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 until 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.[22] At El Dueim, 637 kilometres below the Sobat junction, the summer supply varies between 350 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 is reduced by an amount varying from 30 to 60 per cent., and this holding back continues until the Blue Nile falls 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 are from 300 to 1,500 metres cube per second, and in one of high flood from 400 or 500 to 1,700 metres cube per second.[23] 7. THE BLUE NILE.