How Environmentally Friendly Is Hydropower?

How environmentally friendly is hydropower?
Hydropower is a renewable energy source that utilizes the natural water cycle to generate electricity. It is one of the oldest and widely used forms of renewable energy. Hydropower plants capture the energy of flowing water by storing it in a reservoir and then releasing it through turbines to generate electricity. The kinetic energy of the moving water rotates the turbines, which then spin magnets within coiled copper wire inside electric generators to produce electricity.

Globally, hydropower supplies over 16% of electricity production and over 70% of all renewable energy. In 2020, hydropower generated around 4,418 TWh of electricity worldwide. The top producing countries are China, Brazil, Canada, the United States, and Russia. Due to advantages like low operating costs and greenhouse gas emissions compared to fossil fuels, hydropower capacity and generation continues to expand globally. The International Energy Agency (IEA) projects hydropower to have average annual growth around 3.5% through 2030.


Hydropower’s Environmental Benefits

Hydropower is considered a renewable and sustainable energy source because it relies on the water cycle and gravity to generate electricity. Once a hydroelectric dam and power plant is built, it can produce electricity for decades with minimal impact on the environment (1).

Compared to fossil fuel power plants, hydropower emits very low amounts of greenhouse gases and air pollutants. According to the U.S. Department of Energy, hydropower produces fewer emissions than any other electricity generation source (2). This makes it one of the cleanest energy sources available today.

Hydropower facilities with dams and reservoirs provide energy storage capabilities. The reservoir acts as a battery, storing energy in the form of water that can be used to generate electricity on demand (3). This makes hydropower more reliable than some other renewable sources like wind and solar power.

Many hydropower projects are designed and operated with environmental concerns in mind, including installing fish ladders and minimizing impacts on wildlife. However, there are still significant environmental tradeoffs with large hydroelectric dams, which will be discussed in the next section.





Hydropower’s Environmental Concerns

While hydropower provides clean energy, some environmental concerns arise from building dams and reservoirs. Dams impact fish migration and ecosystems by blocking access to spawning grounds and disrupting river connectivity. This can harm populations of migratory fish like salmon (UCSUSA). Reservoirs themselves change natural water flow and transform river habitats into stagnant artificial lakes.

Reservoirs can also contribute to greenhouse gas emissions from decomposing organic matter (EIA). Some estimates calculate the lifecycle emissions from reservoirs as comparable to fossil fuel plants. Dams also hold back sediments and interrupt the natural transport downstream, resulting in increased erosion.

Fish and Wildlife Impacts

Hydropower dams can have significant impacts on fish and wildlife populations, especially migratory fish species like salmon and steelhead trout. One major impact is blocking fish migration routes and access to spawning grounds. To help mitigate this, many hydropower facilities now incorporate fish ladders and other passage structures. These allow fish to safely bypass the dam and continue migrating upstream and downstream.

Another mitigation strategy is redesigning turbine systems to be more fish-friendly with advanced turbine designs and screening systems. This helps prevent injury and mortality from fish passing through the turbines.

Managing reservoir water levels and maintaining sufficient downstream flows is also important. This provides adequate habitat for fish during spawning and migration seasons. Many hydropower facilities now have to comply with regulations like Federal Energy Regulatory Commission (FERC) license conditions to release certain minimum flows.

There are also various habitat protection regulations in place, such as designating critical habitat and management zones. These help minimize the impacts of hydropower dams on sensitive aquatic ecosystems. Overall, while hydropower can negatively affect fish and wildlife, new designs and regulations have helped make progress in mitigating many of these impacts.

Greenhouse Gas Emissions

Hydropower reservoirs emit greenhouse gases like carbon dioxide and methane, but overall lifecycle emissions are generally lower than fossil fuel power plants. According to the IPCC, hydropower’s median greenhouse gas emission intensity is 24 gCO2-eq/kWh, compared to 820 for coal and 490 for natural gas [1]. Reservoirs emit methane from the decay of vegetation and carbon dioxide from decomposing organic matter.

However, methane has a global warming potential 25 times greater than CO2, so methane emissions are a significant concern. A 2012 study by the U.S. Nuclear Regulatory Commission reviewed reservoirs globally and found a wide range of emission rates depending on the climate and characteristics of the reservoir. Proper site selection and management strategies can help minimize greenhouse gas emissions from hydropower reservoirs [2].

Overall, analyses show hydropower has much lower lifecycle greenhouse gas emissions compared to fossil fuel alternatives. According to the U.S. Department of Energy, the lifetime emissions of hydroelectricity are about one-tenth the emissions of electricity generated from fossil fuels [3].

