Hydropower turns the energy of moving water into useful mechanical energy or electricity. Humans have used waterwheels for centuries, but modern hydroelectric plants use turbines and generators to convert flowing or falling water into electrical power.
Hydropower is renewable because its energy source depends on water continually moving through Earth's hydrologic cycle. Rain and snow replenish rivers and reservoirs, while gravity moves water downhill.
Quick answer: A hydroelectric plant directs moving water through a turbine. The water spins the turbine, the turbine turns a generator, and the generator converts mechanical energy into electricity. The available power depends mainly on the flow rate and the vertical drop (head).
Hydropower can provide more than electricity. Reservoir-based plants can offer flexible generation, water storage, flood-management benefits, irrigation support, and recreation, while pumped-storage hydropower plays an important role in grid-scale energy storage.
In This Guide
- What Is Hydropower?
- Hydropower & the Water Cycle
- How Hydropower Works
- Head & Flow
- History of Hydropower
- Types of Hydropower Plants
- Impoundment & Dams
- Run-of-River
- Pumped Storage
- Small & Micro Hydropower
- Hydropower in the United States
- Hydropower Around the World
- Benefits of Hydropower
- Environmental Impacts
- How Much Does Hydropower Cost?
- Hydropower vs. Marine Energy
- The Future of Hydropower
- Frequently Asked Questions
What Is Hydropower?
Hydropower, also called hydroelectric power when electricity is being generated, uses the energy of moving water.
The energy can come from:
- Water falling from a higher elevation to a lower one
- Water flowing through a river
- Water released from a reservoir
- Water moved between upper and lower reservoirs in pumped-storage facilities
Modern hydropower plants usually convert this energy through a turbine and generator.
Water is not “burned” as a fuel. The plant extracts part of the water's gravitational or kinetic energy while the water continues downstream or enters another reservoir.
How Hydropower Depends on the Water Cycle
Hydropower is closely connected to the water cycle.
Solar energy drives evaporation from oceans, lakes, rivers, soil, and vegetation. Atmospheric water later returns as rain or snow.
That precipitation may:
- Flow into streams and rivers
- Accumulate as snowpack
- Collect in reservoirs
- Recharge groundwater
Gravity then moves surface water downhill, creating the potential energy hydropower systems can use.
In simple terms: solar energy helps lift water through evaporation and atmospheric circulation, and gravity brings water back downhill. Hydropower captures part of the energy available during that downhill journey.
How Does Hydropower Generate Electricity?
Although hydropower facilities vary greatly in size and design, the basic conversion process is straightforward.
1. Water Is Directed Toward a Turbine
Water may enter through an intake and travel through a pipe or enclosed passage called a penstock.
2. Moving Water Spins the Turbine
The force of the water turns turbine blades.
3. The Turbine Turns a Generator
The turbine shaft is connected to an electrical generator.
4. The Generator Produces Electricity
The generator converts the turbine's mechanical rotation into electrical energy.
5. Electricity Enters the Grid
Transformers and transmission equipment deliver the electricity at appropriate voltages to the power system.
6. Water Continues Downstream
After passing through the turbine, the water normally returns to the river or enters a lower reservoir.
Why Head and Flow Matter
The amount of power available from a hydropower site depends largely on two factors:
Head
Head is the effective vertical distance the water falls.
A greater head generally means that each unit of water contains more gravitational potential energy.
Flow
Flow is the volume of water moving through the turbine over time.
A high-flow river can produce substantial power even with a moderate head, while a high-mountain installation may work with lower flow but a much greater vertical drop.
Bigger dam does not automatically mean more efficient hydropower. Plant output depends on the combination of usable head, flow, turbine efficiency, generator efficiency, operating restrictions, and water availability.
A Brief History of Hydropower
Humans used flowing water long before electricity existed.
Ancient waterwheels converted the motion of rivers and streams into mechanical power for tasks such as:
- Grinding grain
- Pumping water
- Sawing wood
- Operating machinery
During the 1800s, improvements in turbines and electrical generators made it possible to convert water power into electricity.
