In the 1970s, environmental scientists declared the Seine biologically dead. More than half the region's wastewater flowed untreated into the river; oxygen levels dropped to values where fish could not survive. Today, fifty years later, three official swimming spots in central Paris open their doors. What bridges that gap: 1.4 billion euros in investment, two decades of wastewater treatment modernization, and an Olympics deadline that pulled the project out of political limbo.
1923: The ban on free swimming
The Seine was a fixture of daily life in Paris until the early 20th century. Bathing barges lined the banks, floating pools did brisk business, Parisians jumped directly from towpaths into the water. Then came the prohibition. In 1923, city authorities banned swimming for two reasons: growing ship traffic on the Seine had increased drowning accidents and collisions, and water quality had become dangerous as Paris discharged most sewage untreated into the river.
Over the following decades, conditions deteriorated sharply. By the 1970s, the Seine was biologically dead over wide stretches. More than half of regional wastewater flowed untreated into the river. Oxygen levels fell to concentrations where fish could not survive.
1.4 billion euros and an underground reservoir
The turning point came slowly and cost heavily. Paris invested 1.4 billion euros in cleanup through the 2024 Olympic Games. The project's centerpiece is an underground storage basin that captures combined sewer overflows during heavy rain before they flow into the river. Simultaneously, the city upgraded wastewater treatment plants and renovated large sections of sewage infrastructure across the Paris metropolitan area.
The system has one weakness that persists today: during heavy rain, swimming spots must be preemptively closed for 24 to 36 hours until water quality can be reassessed. Daily monitoring runs through water samples, with authorities posting results using a flag system at entry points.
The Olympic test run in 2024
In summer 2024, Paris tested its cleanup performance before the eyes of the world. The Olympic Games added triathlon and open water swimming to the Seine. That this plan succeeded was far from certain. Previous test events had been cancelled due to poor water quality. The Olympics unfolded differently. Athletes actually swam in the Seine, and Paris authorities declared it proof that cleanup measures were working.
In 2025 came the first summer the public could swim. Images of Parisians taking their first swim in the Seine in over a century went global. 2026 is the second season.
Grenelle, Bras Marie, Bercy: three free entry points
Since July 4, 2026, three official swimming spots have been open, all staffed daily with lifeguards. At Bras de Grenelle in the 15th arrondissement, swimmers have a view of the Eiffel Tower. The spot at Bras Marie in the 4th arrondissement sits near Notre-Dame Cathedral. Eastern Bercy in the 12th arrondissement offers a less touristy atmosphere with more space.
Entry is free and requires no reservation. A mandatory shower before entry is required; the city recommends washing with soap afterward as well. The season runs through August 30, 2026. A green flag means open, red flag means closed.
In comparison: The Thames and the Rhine
Paris is not the first example of successful urban river cleanup in Europe. The Thames in London was declared biologically dead in 1957 by the Natural History Museum. Legal regulations from 1961 and full wastewater treatment requirements from 1976 triggered the turnaround. Today around 125 fish species are documented in the Thames, including salmon and seahorses. The Zoological Society of London officially declared the river ecologically active again in 2021.
The Rhine offers another parallel. After the 1986 Sandoz chemical accident near Basel, when pesticide-contaminated firefighting water killed fish over hundreds of kilometers, the riparian countries coordinated joint cleanup efforts. The Rhine Action Plan brought salmon and other species back by 2000, creatures unseen in the river for decades.
Two decades of patience and an Olympic deadline
The Paris project illustrates the conditions for successful urban river restoration. First, it requires political continuity across electoral cycles. The 1.4 billion euros were not decided in a single term but built up over roughly two decades, pursued regardless of changes in city government. Second, an external deadline helped: the 2024 Olympic Games forced the project into a concrete timeline by which it could be publicly measured.
The result: for the first time in 103 years, the river flowing through historic central Paris is again part of urban summer life. Across the metropolitan area, Paris has now opened eleven supervised swimming spots, three directly in the Seine.