Hydropower Basics

Many terms used in this article are defined in the glossary at the end of the page.

How Hydropower Works


This illustration of the water or hydrologic cycle shows how water evaporates into clouds, precipitates, forms rivers that flow downhill into the ocean, and begins the cycle again.

The Water (Hydrologic) Cycle

Hydropower is using water to power machinery or make electricity. Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, then flowing back down to the ocean. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain. Hydropower uses a fuel—water—that is not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy.

When flowing water is captured and turned into electricity, it is called hydroelectric power or hydropower. There are several types of hydroelectric facilities; they are all powered by the kinetic energy of flowing water as it moves downstream. Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and by industry.

Types of Hydropower Plants

There are three types of hydropower facilities: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not. The images below show both types of hydropower plants.

Drawing showing a cross section of an impoundment dam and hydropower plant. Transmission lines conduct electricity to homes and businesses. Dam stores water. Penstock carries water to the turbines. Generators are rotated by the turbines to generate electricity. Turbines are turned by the force of the water on their blades.

An impoundment hydropower plant dams water in a reservoir.

Photo of an aerial view of a river with a waterfall and no dam. The hydropower intake and outlet are labeled. The intake is above the waterfall; the outlet is below it.

The Tazimina project in Alaska is an example of a diversion hydropower plant. No dam was required.

Drawing shows a micro hydropower plant. Intake gates allow water to flow through the Penstock Powerhouse to the turbine.

A Micro Hydropower Plant

Many dams were built for other purposes and hydropower was added later. In the United States, there are about 80,000 dams of which only 2,400 produce power. The other dams are for recreation, stock/farm ponds, flood control, water supply, and irrigation.

Hydropower plants range in size from small systems for a home or village to large projects producing electricity for utilities.


The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.


A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.

Pumped Storage

When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.

Sizes of Hydroelectric Power Plants

Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities. Although definitions vary, the Department of Energy defines them as follows:

Large Hydropower — facilities that have a capacity of more than 30 megawatts.

Small Hydropower — facilities that have a capacity of 100 kilowatts to 30 megawatts.

Micro Hydropower — facilities that have a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch, or village.

Types of Hydropower Turbines

There are two main types of hydro turbines: impulse and reaction. The type of hydropower turbine selected for a project is based on the height of standing water—referred to as "head"—and the flow, or volume of water, at the site. Other deciding factors include how deep the turbine must be set, efficiency, and cost.

Impulse Turbine

The impulse turbine generally uses the velocity of the water to move the runner and discharges to atmospheric pressure. The water stream hits each bucket on the runner. There is no suction on the down side of the turbine, and the water flows out the bottom of the turbine housing after hitting the runner. An impulse turbine is generally suitable for high head, low flow applications.

Pelton hydropower turbine

Pelton hydropower turbine


A pelton wheel has one or more free jets discharging water into an aerated space and impinging on the buckets of a runner. Draft tubes are not required for impulse turbine since the runner must be located above the maximum tailwater to permit operation at atmospheric pressure.

A Turgo Wheel is a variation on the Pelton and is made exclusively by Gilkes in England. The Turgo runner is a cast wheel whose shape generally resembles a fan blade that is closed on the outer edges. The water stream is applied on one side, goes across the blades and exits on the other side.


Propeller hydropower turbine

Propeller hydropower turbine

A cross-flow turbine is drum-shaped and uses an elongated, rectangular-section nozzle directed against curved vanes on a cylindrically shaped runner. It resembles a "squirrel cage" blower. The cross-flow turbine allows the water to flow through the blades twice. The first pass is when the water flows from the outside of the blades to the inside; the second pass is from the inside back out. A guide vane at the entrance to the turbine directs the flow to a limited portion of the runner. The cross-flow was developed to accommodate larger water flows and lower heads than the Pelton.

Reaction Turbine

A reaction turbine develops power from the combined action of pressure and moving water. The runner is placed directly in the water stream flowing over the blades rather than striking each individually. Reaction turbines are generally used for sites with lower head and higher flows than compared with the impulse turbines.

Bulb hydropower turbine

Bulb hydropower turbine


A propeller turbine generally has a runner with three to six blades in which the water contacts all of the blades constantly. Picture a boat propeller running in a pipe. Through the pipe, the pressure is constant; if it isn't, the runner would be out of balance. The pitch of the blades may be fixed or adjustable. The major components besides the runner are a scroll case, wicket gates, and a draft tube. There are several different types of propeller turbines:

  • Bulb turbine

    The turbine and generator are a sealed unit placed directly in the water stream.

  • Straflo

    The generator is attached directly to the perimeter of the turbine.

  • Kaplan hydropower turbine

    Kaplan hydropower turbine

  • Tube turbine

    The penstock bends just before or after the runner, allowing a straight line connection to the generator.

  • Kaplan

    Both the blades and the wicket gates are adjustable, allowing for a wider range of operation.


A Francis turbine has a runner with fixed buckets (vanes), usually nine or more. Water is introduced just above the runner and all around it and then falls through, causing it to spin. Besides the runner, the other major components are the scroll case, wicket gates, and draft tube.

Francis hydropower turbine

Francis hydropower turbine


Kinetic energy turbines, also called free-flow turbines, generate electricity from the kinetic energy present in flowing water rather than the potential energy from the head. The systems may operate in rivers, man-made channels, tidal waters, or ocean currents. Kinetic systems utilize the water stream's natural pathway. They do not require the diversion of water through manmade channels, riverbeds, or pipes, although they might have applications in such conduits. Kinetic systems do not require large civil works; however, they can use existing structures such as bridges, tailraces and channels.

Benefits of Its Use

Hydropower offers advantages over other energy sources but faces unique environmental challenges.

