Banks.The average height of the banks over low-water level is from 26 to 30 feet for the first 150 miles up-stream from Khartoum. Further south they are higher, and average over 33 to 39 feet above low-water level. The difference in level between flood and low-water is 20 to 23 feet. In the first quarter of the year, the river is reduced to a succession of deep pools, connected by very shallow reaches. Even native boats can with difficulty navigate the distance between Sennar and Khartoum during this season. The Blue Nile is at its lowest in April, but during the latter half of May the first or false rise begins. The real rise begins in June, and the maximum height is attained in August. In the latter half of September it begins to fall rapidly.
Navigability. Cataract.Navigation is simple enough at high Nile. As far up-stream as Roseires, 405 miles above Khartoum, the river is navigable by the ordinary Nile steamers from the middle of June till the end of November. Just above Roseires, however, there is a cataract about 6 miles long. This cataract has never been navigated by steamer, but it is said that previous to 1881, sailing boats passed regularly up and down it. Rafts occasionally navigate it successfully on their way down stream. During the last two years a small launch and a few sailing boats have been passed up and down, but there is a dangerous reach for sailing boats above the cataract.
During November and December the water falls rapidly, and sandbanks appear in quantities, the rush of water through the narrow channels being very great. The worst part of the river is near Abdin and Sennar, but there is little rock anywhere. The water for 5 miles below Roseires is bad, and in places dangerous from rock. Steamers with barges lashed alongside, at the end of December in most years, can get through, except at one point some 20 miles south of Sennar, near Abdin, where a reef of rocks extends almost entirely across the river. Steamers have to be steadied over this place by ropes in December, and the barges passed up and down by ropes.
Discharges.[71]Sir W. Garstin calculates the average discharge of the river at Khartoum to be:—
| At low-water (May) | 200 | metres cube per second. |
| In flood (August) | 11,000 | „ „ „ |
Velocity.The velocity of the stream is very great: even in February it is not less than 3 miles an hour, while in full flood it must be considerably over 6 miles an hour. In winter the water is very clear, and of a beautiful limpid blue. In flood, being charged with the scourings of the Abyssinian mountains and forests, it is heavily charged with deposit, and the water is of a deep chocolate colour. The Blue Nile is considered the chief fertilizing agent of Egypt.
ON THE BLUE NILE.
FOREST SCENERY, WEST BANK, UPPER BLUE NILE.