Sediment and Water Flow

The construction of dams and operation of hydropower plants can significantly impact the natural sediment transport and water flow rates downstream, leading to issues like scouring, erosion, and ecological damage. When reservoirs trap sediment that previously traveled downstream, it can cause a lack of sediment in the water flow below the dam, resulting in channel degradation through increased scouring and erosion of the riverbed and banks (Source). This can undermine bridges, pipelines, and other infrastructure. It can also disrupt river habitat and spawning areas relied on by fish, invertebrates, and other wildlife (Source).

Altering natural water flow rates below dams through hydropower operations like hydropeaking can cause stranding of fish when flows quickly increase or decrease. Unnatural flow regimes also impact riparian vegetation, wetlands, and habitat connectivity for wildlife (Source). However, solutions exist like sedimental flushing, environmental flow requirements, and retrofitting dams and powerplants to restore more natural sediment and flow dynamics.

Social and Cultural Impacts

The flooding caused by hydropower dams can drastically alter land use and displace indigenous populations. According to the UN, two categories of cultural assets may be lost in dam construction: underground remains of significant historical importance and buildings or places of cultural, religious, or historical value.

For example, a study by Michigan State University found that dam construction impacted local communities by causing population loss and less economic development in the surrounding areas (Source).

Cultural sites of significance to indigenous communities are also often lost when land is flooded for dam reservoirs. As noted by, “Communities of people living alongside rivers stand to lose much more than land, food and income when dams interrupt natural river flows,” including cultural and sacred sites (Source).

Dam Removal and Decommission

Dam removal is increasingly being recognized as a viable option to restore rivers and ecosystems damaged by outdated dams. According to a case study by Headwaters Economics, over 1,300 U.S. dams have been removed in recent decades, with benefits including restored fish migration routes, improved water quality, and economic revitalization around the former reservoir area. However, removing a longstanding dam does not come without challenges.

A key concern with dam removal is the large amount of built-up sediment trapped behind the dam, which can be released downstream and negatively impact water quality and fish habitat if not properly managed. Strategies exist to mitigate sediment release, such as slowly drawing down reservoir levels over multiple years or seasons, dredging built-up sediment, and phasing removal during natural high-flow events (Dam Removal EU). Careful sediment testing and modeling is critical.

Despite short-term impacts, dam removal and river restoration efforts often lead to long-term habitat improvements. An example is the removal of two century-old dams on Washington state’s Elwha River, which opened up over 70 miles of salmon habitat according to a Riverwatch case study. Salmon populations rebounded strongly within just a few years following removal. With mindful planning and mitigation, dam removal can successfully restore rivers and ecosystems.

Future Outlook

Hydropower is expected to play a major role in the growth of renewable energy globally. According to the International Energy Agency, hydropower currently generates more electricity than all other renewable technologies combined, and is projected to remain the world’s largest source of renewable electricity in the coming decades.

In the United States, the Department of Energy’s Hydropower Vision report forecasts that hydropower capacity could grow from 101 GW in 2015 to nearly 150 GW by 2050. This growth will be driven by upgrades and optimization at existing plants, adding power generation to non-powered dams, and new stream-reach development. New technologies and operational practices will enable greater grid integration and flexibility.

Globally, an estimated 1,246 GW of new hydropower capacity is either under construction or planned, mostly in developing countries. The International Hydropower Association projects the vast majority of growth occurring in Asia, Africa, and Latin America. However, environmental and social concerns remain a major challenge. Sustainable development and management will be needed to minimize ecological impacts.

Overall, hydropower is poised to meet a substantial portion of the world’s growing electricity demand. But realizing its full potential will require balancing renewable energy goals with environmental stewardship and social needs.



In summary, hydropower can be a sustainable source of renewable energy compared to fossil fuel alternatives, but proper siting, management and regulations are crucial. While hydropower dams provide carbon-free electricity and help meet renewable energy goals, they still have significant environmental and social impacts that must be carefully weighed. Issues like disrupted fish migration, altered water flows, methane emissions from reservoirs, and displacement of communities should factor into whether and how new dams are built. With thoughtful planning and mitigation strategies, some impacts can be minimized. Forward-looking management approaches that balance economic, social and ecological considerations are key. As countries work to transition their energy supplies, hydropower will likely play an important role, but its expansion should be guided by comprehensive assessments and policies that protect people, wildlife and ecosystems.

Ultimately, determining hydropower’s sustainability requires examining the tradeoffs for each unique project and location. With care and oversight, hydropower can be harnessed as one of many diverse energy solutions to build a cleaner electric grid. But mismanagement of dams and failure to account for environmental impacts could make hydropower’s ecological footprint too costly.

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