WESTERN AREA POWER ADMINISTRATION

HARNESSING

HYDROPOWER

The Earth’s 

Natural Resource
How Electricity Reaches You

The hydropower system begins at a dam, where the powerplant converts the 

force of flowing water into electricity. Transmission lines carry electricity from 

the dam to a switchyard, which increases voltage for transmission across the 

country. Electricity is delivered to local utilities at substations, which decrease 

voltage for safe distribution to homes and businesses in cities, towns and rural 

areas. It provides light, runs electric motors, computers and appliances, and 

operates pumps to irrigate crops, using the same water originally used to 

generate that power.

DAM AND HYDRO POWERPLANT

SWITCHYARD

INDEPENDENT POWER PRODUCER 

OR INVESTOR-OWNED UTILITY

GENERATION

ELECTRIC COOP-OWNED OR 

MUNICIPAL UTILITY-OWNED 

GENERATION

DISTRIBUTION UTILITY SUBSTATION

FARM HOME SMALL BUSINESS

HIGH-VOLTAGE TRANSMISSION
1

Introduction

T

he growth of civilization has been linked to our ability to 

capture and use the force of flowing water to our benefit. 

Power from rushing water was first harnessed to process 

food and provide energy for factories and plants. Over 

time that water power also became useful for efficient hydroelectric 

generation.

Because the fuel for hydropower is supplied by nature’s own 

self-renewing hydrologic cycle—rain and snow filling the rivers, 

rivers rushing to the seas and water 

evaporating again to form clouds—it 

is an environmentally clean, renewable 

and economical energy source. Today 

hydroelectric plants use water power to 

produce seven percent of the nation’s 

electrical supply.

History of 

Hydropower

The water-driven machine is an 

ancient invention. In fact, the waterwheel was the first application of powerdriven machinery. More than 2,000 years 

ago, the Romans developed a waterwheel 

for milling flour. This invention became 

popular in Europe and was used widely 

by the 11th century.

Hundreds of years later, the waterwheel had increased its efficiency. 

However, before the discovery of electricity, machines powered by the wheel 

had to be directly coupled to it by shafts 

or belts. Thus, factories were built near 

rivers.

Even with this limitation, dependence on water power increased. In the United States, water supplied energy for textile industries in New England and the Niagara 

Falls power system—both of which were constructed before falling 

water was used to produce electric power.

Waterwheels convert the energy of 

motion to useful energy, allowing 

people to put power to work.
2

In its earliest days, the Niagara Falls Power Company channeled 

water to small wheels of several manufacturers. Within 10 years, 

the operation grew to become a central powerhouse, transmitting mechanical energy to nearby customers using ropes, belts and 

shafts. Later, Niagara Falls became the site for the first large hydroelectric installation in the United States.

Then in 1831, Michael Faraday discovered the principles of electromagnetic induction, the process of electrical current flowing in a 

closed loop of wire as it moves through a magnetic field. This led to 

the invention of the electric generator and the electric transformer. 

Faraday found that electrical current flowed in a closed loop of wire 

as it was moved through a magnetic field.

After 50 years of development, an electric generator directly produced hydroelectric power at an English plant in 1881. This hydroelectric facility in Goldaming, England, supplied power for three 

street lamps.

The first U.S. hydroelectric 

powerplant began operating 

in 1882 in Appleton, Wisc. 

This facility was rated at one 

horsepower, or 746 watts. It 

served two paper mills and 

a residence. Electricity was a 

new source of power, but how 

to control this resource was 

not well understood. Factories 

remained clustered beside the 

old waterwheel until ways to 

transmit hydroelectricity were 

developed.

In the United States, the 

first hydroelectric alternating current was transmitted in 1889 along 

power lines from Oregon City to Portland, Ore., 13 miles away. By 1896, 

300 hydroelectric powerplants were operating in the United States.

As the population increased and industry grew, the value and 

application of this resource became increasingly apparent. Today, 

more than 1,400 hydroelectric plants in this country supply seven 

percent of the nation’s electrical energy. Modern turbines and 

generators efficiently convert water’s energy into electricity. Highvoltage powerlines span the nation, carrying electrical energy to 

even the most remote areas.

By 1896, 300 hydroelectric powerplants were 

operating in the United States.
3

Nature’s

Water Cycle

The fuel for hydropower—

water—is supplied by nature’s 

own hydrologic cycle. Rain and 

melting snowpack fill rivers. Rivers flow to oceans, and the heat 

of the sun causes evaporation. 

