At a point in its course between Lake Albert Nyanza and Gondokoro the Nile makes a descent of several hundred feet, from an elevation some fifteen hundred feet above sea level. This descent in the current causes a series of rapids and cataracts.
As the stream leaves Gondokoro it flows tortuously on, winding first to the northwest, then turning to the northeast, during its journey of five hundred miles.
Later, the river is joined by its first large tributary, the Bahr-el-Gazal, which does not seem to make any noticeable difference in the current as it enters it from the west. A second tributary, the Giraffe River, flows into the main stream from the east.
From its junction, with its first great tributary, the Nile flows on for about eighty miles in an easterly direction. It then changes its course, winding south for about thirty miles, where it is joined by a third tributary, which flows into it from the east. This tributary, the Sobat River, has quite a full, navigable current.
Beyond the junction of the Nile with these tributaries the river flows on for a distance of upwards of four hundred miles. It flows through this section in a northerly direction, attaining width varying from one to two miles. At intervals along its course several streams flow into it from the east before it reaches Khartoom.
The town of Khartoom is situated just at the junction of the Blue and the White Nile streams.
The Blue Nile was for many years supposed to be the main stream of the Nile. It is, however, formed by the union of two rivers, the Abai and the Blue River.
The Abai River rises in Abyssinia. Its source lies about fifty miles from Lake Dembea.
It enters the lake from the southwest and emerges from it at the southeast. For about ninety miles it flows on in this direction, and finally describes a semi-circular course around the peninsula of Godjam.
Leaving Godjam, the river winds on in a northwesterly direction for about one hundred and fifty miles, where it is joined by the Blue River from the south.