Understand Hydropower
Happy Earth Month! This edition of Stanford University’s Understand Energy Learning Hub Energy Spotlight covers hydropower, the world’s largest source of renewable electricity generation. If you like what you see, please share widely and encourage others to subscribe. You can also check out all of our past issues!
What you need to know
Significance: Hydropower, also known as hydroelectricity, accounted for 14% of global electricity generation in 2024, more than any other renewable resource. It’s the third-largest source of electricity after coal and natural gas. Hydropower systems can range in size from huge (utility scale) to quite small (microhydro). Seven of the ten largest power plants in the world by capacity are hydropower facilities. Hydropower is a highly efficient resource with relatively low greenhouse gas (GHG) emissions on average and no air pollution.
Hydropower facilities tend to have high up-front costs (e.g., dam construction). However, once a hydropower facility is built, it can operate for 50–100+ years with relatively low variable costs (maintenance and operation). Plus, the fuel is free! Hydropower is a flexible resource that can support the integration of intermittent renewable energy sources like solar and wind.
Where is hydropower used? China is the largest producer of hydropower, accounting for ~30% of global hydroelectricity generation. Hydropower currently provides 13% of China’s electricity. China is rapidly expanding its hydropower capacity (and other clean energy sources) to meet energy demand and attempt to reduce its reliance on coal.
Hydropower generation by region for top seven countries and rest of world
(1965–2024)
Hydropower is a critical resource in many low- and middle-income countries. For example, Bhutan, Paraguay, Albania, and Ethiopia generate more than 95% of their electricity from hydropower. Hydropower also plays an important role in Central and South America, where about half of electricity generation comes from hydro. In the United States, hydropower supplies ~6% of electricity generation. Washington state, home of the Grand Coulee Dam and many other hydropower facilities, generates more hydroelectricity than any other U.S. state.
What is hydropower? Like many renewable resources, hydropower is powered by the sun. In the hydrologic (water) cycle, the sun elevates water through evaporation. Winds transport the water, which then condenses to form clouds and precipitates as rain or snow at high elevation. The movement of water to higher elevation gives it potential energy, which is converted to kinetic energy when the water flows downhill. Some of that kinetic energy can be used to create electricity.
We consider hydropower a semi-renewable resource because it must be carefully managed to ensure we are not using the water faster than nature can replenish it.
The power of flowing water: Flowing water is an energy carrier. The power of flowing water is determined by:
- Flow rate (Q): the volume of water moving per unit of time
- Head (h): the energy per unit weight of flowing water (dependent on elevation change, pressure, and velocity)
- Gamma (γ): the unit weight of water
P = γQh
The greater the flow rate or head, the greater the available power, making large elevation changes and high flow rates attractive for hydropower development.
How does hydropower work? In hydropower plants, water flows through a penstock (pipe), then rotates turbine blades, spinning a shaft in a generator to produce electricity.
The two major configurations for generating electricity are storage hydroelectric systems and run-of-river systems.
1. Storage hydroelectric systems (dams) store water in reservoirs (artificial lakes). Storage provides flexibility for when electricity generation occurs but can be disruptive to the local environment. For example, the creation of a reservoir changes the river’s natural flow and results in the flooding of large areas of land, impacting ecosystems and communities.
Storage hydroelectric system
Dams also provide elevation change, creating potential energy that can be captured for electricity generation. For example, the 3.6 GW Jinping-I Dam in China is the world's tallest hydroelectric dam with a maximum elevation change of 305 meters (1,001 feet). The Three Gorges Dam, also in China, has the highest maximum water flow capacity for electricity generation (>30,000 m3/second) and at 22.5 GW is the world’s largest hydropower plant.
Three Gorges Dam
The Oroville Dam in California is the tallest hydroelectric dam in the U.S., with a maximum elevation change of 235 meters (770 feet). The U.S.'s largest hydropower plant is the Grand Coulee Dam (6.8 GW), which has a maximum water flow capacity for electricity generation of ~8,000 m3/second.