Water vapor rises into the atmosphere where it forms clouds. 

When this water vapor condenses, rain and snow fall again, and 

the process repeats itself.

As water flows, it becomes 

a source of kinetic energy, the 

energy of motion. The old waterwheel transformed the kinetic energy of flowing water into 

mechanical (useful) energy.

Today, flowing water turns 

turbines. These rotating devices convert falling water into 

mechanical energy and are connected to power generators, which 

change mechanical energy into electrical energy. 

Water Into Energy: 

Hydropower Plants

Two types of hydropower plants exist. The first is a run-of-theriver plant that uses little or no stored water to provide flow through 

the turbines. Seasonal changes in stream or river flow and weather 

conditions affect the plant’s output. 

The second type, a storage plant or reservoir (dam), offers a more 

constant supply of electricity. A dam on the river stores the water 

flowing down from the mountains, creating a reservoir. This manmade lake acts much like a battery, holding the power of water in 

reserve. 

Dams do more than just supply water for hydroelectric power, 

however. Dams regulate rivers for navigation, provide flood control 

Snow and rain fuel the hydrologic cycle by 

flowing downstream to dams where the water 

power is harnessed for our use.
and store water for irrigation, recreation, industry and household uses.

To generate power, this water is released through a turbine in the powerplant that spins a generator and creates electricity.

The energy-producing potential of a 

hydropower plant depends on the difference in elevation between the reservoir (forebay) and the water below 

the dam (tailwater) and the volume of 

water available for release. The greater 

the volume of water stored in the reservoir and the greater the difference in 

elevation, the more potential for energy production.

In power generation, water must fall from a higher elevation to a 

lower one to release its stored energy. The difference between these 

elevations is called head. Dams are divided into three categories: 

y high-head (800 or more feet), 

y medium-head (100 to 800 feet)

y low-head (100 feet or less). 

Some plants may operate as a medium-head plant part of the year 

and as a low-head plant other parts of the year, depending on the 

amount of rainfall and snow melt.

Turbines convert falling water into mechanical energy. 

Generators then change it to electrical energy.

Run-of-the-river plants rely on seasonal 

water flow instead of stored water to 

supply electricity.

Storage project

Run-of-the-river 

project
5

Turbines and Generators

As energy is needed for 

power generation, water stored 

behind the dam flows through 

a penstock, or tunnel, to a 

turbine-driven generator below 

the dam. A turbine converts 

falling water into mechanical 

energy. The force of water on 

the turbine blades spins the 

turbines, which, in turn, drive 

a rotor, the moving part of a 

generator. The rotor contains 

coils of wire wound on an 

iron frame to create a strong 

magnetic field. As the rotor’s 

magnetic field sweeps past the 

generator’s stationary coil, it 

converts mechanical energy 

into electrical energy. 

After the water passes over 

the turbine blades, it exits 

through an exhaust structure under the turbine called the draft 

tube. The water then flows back into the riverbed in an area called 

the tailrace or afterbay and on down the river.

Flowing water turns the turbine, driving the 

generator and converting mechanical energy 

into electrical energy.

Headwater

Tailwater

Afterbay

Penstock

Generator

Turbine

Forebay

Dam

Power is generated 

when water behind the 

dam flows through a 

pipe called a penstock. 

The water then turns 

a turbine that drives a 

generator.
From the Powerplant

to the Line

Hydropower plants generate electricity. A transmission system 

carries it from the plant to homes and businesses that need electricity to operate. 

Electricity undergoes several changes along the way. The lowvoltage electricity, for example, 13 kilowatt volts produced by the 

generator at the powerplant flows through a large electrical conductor, or line, to a powerplant switchyard. In the switchyard, 

devices called transformers increase voltage to a level required for 

efficient transmission. 

High voltage (the pressure of the electrical current) is needed 

to transmit power efficiently over long distances. As the electrical 

charge, known as current, flows through the conductor, some of 

it gets used up as heat. This energy converted to heat is known as 

“losses.” The greater the current, the higher the losses will be. Power 

is the product of voltage times current, and losses are related to current (not voltage); thus, it is more efficient to transmit power with 

higher voltages and lower current. 

Transmission lines carry electricity from the dam to a switchyard that increases 

voltage for transmission across the country.
7

To keep losses small, transformers in 

switchyards increase electrical voltage. The 

transformers in the switchyard “step up” the 

voltage to 115 kV or higher for cross-country transmission to homes and businesses. Stepping up voltage does 

not increase the amount of 

power. The electrical wattage, or power level, stays 

constant.

