Hydropower (Water Science)

Hydropower is energy that is generated by moving water. Today, hydropower facilities make electricity by converting kinetic (moving) energy into mechanical (machine) energy as water flows in a river or over a dam. Electricity made at hydropower facilities can be carried away, via power transmission lines, and sold to homes and businesses. Hydropower is a relatively inexpensive, non-polluting form of renewable energy.

Canada and the United States are currently the world’s top hydroelectric producers. Other countries that use hydropower on a large scale include Brazil, China, Russia, Norway, Japan, India, Iceland, Sweden, and France. Hydropower produces about 10% of United States’ electricity, in contrast to Norway, who generates nearly 99% of its electricity from hydropower. Hydropower is used nationwide, but is primarily used in the western coastal United States where other energy resources such as coal are limited. Hydropower is important to the United States economy because it supplies electricity to a growing population and industry.

Hydropower in history

Humans have harnessed water power for thousands of years, using the mechanical energy of moving water to turn wooden wheels to power mills that sawed lumber and processed grains. Water either fell onto the wheel and caused it to turn or the wheel was placed in the river and the river’s current (a steady flow of water in a prevailing direction) turned the wheel. The wheel was attached to other levers and gears inside the mill that did the work needed. During the 1700s to 1800s, mechanical hydropower was used extensively in the United States and elsewhere for milling and pumping. It began to be widely used to supply electricity in the late 1800s. In 1882 the world’s first hydropower facility was built on the Fox River in Appleton, Wisconsin. The Fox River facility generated electricity for local industries.

By the 1920s, following the development of the electrical motor and the demand for electricity that followed, hydropow-er accounted for about 40% of the U.S. electricity supply. Since then, other energy technologies have developed that are less expensive than hydropower.

In 1933, President Franklin D. Roosevelt (1882-1945) signed the Tennessee Valley Authority (TVA) Act. The purpose of the TVA was to construct dams on the Tennessee River that would aid in river navigation, control flooding, provide water for irrigation (watering crops) and drinking, and provide hydropower to Tennessee River Valley residents. In the American West, hydropower aided in the production of the  dams themselves, moving and lifting construction materials and providing energy for lights to make round-the-clock work possible. Surplus energy was sold to neighboring farms and homes, which in turn paid for the operation and building costs of the dams. Hydropower facility development was at its peak during the 1930s and 1940s. The Hoover Dam on the Nevada-Arizona border and the Grand Coulee dam located on the Columbia River in Washington state were constructed during this period. The Grand Coulee dam remains the largest concrete structure in the United States.

Swimmers enjoy geothermally heated pools at Svartsvehring, Iceland.

Swimmers enjoy geothermally heated pools at Svartsvehring, Iceland.

Hydroelectric technology

Electricity is one of the most important types of energy because it allows people to perform the work needed to light homes and power appliances and computers. Hydro-power generation utilizes the principles of electromagnetic induction (creating an electrical current, the flow of electricity, by moving a magnet through a wire coil), first described by English chemist and physicist Michael Faraday (1791-1867) in 1831 when he made the first generator. A generator is a machine that converts mechanical energy to electrical energy. Energy is the power to do work. Energy cannot be created or destroyed; it merely changes state, as occurs when electrical energy is harnessed from the mechanical energy of moving water.

No matter the type or size of the hydropower facility, electricity is generated in much the same way in each one. The dam or the natural elevation drop in a river creates head, or a certain height over which the water flows as it is released from the dam or as it flows downstream. As water is released from a reservoir in an impoundment or pumped storage facility, or diverted from a river through a control gate, it flows by gravity (the attraction between two masses) to a turbine. A turbine is a device that transforms the energy in moving fluid or wind to rotary mechanical energy. As the water flows past the turbine blades, the turbine blades spin and turn a rotor (the moving part of an electric generator) much like wind spins a pinwheel. Giant magnets inside the rotor move past coils of copper wire and create an alternating current. An alternating current (AC) is produced when electrons wiggle back and forth between atoms. The used water returns to the river through pipes.

