Debate rages over whether or not hydroelectric projects—turbines turned by flowing water—qualify as clean, renewable and sustainable. Most experts say large-scale impoundment dams, such as the Hoover Dam or Three Gorges Dam, do not meet the criteria.
That’s because when a large dam is built to facilitate a hydroelectric project, it disrupts or destroys ecological systems that had existed in the formerly free-flowing river. It devastates downstream land that was dependent on seasonal flooding.
The damage upstream is generally more devastating. When the reservoir first floods, acres of suddenly drowned vegetation rot, releasing huge quantities of carbon into the air. Following this first burst of decay, plant matter settles on the bottom of the reservoir and rots without oxygen. Dissolved methane accumulates, which then releases into the atmosphere after the water flows through the turbines. “Hydroelectric dams … in some cases produce more of these greenhouse gases than power plants running on fossil fuels,” wrote New Scientist (Feb. 24, 2005).
On the other hand, small “run of river” hydroelectric projects are some of the best energy sources on the planet. “[W]hen constructed with local environmental sensitivity, they are the largest and cleanest energy source on the planet,” said Eric Sharpe in Energy Ink Magazine (July 10, 2014).
Is hydropower clean? Yes—but generally only if it is generated on a small scale. Otherwise, not so much.