How did the Virunga volcanic dam change river systems in northern Rwanda?

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Cluster of small houses in a village nestled among green hills beside the wide Congo River, with more hills visible in the background, Democratic Republic of Congo. — Central Africa A small village in the hills along the banks of the Congo River, Democratic Republic of Congo, Central Africa.

Volcanic outpourings from the Virunga Mountains created a dam along the northern shore of Lake Kivu, which fundamentally altered the region's drainage patterns.12

Prior to this geologically recent volcanic activity, the drainage of Lake Kivu was directed northward toward Lake Edward and into the Nile river system. The formation of the volcanic dam blocked this northern outflow, forcing the lake's drainage to reverse and flow southward through the Ruzizi River into Lake Tanganyika.12