. The Nile in flood.

—We are now in a position to apply our knowledge of the Nile and its tributaries to an examination of the behaviour of the rivers in flood and in time of low supply. Lake Victoria, the Victoria Nile, and Lake Albert may all be considered as the great equatorial regulators of the Nile. The river, as a river, begins at the outlet of Lake Albert, i.e., at the head of the Albert Nile. Generally at its lowest in April, it rises gradually and reaches its maximum in November. The mean minimum of 600 cubic metres per second is gradually increased to its mean maximum of 900 cubic metres. The regulating effect of the lakes is very evident.

Between Lake Albert and Gondokoro the heavier rains begin late in April and with a break in June and July continue to November. The mean minimum discharge of 600 cubic metres per second in April is increased by alternating rises and falls to the mean maximum of 1600 cubic metres per second in September, which has disappeared by the end of November, when the water of Lake Albert alone remains in the river.

The Gazelle river in no way affects the flood or the low supply. Its great function is to maintain the levels of the great swamps between latitudes 7° and 9°, saturate the soil, and prevent the complete disappearance of the waters of the Albert Nile between January and May. The functions this river performs are humble ones, but deprived of its aid, the Nile north of Khartoum would frequently be dry in April and May.

The Albert Nile at its tail just upstream of the mouth of the Sobat is at its lowest in April and May with a mean low discharge of 375 cubic metres per second, when it is joined by the Sobat river with an approximate mean low discharge of 125 cubic metres per second; making a joint discharge for the head of the White Nile of 500 cubic metres per second as a mean minimum. Now begins one of the most interesting operations of any in the whole valley of the Nile, exceeded only in interest by what happens at Khartoum lower down. The Albert Nile and the Sobat river both rise together, the Albert Nile on a very gentle slope freely overflowing its banks in the Sudd region, and the Sobat river confined within its channel during its highest floods. The White Nile has a very gentle slope, little carrying capacity and is quite incapable of taking on both floods. The water rises at the junction and the Sudd region becomes a reservoir flooded to a depth of 3 metres. As the Sobat river increases its discharge gradually from 75 cubic metres per second in April to 1000 cubic metres per second in October and November (for it is confined to its channel), the Albert Nile decreases the actual discharge it sends down the White Nile and increases what it spreads over the Sudd region. The Albert Nile, having increased its quota for the White Nile from 375 in April to 450 cubic metres per second in September, gives less in October and November and gradually passes on its waters in December, January and February when the Sobat has fallen.

The White Nile at its head near Tewfikieh has its mean minimum of 500 cubic metres per second in April, and increases slowly to its mean maximum of 1500 cubic metres per second in December. During this interval its water surface is raised by 3·50 metres. This water travels very slowly on to Khartoum, where the mean minimum is 450 cubic metres per second in May, the slope is very insignificant, and the trough of the river is 1500 metres wide.

At Khartoum the White Nile meets the Blue Nile. No greater contrast exists in the world. If maximum discharges are alone considered, the little finger of the Blue Nile is thicker than the loins of the White Nile.

The Blue Nile is at its lowest on the 1st May with a mean minimum supply of 200 cubic metres per second rising to a mean maximum flood of 10,000 cubic metres per second on the 1st September. The flood has fallen to 2000 cubic metres per second by the middle of November.

Up to the middle of July the Blue and White Niles keep increasing their discharges steadily at Khartoum, but after that date the Blue Nile gauge and discharge rise rapidly together, and the Blue Nile not only feeds the Main Nile, but flows up the White Nile and arrests its discharge, so that at Duem, 200 kilometres above Khartoum, the White Nile discharge decreases in July and August while the Blue Nile is steadily flowing up the White Nile valley and converting it into a reservoir for the Nile in winter. It is only after the 15th September, when the Blue Nile has begun to fall steadily and continuously that the White Nile discharge really commences and reaches its mean maximum of some 2000 cubic metres per second in October.

The mean minimum discharge of the Nile of 650 cubic metres per second at Khartoum is obtained on the 1st May and the mean maximum of 9000 cubic metres per second on the 1st September. Fed by the White Nile reservoir the river falls comparatively slowly. Whether this peculiar relation of the two rivers to each other could not be taken advantage of to increase the supply in December, January and February, and decrease it in October and November by means of a regulating dam built across the White Nile at Khartoum is worthy of study.