The ‘ Sadd * of the Upper Nile: its Botany 
compared with that of similar Obstruc- 
tions in Bengal and American waters. 
BY 
C. W. HOPE. 
Introductory. 
T HE Cataracts on the Nile are well-known obstacles to 
navigation between its mouth and the Soudan, but they 
are beginning to yield to the attacks of modern engineers, 
who are throwing dams across the river, and providing side 
channels through which navigation will be carried on by 
means of locks. These cataracts are caused by barriers of 
granite rock which cross the bed of the river. But it is not 
so well known that an almost more serious obstruction to 
navigation is caused by the accumulation of a few species 
of plants floating in the Bahr-al-Jebel, or Mountain Nile, 
beginning about 435 miles south of Khartum, and extending 
thence southwards for about 250 miles ; and that this 
accumulation also seriously reduces the flow of water north- 
wards to the Lower Soudan and Egypt. The great Equatorial 
Lakes store the rainfall of vast catchment basins, and so 
regulate its off-flow northwards by means of the Mountain 
Nile ; but this function is greatly neutralized by the vegetable 
accumulation which begins 71 5 miles northward, in a com- 
paratively flat country, and which reduces the velocity of the 
current, and also causes the water of the river to spill right 
and left over the country and go to waste in shallow lakes 
and lagoons, where it is subject to evaporation to a serious 
extent. 
L Annals of Botany, Vol. XVI. No. LXIII. September, 1902.] 
