In this case, you can rest assured that they’ve been doing a huge amount of work (both politicians enacting regulations, although funding could always have been better, local councils who worked on policies and outreach, scientists, and engineers) for the sake of having a clean river and more biodiversity. This effort more or less started in the 1990s. Chirac, who used to be Mayor of Paris, promised to swim in the Seine within 3 years when he unveiled a cleaning up effort in 1988. Which was obviously propaganda, but still the will was there.
Agree that it’s a great result overall!