Photo of  the fish ladder on the Ice Harbor Dam..

This fish ladder on the Ice Harbor Dam on the lower Snake River provides safe passage for migrating fish.

Hydropower is a fueled by water, so it's a clean fuel source. Hydropower doesn't pollute the air like power plants that burn fossil fuels, such as coal or natural gas.

Hydropower is a domestic source of energy, produced in the United States.

Hydropower relies on the water cycle, which is driven by the sun, thus it's a renewable power source.

Hydropower is generally available as needed; engineers can control the flow of water through the turbines to produce electricity on demand.

Hydropower plants provide benefits in addition to clean electricity. Impoundment hydropower creates reservoirs that offer a variety of recreational opportunities, notably fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities. Other benefits may include water supply and flood control.

Hydropower Use Throughout History

Humans have been harnessing water to perform work for thousands of years. The Greeks used water wheels for grinding wheat into flour more than 2,000 years ago. Besides grinding flour, the power of the water was used to saw wood and power textile mills and manufacturing plants.

For more than a century, the technology for using falling water to create hydroelectricity has existed. The evolution of the modern hydropower turbine began in the mid-1700s when a French hydraulic and military engineer, Bernard Forest de Bélidor wrote Architecture Hydraulique. In this four volume work, he described using a vertical-axis versus a horizontal-axis machine.

During the 1700s and 1800s, water turbine development continued. In 1880, a brush arc light dynamo driven by a water turbine was used to provide theatre and storefront lighting in Grand Rapids, Michigan; and in 1881, a brush dynamo connected to a turbine in a flour mill provided street lighting at Niagara Falls, New York. These two projects used direct-current technology.

Alternating current is used today. That breakthrough came when the electric generator was coupled to the turbine, which resulted in the world's, and the United States', first hydroelectric plant located in Appleton, Wisconsin, in 1882.

B.C. Hydropower used by the Greeks to turn water wheels for grinding wheat into flour, more than 2,000 years ago.
Mid-1770s French hydraulic and military engineer Bernard Forest de Bélidor wrote Architecture Hydraulique, a four-volume work describing vertical- and horizontal-axis machines.
1775 U.S. Army Corps of Engineers founded, with establishment of Chief Engineer for the Continental Army.
1880 Michigan's Grand Rapids Electric Light and Power Company, generating electricity by dynamo belted to a water turbine at the Wolverine Chair Factory, lit up 16 brush-arc lamps.
1881 Niagara Falls city street lamps powered by hydropower.
1882 World's first hydroelectric power plant began operation on the Fox River in Appleton, Wisconsin.
1886 About 45 water-powered electric plants in the U.S. and Canada.
1887 San Bernardino, Ca., opens first hydroelectric plant in the west.
1889 Two hundred electric plants in the U.S. use waterpower for some or all generation.
1901 First Federal Water Power Act.
1902 Bureau of Reclamation established.
1907 Hydropower provided 15% of U.S. electrical generation.
1920 Hydropower provided 25% of U.S. electrical generation. Federal Power Act establishes Federal Power Commission authority to issue licenses for hydro development on public lands.
1933 Tennessee Valley Authority established.
1935 Federal Power Commission authority extended to all hydroelectric projects built by utilities engaged in interstate commerce.
1937 Bonneville Dam, first Federal dam, begins operation on the Columbia River. Bonneville Power Administration established.
1940 Hydropower provided 40% of electrical generation. Conventional capacity tripled in United States since 1920.
1980 Conventional capacity nearly tripled in United States since 1940.
2003 About 10% of U.S. electricity comes from hydropower. Today, there is about 80,000 MW of conventional capacity and 18,000 MW of pumped storage.

U.S. Hydropower Resource Potential

The US Department of Energy conducted a resource assessment for 49 states (no report was generated for Delaware because of scarce resources). The completed work identified 5,677 sites in the United States with undeveloped capacity of about 30,000 MW. By comparison, today there is about 80,000 MW of hydroelectric generating plants in the United States.

To learn more about how much electricity hydropower could potentially generate in the United States, please see the State Resource Assessment Report on the Idaho National Laboratory Web site.

Glossary of Hydropower Terms

Alternating current (AC) — Electric current that reverses direction many times per second.

Ancillary services — Operations provided by hydroelectric plants that ensure stable electricity delivery and optimize transmission system efficiency.

Cavitation — Noise or vibration causing damage to the turbine blades as a results of bubbles that form in the water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, and the cavity or bubble collapses when they pass into higher regions of pressure.

Direct current (DC) — Electric current which flows in one direction.

Draft tube — A water conduit, which can be straight or curved depending upon the turbine installation, that maintains a column of water from the turbine outlet and the downstream water level.

Efficiency — A percentage obtained by dividing the actual power or energy by the theoretical power or energy. It represents how well the hydropower plant converts the energy of the water into electrical energy.

Head — Vertical change in elevation, expressed in either feet or meters, between the head water level and the tailwater level.

Flow — Volume of water, expressed as cubic feet or cubic meters per second, passing a point in a given amount of time.

Headwater — The water level above the powerhouse.

Low Head — Head of 66 feet or less.

Penstock — A closed conduit or pipe for conducting water to the powerhouse.

Runner — The rotating part of the turbine that converts the energy of falling water into mechanical energy.

Scroll case — A spiral-shaped steel intake guiding the flow into the wicket gates located just prior to the turbine.

Small hydro — Projects that produce 30 MW or less.

Tailrace — The channel that carries water away from a dam.

Tailwater — The water downstream of the powerhouse.

Ultra low head — Head of 10 feet or less.

Wicket gates — Adjustable elements that control the flow of water to the turbine passage.

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