Green water.An odd phenomenon, the source of which has not been satisfactorily explained, is the “green water,” which makes its appearance at Dueim about the middle of May. The colour is owing to the mass of minute algæ in the water, which subsequently putrify and stink, but the origin of the water has not been definitely traced. It is believed to come from the upper Sobat, or Bahr El Ghazal, and not from the Bahr El Jebel. This green water reaches Cairo towards the end of June.
Junction of the Niles at Khartoum.At Omdurman, half Nile usually occurs about the middle of July, high Nile at end of August or beginning of September, lasting about a month; half Nile end of November, lowest Nile end of April. The Blue Nile is at its lowest in May, and highest about the end of August. The result of careful measurements during the last 3 years has been to prove that when the Blue Nile is in flood, and generally when its discharge exceeds 5,000 metres cube per second, its waters hold back those of the White Nile and, owing to the increased depth in this river, due to the rise in the water levels, the volume coming from the south floods the Sudd marshes right and left of the channel and thus reduces the discharge of the White Nile by about 50 per cent.
The average difference between low and high Nile here is 22 feet (17½ to 26 feet).
Below Khartoum.The river below Khartoum flows steadily along till it reaches the 6th or Shabluka cataract (see [p. 47]). Below this it is not disturbed, except for the rapids of the so-called 5th or El Homar and Bagara cataracts, till it passes Abu Hamed and enters the 4th or Belal cataract some way below this point. This cataract, forming the most complicated and dangerous rapids on the Nile, is for ordinary purposes unnavigable (see [p. 36]). Below it there is an open stretch past Dongola to the 3rd or Hannek cataract, shortly followed by the Kajbar rapids (see [p. 26]). A rocky channel full of small islands and small rapids, increasing in size and importance as the river proceeds, is entered some 80 miles below Kajbar, and through the Batn El Hagar the river rumbles along till, after passing the 2nd or Amka cataract, it emerges past Halfa in a broad and steady stream which lasts till it arrives at the dam and 1st cataract (Shellal) of Aswan. From here onwards there are no further obstacles (except sand-banks and canals) till, viâ the Delta of Egypt, the Mediterranean Sea is gained.
(iii.)—RIVER DISCHARGES.
The following, taken from Sir W. Garstin’s Report above mentioned, gives a summary of results respecting discharges in the basin of the Upper Nile:—
1. The Victoria Nile.
The discharge at the Ripon Falls varies between 500 and 650 metres cube per second with a range of 1·1 metres. Downstream of the Murchison Falls the range is probably 1 metre, and the maximum and minimum discharges 1,000 and 400 metres cube per second respectively. The increase in flood is due to the rainfall throughout the catchment area of the river between these two points, while the decrease during the low season is due to the Choga Lake, which undoubtedly has a regulating effect upon the supply issuing from Lake Victoria. Lastly, the volume which enters Lake Albert by this river is generally, in flood, greater than that which leaves it by the Bahr El Jebel.
2. The Bahr El Jebel.
At Wadelai, the first discharge site, the range of the river is about 1·11 metres while the discharge varies from 550 to 950 metres cube per second. The increase brought in by the streams which feed this river between Lake Albert and Wadelai is compensated by the loss of water due to a portion of the discharge of the Victoria Nile passing south up the lake during the flood season.