The Kasai River originates from the highlands of central Angola and stretches 1,800 km before emptying its waters into River Congo. It is the longest tributary of River Congo, emptying the largest volume of water into it yearly. The confluence of these two rivers is at Kwamouth near Kinshasa. For a stretch of about 720 kilometres, it flows in the North Western direction through Congo, forming a natural border between Congo and Angola.
The Kasai River, known as Cassai in Angola, has numerous tributaries feeding it. These include the Fimi, the Kwango and the Sankuru rivers. The stretch of river that begins from the confluence of Kasai and the Fimi to where it drains into the Congo River has a different local name, River Kwah. It is after this stage that the name River Kasai becomes prominent. The watershed of the tributaries feeding River Kasai is an equatorial forest with sufficient rainfall all year round that keeps the flow of Kasai and Congo rivers consistent.
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. One can’t help but question the interest of their leaders, since they sacrificed the very people they were supposed to protect from injustices like inter-community clashes, or slavery. You would be shocked to realize that 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. Today, it is an important source of foreign exchange for Angola. Most people would call it compensation for the poor soils in the country that barely support agriculture. My honest opinion, stumbling upon diamond-filled alluvial beds is way better than compensation. Angola is one of the richest countries in Africa, in spite of political turmoil that often sets its economy off balance.