The Albert Nile at its tail just upstream of the mouth of the Sobat is at its lowest in April and May with a mean low discharge of 375 cubic metres per second, when it is joined by the Sobat river with an approximate mean low discharge of 125 cubic metres per second; making a joint discharge for the head of the White Nile of 500 cubic metres per second as a mean minimum. Now begins one of the most interesting operations of any in the whole valley of the Nile, exceeded only in interest by what happens at Khartoum lower down. The Albert Nile and the Sobat river both rise together, the Albert Nile on a very gentle slope freely overflowing its banks in the Sudd region, and the Sobat river confined within its channel during its highest floods. The White Nile has a very gentle slope, little carrying capacity and is quite incapable of taking on both floods. The water rises at the junction and the Sudd region becomes a reservoir flooded to a depth of 3 metres. As the Sobat river increases its discharge gradually from 75 cubic metres per second in April to 1000 cubic metres per second in October and November (for it is confined to its channel), the Albert Nile decreases the actual discharge it sends down the White Nile and increases what it spreads over the Sudd region. The Albert Nile, having increased its quota for the White Nile from 375 in April to 450 cubic metres per second in September, gives less in October and November and gradually passes on its waters in December, January and February when the Sobat has fallen. The White Nile at its head near Tewfikieh has its mean minimum of 500 cubic metres per second in April, and increases slowly to its mean maximum of 1500 cubic metres per second in December. During this interval its water surface is raised by 3·50 metres. This water travels very slowly on to Khartoum, where the mean minimum is 450 cubic metres per second in May, the slope is very insignificant, and the trough of the river is 1500 metres wide. At Khartoum the White Nile meets the Blue Nile. No greater contrast exists in the world. If maximum discharges are alone considered, the little finger of the Blue Nile is thicker than the loins of the White Nile. The Blue Nile is at its lowest on the 1st May with a mean minimum supply of 200 cubic metres per second rising to a mean maximum flood of 10,000 cubic metres per second on the 1st September. The flood has fallen to 2000 cubic metres per second by the middle of November.