During floods they pour over their banks upon the surrounding valleys, by a thousand channels which interlace and establish communication between the main streams. After numerous bifurcations they find their way into the sea by three principal mouths.
The purpose was to decrease the floods which sometimes threaten to entirely submerge the city.
For the next sixty years an urgent question was the prevention of floods in the capital.
The miasmatic exhalations caused by the sun playing on stagnant waters after the floods give rise to the "Sennar fever," which drives even the natives from the plains to the southern uplands.
There are no glaciers near its sources, although they must have existed there in geologically recent times, but masses of melting snow annually give rise to floods, which rush through the midst of the valley in a turbid red stream, frequently rendering the river impassable and cutting off the crazy brick bridges at Herat and Tirpul.