KASAI or CASSAI, a river of Africa, the chief southern affluent of the Congo. It enters the main stream in 3° 1o' S. 16° 16' E. after a course of over Boo miles. The Kasai river-system as a whole consists of innumerable consequent streams which have re markably parallel courses and flow from south to north rising in the highlands which stretch from Angola along the southern boundary of the Belgian Congo and which form the watershed between the Zambesi and the Congo. The chief rivers of the Kasai system which flow in this direction are the Sankuru, Lulua, Kasai, Luembe, Chiumbe, Longachimo, Chikapa, Lovua, Loange, Kwilu, Kwengo, India, Wamba and the largest, the Kwango. In their upper courses, as they flow over the Archaean massif, these streams occasionally assume a subsequent direction, but it is not until they reach the northern plain that this direction becomes dominant. The Sankura, whose waters are a bright yellow colour, is the first to flow westward near Bena Dibele and this stream, which at Basongo is joined by the Kasai and by which name it is henceforward known, intercepts the others or those of them which have been formed by the confluence of several.
Numerous falls and rapids occur in the upper reaches of all the streams thus preventing navigation upon most of the tribu taries. The Kasai and its tributaries are navigable for over 1,5oo m. by steamer. Navigation is possible on the Sankuru to above the town of Lusambe; on the Kasai it is stopped by the Wissmann falls a few miles below its confluence with the Lovua; the other tributaries are navigable for only short distances whilst even on the Kwango navigation does not proceed very far. Apart from those already mentioned the only important right bank tributary is the Mfini, known above Lake Leopold II. into which it flows,
as the Lukenie, which, rising in the eastern Belgian Congo, flows westward and joins the Kasai 450 m. from its mouth. Near its mouth, the Kasai, in its lower course a broad stream strewn with islands, is narrowed to about half a mile on passing through a gap in the inner line of the West African highlands, by the cutting of which the old lake of the Kasai basin must have been drained. The Kasai enters the Congo with a minimum depth of 25 ft. and a breadth of 700 yd., at a height of 942 ft. above the sea. The confluence is known as the Kwa mouth, Kwa being an alternative name for the lower Kasai.
The Kwango affluent of the Kasai was the first of the large affluents of the Congo known to Europeans, being reached by the Portuguese in the 16th century, but of its lower course they were ignorant. The first accurate knowledge of the river basin was obtained by David Livingstone, who reached the upper Kasai from the east and explored in part the upper Kwango 1855). V. L. Cameron and Paul Pogge crossed the upper Kasai in the early '7os. The Kwa mouth was seen in 1877 by H. M. Stanley, who in 1882, ascended the river to the Kwango-Kasai confluence and then proceeded up the Mfini and discovered Lake Leopold II. In 1884 George Grenfell journeyed up the river beyond the Kwango confluence. The systematic exploration of the main stream and its chief tributaries was, however, mainly the work of Hermann von Wissmann, Ludwig Wolf, Paul Pogge, Major Von Mechou, Grenfell and Holman Bentley, Captain C. Lemairc and E. Torday. See CONGO.