For many of these swimmers and paddleboarders, the rising tide of river pollution is a grave concern. A survey of over 4,500 people by Outdoor Swimmer magazine found that 20 per cent of swimmers in England would describe water quality in their local swimming spot as poor. The majority of respondents said worries about pollution and water quality prevented them from swimming outdoors as much as they would like, and the majority said water quality had worsened in recent years.

Simon Griffiths, editor of the magazine, says: “I live by the Thames, and there are days when I won’t go in or will only swim head-up breaststroke. I’ve got friends who have become ill after swimming here. We joke about it, but it’s happened enough – now and again someone gets ill.”

Only 14 per cent of English rivers are of ‘good’ ecological standard and not a single river is free from chemical contamination, according to the Environmental Audit Committee. Its report also states that water quality monitoring routinely fails to identify microplastics, persistent chemical pollutants and antimicrobial resistant pathogens in rivers. 

The biggest culprits in Britain’s river pollution problem are sewage effluent and agricultural waste. Raw sewage spilled into waterways over 400,000 times in 2020 (the actual figure is thought to be higher, and data for 2021 has not yet been published). The Environment Agency allows companies to discharge untreated waste in exceptional circumstances, such as extreme weather events, but the Environmental Audit Committee’s report states that water companies are dumping raw sewage “regularly” and “often breaching the terms of permits”. 

Bacteria from human and animal waste can cause sickness and diarrhoea if river water is swallowed by swimmers. The build-up of nitrates and phosphorus from agricultural run-off “chokes” rivers with algal blooms that kill wildlife and, in some cases, cause sickness and skin irritation in humans.

Dr Christian Dunn of Bangor University, director of the Plastic Research Centre of Wales, suggests that river pollution could also have unforeseen effects on our long-term health due to microplastic and chemical contamination. 

“We did research three or four years ago that found, for the first time, that there are microplastics in every inland water we tested in the UK,” he says. “There’s no complete answer to say how microplastics harm human health yet, but we know they leech some harmful chemicals and that microplastics, in particular, can act as vectors for harmful bacteria and viruses.”

Kate Rew, founder of the Outdoor Swimming Society, says concerns over pollution shouldn’t dissuade swimmers from taking a dip. “We’re not actually getting that ill from rivers, even if they’re not as clean as they could be,” she argues. “I would say the swimming community is more concerned with the environmental impact rather than how we suffer from pollution ourselves.”