Hydroelectricity comes with a number of environmental costs. We discussed several of these impacts in previous chapters. As we saw in Chapter 6, dams harm aquatic biodiversity by trapping nutrient-rich sediments in reservoirs; in Chapter 8, we reviewed how dams restrict migrations of fish such as salmon. Dams also present social and environmental challenges for people.

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Reservoirs built for hydroelectric power can negatively impact water quality. Building dams can turn crystalline rapids into murky, polluted reservoirs. The decomposition of organic matter in reservoirs can deplete oxygen, creating anoxic conditions in deeper waters. When these anoxic waters are released downstream, it restricts the types of organisms that can inhabit them. Large reservoirs with anoxic layers also release climate-altering greenhouse gases (see Chapter 14, page 431), particularly in tropical regions such as Brazil. In addition, the decomposition of flooded forests releases mercury, which enters the food chain and is eventually ingested by fish (see Chapter 11, page 339). In places where fish have high levels of mercury, people eating them can suffer brain and liver damage.

Hydroelectric installations are constructed to generate and deliver dependable electrical energy to people, often at distant locations, which is a clear social and economic benefit. However, these benefits should be weighed against the impact that such construction has on local populations. For example, building the Three Gorges Dam project on the Yangtze River in China (see Figure 10.13) flooded 13 cities, 140 towns, and 1,350 villages. The Chinese government estimates that 1.3 million people were relocated as a result and that an additional half million people may yet be relocated. Furthermore, the dam has harmed migratory fish populations and rare species that rely on wetlands, such as the endangered Siberian crane.

Another example of how dam-building impacts local populations is found in Brazil. About 2,000 indigenous people in Brazil consider the Xingu River a lifeline, providing their domestic water supply and fishing grounds (Figure 10.34). But the Belo Monte dam project, which would be the second-largest hydroelectric dam in the world and which received a provisional approval in 2010, includes canals that will reroute most of the Xingu River away from its natural channel. It will also flood approximately one-fourth of the city of Altamira, Brazil, displacing about 20,000 people.

The Brazilian government did not consult the indigenous people about the impacts of the Belo Monte project—reportedly because they were told they would not be displaced by the flooding of the reservoir. Later, the human rights commission of the Organization of American States ruled that this lack of consultation was a violation of the international accords Brazil had agreed to.