Meghan Hodges
Hydroelectric energy is often regarded as a clean energy source, but it can have some negative impacts on the environment. The construction of dams and reservoirs can lead to the destruction of forests, wildlife habitats, agricultural land, and scenic areas. In addition, the operation of hydroelectric generators can affect the natural water temperatures, water chemistry, river flow characteristics, and silt loads, which in turn can harm native plants and animals. Another significant concern is the release of greenhouse gases, particularly methane, during the generation of hydroelectric power. While hydropower does not directly emit air pollutants, the decomposition of organic matter in reservoirs and the turbulence caused by turbines can result in the emission of methane and carbon dioxide. These gases contribute to global warming and climate change. Therefore, it is crucial to carefully consider the potential environmental consequences and take appropriate measures to minimize the negative impacts of hydroelectric energy projects.
| Characteristics | Values |
|---|---|
| Greenhouse gas emissions | Carbon dioxide and methane are released during the creation of a reservoir when vegetation decomposes underwater. Methane is a more potent greenhouse gas than carbon dioxide over a 20-year lifespan. |
| Global warming emissions | Produced during the installation and dismantling of hydroelectric power plants, as well as during their operation. |
| Impact on aquatic ecosystems | Turbine blades can injure and kill fish and other organisms. |
| Impact on downstream plants and animals | Reservoir water is typically colder and has lower dissolved oxygen than normal river water, which can negatively impact downstream plants and animals. |
| Impact on wildlife | Flooding land for reservoirs can destroy forests, wildlife habitats, agricultural land, and scenic lands. |
| Impact on river characteristics | Dams and reservoirs can change natural water temperatures, water chemistry, river flow characteristics, and silt loads, affecting the ecology and physical characteristics of the river. |
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What You'll Learn
Methane emissions
Although hydropower is a widely available, reliable, and renewable resource, it is associated with methane emissions. Methane is released from reservoirs, which are created by damming rivers to generate hydroelectricity. The methane is produced by the decomposition of organic matter, such as vegetation, that has been submerged under water. This process is similar to the production of methane in natural lakes and wetlands.
When a river is dammed, the resulting reservoir changes the way carbon is emitted and stored in the river system. While reservoirs can sequester carbon, they also release embedded carbon in the form of methane emissions. The exact amount of methane emitted depends on various factors, including water temperature, sedimentation rate, organic matter inputs, and the shape, size, and depth of the reservoir.
The methane emissions from reservoirs occur through several mechanisms. One significant way is through degassing when water passes through the turbines of a hydroelectric dam. As the water churns through the turbines, the pressure changes, causing methane to be released from the water in the form of bubbles. This process is similar to the effervescent "degassing" of a carbonated drink when it is opened or shaken.
In addition to degassing, methane can also be released from reservoirs through diffusion, ebullition (bubbles), transmission via vegetation, and when water passes through a pump house. These processes contribute to the overall methane emissions associated with hydroelectricity.
The methane emissions from hydroelectric dams have been a hidden or underappreciated source of pollution. Recent studies have estimated that the world's hydroelectric dams generate a significant amount of methane, equivalent to almost one billion tonnes of carbon dioxide. This has led to growing concerns about the greenhouse gas (GHG) emissions associated with hydropower reservoirs and has motivated further research into the carbon footprint of hydropower.
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Carbon dioxide emissions
Hydropower is a low-carbon source of renewable energy and a reliable alternative to electricity generation by fossil fuels. According to the IPCC, hydropower has a median greenhouse gas emission intensity of 24 gCO₂-eq/kWh, which is significantly lower than the median figure for gas at 490 gCO₂-eq/kWh.
However, hydropower is not without its carbon dioxide emissions. The creation of a reservoir for hydropower facilities changes the way carbon is emitted and stored in river systems. When a reservoir is created and filled with water, vegetation and other organic matter become submerged and decompose, releasing carbon dioxide and methane. These gases can reach the atmosphere in multiple ways, including diffusion, ebullition (bubbles), transmission via vegetation, and degassing when water passes through a pump house or turbine.