The alternating current moves through a series of devices called transformers that can increase the voltage (energy required to move a charge from one point to another, similar to pressure). The increase in voltage allows the electricity to travel faster and more efficiently through power lines (important when the hydropower facility is in a remote location) from the hydropower facility to a town’s electricity facility. At the local facility transformers are used again to reduce the voltage to a level that is safe for the electricity to be used in homes and businesses.

Types of hydropower facilities

There are three types of hydropower facilities: impoundment, pumped storage, and diversion. Impoundment and pumped storage facilities require dams. In the United States, hydropower is a very small percentage of the primary purpose of dams. Usually a dam is built first for other reasons, such as water storage and flood control, and a hydropower facility is incorporated later if there is a demonstrated need for electricity.


The northern island of Iceland is known for generating geothermal power (power derived from heat found under Earth’s surface) rather than hydropower. Yet when large glaciers (large, slow-moving mass of ice) melt in the spring it supplies water to large rivers and provides favorable conditions for hydropower development. Long, dark, and cold winters contribute to Iceland’s high rate of energy consumption, second only to Norway.

In the early 1900s, Icelanders primarily used peat (plant debris and moss from bogs that is dried) for fuel. By 1940 imported coal and oil were used. Geothermal and hydropower accounted for only approximately 9% of Iceland’s energy. About 87% of houses in Iceland are currently heated with geothermal energy. The rest are heated with electricity, 83% of which is generated using hydropower. Orkustofnun, Iceland’s National Energy Authority, estimates that only about 10-15% of Iceland’s potential hydropower sites have been developed.

Impoundment facilities require a large dam that allows river water to be stored in a reservoir. When water is released from the reservoir, the water flows downward through a penstock (pipe) in the dam and spins turbines, thereby creating mechanical energy that is then used to power electric generators. Transmission lines carry electricity away from the impoundment facility to local distributors who sell the electricity to homes and businesses.

Pumped storage hydropower facilities also require dams to operate. In periods of low electricity demand water is pumped from a lower reservoir to a higher reservoir. When electrical demand increases, water is released from the higher reservoir back into the lower reservoir through a penstock, turning turbines that power electric generators. Pumped storage and impoundment hydropower facilities provide a reliable source of electricity because water flow from reservoirs can be controlled so that electricity production meets demand.

Diversion hydropower facilities (sometimes called run-of-river systems) are smaller than impoundment facilities and do not require a dam. Instead, diversion facilities use a river’s natural flow to generate electricity. The amount of electricity produced depends upon the river’s rate of flow (volume of water flow within a period of time) and the river’s elevation change at the diversion facility site. In diversion hydropower facilities, river water is channeled through a canal (artificial waterway that controls water flow, in this case, to a turbine) or penstock. Diversion facilities are less reliable than impoundment or pumped storage facilities because flow rates in rivers can change drastically depending on the amount of rainfall or spring meltwater that supplies the river.

Tennessee Valley Authority

Prior to 1933, residents of the Tennessee River Valley lived without water and power. Many people lived in poverty without jobs following the Great Depression, which began in 1929. In 1933, President Franklin D. Roosevelt signed the Tennessee Valley Authority Act as part of his New Deal plan, a series of social programs that reformed American financial practices and offered relief and jobs to struggling Americans during the Depression. This act created the Tennessee Valley Authority (TVA), a federally funded utility company incorporating approximately 80,000 square miles (207,200 square kilometers) in Tennessee and parts of Kentucky, Virginia, North Carolina, Georgia, Alabama, and Mississippi.

The TVA was initially charged with making rivers more navigable and bringing water and electricity to homes and farms throughout the seven southern states. Since its founding, the TVA has also been responsible for developing flood control through dam construction, improving and maintaining water quality, replanting forests, developing roads and providing recreation opportunities on lakes and rivers. The TVA improved the Tennessee River Valley economy by providing construction and maintenance jobs to the region and supporting farm and industry development.

Originally, tax dollars provided funding for TVA projects, but today the TVA supports its staff and maintains its facilities by making and selling electricity. Today the TVA is the largest public power company in the United States, supplying power for more than eight million people. The TVA currently operates twenty-nine hydroelectric and one pumped-storage dam, eleven coal-fired plants, three nuclear power plants, and several solar, wind, and other renewable energy sites.

