The Black Volta rises in the Cascades Region of southwestern Burkina Faso, near Mount Tenakourou -- at 749 meters, the highest point in the country. The landscape here is green and hilly, a surprise in a nation most people associate with flat Sahelian dust. Water seeps from the highlands and gathers into a stream that quickly becomes something more consequential. For its first 200 kilometers the river flows roughly north and east across Burkina Faso, carving through savanna woodland and collecting tributaries from the surrounding hills. Then it turns south, as if remembering where the ocean is, and begins the long journey that will carry it through three countries and two dams before it merges with the White Volta to feed Lake Volta, the largest man-made reservoir by surface area on Earth.

Rivers make convenient borders, and the Black Volta has been pressed into that service more thoroughly than most. After crossing western Burkina Faso, the river becomes the international boundary between Ghana and Burkina Faso for about 100 kilometers, a stretch where fishermen cast nets on one side and haul their catch onto another country's bank. Further downstream, the river briefly marks the border between Ghana and Ivory Coast before entering Ghana entirely. Inside Ghana, it continues to serve as a dividing line -- this time between the Savannah and Bono regions. For the communities that live along its banks, these borders are administrative abstractions; the river is the constant, the thing that waters crops, carries canoes, and floods predictably each rainy season.

Where the Black Volta cuts through rocky terrain near Bui in Ghana's Bono Region, the river narrows into a gorge that once sheltered one of West Africa's most significant hippo populations -- an estimated 250 to 350 animals, the only two populations of hippopotamus in Ghana. In 2008, the Ghanaian government began construction of the Bui Dam at this gorge, a 400-megawatt hydroelectric project built in collaboration with Chinese state-owned firm Sino Hydro. The dam reached full operation in December 2013, but its reservoir inundated 21 percent of Bui National Park and forced the relocation of over 1,200 people. The fate of the hippos became an international concern; it remains unclear whether viable habitat exists nearby for animals that cannot simply be moved. The dam generates power, but the gorge that made it possible held its own kind of irreplaceable energy.

The Black Volta does not reach the sea alone. In the ancient kingdom of Dagbon in northern Ghana, it merges with the White Volta to form the upper end of the Volta River system -- a drainage network that defines Ghana's geography as completely as the Nile defines Egypt's. The Red Volta, the third and smallest tributary, joins from the east. Together these three rivers feed Lake Volta, the 8,502-square-kilometer reservoir created by the Akosombo Dam in the 1960s. Burkina Faso was once called Upper Volta, named after this river system by French colonial administrators. The country changed its name in 1984, but the rivers kept theirs. From Mount Tenakourou to the confluence at Dagbon, the Black Volta carries more than water: it carries the sediment of centuries, the boundaries of nations, and the quiet insistence that geography, not politics, has the final word.