The operating head and water flow of hydroelectric dams are usually lower than the maximum due to water availability and because other needs often take priority over electricity generation. Many dams are built for reasons like flood control, irrigation, drinking water supply, recreation, and navigation facilitation. Electricity generation is sometimes included to generate a revenue stream and enhance the financial viability of a project. In the U.S., for example, fewer than 3% of the more than 90,000 dams are hydroelectric facilities.
2. Run-of-river systems primarily use the river’s natural flow to generate electricity. Some store small amounts of water (pondage) and some don’t store water at all, generally resulting in less disruption to the natural river system. However, they can still block fish passage, alter water temperature, and flood vegetation. Run-of-river systems are less flexible than storage systems because their electricity output is more tied to the rises and falls in the natural river flow.
Run-of-river system (no pondage)
Run-of-river systems are typically smaller than storage systems but can vary widely in size, from small community-level projects to large industrial installations. For example, the small Tazimina Falls system in Alaska has a capacity of 0.8 MW and supplies power to the villages of Iliamna, Newhalen, and Nondalton.
Tazimina Falls run-of-river (no pondage) facility
The larger Ice Harbor run-of-river system on the Snake River in Washington includes pondage and has a capacity of 603 MW. It’s also used for navigation. Ice Harbor Dam navigation lock is the fifth of eight locks encountered in the Columbia-Snake Inland Waterway, a 465-mile river highway that allows barge transport of commodities between the Pacific Ocean and Lewiston, Idaho.
Ice Harbor run-of-river (with pondage) facility
Pumped storage hydropower facilities are designed to store electricity, not generate electricity. Operators use electricity to pump water up to a higher elevation reservoir to store potential energy for use at a later time, and then release the water to generate electricity when needed. Closed loop pumped storage systems have two stand-alone reservoirs—an upper reservoir and a lower reservoir. In an open loop pumped storage system, the upper or lower reservoir is continuously connected to a naturally flowing water source (e.g., river or lake).
Pumped storage hydropower system
Pumped storage systems help maintain the balance between electricity supply and demand, which varies hourly, seasonally, and by location. Depending on market conditions, pumped storage systems can also participate in energy arbitrage—storing energy when electricity prices are low and selling when electricity prices are high (e.g., using cheap excess solar energy to pump water up during the day, then generating electricity after the sun sets and electricity prices go up).
Pumped storage systems have a round-trip efficiency of about 80%, which is competitive with battery storage. Some pumped storage facilities are optimized for short-duration storage (<4 hours) while others can be used for long-duration storage (up to 100+ hours). Pumped storage represents the largest share of global energy storage capacity today (54%) but is only growing modestly, primarily driven by China. In contrast, electrochemical energy storage (mainly lithium-ion batteries) is rapidly expanding.
Why hydropower matters for the grid: Hydropower plants are highly flexible and uniquely suited to provide many ancillary services that help keep the electricity grid stable and reliable. For example, hydropower is one of the only resources with black-start capabilities, meaning that it can restart the grid after a blackout without needing electricity. All it takes is a person opening a valve to start the flow of water and generate electricity! Another critical service hydropower can provide is ramping, the ability to ramp electricity generation up and down quickly to meet demand, which can help with solar and wind integration on the grid.
Ancillary grid services provided by hydropower
Social and environmental impacts
Electricity generation: Two significant impacts of hydropower facilities used for generating electricity are downstream fish passage and altered river flow patterns.
- Downstream fish passage through or around the powerhouse can injure or kill fish. Mitigation measures include fish screens, "fish-friendly" turbines, and bypass channels.
Potential impacts on fish passing through a turbine
- Downstream river flow patterns are often significantly altered to meet changing electricity demand with hydropower. This can cause rapid changes to water levels, fluctuating water temperatures, and unnatural sediment and nutrient fluxes that disrupt ecosystems.