One of the earliest U.S. hydroelectric projects began operating in Appleton, Wisconsin, in 1882.
As long-distance electrical transmission improved, hydropower plants no longer needed to serve only customers located immediately beside the generating site.
Large U.S. projects followed during the twentieth century, including:
- Hoover Dam
- Grand Coulee Dam
- Major Columbia River projects
- Tennessee Valley Authority projects
These facilities combined electricity generation with other purposes such as irrigation, navigation, flood management, and water storage.
What Are the Main Types of Hydropower Plants?
Modern hydropower can be divided into three broad configurations:
| Type | How It Works | Key Characteristic |
|---|---|---|
| Impoundment | A dam stores water in a reservoir and releases it through turbines | Can provide substantial control over when electricity is generated |
| Diversion / Run-of-River | Channels part of a river through a canal or penstock | Usually has little or no large storage reservoir |
| Pumped Storage | Moves water between lower and upper reservoirs | Stores energy for later electricity generation |
Impoundment Hydropower and Dams
An impoundment facility uses a dam to create a reservoir.
When electricity is needed, water is released through an intake and penstock toward turbines located at a lower elevation.
The height difference creates pressure that helps turn the turbine.
Why Reservoir Storage Is Valuable
A reservoir can give operators some control over when water is released for generation.
This allows certain plants to:
- Increase output during periods of high electricity demand
- Reduce output when demand is lower
- Provide grid-balancing services
- Respond relatively quickly to changing system conditions
Many reservoirs serve multiple purposes beyond electricity generation, including:
- Flood management
- Water supply
- Irrigation
- Navigation
- Recreation
Examples
Well-known U.S. hydropower dams include Hoover Dam on the Colorado River and Grand Coulee Dam on the Columbia River.
What Is Run-of-River Hydropower?
A run-of-river or diversion hydropower facility channels part of a river through a canal, penstock, or powerhouse and then returns the water downstream.
Some run-of-river facilities use a small dam or weir, while others may require little impoundment.
How It Differs From Reservoir Hydropower
Run-of-river plants generally have less ability to store water and shift electricity generation to a different time.
Their output therefore tends to depend more closely on current river flow.
Run-of-river does not mean environmentally impact-free. Diversions, altered river flow, barriers, turbines, and supporting infrastructure can still affect fish, habitat, sediment, and aquatic ecosystems.
The old version of this article described run-of-river hydropower as inherently inefficient and broadly “better for the Earth.” Neither statement is reliable. Its advantages and impacts depend heavily on the particular site and design.
What Is Pumped-Storage Hydropower?
Pumped-storage hydropower, or PSH, differs from conventional generation in that its primary function is energy storage.
It uses two reservoirs at different elevations.
When Electricity Is Plentiful
Electricity powers pumps that move water from the lower reservoir to the upper reservoir.
When Electricity Is Needed
Water is released downhill through turbines, generating electricity and returning to the lower reservoir.
Pumped storage works somewhat like a rechargeable battery. It consumes electricity to lift water to a higher elevation and later recovers much of that energy as the water flows downhill through turbines.
Why Pumped Storage Matters
Modern electric grids increasingly combine power sources with different operating characteristics.
Pumped storage can help by providing:
- Large-scale energy storage
- Grid balancing
- Fast response to changing demand
- Support for variable wind and solar generation
- Long-duration storage in suitable designs
In the United States, pumped storage remains the dominant form of utility-scale energy storage by stored-energy capacity.
Open-Loop vs. Closed-Loop Pumped Storage
Open-loop systems maintain an ongoing hydrologic connection with a natural body of water.
Closed-loop systems use reservoirs that are not continuously connected to a natural river or lake.
Small and Micro Hydropower
Hydropower does not always require a massive concrete dam.