To understand how this 

works, lets compare volts, 

amps and watts to water 

running through a hose. The 

voltage is the water pressure. 

The amps, or current, is the 

flow of water, and watts are 

the amount of water. So, if 

you increase the size of the 

hose (current or amps), the water pressure (voltage) would drop if 

the amount of water (watts) running through the hose remains the 

same.

Transmission Systems

High-voltage transmission lines act much like a highway system. 

Instead of cars, the lines carry electrical energy from hydropower, 

wind farms, fossil fuel and nuclear plants and other generators to 

local utilities across the country. These utilities distribute electricity 

on to homes and businesses.

A transmission system includes more than just transmission 

lines. Like a road system, the traffic on a transmission system must 

be controlled. Relays, circuit breakers and other types of equipment 

act as traffic control devices.

Relays react quickly to problems on the transmission line. They 

work with circuit breakers to protect the power grid from damage. 

Lightning strikes, downed lines and other conditions that cause sudden increases or decreases in voltage can damage other parts of the 

system if they aren’t isolated quickly.

To understand how voltage works, think 

of water running through a hose. If you 

increase the hose size (current or amps), 

the water pressure (voltage) would drop 

if the amount of water (watts) through 

the hose remains the same.
8

When a circuit breaker is closed, it provides a connection 

between two sections of transmission line. If lightning strikes a line, 

for example, the relay will note the change in voltage and current 

and then close. As the relay closes, it opens a circuit breaker, stopping the flow of electricity between sections of a transmission system. Opening the breaker isolates the line damaged by the lightning 

from the rest of the system, protecting the flow of power.

Of course, if a circuit breaker opens on a line serving a city or 

town, it can cause a loss of electrical service. When that happens, 

power system dispatchers must work to provide power through 

an alternate route. Just as highway repairs can cause cars to take 

a detour, faults in a power system mean dispatchers have to find a 

different way to re-route electricity.

The System’s Nerve Center

A power dispatching center serves as the brain of a transmission 

system. Dispatchers monitor demand for electricity and potential 

water power available to supply it. They communicate with powerplant operators and tell them to increase or decrease generation 

based on demand. 

Circuit breakers help protect the power grid from damage.
9

Each dispatch center controls electrical transmission in a specific 

area. Marketing staff sell and buy power to meet the needs of local 

utilities and schedule generation and transmission. Meters, graphs, 

displays and computer terminals tell dispatchers about generation 

at various plants, load demand and problems affecting the system. 

If part of the system is taken out of service resulting in an interruption, dispatchers work to restore power to customers. 

Through regular maintenance, many problems can be prevented 

before they happen. Maintenance staff routinely check all substation equipment and perform maintenance according to National 

Electric Reliability Corporation requirements or national standards. 

They also X-ray cables, check for deterioration of wooden poles, test 

connections and perform many other maintenance functions.

At a dispatch center, dispatchers monitor the demand for electricity and the 

power available to meet the demand.
10

From the Utility to You, 

the Consumer

Before electricity reaches homes or businesses, substation transformers reduce voltage from transmission levels to distribution levels. Substations work like a railroad switchyard, directing electrical 

current toward its destination. At a substation, a transmission line 

leads to a high-voltage bus, which is a rigid line that connects equipment of the same voltage. Step-down transformers connected to the 

bus lower voltage to a safe level for transporting through city streets. 

Another line carries electricity from a high-voltage bus to a low-voltage bus connected to the utility’s main distribution system. From 

there, lines carry the power through the utility’s service area. 

The wires of the primary system may be carried on poles above 

ground or burried underground through a system of cables and 

ducts. The method depends on the number of customers served, the 

number of transformers needed, necessary access for operating and 

repairing lines, cost and the appearance of the area served.

These main distribution lines carry power to secondary lines. At 

the connection between the primary and secondary lines, a small 

transformer immediately outside the residence or business steps 

down the voltage one last time—to 240/120 volts for homes and 

up to 600 volts for industrial and commercial customers, depending on their needs.

At substations, voltage is reduced for local distribution.
Wires leading to various buildings tap the 

secondary circuits. For an overhead system, 

the tap occurs at the nearest pole. Current 

flows through wires from the pole to customers’ homes and businesses. These wires pass 

through a metal conduit, or tube, to the customer’s meter and fuse (or circuit breaker) 

box.

In an underground distribution system, 

the customer’s line taps the circuit at the 

nearest connection box or manhole. The current goes through wires in a duct to the customer’s home. The duct may pierce the basement wall, or it may go up through the surface 

of the ground and connect to a box on the 

wall. The wires in the duct connect with the meter and fuse box.

