Will the flow and quality of the Seine’s waters allow athletes to swim there during the Paris Olympic and Paralympic Games? Before then, what will be the outcome of Paris Mayor Anne Hidalgo taking a plunge, on Wednesday, July 17? And in the future, will it be possible to make the Paris river swimmable, as it was a century ago? Other major cities, such as Lyon, Bordeaux and Toulouse, have made urban swimming a short- or medium-term objective, while wild diving is on the increase in the hot summer months.

But what exactly are the risks associated with exposure to water not intended for swimming?

There are three main categories of risk:

• Topological (linked to currents and site configuration);
• Bacteriological (linked to contamination by harmful micro-organisms);
• Chemical (linked to different types of pollution).

Beware of undetectable surface currents

The topological risk is the most obvious. It depends directly on the configuration of the site (water depth, bank layout, current strength, etc.) and can lead to fatal accidents. According to France’s public health agency, accidental drowning causes around 1,000 deaths a year in the country (around a quarter of them in rivers and lakes), making it the leading cause of accidental death in everyday life among the under-25s. “We strongly advise against swimming in strong currents,” said Marc Valmassoni, campaign coordinator for the non-governmental organization Surfrider, which monitors recreational waters and campaigns to improve their quality.

Some rivers, which are otherwise prohibited for swimming, are particularly dangerous. Such is the case of the Loire, with its major flow variations, loose sands and whirlpools that can trap swimmers. Tragedies are also common on the Rhône and Saône rivers in Lyon, due to powerful underwater currents undetectable from the surface. As a general rule, swimming should be avoided at all costs in the vicinity of engineering structures such as bridges, which can create eddies, or near dams and locks, where boat maneuvers generate strong currents that can trap swimmers.

At sea, particularly on the southwest coast of France, baïnes – water formations created by the movement of the tides – also drag swimmers out to sea. To avoid rapid exhaustion, don’t try to swim against the current, but rather swim out to sea and parallel to the beach when the current weakens, in order to escape. Inland, the risk is also higher in gravel pits and quarries, where bottom collapses can create whirlpools that suck swimmers in.

Bacteriological risks leading to poisoning

The second category of risks concerns the micro-organisms that multiply in water. “We are much more demanding for drinking water and swimming pool water than for swimming water,” warned Bernard Legube, emeritus professor at the University of Poitiers and a specialist in water physico-chemistry and water treatment.

Their concentration in rivers increases after heavy rainfall. These rains can cause sewage treatment plants to overflow, resulting in the discharge of wastewater and surface water runoff and their wastes. Marseille’s Huveaune beach, near the mouth of the coastal river of the same name, is even nicknamed the “peelings beach.” “As soon as it rains, the debris is washed away, it’s dirty. You shouldn’t swim in a coastal river after it rains,” advised Marc Valmassoni.

What’s more, algae and microalgae multiply in hot weather, a phenomenon aggravated by climate change. The consequences vary according to the micro-organisms and the duration of exposure.

Gastroenteritis is most often caused by fecal bacteria such as Escherichia coli (E. coli) and enterococci, which can be found in river water, especially after heavy rainfall. At the end of June, the Seine in Paris showed levels of E. coli and enterococci above the threshold set by the International Swimming and Triathlon Federation, currently preventing these Olympic events from taking place. “But I’m not worried: the authorities have set up reprocessing and storage systems that will be used very soon,” reassured Legube.

The French Ministry of Health maintains an up-to-date website where it is possible to check the quality of swimming water, according to the results of the latest analyses carried out.

• Bather’s dermatitis, or ‘duck flea’

After a summer swim in a body of water populated by waterfowl and snails, you may experience a mild but annoying itchy skin rash. They are caused by cercarial larvae, a parasite found particularly in mallards and swans. Bather’s dermatitis is quite rare. Shower immediately after swimming and dry off thoroughly to limit the risks.

Leptospirosis is transmitted by the urine of rodents that contaminate rivers and ponds. Most often benign in humans, this disease can in some cases lead to severe kidney failure, according to the Institut Pasteur. The most common symptoms are vomiting, fever, headaches and diarrhea. If you’re involved in freshwater activities, it’s advisable to disinfect and protect wounds; wash your hands; avoid contact with eyes, mouth and nose; and wear protective equipment.

Skin irritation, digestive problems and even nervous disorders – these are the signs of intoxication by cyanobacteria, bacteria that proliferate in the heat. Several cases of death have been reported in dogs. It’s best to avoid swimming if the water turns green or accumulates a brown film. Cyanobacteria can also form algae-like clusters. The risk is particularly high in low-oxygen lakes, notably in the south of France, but they also thrive in slow-flowing rivers, such as the Tarn.

Pollution, the most poorly understood risk category

Whether chemical discharges, pesticides, drug molecules, many human activities have a direct impact on the quality of watercourses, with two major unknowns: at what concentration levels and with what consequences?

In some extreme cases, we know that river pollution is linked with serious health consequences. This is the case in China, where over 450 villages lived for years with drinking water contaminated by upstream factories. These polluted waters led to cancer rates 50% higher than in the rest of the country.

In France, such extreme situations are rare. “In the Seine, there are a few traces of pesticides, mainly herbicides, but they are very low,” explained Legube. The risk is inversely proportional to the flow rate. In flowing waters, between currents, oxygenation and dilution in rivers, pollution doesn’t stay for long, noted the researcher.

At present, there is no precise national monitoring system for chemical pollution. “It’s costly to set up, and the stakes are not the same from one region to another,” explained Valmassoni. The NGO Surfrider is advocating for these issues to be taken into account in the next revision of the European directive.