The Paulo Afonso Falls were not a single sheet of water but a staircase — a series of tiered cascades dropping around 80 metres in total, the longest single plunge close to 59 metres, funnelling into a gorge that narrows to an average of about 18 metres across. Before the dams, the flow through that gap was estimated at some 2,832 cubic metres per second, rising in flood to at least 14,158. Compressed into a slot that width, the effect was violent.

People travelled a very long way to look at it. The Dutch landscape painter Frans Post put the falls on canvas in 1649. Emperor Pedro II came in 1852. Marc Ferrez photographed the site in 1875. And the Bahian poet Castro Alves wrote A Cachoeira de Paulo Afonso, published in the posthumous 1876 collection Os Escravos, using the falls as an image for the violence of slavery — the abolitionist poet reaching for the loudest thing in his country to say what he meant.

The first person to take power out of the falls was not a state agency but a merchant from Ceará. Delmiro Augusto da Cruz Gouveia, born at Ipu on 5 June 1863, built the small Angiquinho plant at the foot of the cataract and brought it online in January 1913 — 1,500 horsepower, and the first hydroelectric station in northeastern Brazil.

He was not doing it for the novelty. In 1914 he opened the Fábrica da Pedra, a sewing-thread mill that by 1916 was turning out more than half a million spools a day and undercutting the British thread houses that had held the Brazilian market. On 10 October 1917 Gouveia was shot dead at Pedra. His heirs sold the factory to Machine Cotton, the British conglomerate that had been trying to buy it, and the new owners destroyed the machinery, pulled down the buildings and threw the wreckage into the São Francisco. Angiquinho itself passed to the state utility in 1957, stopped generating in 1960, and was given heritage protection by Alagoas in 2006.

Brazil's Ministry of Agriculture had recognised by the 1940s that harnessing the São Francisco was the lever for developing the semi-arid Northeast. Paulo Afonso I was authorised on 23 May 1944 with two generators, and the Companhia Hidro-Elétrica do São Francisco — CHESF — was created in 1945 to build it.

Construction began in 1948 and immediately ran into the river. Diverting the São Francisco at a point where it was both deep and fast took until 1954. Because there was no room for a conventional powerhouse, engineers excavated tunnels and a cavern in the rock and built Brazil's first underground power plant inside it: a chamber 60 metres long, 31 high and 15 wide, roughly 82 metres beneath the surface, holding three 60-megawatt Westinghouse generators on Francis turbines. President João Café Filho inaugurated it on 15 January 1955. It was the first large power plant Brazil had ever built.

Nobody stopped. Paulo Afonso II came on line between 1961 and 1967, six generators in a longer cavern alongside the first, totalling 443 megawatts. Paulo Afonso III followed from 1967 to 1974, four Siemens machines of 198.55 megawatts each. In 1971 work shifted four kilometres upstream to the Apolônio Sales Dam — originally called Moxotó, later renamed for the founder of CHESF — a 2,825-metre embankment whose reservoir spreads across 100 square kilometres and regulates the water reaching the plants below; its four Kaplan units, online in April 1977, add 400 megawatts.

The last and largest is Paulo Afonso IV, two kilometres southwest of the falls: a 7,430-metre dam, an underground hall 210 metres long and 52 high, and six generators of 410.4 megawatts apiece. The first spun up on 1 December 1979, the last on 28 May 1983. Altogether the complex is three dams, five power plants and 23 generators producing 4,279.6 megawatts — the densest concentration of dams in Brazil, and behind only Belo Monte and Tucuruí among the country's wholly national hydroelectric plants.

The gains were real. Electricity from these caverns underwrote the industrialisation of the Northeast, and the town that grew around the works became the city of Paulo Afonso in 1958, home to about 113,000 people at the 2022 census.

The costs were also real. Building the dams took roughly 1,600 hectares of land and displaced 52,000 people, most of them river-dwellers with no leverage over the decision. Along the São Francisco, dams have blocked migration routes of fish people had lived on for generations, and mass die-offs have been recorded. And the falls that drew Pedro II and Frans Post and Castro Alves are now an operating decision — magnificent when the gates come up, and absent the rest of the time.