In either case, the customer’s wiring finishes the delivery from 

the fuse box, carrying electricity to wall plugs, light fixtures and 

other outlets.

Small transformers outside a 

home or business step down 

voltage one last time.

The Power Grid

Transmission lines throughout the United 

States are connected to each other. Your 

utility’s power comes from the same lines 

as the power in neighboring communities 

and even neighboring states. This system 

of transportation for electricity is called the 

“power grid.”

Hydropower plants provide only part of the 

nation’s electricity. Powerplants that burn fossil fuels create steam to power turbines and 

run generators. Nuclear plants use energy 

from splitting atoms to produce electricity. 

Solar and wind-powered systems convert 

energy from the sun and wind to produce 

electricity. These sources all use the same 

transmission lines to bring power to your 

home. This power grid allows electricity to be 

shared so that varying demands can be met.

The United States has three main power 

grids. One serves the eastern states, one 

serves the western states and one serves Texas. Utilities in each of these areas have 

access to the same high-voltage lines. If one utility’s generation falls short of customer 

demand, another utility provides additional power. Linking utilities together provide 

more reliable electrical service.

The system for transporting electricity is 

called the power grid.

11
12

Hydropower in the 21st Century

Hydroelectric generation has evolved into a sophisticated and 

complex science. Weather satellites provide data to predict rainfall and stream flow long before water actually reaches reservoirs. 

Snowfall and stream flow measurements also indicate the amount 

of water expected during spring runoff. Although meteorologists 

compile plans and forecasts months in advance, some factors affecting water levels remain unpredictable. Rain, temperature and wind 

all influence reservoir levels, and these elements may change from 

day to day.

Water levels in each storage reservoir are planned far in advance, 

and the process is complex; sometimes it involves interstate and 

international agreements. When planning water releases, engineers 

must consider that irrigation, recreation, navigation, domestic uses 

and wildlife conservation also depend on the flow of rivers and the 

depths of reservoirs.

Additionally, computers have become vital tools for power systems. Today, controlling transmission and finding problems quickly 

are exact sciences. Previously, dispatchers made these transactions 

over the telephone. Now, microwave transmitters and other communication equipment send split-second messages to control generation and report and isolate trouble.

Computers also predict how much power 

consumers will use. 

They process data about 

supply and demand and 

the economics of using 

one power supply over 

another. This system in 

the power-control center 

then gives instructions 

to remote powerplants 

or substations.

Computers process data about supply and demand and the system 

then gives instructions to remote powerplants or substations.
13

Energy and the Environment

Energy is essential to the world today. We fuel our energy needs 

with natural resources, such as coal, nuclear fuel, oil, gas and hydropower. Most of these sources are fossil fuels, which are not renewable and exist in limited amounts. To reach them, we drill oil wells, 

tap natural gas supplies or mine coal and uranium. To put water to 

work on a large scale, we need storage dams. 

Americans want clean air and water 

and a pleasing environment, but we 

also need energy to heat and light 

homes and run machines. Balancing 

these two goals is a challenge.

The situation seems simple: Either 

the demand for electrical power must 

be curbed, or more power must be 

produced in ways that don’t destroy 

the environment. Neither solution is 

easy.

Conservation helps us save electricity, but our population continues 

to grow. Growth increases demand 

for electrical power. Natural resources 

will continue to be used, so the wisest solution is continued conservation 

combined with careful planning—or 

assessing all the choices and picking 

the best ones.

Hydroelectric generation is an economical, renewable-energy source that 

has been proven safe and nonpolluting. 

Because of the hydrologic cycle, it is the 

most efficient converter of energy presently available and can provide energy 

upon a moment’s notice.

Hydropower offers many advantages over other power sources:

y Hydroelectric powerplants use renewable resources. 

y Hydropower does not contribute to air, land or water 

pollution.

y Hydropower generators have low outage rates and low 

maintenance and operating costs. 

Reservoirs offer many recreational 

opportunities, such as boating. 

Balancing the need for a healthy 

environment and the energy we need 

for everyday functions is a challenge.
y Hydro turbines can provide startup power quickly if a system-wide failure occurs. They can be operated automatically 

and by remote control.

y Hydropower generators have a long life expectancy.

The reservoir of a hydroelectric project can provide many benefits 

not possible with other forms of generation. Reservoirs have scenic 

and recreation value for campers, fishing enthusiasts and those 

who enjoy water sports. The water is home to fish and wildlife, and 

reservoirs provide domestic water supplies, irrigation for agriculture 

and flood control. Dams can improve downstream conditions by 

allowing mud and other debris to settle out.