Smaller installations can serve:
- Remote communities
- Industrial sites
- Farms
- Water infrastructure
- Individual properties with suitable water resources
Small projects may use:
- Natural stream elevation change
- Irrigation canals
- Municipal water conduits
- Existing non-powered dams
“Small hydropower” does not have one universal size definition. Different agencies and countries use different thresholds, so a fixed statement such as “all small hydro is below 30 MW” should be avoided unless a particular classification system is being cited.
Can You Generate Hydropower at Home?
Microhydropower can be practical on some properties with dependable flowing water and sufficient vertical drop.
But feasibility depends on:
- Year-round flow
- Available head
- Water rights
- Environmental permits
- Local regulations
- Transmission distance
- Installation and maintenance costs
A stream beside a property does not automatically make home hydropower practical or legally permissible.
How Much U.S. Electricity Comes From Hydropower?
Hydropower remains an important part of the U.S. electricity system, although its share is lower than historically because other generation sources have grown.
In 2025, conventional hydroelectricity produced approximately:
- 5.6% of total U.S. utility-scale electricity generation
- 23.1% of U.S. utility-scale renewable electricity generation
Hydropower output also varies from year to year because rainfall, snowpack, drought, reservoir conditions, and river flows change.
Where Is U.S. Hydropower Concentrated?
The Pacific Northwest is especially important.
In 2025, Washington alone accounted for about 26% of U.S. conventional hydroelectric generation.
Other major producing states include:
- Oregon
- California
- New York
- Montana
How Important Is Hydropower Globally?
Hydropower remains one of the world's largest electricity sources and the largest individual renewable electricity technology by generation.
In 2024, hydropower generated roughly:
- 4,500 terawatt-hours of electricity worldwide
- 14% of total global electricity generation
China is the world's largest producer of hydroelectricity.
The old article stated that hydropower supplied 17% of world electricity. That figure is no longer current; the recent global share is closer to 14%.
What Are the Advantages of Hydropower?
Renewable Energy Source
Hydropower relies on water continually replenished through the water cycle.
Low Direct Operating Emissions
Hydroelectric turbines do not burn fossil fuel while generating electricity.
Flexible Generation
Reservoir plants can often increase or decrease output more quickly than many large thermal power plants.
Long Operating Life
Hydropower facilities can operate for many decades when maintained and modernized.
Grid Support
Hydropower can provide:
- Frequency response
- Operating reserves
- Voltage support
- Rapid ramping
Energy Storage
Pumped-storage plants can store very large amounts of energy for later use.
Multipurpose Infrastructure
Some hydropower reservoirs also support:
- Water supply
- Irrigation
- Flood management
- Navigation
- Recreation
What Are the Environmental Impacts of Hydropower?
Renewable does not mean impact-free.
The environmental consequences of hydropower vary enormously by:
- Site
- Dam size
- Reservoir size
- River ecology
- Operating practices
- Climate
- Mitigation measures
Fish Migration
Dams and other structures can interfere with the movement of migratory fish.
Mitigation approaches can include:
- Fish ladders
- Fish lifts
- Bypass channels
- Fish-friendly turbine designs
- Operational changes
The effectiveness of these measures varies by species and facility.
Changed River Flow
Dams can alter:
- Timing of downstream flows
- Water temperature
- Sediment movement
- Water chemistry
- Flood patterns
Those changes can affect aquatic and riverbank ecosystems.
Flooded Land
Creating large reservoirs can inundate:
- Forests
- Farmland
- Wildlife habitat
- Archaeological sites
- Communities
Reservoir Greenhouse-Gas Emissions
Organic material decomposing in reservoirs can produce carbon dioxide and methane.
The amount varies widely with:
- Climate
- Reservoir depth
- Flooded vegetation
- Water chemistry
- Reservoir age
It is misleading to say hydropower has “no emissions.” Turbines generate electricity without combustion, but full environmental assessment can include construction, reservoir emissions, land-use changes, and ecosystem impacts.
Is Hydropower Renewable?
Yes. Hydropower is classified as renewable because flowing water is continually replenished by the hydrologic cycle.
However, "renewable" describes the energy resource—not the absence of environmental trade-offs.
