Before dawn on 28 July 1938, a police flying column under João Bezerra found Virgulino Ferreira da Silva — Lampião, born 7 July 1897 — camped with his band at Grota do Angico, near Poço Redondo across the river in Sergipe. The machine guns did their work in minutes. Lampião died there, along with Maria Déia, known as Maria Bonita, and nine others; some forty people escaped into the caatinga. The dead were beheaded. Eleven heads were carried to Piranhas and set out on the steps of the town hall, arranged with their hats and rifles for the photographers, and the picture that resulted is one of the most reproduced images in Brazilian history. The heads went on to a forensic institute in Salvador, where they stayed on public display for three decades. They were finally buried in the Quintas cemetery there in 1969.

The reason a town exists here at all is that the river stopped being useful just upstream. Impassable rapids begin above Piranhas, and sixty kilometres beyond them came the great falls at Paulo Afonso; above the falls the São Francisco runs navigable again for more than 1,500 kilometres. Piranhas was the upstream limit of the lower river, and steam navigation from Penedo reached it in August 1867, which is what turned a fishing spot — named for the carnivorous fish the fishermen pulled out of it — into a port. The district was created in 1885 and separated from Pão de Açúcar as a vila in 1887. In the early republic it briefly carried the name Floriano Peixoto, after the marshal from Alagoas who became Brazil's second president.

In 1881 Piranhas became the lower terminus of the Estrada de Ferro Paulo Afonso, built for one purpose: to carry goods around the rapids and the falls. The line climbed out of the gorge, ran northwest a few kilometres back from the river through Olho d'Água das Flores and Delmiro Gouveia, and rejoined the São Francisco at Petrolândia, some eighty kilometres upstream. It was single track from end to end and never physically connected to any other railway in Brazil — an isolated eighty-kilometre bypass around a waterfall. It kept its own name until 1903, passed through two later companies, and closed for good in 1964. The abandoned trackbed still climbs the valley wall out of the town centre, and walking it gives you the river below and the Xingó dam upstream.

Xingó changed the view. Construction started in March 1987 and the plant reached full output in August 1997 at 3,162 megawatts, roughly a quarter of the electricity in Brazil's Northeast. The reservoir it created filled the gorge with startlingly green, still water between walls of bare rock — an improbable sight in the caatinga, which is thorn scrub and dust for most of the year. The canyon stretch between the Paulo Afonso complex and Xingó is protected as the 26,736-hectare Rio São Francisco Natural Monument, part of it in this municipality. Catamarans, sailboats and canoes work the water now, along with tourist runs on paddle-wheel boats built in the style of the old gaiolas.

Piranhas never grew much — 25,183 people at the 2020 count, spread over 407.65 square kilometres, 291 kilometres by road from Maceió, and hot enough to average 28°C through the year. What it kept instead was its buildings. The centre is a compact set of late-imperial northeastern architecture that lost its economic reason for existing when the railway closed and simply stayed standing. IPHAN listed it in 2004, protecting the historic core, the Entremontes district and thirteen kilometres of riverfront. The trains are gone, the river boats are tourist craft, and the town's most famous day is one nobody there chose. But the stairs still go down to the water, and the water is still the reason.