The idea was born of drought. In 1972, as the Sahel suffered, Mali, Mauritania, and Senegal joined together as the OMVS, the Organization for the Development of the Senegal River, to harness the basin's agricultural and hydropower potential. The Bafing, a headwater of the Senegal, would be dammed at Manantali. The World Bank looked at the numbers in 1979 and declined to fund it, judging the investment unreasonable. Europe stepped in instead, and construction began in 1982. By 1988 the dam stood completed, but without its power plant. A Swiss journalist who visited that year called it a luxury car without a motor. When the Mauritania-Senegal Border War erupted in 1989, all work stopped, and the half-finished project sat idle while the politics caught up.

The dam's full cost ran to more than a billion euros, financed by sixteen donors, from German and French development agencies to the African Development Bank, from Saudi Arabia and Kuwait to the United Nations. Norway declined to take part, worried about the project's health effects. In 1993, Germany's own minister for development assistance called Manantali an act of economic and environmental nonsense. The economics never quite worked. Even when the hydropower plant was finally funded in 1997 and brought online, the electricity sold for about half what it cost to make, and the national utilities that bought it routinely paid only half their bills. The ledger told one story. The valley told another.

Ten thousand people lived in the land the reservoir would drown. In 1986 and 1987, ahead of the rising water, around eight thousand of them were resettled, lifted out of the country their families had farmed and moved to make way for the lake. These were not statistics. They were households with fields, graves, and histories in soil that would soon lie beneath the water, asked to begin again in unfamiliar ground. The irrigated plots they were offered often yielded less than the land they had lost. Only a small share of the farmers affected received any irrigated land in compensation at all. The reservoir filled. The villages did not move back.

Downstream, the loss spread far beyond those who were moved. The annual flood that had defined the Senegal valley was now held back behind the wall, reduced to a managed, artificial release of about two weeks. Where the river had once spread across 150,000 hectares in an average year, and up to 350,000 in a wet one, the inundation shrank toward a fraction of that. Some 370,000 people had lived by flood-recession farming in the valley. That way of life has now disappeared entirely. In the new irrigated zones, standing water bred disease, and cases of bilharzia climbed; the river fisheries that had fed families fell away. The dam delivered power and stored water, as promised. But it also taught a hard lesson about what is lost when a river's oldest rhythm is broken, and who pays the price.