Hoover Dam

The Hoover Dam hydropower plant is driven by waters of the Colorado River.

The Hoover Dam hydropower plant is driven by waters of the Colorado River.

Hoover Dam was built on Black Canyon, about 30 miles (48 kilometers) southeast of Las Vegas on the Colorado River. The dam took less than five years (1931-36) to complete and employed an average of 3,500 people each year. At 726.4 feet (221.4 meters) tall, Hoover Dam was the largest dam of the time. Today, Hoover Dam is a National Historic Landmark and heralded as one of America’s Seven Modern Civil Engineering Wonders. The dam and Hoover Power plant (the hydropower facility) are currently operated and maintained by the Bureau of Reclamation. The reservoir (Lake Mead) is maintained jointly by the Bureau of Reclamation and the National Park Service.

Hoover Dam was built to control waters of the Colorado River Basin, including the Green, San Juan, Virgin, and Gila Rivers, which are all tributaries (smaller streams that feed into a larger stream or river) of the Colorado River. Rivers of the Colorado River Basin drain a 242,000-square-mile (626,777-square-kilo-meter) area of the western United States between Colorado and the Gulf of California. Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming are allocated water from the Colorado River Basin.

The Hoover power plant is a U-shaped facility located at the base of the dam. The power plant was installed in 1961 and upgraded between 1986 and 1993. Water from the river is taken through 30-foot (9-meter) wide penstocks to sixteen 13-foot (4 meter) wide penstocks to the turbines. Electricity generated in the Hoover power plant supplies the operations of the dam and plant and provides low-cost power to Arizona, Nevada, and Southern California. Revenues from the sale of power have allowed for its $165 million dollar dam construction cost to be repaid to the Federal Treasury. Revenue monies now pay for operation and maintenance of the dam.

Sizes of hydropower facilities

The size of a hydropower facility depends upon the amount of energy that can be generated at the facility. Some hydropower plants may produce electricity for only one to a few homes. These facilities are called micro-hydropower plants. In micro-hydropower facilities the total change in elevation of the flowing water is only about 100 feet (30 meters). Energy output can be increased, however, with higher water flow. Larger hydroelectric power facilities such as the Hoover power plant in Nevada, however, can provide electricity to more than one million consumers.

Benefits and drawbacks of hydropower

Hydropower is an efficient, clean, and reliable source of energy. Once hydropower plants are constructed and the technology is in place, the cost of hydropower is the lowest of all energy sources. No fuel is used during hydropower production; water returns to the river. No pollutants are released into the air during electrical generation, so the air around hydropower plants remains clean. Hydropower facilities can respond to high demands for electricity through reservoir storage, even in times of water shortage.

The negative effects of hydropower generally come from large-scale facilities. Impoundment or pumped storage facilities often alter wildlife habitats along rivers because large dams must be built to provide a reservoir. Dams can flood areas close to a river or lake, obstruct fish migration (periodic movement from one region to another), and affect water quality and flow downstream. Humans are sometimes displaced from their homes when dams are built. In diversion facilities, seasonal and annual fluctuations (variations) in water supply can negatively affect electrical production if river flow rates become too low. Regulations and licensing permits for dams and hydropower facilities as well as electrical transportation from remote hydropower are often costly.

The future of hydropower

Recent energy shortages in the United States has spurred the government to study hydropower’s future potential. The Department of Energy has identified 5,677 sites in the United States that have potential for large-scale hydropower development, but many of these sites are not possible because of economic drawback, such as their remote location, environmental issues, and other circumstances. Because a change to hydropower as a primary energy source could help lessen air pollution and reduce demands for fossil fuels (oil and gas), hydropower will likely be further explored.


Canal: Man-made or artificially improved waterway used for travel, shipping, irrigation or hydropower.

Electrical current: Flow of electricity.

Generator: Machine that converts mechanical energy to electrical energy.

Turbine: A device that transforms the energy in moving fluid or wind to rotary mechanical energy; electrical turbines generate electricity by spinning inside a magnetic field.

Voltage: Energy potential from a source that can produce a flow of electricity in an electrical conductor (circuit).

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