It is joined by four main tributaries, the Drina, Bosna, Vrbas and Una.
Farther west, the Vrbas cuts a channel through the Dinaric Alps, and, after passing Jajce and Banjaluka, meets the Save 94 m.
Small steamers ply on the Drina, Save and Una, but the Bosna, though broad from its very source, is, like the Vrbas, too full of shallows to be utilized; while the Narenta only begins to be navigable when it enters Dalmatia.
Pop. (1895) 13,666, of whom about 7000 were Moslems. Banjaluka lies on the river Vrbas, and at the terminus of a military railway which meets the Hungarian state line at Jasenovac, 30 m.
A small stream called the Crkvina enters the Vrbas from the north-east and in the angle thus formed stand the citadel and barracks, with the 16th-century Ferhadiya Jamia, largest and most beautiful of more than 40 mosques in the city.