18 White Nile : River Disclmrges.

At Bor, 5r)9 kilometres, the loss of water in flood is soma 50 p^r cent, of the amount passing Lado, and the discharge here can rarely, if ever, exceed 1,0(X) metres cube per second. This loss is due to the filling up of the entire river valley, which thus forms an immense basin or reservoir, and reduces the discharge passing to the north. This re8er\'oir extends from Lado to the head of the Bahr El Zcraf, a distance of some .'578 kilometres. As the river falls the water of this basin, with the exception of the large amount lost by evaporation, slowly filters back through the marshes into the river during the winter montlis, and thus maintains the constancy of supply.

Throughout the " Sudd " region the loss of water in the Bahr El Jebel, both in summer and in flood, is very considerable. By the time that Lake No (1,156 kilometres from Lake Albert, and 74'J kilometres from Lado) is reached, 85 per cent, of the discharge at Lado has been lost in a high flood, and 70 per cent, in a low one. During the summer months the loss at this point varies between 50 and 60 per cent. Lastly, the discharge which enters the ^\'lute Nile from the Bahr El Jebel is nearly constant at all seasons of the year, and never even in the highest flood exceeds 300 or 320 metres cube per second. The regulating effect of the great marshes is thus very apparent.

3. Thk Bahr El Ghazal.

The discharge of this river, as a feeder of the White Nile, may be neglected entirely. Its summer volume entering Lake No varies from 20 to 30 metres cube per second, while its flood discharge is even less, equalling from 12 to 20 metres cube per second. None of this water enters the White Nile, merely increasing the flooded area of Lake No. It, however, helps to augment the reservoir area of the main stream.

4. The Bahr El Zeraf, This branch of the Bahr EI Jebel adds to the vohmie of the White Nile by an amount varying from 30 to 60 metres cube per second in summer, and from 80 to 160 metres cube per second when in flood. If, however, the Bahr El Jebel is closed by "Sudd" then the discharge of the Bahr El Zaraf increases, possibly to from 30O to 400 metres cube per second during the flood season.

5. The Sobat.

This river is the main supply of the White Nile during the period of flood. The first effects of its waters are felt in May and June, while, as it does not reach its maximum unfil October and November, the volume of the Blue Nile having then been largely reduced, it maintains the discharge passing Khartoum to a very considerable figure. In years of good flood the discharge of the Sobat varies from 900 to 1,000 metres cube per second. In the early months of the year its discharge shrinks to very low limits, its waters being held back by those of the White Nile.

When in flood the reverse is the case. The volume of the Sobat being at that time more than double that of the White Nile causes a rise in the levels of the latter upstream of the junction and holds back its water as far as Lake No.

6. The White Nile.

The discharge of this river below the Bahr El Zeraf junction varies from 300 to 500 metres cube per second according to the season of the year and the nature of the flood. It is probable that the last figure is a maximum and is never surpassed.* At El Dueim, 637 kilometres below the Sobat junction, the summer supply varies between S.^) and 500 metres cube per second. The minimum levels are generally attained in the month of April and the first half of May. The discharge, owing to the Sobat water, gradually increases until the Blue Nile flood exceeds the volume of 5,000 metres cube per second at Khartoum. As soon as this figure is passed the discharge of the White Nile ifl reduced by an amount varying from 30 to 60 per cent., and this holding back continues until the Blue Nile falb again below the figure above given. This reduction of the White Nile discharge takes place in the months of August and September. As soon as the Blue Nile discharge has fallen below 5,000 metres cube per second that of the White Nile rises very rapidly, attaining its maximum in the months of November and December, when as much as from 1,500 to 1,700 metres cube per second have been recorded. This increased discharge is, of course, partly due to the Sobat, but also to the draining off of the water which has been ponded up for so long a period. It seems safe to assume that the White Nile discharge at Khartoum never under any circumstances exceeds 1,800 cubic metres per second.

To sum up : — The White Nile is at its lowest from March to May. It rises in June, is checked again in August and September, and attains its maximum during the months of November and December. Its limits in a low year * The dates of its maximum levels upstream of tlie Sobat coincide with tliose of the Bahr EI Jebel.