Dams

Learn about how a river can be managed…

A dam is a barrier that stops or diverts the flow of water along a river. Humans have built dams for thousands of years.

Dams are built for many purposes. Some dams prevent flooding or allow people to develop or “reclaim” land previously submerged by a river. Other dams are used to change a river’s course for the benefit of development or agriculture. Still others provide water supplies for nearby rural or urban areas. Many dams are used to provide electricity to local communities.

 

Hydroelectric Power

In 1882, the world’s first hydroelectric power plant was built on the Fox River in the U.S. city of Appleton, Wisconsin. Since then, thousands of hydroelectric plants have been built on rivers all over the world. These plants harness the energy of flowing water to produce electricity. About 7 percent of all power in the United States, and 19 percent of power in the world, comes from hydroelectric plants. China is the world’s largest producer of hydroelectric power.

Hydroelectric power is renewable because water is constantly replenished through precipitation. Because hydroelectric plants do not burn fossil fuels, they do not emit pollution or greenhouse gases. However, hydroelectric power does have some negative effects on the environment.

 

Dams and hydroelectric plants change the flow and temperature of rivers. These changes to the ecosystem can harm fish and other wildlife that live in or near the river. And although hydroelectric plants do not release greenhouse gases, rotting vegetation trapped in the dams’ reservoirs can produce them. Decaying plant material emits carbon dioxide, a major greenhouse gas, into the atmosphere.

 

Impacts

Building a dam can have significant effects on ecology, wildlife, infrastructure, and people. The impacts vary depending on the size and type of the dam, as well as the specific environmental and social context. For example, more than 1.3 million people had to move from their homes to make way for China’s Three Gorges Dam and its reservoir. Human rights organisations claim that many of these people did not receive the compensation they were promised in return for being displaced.

Impacts include:

Ecology:

    • Altered Water Flow: Dams can disrupt natural river flow patterns, leading to changes in sediment transport, nutrient cycling, and temperature regimes. This can affect the habitat of aquatic organisms.

    • Habitat Destruction: The creation of reservoirs behind dams often results in the submergence of land, including forests, wetlands, and other ecosystems. This can lead to the loss of biodiversity and displacement of plant and animal species.

Wildlife:

    • Habitat Loss: The inundation caused by reservoir creation can displace or eliminate wildlife habitats, leading to population declines or local extinctions.
    • Fish Migration: Dams can obstruct the natural migration of fish, disrupting their life cycles. 

Infrastructure:

      • Reservoir-Induced Seismicity (RIS): The weight of water in a reservoir can induce seismic activity, which may pose risks to infrastructure, especially in seismically active regions.
      • Sedimentation: Dams can trap sediments, leading to a reduction in downstream sediment supply. This may affect riverbed stability, downstream erosion, and the functioning of estuaries.

People:

      • Relocation: The creation of reservoirs may require the displacement of communities living in the affected areas. This can have significant social and economic implications for the affected populations.
      • Water Management: Dams can alter water availability downstream, affecting agriculture, water supply for communities, and industries.

Water Quality:

    • Stagnation and Eutrophication: Reservoirs can promote the stagnation of water, leading to increased nutrient levels and the potential for eutrophication, negatively impacting water quality and water life.

Sustainability

It’s important to note that modern dam projects often include environmental impact assessments and efforts to mitigate negative effects. Sustainable dam management practices, such as fish-friendly turbines, sediment management, and community engagement, can help minimise the environmental and social impacts of dams.