Helena Joyce
Hydropower is often considered a source of renewable electricity because its fuel, water, is constantly replenished by nature. However, hydropower assets have a very long lifespan, and the emissions associated with construction can be spread over a much longer time compared to technologies with shorter lifespans. The creation of a reservoir can also lead to the obstruction of fish migration and the release of greenhouse gases due to the decomposition of flooded organic material. Additionally, the water quality degradation caused by hydroelectric facilities is a major concern as it affects a wide range of plant and animal life. While hydropower does not directly emit air pollutants, it is important to consider the potential environmental impacts and emissions associated with its use.
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What You'll Learn
Methane emissions
Hydropower is widely considered a "clean" energy source and a low-carbon alternative to electricity generation by fossil fuels. However, it is associated with greenhouse gas emissions, particularly methane.
When human-made reservoirs are constructed for hydropower facilities, they change the way carbon is emitted and stored in river systems. As water levels decline, the hydrostatic pressure on submerged soils also lowers, allowing methane bubbles to escape. These gases reach the atmosphere through diffusion, ebullition (bubbles), transmission via vegetation, and degassing when water passes through a pump house or turbine.
A study from Washington State University found that methane, which is at least 34 times more potent than carbon dioxide, makes up 80% of the emissions from water storage reservoirs created by dams. Another study by ETH Zurich, examining data from over 1,400 dams worldwide, concluded that the "carbon footprint of hydropower is far higher than previously assumed".
The Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report noted that hydropower has a median greenhouse gas (GHG) emission intensity of 24 gCO₂-eq/kWh, which is lower than most other energy sources. However, in rare cases, hydropower reservoirs have been documented to produce significantly higher emissions, with some facilities having carbon footprints equal to or greater than fossil fuels.
To address these emissions, several methods have been proposed:
- Installing aerating devices to increase dissolved oxygen and reduce CH4 release.
- Adding a secondary intake above the thermocline to circulate oxygenated water through turbines and reduce degassing.
- Managing water levels differently in reservoirs to potentially reduce emissions.
- Capturing methane from reservoirs and using it as a source of power, either by burning it or converting it to hydrogen.
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Carbon dioxide emissions
Hydropower is a widely available, reliable, and renewable resource. It is also one of the few types of renewable power that can provide the energy storage and rapid-response generation capabilities needed to stabilize the grid during fluctuations in energy demand. However, despite being considered a "clean" energy source, hydropower can still contribute to carbon dioxide emissions.
The Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report noted that hydropower has a median greenhouse gas (GHG) emission intensity of 24 gCO₂-eq/kWh. This is much lower than the median figure for gas, which is 490 gCO₂-eq/kWh. However, it is important to note that the IPCC cautioned that few studies had assessed the net emissions of freshwater reservoirs, accounting for pre-existing natural sources and unrelated human emissions.
The creation of reservoirs for hydropower facilities can change the way carbon is emitted and stored in river systems. When a reservoir is created and filled with water, submerged organic matter such as vegetation decomposes and breaks down, releasing carbon dioxide. The rate of soil erosion into a reservoir has been identified as a leading predictor of carbon dioxide emissions. The time water spends in the reservoir, the amount of carbon contained in the flooded soil, and the amount of shallow area in the reservoir all contribute to the GHG emissions profile of hydropower reservoirs.
In certain conditions, a reservoir will release greenhouse gases due to the decomposition of flooded organic material. Emissions from reservoirs tend to be highest in the first 10 to 20 years immediately following their creation, then decrease over time. In other cases, a reservoir may act as a carbon sink, absorbing more emissions than it emits. The depth and shape of a reservoir, the amount of sun reaching its floor, and wind speed all affect the different pathways by which CO₂ is created and released into the atmosphere.
While hydropower has helped to avoid more than 100 billion tonnes of carbon dioxide in the past 50 years, independent research suggests that replacing hydropower with coal to generate electricity would result in more than 4 billion metric tonnes of additional greenhouse gas emissions annually. However, it is important to note that the methods for analyzing GHG emissions from hydropower facilities need to be refined to ensure accurate and consistent estimations.
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Impact on aquatic life
Although hydropower is a low-carbon source of renewable energy, it does have some environmental costs. Hydropower plants can have a significant impact on aquatic life in several ways. Firstly, the creation of a reservoir can lead to an initial increase in greenhouse gas emissions from the decomposition of organic matter that was flooded during its formation. This can negatively affect water quality and contribute to climate change. The reservoirs themselves also tend to have more stagnant water than normal rivers, leading to higher levels of sediments and nutrients, which can cause an excess of algae and aquatic weeds. These weeds can then crowd out other plant and animal life in the river.
The presence of algae can negatively impact water quality in several ways. Firstly, it can reduce the oxygen levels in the water, creating hypoxic or even anoxic environments that are not suitable for aquatic life and can lead to fish deaths. Additionally, algae can negatively impact the taste, colour, and odour of water, making it unsuitable for human consumption. Some types of algae are even poisonous and can harm human health if ingested. Furthermore, increased algal concentrations can cause premature clogging of filters and traps, decreased biodiversity, and increased concentrations of harmful substances such as hydrogen sulfide, iron, manganese, and ammonia.