The carbon processing in reservoirs can work both ways, emitting and absorbing emissions. In some cases, reservoirs can act as carbon sinks, absorbing more carbon through photosynthesis by aquatic organisms than they emit through decomposition. The balance between carbon storage and emissions is influenced by factors such as water temperature, sedimentation rate, organic matter inputs, and the shape, size, and depth of the reservoir.
While hydropower does produce carbon dioxide emissions, it has helped to avoid more than 100 billion tonnes of carbon dioxide in the past 50 years by replacing fossil fuels for electricity generation. If hydropower was replaced with burning coal, more than 4 billion metric tonnes of additional greenhouse gases would be emitted annually, resulting in a 10% increase in global emissions from fossil fuels and industry.
To summarize, while hydropower does produce carbon dioxide emissions, it is still a low-carbon energy source compared to fossil fuels and has played a significant role in reducing global carbon dioxide emissions.
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Impact on aquatic ecosystems
Hydroelectric energy has a range of impacts on aquatic ecosystems. Firstly, the creation of reservoirs can lead to the flooding of forests, causing the loss of carbon sinks and the release of greenhouse gases due to the decomposition of organic material. This can result in higher levels of carbon dioxide and methane in the water, which can have negative consequences for aquatic life. The flooding of forests can also lead to the irretrievable loss of wildlife habitats and important natural areas.
Secondly, hydroelectric dams can alter the temperature and chemical composition of water, affecting the biological integrity of river ecosystems. Thermal pollution occurs when the top layers of water in a reservoir closest to the sun absorb heat, while the lower layers remain cold. When water is released from the reservoir, it can alter the temperature of the water downstream, which can directly impact marine populations and alter the metabolic rates, reproduction, and growth of animals.
Additionally, the water in reservoirs tends to be more stagnant than normal river water, leading to higher levels of sediments and nutrients. This can promote the growth of algae and aquatic weeds, which can crowd out other plant and animal life. The water in reservoirs also typically has lower levels of dissolved oxygen, which can negatively impact downstream plant and animal life when released. This reduction in oxygen can have significant effects on the ecosystem, even if the decrease is minimal.
Hydroelectric dams can also obstruct fish migration and passage, impacting fish populations and disrupting the natural flow of rivers. The operation of hydroelectric generators can divert water around sections of rivers, leaving them dry or subject to alternating drought and flood-like conditions. These changes in water flow can have negative effects on native plants and animals, as well as on human activities such as agriculture and recreation.
Overall, the impacts of hydroelectric energy on aquatic ecosystems are complex and varied. While it is important to recognize the benefits of hydroelectric power as a renewable energy source, it is also crucial to address and mitigate its potential negative consequences on the environment.
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![Draft environmental impact statment [sic] of the White River Dam Project](https://m.media-amazon.com/images/I/71vhNfRTvhL._AC_UY218_.jpg)
Reservoir flooding
The creation of reservoirs for hydroelectric energy projects can result in the flooding of large areas of land, which can have significant environmental and social impacts. The size of the reservoir depends on the size of the hydroelectric generators and the topography of the land. For example, the Balbina hydroelectric plant in Brazil, built on a flat area, flooded 2,360 square kilometres of land, equivalent to the size of Delaware, and only provides 250 MW of power-generating capacity. In contrast, a small 10 MW run-of-the-river plant in a hilly location may only use 2.5 acres of land.
The flooding of land for a hydroelectric reservoir can have extreme environmental impacts, including the destruction of forests, wildlife habitats, agricultural land, and scenic areas. The flooding of land can also lead to the displacement of local communities and can impact downstream communities by altering water temperatures, water chemistry, river flow characteristics, and silt loads. These changes can negatively affect native plants and animals in and around the river, and can also increase the risk of flooding downstream.
