As far as Shambe the banks of the river are fairly well defined, but there are numbers of breaches in them, through which the water spills into the marshes. In a distance of seventy-five miles Sir W. Garstin counted 129 of these breaches, usually about 4 yards wide; nearly all of these were on the eastern side. These channels are deeply cut with vertical sides, as if dug by hand, and the loss of water through them is very great. The marshes on the eastern side drain off into the Bahr el Zeraf, which takes off near Shambe, and, cutting off the corner, rejoins the Nile below Lake No. Greatly diminished by this loss, the Bahr el Gebel continues on its way, its bank being often nothing more than a line of papyrus separating it from large lagoons or dense thickets of reeds. As long as the channel is clear, tortuous and winding though it is, a great deal of water is able to pass; but under certain circumstances the channel becomes blocked by bars of solid vegetable matter, called sudd. Upstream of the obstruction the level rises, and, spreading out over the marshes, more and more water is lost in evaporation. Sudd occurs both in the Bahr el Ghazal and the Bahr el Gebel, but it is of different kinds. The Ghazal sudd is very much less substantial than that of the Gebel. It is chiefly composed of masses of the smaller swimming plants, which grow in Lake Ambadi and other shallow lagoons on that river and its affluents. But on the Gebel the principal constituents of the sudd are the papyrus and the ‘um soof’ reed. Both these plants grow with their roots bedded in the soil below the shallow water of the lagoons. Strong gales are prevalent in these latitudes, and these loosen the roots, so that, if the storms are accompanied by flood, large masses of the plants are set free, and begin drifting over the lagoons. Quantities of earth remain clinging to their drifting roots, and thus they are driven backwards and forwards by the wind. The storms come generally just before the flood, and thus there is plenty of water for the reeds to float in. Eventually they are driven, whole acres at a time, into the channel of the river, and are carried down the stream. But the channel being very narrow and winding, they soon get caught up, and a block occurs. New masses come down, and these are sucked by the stream under the original block, till a dense mass is formed, sometimes as much as over 20 feet thick, and so solid that a man can walk over it. If left to themselves, these blocks may last a very long time, and cause the river to change its course; or else by increased pressure the block is burst, and a great wave passes down the river, which sweeps all other blocks away. Years of high flood are generally followed by a great deal of sudd.