Kasai or Cassai, river, c.1,100 mi (1,800 km) long, rising in central Angola, S central Africa, flowing E, N, and NW through W Congo (Kinshasa) to the Congo River; it forms part of the Angola-Congo border. The Kasai, navigable for c.475 mi (760 km) above its mouth, is an important trade artery. Its tributaries include many navigable streams (some of which are rich in alluvial diamonds).
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