The generation of electricity from falling or flowing water — the world's oldest and still its largest source of renewable electricity, and the clearest case of clean power whose cleanliness conceals large human and ecological costs.
The Hoover Dam, completed in 1936, backed up a river across an entire canyon and called the result a lake. It generates enough electricity to power a large city, releases no carbon while running, and flooded farms, homes, and Native land upstream. That tension — firm, low-carbon power purchased at a cost the grid never shows — is the story of hydropower everywhere. Water has been doing work for people for thousands of years, turning mill wheels and grinding grain. The leap to electricity came in the late 1800s, when engineers learned to connect a water turbine to a generator. The dam followed as the device that stored potential energy by holding water high and releasing it through a turbine at will. The result is renewable electricity on demand — and flooded valleys, displaced communities, and silenced rivers downstream.