Dams and reservoirs: Large dams, whether or not they are used to generate electricity, have significant social and environmental impacts. Dam structures change river ecosystems to lake ecosystems, causing changes in water quality and temperature and potential creation of vector (e.g., mosquitos) habitat and waterborne diseases that impact human health. Dams also fragment river habitats, block fish migration, and can submerge vast areas of natural habitat, farmland, cultural sites, and communities. For example, China’s Three Gorges Dam displaced about 1.3 million people, submerged over 1,300 cultural and archeological sites, and resulted in the functional extinction of the Yangtze River dolphin. “Careful, thoughtful design and operation can reduce impacts, but they can’t be eliminated. In other words, impacts are fundamental,” according to Stanford University Professor Emeritus David Freyberg.
Reservoirs created by dams can submerge vegetation, releasing methane (a potent GHG) as vegetation decomposes. Dam construction can also be a significant source of GHG emissions because concrete is an extremely carbon-intensive material. However, the median lifecycle GHG emissions per kilowatt hour (kWh) of electricity generated by hydropower are low, especially compared to biomass and fossil fuels.
Life-cycle CO₂e/kWh by energy resources
Current and future trends
Hydropower is the largest source of renewable electricity generation, accounting for ~45% of global renewable electricity production. However, after decades of being the number one renewable energy source by capacity, hydropower was overtaken by solar photovoltaics (PV) in 2023.
Hydropower growth: Global hydropower capacity is set to increase by 17%, or 230 GW, between 2021 and 2030, according to the IEA, which is a slowdown relative to the previous decade. Storage hydropower systems (dams) are expected to provide half of the new additions and pumped storage about 30%. Growth is primarily driven by projects in Asia, which account for 70% of planned capacity additions. In the U.S. and Europe, most of the best sites for large-scale hydropower have already been developed, and projected growth is small.
Climate change: Hydropower systems depend on water availability. Hydropower facilities have been built to take advantage of past weather patterns, but those weather patterns are evolving both in amount and location of water due to climate change. Therefore, existing hydropower infrastructure may no longer be in the right locations. Overuse of water, more frequent and severe droughts, and shorter winters also impact hydropower availability. Global hydropower electricity generation decreased in 2023 due to continuing droughts reaching levels last observed in 2019.
In the news
News: China’s 60 GW Medog Hydropower Station is poised to become the world's largest hydropower facility, far surpassing China’s 22.5 GW Three Gorges Dam. The estimated cost for the project is ~$167 billion USD (about 4.5 times the cost of the Three Gorges Dam). Construction on the project began in 2025, with commercial operations targeted for 2033.
The Medog project is being built on the Yarlung Zangbo River in Tibet along the Indian border and aims to harness the "Great Bend" of the Yarlung Tsangpo Grand Canyon, where the river undergoes a 2,000-meter elevation drop over 50 kilometers. Learn more in this 14-minute video on the Medog project.
Context: China views the Medog project as important for its economic strategy and decarbonization goals. When completed, it will have the capacity of ~60 nuclear power reactors. However, due to its location, the project is not without controversy. The Yarlung Tsangpo River is the world’s highest (reaching 5,000 meters above sea level) and is considered sacred to Tibetans. Construction of the dam is likely to lead to flooding and the displacement of Tibetan people. Experts warn that the dam will be in a seismically active zone and prone to landslides. The geopolitics of damming this river are also complicated. Downstream nations including India, Bangladesh, and Bhutan have concerns about water shortages and flooding.
Fun Fact
Salmon cannons can launch fish safely over dams with nearly 100% success rates!
Salmon cannons are fish-passage systems designed to transport salmon and other migratory species safely over dams and other barriers in rivers.
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Guest contributor: Dr. David Freyberg, Professor Emeritus, Civil and Environmental Engineering, Stanford University
Understand Energy team contributors: Dr. Diana Gragg, Bria Schraeder, Sharon Poore, and Shirley Chang
The data in this issue are current as of April 2026. For the most current data, visit our Hydropower Fast Facts.
