In the fourth year of this seven-year "Changing Planet" project, Dr M. Sanjayan is investigating efforts to protect and revive our dying rivers. Two ambitious river restoration projects are aiming to bring life back to the Klamath in northern California and the Seine in Paris, France.

Sanjayan returns to northern California, where the largest dam removal project in U.S. history is aiming to resuscitate the river Klamath. Historically, it had one of the largest salmon runs on the West Coast but from the early 1900s a series of giant dams were built to provide hydroelectric power. These dams blocked salmon from reaching their spawning grounds, stopped nutrients reaching downstream and toxic blue-green algae flourished in the warm reservoir. The whole ecosystem was damaged and numbers of Chinook salmon fell by more than 90%.

In France, journalist and Paralympian Ade Adepitan investigates the clean-up of Paris’ iconic river Seine. For decades, raw sewage flowed into the Seine, killing off aquatic wildlife. Now, a mega-engineering project should make the river clean enough to swim in, not only for the 2024 Games but also for public swimming. Wildlife will benefit from a cleaner river too. Ade tracks down bats feeding on insects that start life in the water and he joins local anglers to catch a giant Wels catfish, a good indication of a healthy river.

These pioneering projects offer solutions and hope for other rivers around the world.