A useful distinction is:
- Renewability: Is the underlying energy resource naturally replenished?
- Environmental impact: What effects does obtaining that energy have on ecosystems, communities, land, and climate?
Hydropower is renewable, while individual projects can still have substantial environmental consequences.
How Much Does Hydropower Cost?
There is no single cost for hydroelectricity.
Project economics depend heavily on:
- Whether a dam already exists
- Site geology
- Project size
- Transmission requirements
- Turbine design
- Environmental mitigation
- Permitting
- Construction financing
High Upfront Costs
Large hydropower projects can require enormous capital investment and long development periods.
Low Fuel Costs
Hydropower does not require purchased fuel such as coal or natural gas.
Long Lifespans
Well-maintained facilities can operate for many decades, and existing plants can often be upgraded with newer turbines, generators, controls, and environmental technologies.
Old construction costs should not be used as modern cost comparisons. Quoting Hoover Dam's original 1930s price without accounting for inflation, financing, environmental requirements, and the modern construction context tells readers very little about what a hydropower project would cost today.
How Drought Affects Hydropower
Hydropower is renewable, but that does not mean its electricity output is constant.
Prolonged drought can reduce:
- River flow
- Reservoir levels
- Available head
- Water available for generation
Snowpack is also important in many western watersheds because mountain snow stores winter precipitation and releases it later through snowmelt.
This helps explain why U.S. hydroelectric generation can vary substantially from year to year.
For more context, see our Climate Change and Drought Guide.
Is Tidal or Wave Power Hydropower?
Hydropower and marine energy are related because both extract energy from moving water, but they are usually treated as separate technology categories.
Conventional hydropower generally uses rivers, reservoirs, diversion systems, and elevation differences.
Marine energy includes technologies that extract energy from:
- Ocean waves
- Tides
- Ocean currents
- River currents without conventional dams
- Ocean thermal gradients
I would not place tidal, wave, and ocean thermal energy inside the main “types of hydropower” section. They are better presented as related water-power technologies.
Tidal Energy
Tidal energy devices use the predictable motion of ocean tides.
Systems may use:
- Tidal-stream turbines
- Barrages
- Tidal lagoons
Wave Energy
Wave-energy devices attempt to convert the motion of ocean waves into electricity.
Many different designs are under development, including floating, submerged, and shoreline systems.
Ocean Thermal Energy Conversion
Ocean thermal energy conversion, or OTEC, uses the temperature difference between warm surface seawater and colder deep water to operate a heat cycle.
OTEC generally requires a large temperature difference—roughly 20°C—to be technically attractive.
These marine-energy technologies remain much less commercially mature than conventional hydropower.
What Is the Future of Hydropower?
The future of hydropower is unlikely to consist simply of building huge new dams everywhere.
Important opportunities include:
Modernizing Existing Plants
Older plants can sometimes gain capacity, efficiency, reliability, or environmental improvements through updated:
- Turbines
- Generators
- Digital controls
- Fish-passage systems
- Monitoring equipment
Adding Generation to Existing Infrastructure
Some existing dams were built for irrigation, navigation, flood control, or water supply and do not currently generate electricity.
There may be opportunities to add generation without constructing an entirely new dam.
Pumped-Storage Expansion
As grids add more wind and solar electricity, interest in long-duration storage and grid flexibility increases.
Pumped storage is therefore likely to remain an important part of future hydropower development.
Improved Environmental Performance
Research continues into:
- Fish-friendly turbines
- Improved fish passage
- Better flow management
- Environmental monitoring
- More flexible plant operation
Small and Distributed Hydropower
Some future projects may use existing:
- Canals
- Water pipelines
- Non-powered dams
- Small river drops
Hydropower will probably grow more slowly than solar power globally, but it remains particularly valuable because reservoir hydro and pumped storage can provide flexibility and storage services that variable renewable generation cannot provide by itself.
Frequently Asked Questions About Hydropower
What is hydropower?
Hydropower uses the energy of moving water to perform mechanical work or generate electricity. Modern hydroelectric plants typically use water to spin turbines connected to generators.
