122 (No. 98 ECONOMIC CONDITIONS The design of the White Nile dam is described as being comparatively simple. The maximum head of water required is 11 yds. , and no complications are anticipated regarding waterways or sluices. The cost has been estimated at about £Ě . 1,500,000, though, in view of the recent great increase in the price of labour, thiscannot be regarded as final.

The spill-water channel would run through the central portionsof the Gezira, from the Blue Nile above the Makwa (Sennar) dam , in order to conduct excessive flood water to the White Nilevalley reservoir, whichit would enter some way above the Jebel el-Auli dam . Its construction might be postponed until the completion of the Makwa dam makes it possible to excavate by dredging The Bor- Sobat canal project. The volume of water in the White Nile , at least above the confluence of the Sobat, remains fairly constant at all seasons and under allconditions, no matterwhattherainfall in the Upper Nile region and along the Nile-Congo watershed may be. It would be much larger if the supply from the Bahr el-Ghazal, with its many largetributaries, and from the Upper Nile were not wasted by evaporation and absorption in the sudd region ( see above, p . 75 ) , which acts as a natural reservoir. It has been com puted that , at the point at which the river leaves this region, the volume of water is 50 to 80 per cent. less than that at the point of entrance. The discharge at Mongalla is estimated at from 20,000 to 70,000 cubic ft. per second ,whereas, at the point at which the river enters Lake No , it is said to be only about 9,000 to 10,000 cubic ft . per second. A possible remedy for this would be the cutting of a channel due north from Bor to near the mouth of the Sobat large enough to carry the entire summer dis charge of the Upper Nile. Such a channel would practically be a straight canal, about 210 miles in length , and would reduce the distance for steamer transport between its two extreme points by more than one -half. This great work has been for some time past