— 44 — 
shallow. There are many islands and some of them considerable ones. 
The rivers feeding the lake are the following — the Abai discharging 
9 cubic metres per second in January 1903, the Reb 2 cubic me- 
tres, the Gumara 2, the Magetch 0*3, the Arno Garno 0*3, the Gelda 
0*5, the Unfraz 1*2, and many smaller streams say 1*7, or 17 cubic 
metres per second in all. This of course was in the dry season. Mr. 
Dupuis considers the evaporation as 4 millimetres per day. 
Between Lake Tsana and Rosaires, on a length of about 750 kilo- 
metres the Blue Nile falls some 1260 metres ; between Rosaires and 
Sennar, on a length of 270 kilometres, it falls about 60 metres, and 
between Sennar and Khartoum on a length of 345 kilometres it falls 
about 50 metres. The cross sections of the river at Wad Medani, 
200 kilometres above Khartoum, and at Khartoum are given on 
Plate IX. 
In flood and early winter the river is navigable up to the Rosaires 
cataract. The width of channel may be considered as varying between 
350 and 700 metres with an average width of 500 metres. The river 
rises from 9 to 12 metres in flood and has a velocity in high floods of 
3 metres per second. Rivers with such velocities scour out their beds 
very severely in high floods and deposit silt in low floods, and for the 
discharges below 5 metres, cross sections should be annually established 
and discharge tables made depending on the sections. The summer 
discharge varies from 100 to 300 cubic metres per second and the 
flood from 7,500 to 12,500 cubic metres per second. The behaviour 
of the river is shown in Tables 24 and 25. The beginning of May is 
generally low water and the beginning of September high water. The 
winter discharge may be taken as 500 cubic metres per second. 
The tributaries south of Rosaires are the following : — on the right 
bank, the Folassa, the Durra, the Fatsam, the Bir and the Temsha, 
veritable torrents; and on the left bank, numerous streams from north 
and north-west of Addis Ababa, the Anjur, and the Didessa, the latter 
from 100 to 150 metres wide. North of the last is the Tumat. The 
Didessa is about 350 kilometres long and the Tumat 200. It is not at 
all improbable that in the valley of the Didessa far better reservoir 
sites could be found than at Lake Tsana. Capt. Lyons tells me that 
there are important reaches here with very little slope. North of 
Rosaires there are two important tributaries on the right bank, the 
Dinder, north of Sennar, and the Rahad, just north of Wad Medani. 
Both these streams run only in flqpd and are dry in winter and sum- 
