The Kasai River (called Cassai in Angola) is a tributary of the Congo River, located in central Africa. The river begins in Angola and serves as the border between Angola and the Democratic Republic of the Congo (DRC), then flows into the DRC, where it joins the Congo at Kwamouth northeast of Kinshasa. The Kasai's tributaries include the Fimi, Kwango, and Sankuru rivers. The short stretch of the Kasai from the inflow of the Fimi to the Congo is known as the Kwah River. The Kasai watershed consists mainly of equatorial rainforest areas, which provide an agricultural land in a region noted for its infertile, sandy soil. It is a tributry of Congo river and diamonds are found in this river. Around 60% of diamonds in Belgium go from Kasai river for cutting and shaping.
The tributaries of River Kasai are clear of obstacles like cataracts and river weed, making them very navigable. They facilitate the transport sector and form an important trade artery. The river’s role in transport and trade was more prominent during the pre-colonial period when slave trade was legal. Slave traders used one of its major tributaries, the Kwango River, to navigate the equatorial rain forest, capture slaves and find their way back to the Atlantic Ocean where they had docked their ships. It is greatly controversial that some of the local kingdoms that lived along the Kasai River supported slave trade. The Rund kingdom for instance, readily provided slaves for most notorious slave traders like John Matthews, a renowned British slave vendor. These activities, though they occurred between 18th and 19th centuries, left a lasting impact in the regions where they were most prominent, like between Kwango and Kwilu rivers. The population has never recovered fully, with the population density lower than that of areas that did not experience slave trade. The most probable trigger to British and Portuguese great interests in the Kasai River is the presence of alluvial diamonds lying in rich deposit beds, especially at the river’s mouth. More deposits lie along the beds of its major tributary, the Kwango River. In fact, it is common to hear the phrase “the diamond heartland of North Eastern Angola” used in reference to the Kwango River valley. This is because the diamond alluvial beds found in this region are the richest in Angola.