The idea of staging a long-distance Olympic swimming event on the Seine – with the Eiffel Tower in the background – seems at first glance like a nice idea. But for many athletes, it has turned into a planning nightmare.

"It's exasperating for the athletes and the coaches who have to prepare everything," said German national swimming coach Bernd Berkhahn recently. The biggest obstacle at the moment is the still very swift current of the river.

"It's not a problem to swim with the current, it's the way back that's the problem," Florian Wellbrock, Olympic champion in the 10-kilometer (6.2 miles) distance in Tokyo told DW. "In the current conditions, it's impossible to hold an event in the Seine."

Against the current

The planned route of the swimming marathon would see the athletes swim back and forth between the bridges Pont Alexandre III and Pont de Alma – a one-kilometer stretch of the Seine in the center of Paris. 

If the event is held in the Seine, swimmers would have to complete half the distance (five kilometers) battling against the current. Based on the winning times from Tokyo – 1:48 hours for the men and 1:59 hours for the women – the athletes would wind up swimming against the current for around 50 to 60 minutes.

The river is currently flowing at a speed of 1.2 meters per second. On average, an open water swimmer moves about 1.6 meters per second – when not battling against a current. At the current speed of the water, they would therefore only be able to move 40 centimeters per second against the current.

Improved water quality

For a long time, the poor water quality of the Seine was also a major problem. In the summer of 2023, a World Cup race that was planned as an Olympic test run was canceled. At the time, the Seine was too contaminated with intestinal bacteria. However, there is hope on this front. A spokesperson for the city of Paris said two weeks before the Summer Games were slated to begin that the Seine had been clean enough to host swimming competitions on "11 or 10" of the past 12 days.

The French authorities invested around €1.4 billion ($1.53 billion) in new wastewater treatment plants and sewage systems. If the water quality is not good enough on the competition days, the races could be postponed.

Rowing course as Plan B

Originally, the swimmers were told that there was no alternative venue to the Seine, national team coach Berkhahn said. Now, though, organizers have identified the rowing regatta course in Vaires-sur-Marne, around 20 kilometers from Paris, as a possible alternative.

"The communication was like a rollercoaster ride with lots of ups and downs," said swimming star Wellbrock. He is pleased that there is now an alternative to the Seine and that the competition appears set to take place one way or another.

However, as Berkhahn points out, a race on the river and a race on a regatta course are two "completely different things." The water temperature is different, as are the wind and the limitations.

"For the athletes, it's unspeakable that you don't know what's coming and you don't know what to prepare yourself for mentally for the race."

German Olympic swimmer Oliver Klemet would prefer the rowing course to the River Seine.

"If we change the venue, it will be easier for us," the 22-year-old open water swimmer told DW. "Swimming in a river is much harder than in the sea.

Focus on counter-current training

Olympic champion Wellbrock says it doesn't matter to him where the event is held, but one thing is clear: "This race will be the toughest 10-kilometer race ever. It's all about the preparation."

And training for this event has been particularly tough. National team coach Berkhahn has focused on preparing his athletes to swim against the current in Paris: "Strength and endurance is a focus in training. So they swim more with their arms (than usual)."

To simulate the counter-current, the athletes wore swimming trunks that feature a net-like surface designed reduce glide and increase drag.  In other training sessions, the swimmers pulled a parachute through the water.

The next couple of weeks will show whether this type of preparation was actually necessary. This is because the Seine is unusually high and it current unusually strong for this time of year. Should the weather improve, the level of the Seine – and it's current – can be expected to decrease.

This article was originally published in German.