Below the Kariba wall the Zambezi runs through a gorge so narrow and so deep that the engineers who surveyed it in the 1950s could hardly believe their luck. That is why the dam went there. For the Tonga, who had lived along both banks for as long as anyone could say, it was also why the dam should not have.

Nyaminyami is the spirit of the river: a great serpent with the head of a fish, or in some tellings a river dragon, who lives in the deep water of the gorge with his wife. He is not a monster in the tradition. He is the one who keeps the river, who sends fish, and who in a famine will let the people cut meat from his own flanks and feed themselves. The elders and the spirit mediums spoke to him when he was angry. When the Federation announced in the mid 1950s that it would dam the gorge and drown the valley behind it, the Tonga elders said what would happen. The wall would separate Nyaminyami from his wife, who lived upstream. He would not allow it.

Work started in 1955. Some fifty-seven thousand Tonga were moved out of the valley over the next few years, twenty-two thousand on the southern bank and twenty-nine thousand on the northern, from river gardens and fishing grounds to dry upland where they were told to grow crops. The wildlife rescue from the rising lake, Operation Noah, made newspapers around the world. The people’s removal made fewer.

Then the river answered, or the weather did. In the 1957 rainy season the biggest flood the Zambezi had ever recorded came down the gorge, tore through the coffer dam and the foundations, swept away plant and equipment, and drowned workers. The engineers rebuilt and raised their estimates. In 1958 the flood was bigger still, on a scale the hydrologists put at once in a thousand years. It took the coffer dam, the access bridge and part of the wall, and it killed more of the men working on it, Italian and local. In the tradition, some of the missing were only recovered after Tonga elders sacrificed a black calf on the water, and the bodies came up soon after. There are ordinary reasons a river gives up bodies after a few days. The story does not care about them.

The wall was finished anyway. The Queen Mother opened it in May 1960 and the turbines came on that year, and Kariba has fed power to both banks ever since. The lake behind it is the biggest man-made one on earth by volume. The Tonga were not consulted about any of that, and a good many of them still say that Nyaminyami has not finished with the wall. The small earthquakes that shake the Kariba basin from time to time, which geologists put down to the weight of the water settling on the rock, are explained on the lakeshore differently: that is the river god, on the wrong side of the wall from his wife, trying to get through.

He is everywhere at Kariba now. Carved on walking sticks in the curio shops, cast in pendants, printed on the tourist leaflets. The Tonga remain largely on the poor land they were moved to, and the wall, sixty-odd years on, has needed serious repair to the plunge pool beneath it, which the river has been quietly digging out since 1960.

The engineers call that erosion. Ask on the north shore and you will get a different word.