208 The Sudd.

becomes solid enough to dam up such a mighty river as the Upper Nile, whose breadth, depth, and current would necessarily prevent such a growth. Few rivers in the wrorld have so slight a fall as the Upper Nile, and its western affluent, the Bahr el Grhazal. In its course across many degrees of latitude, the former traverses a uniform level region, and in many places the current is maintained merely by the pressure of the streams descending from the more elevated parts of the Bahr el Jebel basin.

" Through the periodical rise of the Nile, which differs considerably in volume from year to year, being determined by the amount of the rainfall in the tropics, the low-lying riverain tracts are flooded, and these consist in many places of flat depressions where the Nile waters lodge after the general subsidence. Such depressions often continue even at low water to communicate with the main stream, of which they form, as it wrere, so many inlets ; or else they become transformed in the dry season to small lakes and ponds, which resume their connection with the Nile at each returning rise. These are the 'oW or back waters, the Maiyehs of the Arab boatmen, which are constantly changing their level and assuming a different aspect with the rise and fall of the flood-waters. They form hundreds of culs-de-sac of all sizes, by which the difficulties of the navigation is greatly increased. Such conditions do not occur in the Sobat with its high and regular banks, but are partly found in the section between the Sobat and the Moqren el Bahiir, and especially on the Bahr el Jebel, as far as and beyond the station of Bor. Other conditions prevail in the Bahr el Ghazal, where the Maiyehs are doubtless rarer, but where broad flooded expanses are more frequent. " The periodically replenished lateral lagoons and depressions naturally promote a rank growth of aquatic plants, and the Maiyehs are in fact the hot-beds and nurseries of all the grass islands which drift away to the main stream when the communications are re-opened. But at other times the same stagnant waters serve to retain the masses which, being rooted very lightly to the ground, gradually form floating islands. Such islands, continually growing in thickness and solidity, would in fact become stable, and in course of time fill up nearly the whole of the Maiyeh, but for the fact that at each