It wasn’t until the 1960s that restoring the river began to attract political attention, first with the establishment of the French water agency, and later with a pledge by then mayor of Paris (later Prime Minister) Jacques Chirac. “I will bathe in the Seine in front of witnesses to prove that the Seine has become a clean river,” he declared in 1988, promising to complete the stunt by the early ’90s. Chirac, who died in 2019, never did take that public plunge. But his idea would live on in French politics, and the Olympics created a new deadline to complete the cleanup.

Macron has repeated his pledge. “I’ll do it,” he told reporters in March, refusing to be pinned down on a date. Both he and Hidalgo, however, were beaten into…