How does a hydroelectric dam generate electricity?
Water stored at a higher elevation is released through an intake and penstock. The moving water spins a turbine connected to a generator, producing electricity.
What is a hydropower turbine?
A turbine is a rotating machine that converts the energy of moving water into mechanical rotation. That rotation can drive an electrical generator.
What is a penstock?
A penstock is a pipe or enclosed conduit that directs water toward a hydropower turbine.
What does “head” mean in hydropower?
Head describes the usable vertical elevation difference through which water falls or flows. Greater head generally provides more potential energy per unit of water.
Does hydropower require a dam?
No. Some diversion or run-of-river hydropower systems can operate without a large storage dam, although they may use smaller diversion structures.
What is run-of-river hydropower?
Run-of-river hydropower channels part of a river through a canal, penstock, or powerhouse and generally has little or no large reservoir storage.
Is run-of-river hydropower environmentally harmless?
No. It may avoid some impacts associated with large reservoirs, but diversions, altered flow, turbines, and structures can still affect aquatic ecosystems.
What is pumped-storage hydropower?
Pumped storage moves water from a lower reservoir to an upper reservoir using electricity, then releases it through turbines to generate electricity. It functions as large-scale energy storage.
Is pumped-storage hydropower a source of energy?
It is primarily an energy-storage technology rather than a net energy source. Pumping consumes more electricity than the system later returns, but it shifts electricity from one time to another and provides valuable grid services.
Is hydropower renewable?
Yes. Hydropower is considered renewable because flowing water is continually replenished through the water cycle.
Is hydropower clean energy?
Hydropower produces electricity without burning fuel at the turbine, so direct operating emissions are low. However, dams, reservoirs, construction, land-use change, and greenhouse-gas emissions from reservoirs can have environmental impacts.
Does hydropower release carbon dioxide?
Hydroelectric generation does not require combustion, but the overall project can produce greenhouse gas emissions from construction and, in some reservoirs, from the decomposition of organic material.
How does hydropower affect fish?
Dams, turbines, and altered river flows can affect fish migration and habitat. Fish passages, bypasses, turbine improvements, and operational changes are among the techniques used to reduce those impacts.
How much U.S. electricity comes from hydropower?
In 2025, conventional hydroelectricity supplied about 5.6% of U.S. utility-scale electricity generation and about 23.1% of utility-scale renewable electricity generation.
How much of the world's electricity comes from hydropower?
Hydropower generated roughly 14% of global electricity in 2024.
Which country generates the most hydropower?
China is the world's largest producer of hydroelectricity.
Can drought reduce hydropower generation?
Yes. Lower river flow and reservoir levels can reduce the water and usable head available for electricity generation.
Can a home use microhydropower?
Some properties with reliable flowing water and suitable elevation change can use microhydropower. Water rights, environmental regulations, permits, seasonal flow, and economics must also be considered.
Are wave and tidal power the same as hydropower?
They are related water-power technologies but are generally classified as marine energy rather than conventional hydropower.
Why is hydropower useful with solar and wind?
Reservoir hydropower can often adjust output relatively quickly, while pumped-storage hydropower can store electricity for later use. Those capabilities can help balance variable wind and solar generation.
Final Thoughts
Hydropower is one of the oldest renewable-energy technologies, but its role in the modern electricity system extends far beyond the traditional image of a giant dam.
Today's hydropower includes:
- Large reservoir plants
- Run-of-river facilities
- Small and microhydropower
- Generation added to existing water infrastructure
- Pumped-storage energy systems
Its strengths include long operating life, renewable generation, flexible output, and large-scale energy storage.
Its limitations are equally important: hydropower depends on local water conditions, suitable geography, complex infrastructure, and responsible management of river ecosystems and surrounding communities.
The best way to understand hydropower is to view it as both an energy technology and a water-management technology. Its value—and its environmental impact—depends not only on the turbine, but also on how water is stored, released, diverted, and managed throughout the surrounding river system.