Others came from much farther away. Paulo Antonialli and Rachel Schroeder, a couple from Brazil, planned a Paris holiday for Rachel's birthday, and by chance, they arrived just in time to swim in the Seine's clean waters. "There's only one Paris, and only one Seine," Paulo laughed, as they inched closer to the front of the line, without caring the length of the queue.

On the river itself, the mood is part festival, part history lesson. Lifeguards patrol the edges of the cordoned-off sections, keeping watch over a crowd of swimmers, many wearing inflatable rings for safety. Some older locals prefer to wade in slowly, chatting with neighbors who they haven't seen since the days when swimming here was just a childhood memory.

Each of the three sites will remain open daily during the summer, with seasonal swimming hours and water quality checks every morning. The city has pledged that these new swimming spots will return every summer from now on, a new Paris tradition built on old memories.

The reopening is a milestone long in the making. For more than a century, the Seine was off-limits for swimmers due to industrial pollution and poor urban sewage systems. But Paris's Olympic bid brought political will and more than 1.4 billion euros ($1.63 billion) in investment to overhaul wastewater pipes, upgrade treatment plants, and reimagine how the city uses its river.