The operation of hydropower plants can also directly impact aquatic life. Turbine blades can injure or kill fish and other organisms, despite the use of mitigation methods such as fish ladders and intake screens. The release of water from reservoirs can affect downstream plant and animal life, as it is typically colder and has lower dissolved oxygen levels than normal river water. If not managed properly, water levels downstream can drop, harming aquatic ecosystems. Additionally, hydropower dams can obstruct fish migration and alter natural water temperatures, water chemistry, river flow characteristics, and silt loads, impacting the ecology and physical characteristics of the river.
The flooding of land for reservoirs can also have indirect effects on aquatic life by drowning terrestrial ecosystems and important natural areas, including forests and archaeological sites. This can lead to the loss of carbon sinks and the degradation of freshwater habitats, further impacting water quality and aquatic biodiversity. Overall, while hydropower has a lower environmental impact than fossil fuels, it is important to recognise that it is not without its costs, particularly when it comes to the health of rivers and the aquatic life that depends on them.
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Water quality degradation
Hydropower dams can also cause the flooding of natural areas, agricultural land, and archaeological sites. The flooding of land for reservoirs can lead to biodiversity loss. This includes the extinction and near-extinction of species, as well as the loss of carbon sinks, fisheries, and recreational opportunities. The construction and operation of dams and reservoirs can also result in the relocation of people, with the World Bank estimating in 2000 that between 40 and 80 million people had been directly displaced.
The creation of reservoirs can also lead to the release of greenhouse gases due to the decomposition of flooded organic material. While emissions tend to decrease over time, they can be significant in the first 10 to 20 years after a reservoir is impounded. The accumulation of thermal energy in the top layers of reservoirs closest to the sun can also lead to thermal pollution, altering the temperature of water downstream.
The operation of hydropower dams can also affect fish migration and wildlife displacement. The diversion of water around sections of rivers can leave them dry or subject to unnatural drought and flood-like conditions. These environmental impacts can have spillover effects on humans, with people downstream from large dams suffering from reduced food security, regular flooding, or negative impacts on their livelihoods.
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Reservoir flooding
A reservoir is an artificial lake where water is stored. Reservoir flooding occurs when excess rainfall causes the lake level to rise or floodwater spills downstream. Climate change, which is causing increasing rainfall, may increase the risk of reservoir flooding.
Reservoirs are usually formed by building a dam across a river. When a dam fails, a large volume of water is suddenly released from the reservoir, resulting in downstream land or properties being flooded. This can cause economic losses, environmental impacts, loss of cultural resources, and loss of life. The effects of flooding on health are extensive and significant, ranging from mortality and injuries resulting from trauma and drowning to infectious diseases and mental health problems.
The likelihood of a reservoir failing and causing flooding is low if it is properly maintained. However, in the unlikely event of a dam collapse, a large volume of water could be released, quickly flooding a large area and possibly causing significant property damage or loss of life. Reservoir flooding can also have long-term health effects, such as those due to displacement, destruction of homes, delayed recovery, and water shortages.
Hydropower generation is a low-carbon source of renewable energy and a reliable and cost-effective alternative to electricity generation by fossil fuels. However, the creation and operation of reservoirs can have environmental impacts. For example, a dam and reservoir can obstruct fish migration, change natural water temperatures, water chemistry, river flow characteristics, and silt loads. These changes can affect the ecology and physical characteristics of the river, with negative effects on native plants and animals.
The construction and operation of reservoirs can also emit greenhouse gases, particularly in the first 10 to 20 years after impoundment. This is due to the decomposition of flooded organic material, vegetation, and soil. However, in some conditions, a reservoir may act as a carbon sink, absorbing more emissions than it emits. The size of the reservoir and the nature of the land that was flooded also affect the amount of emissions produced.
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Frequently asked questions
Hydropower is often considered a source of clean, renewable electricity because it does not directly emit air pollutants. However, the construction and operation of dams, reservoirs, and hydroelectric generators can have significant environmental impacts.
Hydropower facilities can affect aquatic ecosystems, including fish migration and other organisms, which can be injured or killed by turbine blades. The creation of reservoirs can also lead to changes in water temperatures, water chemistry, river flow characteristics, and silt loads, impacting native plants and animals.
While hydropower has a lower median lifecycle greenhouse gas emission intensity compared to most other energy sources, the creation and operation of reservoirs can release carbon dioxide and methane, potent greenhouse gases. The impact varies depending on the size and location of the reservoir.
Water quality degradation is a major concern with hydropower development. The formation of reservoirs can lead to oxygen stratification, with lower oxygen concentrations at deeper levels, impacting aquatic life. Additionally, the stagnant water in reservoirs can promote algae growth, negatively affecting water taste, colour, odour, and suitability for human consumption.
Yes, renewable energy sources such as wind, solar, and geothermal power provide substantial benefits for the climate, health, and economy while typically having lower pollution impacts than hydropower. However, it is important to note that all energy sources, including renewables, produce some carbon emissions during their lifecycle.
