Since the 20th century, European rivers have been dammed and diverted for the sake of generating renewable energy. But the concrete walls have left an indelible mark, upending entire ecosystems. Migratory fish species in particular are paying the price. Atlantic salmon can no longer find they way back to their home streams to spawn, pushing the species to the brink of extinction. Should dams be torn down in the name of biodiversity? We take a closer look in this edition of Down to Earth.