The colony contains no navigable streams. For a short distance the Setit (known in its upper course as the Takazze), a tributary of the Atbara, forms the frontier, as does also in its upper course the Gash or Mareb (see Abyssinia).
Such are the Takazze in the north, the Abai in the centre, and the Sobat in the south, and through these three arteries is discharged about four-fifths of the entire drainage.
The Takazze, which is the true upper course of the Atbara, has its head-waters in the central tableland; and falls from about 7000 to 2500 ft.
During the rains the Takazze (i.e.
The Mareb is dry for a great part of the year, but like the Takazze is subject to sudden freshets during the rains.
At the south-east corner the rim of the crater is, as it were, breached by a deep crevasse through which the Abai escapes, and here develops a great semicircular bend like that of the Takazze, but in the reverse direction - east, south and north-west - down to the plains of Sennar, where it takes the name of Bahr-el-Azrak or Blue Nile.
In the deep valleys of the Takazze and Abai, and generally in places below 4000 ft., the conditions are tropical and fevers are prevalent.
The rain is heaviest in the Takazze basin in July and August.