The region's 400 thousand square kilometers are centered on the Congo basin's interior lowlands--above 200 meters like the entire plateau, but lower than the surrounding lands that exceed 500 meters. The regions's higher places are found across most of the north, and in the southeast.
The Congo River(2), locally called the Zaïre, flows west from north of the region's east-center, then turns southwest, forming the national border after receiving the Ubangi(3).
The Ubangi is formed in the region's northeast by the joining of the Bomu(4) and the Uele. It then follows the nation's north and northwest bound before reaching the Zaïre in the middle of a swamp.
Between the two are the Lua and the ill-defined Guri, adding to the Ubangi, and the Mongala with its headwater, the Ebola.
Left bank feeds of the Zaïre are the Lulonga, the Ikelemba, and--most importantly--the Ruki. The Ruki is formed from the Busira and the Momboyo. The Busira in turn is formed by the Tshuapa and the Lomela.
No one language accounts for a majority. Lingala serves as a lingua fraca, with French a distant second. Lingala as a first language accounts for nearly four in ten, and belongs to the the C Group of Narrow Bantu languages. These include nearly two in three first language speakers. The only other important one of these is Mongo-Nkundu (one in 20)(5). But the region's second most important tongue is outside the C group. Ngbaka is native to about two in ten and belongs to the Ubangi group. Another member of that group, Mbanja, is spoken by more than one in 20.
The region's capital, Mbandaka, has nearly 200 thousand residents, most of them fluent in Mongo and Lingala, some in French and English. It is an important port at the junction of the Zaïre and Ruki Rivers. During the first Congo War, non-local Tutsi genocided non-local Hutus as they tried to escape by barge.
Bantu speakers arrived from the north within the last 4,000 years. The Ubangi speakers have been in the area for a long time, perhaps 3,000 years, perhaps coming from the northwest.
Lingala evolved locally--near Mbandaka--from Mangala, the language of the Ngala trading people.
Christianity in Kinshasa Congo
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(1) Formerly Zaïre. Also known as Kinshasa Congo.
(2) Africa's second longest, the world's eighth.
(3) Also spelled Oubangui.
(4) Or Mbomou.
(5) Some would consider Ngando and Mongo-Nkundu (sometimes just called Mongo) as one language. This would not raise its regional proportion much.