Paris is working hard to be able to host some of the Olympic games using the River Seine. I believe even the opening is meant to feature swimming there.
Now the current issue is that sewage, and other contaminants, too often goes into the Seine. The authorities have built a storm overflow vessel that reduces the probability of pollutants discharging into the river. However a heavy storm would overwhelm this defence, meaning that it would be unsafe to swim in the Seine for some days after.
So I'm asking if some event means that the river water is deemed not safe enough for swimming during the Paris Olympics that an event has to be delayed or cancelled. If the event gets moved from the Seine to a swimming pool or some other venue this also counts. Will check reliable news sources.
Some background on the matter:
