On the first Saturday of July, under a sky already heavy with the threat of another heatwave, a woman named Lauriane Fiorentino bobbed in the Seine at the foot of the Eiffel Tower, gripping a mandatory yellow floatie. “It’s better than a pool, not quite the beach, but it gives you the feeling of already being a little on vacation,” she told onlookers, as the elevated metro rumbled across the Pont de Bir-Hakeim behind her. Nearby, an American visitor, Benjamin Doncan, gestured toward the water and the iron lattice of the tower: “Frankly, could it be more beautiful than this?” So began the second summer of legal swimming in the Seine within Paris, a century after the city banned the practice for fear of pollution.

The return of the river baths is the most tangible legacy of the 2024 Olympic Games, for which France poured €1.4 billion into modernising wastewater treatment and sewage systems. The effort has revived not just a summer ritual but an ecosystem: over 30 fish species now swim in waters that, a few decades ago, hosted barely three, and even endangered mussels have reappeared, according to local health authorities. Three open-air sites—at Grenelle, Bercy, and the Bras Marie—operate free of charge through August, their water quality tested multiple times daily and communicated to bathers via a simple flag system: green, orange, red.

Yet the Seine’s reinvention as a summer playground is not without shadows. On the same Saturday morning, hours before the swimming areas opened, emergency workers pulled the bloated body of a man in his thirties from the Canal Saint-Martin, the other Parisian waterway that had been opened for bathing weeks earlier during a heatwave. Officials said he had been in the water for days, his identity unknown. Just eight days before, another man had drowned in the canal after swimming outside the permitted zone, one of more than ninety drowning deaths nationwide since mid-June, a figure the French sports minister called ‘alarming.’ The canal’s bathers also included dozens of youths injured jumping from bridges—an act strictly prohibited but difficult to prevent in a city thirsty for cool relief.

Paris is not alone in rethinking its rivers as public commons. In Basel, swimmers float down the Rhine using bright waterproof bags as both buoy and locker, paying a modest entry fee for a wooden pontoon turned summer refuge. Munich’s Eisbach wave, officially sanctioned since 2010, sees surfers carving a stationary rapid in the English Garden from dawn until 10 p.m., while bathers find less formal plunges in the Isar. London, too, opened its first official Thames bathing site this spring, near Ham. Yet viewed from Moscow, the trend looks different: authorities there have steadily shrunk the number of approved urban swimming zones to just five, as microbiological contamination closes lake after lake. Such tightening stands in contrast to the Parisian experiment, where swimmable rivers are cast as a democratic luxury, accessible without ticket or reservation to anyone taller than 1.2 metres.

On the wooden quay at Grenelle, as the afternoon heat climbed, lifeguards in fluorescent shirts watched the bathers. The swimmers, suspended in the greenish water with their bright yellow floats, drifted in a scene that felt both ancient and utterly new. Above, the métro trains clattered past, but for a while, the city seemed to float, too, in an unsteady truce between the elemental and the engineered.