Reservoirs themselves can also have environmental impacts. Reservoir water is usually more stagnant than normal river water, leading to higher levels of sediments and nutrients, which can promote the growth of algae and aquatic weeds. These weeds can crowd out other aquatic plant and animal life and must be manually controlled. Additionally, the creation of a reservoir can change the way carbon is emitted and stored in river systems, releasing embedded carbon in the form of methane (CH4) and carbon dioxide (CO2) emissions. These gases are released through the decomposition of submerged organic matter, such as vegetation, and can contribute to atmospheric warming and climate change.
However, reservoirs can also provide benefits in terms of flood control. Regulated watercourses and reservoirs can be effective protection measures against flood damage. Hydro power plants can help to control and prevent floods by allowing for the maintenance and monitoring of a single body of water. Even small retention volumes can reduce the likelihood of flooding. For example, the construction of a hydro power plant with a reservoir in Austria reduced the probability of a major flood occurring from once every 10 years to once every 75 years.
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Manufacturing emissions
While hydropower is often considered a "'clean' energy source, it can still have a significant environmental impact. Hydroelectric companies often emphasise their "clean" manufacture of electricity, neglecting the long-term environmental hazards. The manufacturing of concrete and steel in hydropower dams requires equipment that may produce emissions. If fossil fuels are the energy source for making these materials, then the emissions from the equipment could be associated with the electricity that hydropower facilities generate. However, given the long operating lifetime of a hydropower plant (50–100 years), these emissions are offset by the emissions-free hydroelectricity generated.
The creation of a reservoir can also have a significant environmental impact. Flooding land for a hydroelectric reservoir can destroy forests, wildlife habitats, agricultural land, and scenic lands. In some cases, entire communities have had to be relocated to make way for reservoirs. The damming of a river can also obstruct fish migration and change natural water temperatures, water chemistry, river flow characteristics, and silt loads. These changes can negatively affect native plants and animals in and around the river.
The temperature of reservoir water can also influence the production of methane emissions, with higher temperatures resulting in increased emissions. When a reservoir is created and filled with water, submerged organic matter such as vegetation decomposes, releasing carbon dioxide (CO2) and methane. These gases can reach the atmosphere through diffusion, ebullition (bubbles), transmission via vegetation, and degassing when water passes through a pump house or turbine. Methane is a more potent greenhouse gas than CO2, with over 80 times the warming power in the first 20 years after its release.
Overall, while hydropower does not produce pollution during its operation, the construction and installation of hydroelectric power plants can result in significant emissions and environmental impacts. It is important to carefully consider the design, construction, and geographic placement of new hydropower facilities to ensure they deliver climate benefits and minimise potential negative consequences.
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Frequently asked questions
Hydroelectric energy is often regarded as a clean energy source as it does not directly emit air pollutants. However, it does produce some pollution, including greenhouse gases such as carbon dioxide and methane. These gases are released due to the decomposition of organic matter in the water, and their emission is influenced by factors such as water temperature and the size and shape of the reservoir. Additionally, the construction and operation of hydroelectric generators can impact the environment, including native plants and animals.
Methane is released from water bodies, including hydroelectric reservoirs, when methane gas escapes from the water as bubbles. This can occur due to the turbulence created by the turbines in hydroelectric dams. Methane is a potent greenhouse gas that contributes significantly to global warming.
Carbon dioxide is produced when organic matter, such as vegetation, decomposes under water in reservoirs. The creation of a reservoir for hydropower can change the way carbon is emitted and stored in river systems, releasing carbon dioxide into the atmosphere.
Hydroelectric dams can have significant environmental impacts, including the disruption of aquatic ecosystems and the obstruction of fish migration. The flooding of land for reservoirs can also destroy forests, wildlife habitats, agricultural land, and scenic lands. Additionally, changes in water temperatures, water chemistry, and river flow characteristics can affect native plants and animals.
Hydroelectricity has a smaller carbon footprint per unit of energy than electricity generated from fossil fuels. However, it is important to consider the full lifecycle of hydroelectric power plants, including construction, operation, and dismantling, which can produce emissions. In certain cases, new hydropower facilities may generate electricity that leads to more warming than fossil fuels in the near term due to the release of methane and carbon dioxide from reservoirs.