Meeting Peak Demands

Millions of barrels of oil are saved by “peaking” with hydropower. 

Demands for power vary greatly during the day and from season to 

season. Often, energy use reaches its highest level, or peak, during 

summer daylight hours when air conditioners are running. In farm 

communities, peaks can occur when grain-drying machinery operates or irrigation pump use is great. 

Because they start easily and quickly and change power output 

rapidly, hydroelectric plants are ideal for meeting demand. They also 

complement large thermal plants (coal and nuclear), which are most 

efficient in serving base power loads.

Responding to Environmental Concerns

Western has participated in many innovative 

programs in recent years to ensure that hydropower generation protects the downstream 

environment, such as fish and wildlife habitat. 

For example, fish screens have been added to 

prevent fish from swimming through dam turbines and fish ladders have been constructed to 

assist salmon in migrating upstream. 

Western is committed to continue searching for 

innovative ways to protect the hundreds of plant 

and animal species that depend on the rivers, 

lands and reservoirs. 

14

Fish ladders help salmon 

migrate upstream.
15

Usually, a utility company 

would have to build additional 

fossil-fueled powerplants (oil 

or gas) to meet peak demand. 

Through planning and cooperation between public and private utilities and the government, hydropower may eliminate the need for more plants 

by providing needed power 

during peak periods.

Lighting

the Future

Currently, we’re using only 

a part of hydropower’s potential. Studies show that many 

potential sites for conventional hydropower could be developed. If all these sites could be used, 

hydropower output might double.

However, not all these sites can be developed. Unfavorable terrain eliminates some; society’s concern about the environment eliminates others. Public resistance to building dams that would flood 

scenic or agricultural land is increasing. Sites for new, large dams are 

becoming non-existent.

Other ways to increase hydropower output include:

y Increasing output at existing plants

y Using pumped storage systems

y Tying hydropower to other forms of energy (such as solar)

y Improving existing equipment at many existing hydropower 

plants to increase capacity and, sometimes, lower generating costs—known as “uprating,” generators in this way can 

made make the most of available stored water, increasing 

output. 

y Adding generating units at existing plants 

On farms, peak demands for power can occur 

when irrigation pumps are running.
16

Another promising source of energy is low-head hydropower. 

Large, high-head dams produce more power at lower costs than lowhead dams, but construction may be limited by lack of suitable sites, 

environmental concerns or cost. 

In contrast, many existing small dams offer ideal sites for small 

generating plants. New low-head dams also could increase output. 

The ability to generate power where it is needed, reducing losses 

from transmission, makes these plants useful.

An Important Source of Energy

Hydroelectric generators use a renewable resource. They do not 

pollute the air. They operate reliably, adapt easily to different needs 

and require limited manpower. Low maintenance costs and outage 

rates and long life expectancy are other benefits.

Hydropower is a valuable resource that has evolved from the simple idea of the waterwheel. Through the years, technology improvements have made it efficient and reliable. Today, hydropower generation remains one of our nation’s important sources of energy. 

Through careful planning, use and management, Western is committed to seeing that consumers receive the benefits of this natural energy resource.

Photovoltaic panels, like the ones pictured on the left, harness the sun’s 

energy and provide one alternative to meeting the increasing demand for 

electricity. Low-head dams like the one at Nimbus in California, pictured 

right, are also a promising source of energy.
The constant

renewal of the

hydrological cycle

through rain and

snow makes water

a valuable energy 

resource for today 

and tomorrow.

This brochure was published by Western Area Power Administration, a Department of 

Energy agency that markets and transmits Federal hydroelectric power in 15 western 

and central states. Western operates and maintains the transmission system from its four 

regional offices in Billings, Mont.; Phoenix, Ariz.; Loveland, Colo.; and Folsom, Calif. Power 

is marketed from these regions and our Colorado River Storage Project Management 

Center in Salt Lake City, Utah.
For more information about Western, call or write:

CORPORATE COMMUNICATIONS

WESTERN AREA POWER ADMINISTRATION

P.O. Box 281213

Lakewood, CO 80228-8213

720-962-7050

Visit Western’s website at www.wapa.gov

Send e-mail to: CorpComm@wapa.gov

For energy-related technical assistance, call 

1-800-769-3756

PRINTED ON

RECYCLED PAPER

ENERGY

U.S. DEPARTMENT OF

4/2011