On a summer morning in the Swiss city of Basel, groups of commuters bob merrily down the Rhine. They’re not on boats but in their trunks, clutching fish-shaped waterproof bags that double as floats as they drift to work alongside cargo ships and gravel barges.

At lunchtime in Copenhagen, the harbour walls are packed with bronzed bodies tanning on tiered decks, and launching themselves into the water from daring wooden platforms. Office workers stop for a quick dip between meetings, while ferries cruise by. After work in Vienna, the grassy banks of the Danube throng with swimmers lounging in the dappled shade, fresh from changing in multistorey locker room towers, as a metro train rumbles across a nearby bridge.

After a century of ignoring the very arteries that allowed them to grow in the first place, cities are learning to love their rivers again. Around the world, as global heating causes summer temperatures to soar, people are flocking to urban waterways and reclaiming these once polluted, poisoned gutters as indispensable places to cool off and unwind.

Last month, the urban swimming movement made its biggest splash yet, when 110 athletes dived into the River Seine for the Olympic triathlon. The televised spectacle of swimmers front-crawling their way through Paris, flanked by beaux-arts bridges, offered a glimpse of what all our urban waterways could look like. Might these dangerous arteries of cargo and sewage be reborn as the great free public spaces that they could be? Could taking a plunge in the Thames, Hudson or Tiber one day be as common as going for a stroll in the park?

“What’s happening in Paris represents a generational baton change,” says Matt Sykes, an Australian landscape architect and the convener of the Swimmable Cities Alliance, a global network of urban swimming campaigners pushing to make the scenes in the Seine an everyday reality for us all. “With climate change, cities are being forced to adapt. Swimming access will become an inevitable part of the urban design vocabulary. The next generation are ready – kids will be watching the Olympic triathlon on TV and asking: ‘Why can’t we swim in our river?’”

In Sykes’s eyes, floating pontoons and riverside showers should be as common a part of the cityscape as bike lanes and benches – and he and his fellow advocates are pushing to make it a reality. To coincide with this summer’s Olympics, the alliance published a charter, signed by a host of municipalities, government agencies, community groups and cultural institutions from 31 cities around the world, in a bid to create safe, healthy and swimmable waterways, accessible to all. The hope is to have 300 new cities starting their journey towards “swimmability” by 2030.

The alliance is already making headway. In the Dutch city of Rotterdam, a masterplan for the Rijnhaven dock includes a new permanent beach and a tidal park. In Sydney, the Urban Plunge programme has plans that include floating pools, and riverside ladders and lockers. By next summer, if all goes according to plan, New Yorkers will be swimming beneath skyscrapers in the safe surrounds of a floating, filtered pool in the East River.

“This is going to be the cleanest water anyone ever swims in,” says Kara Meyer, the managing director of Plus Pool, a project which began in 2010 as a Kickstarter campaign by four young designers. Fourteen years on, New York State and New York City have announced changes to regulations that finally make the project possible, and committed $16m (£12.4m) to see a prototype pool realised by 2025.

“The original idea was: ‘What if you just dropped a big strainer in the river?’” says Meyer. “Now, we’re essentially building a floating wastewater treatment facility.” Engineered by Arup, the pool will pass the river water through a series of filtration membranes and blast it with UV disinfectant, in order to meet stringent water quality standards.

It will be a far cry from the floating bathhouses that used to be docked on the city’s riverbanks in the 19th century. These rectangular wooden slatted structures, which allowed the river to flow freely through, were gradually decommissioned in the 1930s as water quality declined. The Clean Water Act, passed in 1972 with the ambitious goal of making all US rivers and lakes swimmable by 1983, set the wheels in motion, but that target is still a way off.

“The pandemic was a real catalyst,” says Meyer. “There’s been a realisation that we need far more public space, and much better access to our natural environment.” She says a recent rise in drowning deaths, after decades of decline, underlines the importance of access to water and basic swimming skills – a need exacerbated by a shortage of lifeguards, after decades of pool closures. “It’s taken getting to this point of crisis for people to pay attention and understand the value of projects like these.”

Along with Switzerland – where Rheinschwimmen has been a tradition since the 1980s, after wastewater treatment reforms – Denmark is leading the way. Thirty years ago, Copenhagen’s harbour was a polluted mess of sewage and industrial waste. Now Danes are spoilt for choice of architect-designed bathing structures, and water quality is constantly monitored on a dedicated app. The Islands Brygge harbour baths, designed in 2002 by then-little-known architects Bjarke Ingels and Julien De Smedt, launched a generation of increasingly expressive timber platforms for diving, lounging and people-watching. They will soon be joined by Water Culture House, a temple to urban swimming by Kengo Kuma at the heart of a new waterfront development.

Elsewhere in Europe, the Fluss Bad campaign in Berlin organises an annual swim in the Spree canal, seeing swimmers splashing past the cultural palaces of museum island. The group is pushing for local bylaws to be changed to permit swimming, and has launched a water quality monitoring website to show the canal is clean enough to swim in 90% of the time. In Brussels, a city without a single outdoor swimming pool, the Pool Is Cool campaign operates a temporary pool each summer, as a prelude to future plans for swimming in the canal. In the bathing capital of Budapest, the Valyo group wants to see the city’s history of floating wooden pools return to the Danube. Swim fever is rampaging across the continent. So why is the UK lagging so far behind?

“There is an inherent fear in this country of getting anyone near water,” says architect Chris Romer-Lee of Studio Octopi, who has been battling to realise his floating Thames Baths project for over a decade. “Which is ridiculous, given we’re an island.”

Might our rivalry with the French be the catalyst we need to force our statutory agencies to work together? Following Paris’s €1.4bn (£1.2bn) clean-up operation of the Seine, and scenes of mayor Anne Hidalgo boldly plunging into the river, London’s mayor, Sadiq Khan, hastily pledged to make the UK capital’s rivers “swimmable by 2034”. He faces an uphill battle. Recent City Hall analysis found a five-fold annual increase in sewage entering London’s rivers, with spills lasting nearly 7,000 hours between April and December 2023.

Pollution isn’t the only barrier. The Port of London authority, in charge of Thames traffic, shows no sign of lifting its ban on swimming east of Putney. They cite “fast-flowing tides, undertows, underwater hazards and the heavy presence of commercial vessel traffic” – even if the £5bn super sewer eventually makes Thames water clean enough to swim in. While Basel’s bathers enjoy broad banks of stone steps leading down to the Rhine, London continues to treat its majestic river as a filthy foe. A brand new series of riverside public spaces, currently being completed as part of the Tideway sewer project, sadly greet the Thames with the same suspicion as our Victorian forebears, with defensive walls and high railings.

Paris shows another way is possible. The Olympic triathlon wasn’t a stunt but the culmination of a 30-year shift in public policy. Next summer will see four new swimming pools floating in the Seine, built for €10m, allowing Parisians to plunge directly into river water, with changing rooms, showers and lockers all provided for free. Boat traffic and currents are just as much of a hazard there, but through careful negotiation and political will, they’ve made it work.

“We lost the tradition of river swimming in the last century,” says Julien Laurent, who has the enviable job title of head of swimming in the Seine River. “But it’s not so new, or so radical. It happened for centuries, before boat traffic took over. We’re just bringing it back.”