Disadvantages of hydroelectricity Large dams are expensive and slow to build. It can take over ten years between the decision to build one and the time the electricity flows. There are few sites for building large dams. In North America, most sites suitable for large dams are already used. The sites that are left are mostly in the far north.
Dams disrupt the natural flow of water and disrupt ecosystems upstream and downstream.
Creatures that live in the water and animals that live on land can be seriously harmed by changes in the flow of the river. Reservoirs flood people's homes, forests and farmland. The people who live in the settlements and who use the forests and land may not want a reservoir there, even if they want electricity.