hydroelectric power

While hydropower is often widely touted as being a green and clean way to generate energy – it also draws criticism. There are pros and cons of hydroelectric power, but because reservoirs and hydropower plants don’t burn fossil fuels to create electricity, the many downsides to hydropower are often overlooked.

Regardless of where opinion falls when it comes to determining just how ‘green’ hydropower is, it’s important to be clear on what, exactly, hydroelectric energy entails, to better understand its wider impact on the environment.

What is hydroelectric power?

Hydroelectric power refers to using the kinetic energy of moving water or the potential energy of unmoving water to generate electricity.

water moving through a hydroelectric power plant

In most types of hydropower generation, the process occurs in five steps:

  1. A water flow turns a turbine connected to a generator in a powerhouse.
  2. The generator’s rotational energy is converted to electricity.
  3. Electricity flows from the generator to a utility substation (transmission yard) where its voltage is increased.
  4. Then electricity travels long distances via transmission lines to the power grid.
  5. Electricity is delivered to businesses and consumers like you.

History of hydroelectric power

As described in the U.S. Federal Energy Regulatory Commission (FERC) document titled Hydropower Primer, water has been used for thousands of years in many parts of the world to create mechanical power to grind grain.

Over 200 years ago in the United States, the first dams were constructed to generate mechanical power at grain and paper mills as well as other industries.

The world’s first hydroelectric power plant was built in 1882 on the Fox River in Appleton, WI.

In the early to mid-1900s, as funded under FDR’s New Deal, many dams used for mechanical power were converted to hydroelectric plants. Additionally, hundreds of new plants were built just as the national electric grid was being developed.

The 1940s saw the biggest contribution of hydroelectric power to electricity generation in the U.S.: 40%.

Below is a map of where the federally-owned hydroelectric projects are located along with the hydropower plants owned by private companies, municipalities, electric co-ops, or private citizens. (The majority of all hydropower plants in the U.S. are non-federal.)

Hydroelectric projects in the US map
Source: FERC

Current state of hydroelectric power in the United States

In 2021, according to the U.S. Energy Information Administration (EIA), only 6.5% of all utility-scale electricity produced in the U.S. came from conventional hydropower, defined as hydroelectricity generated from “natural streamflow.” 

With the exception of Mississippi and Delaware, all U.S. states have some form of conventional hydroelectric power:

hydroelectricity generation by state in the USA
Source: EIA

To find out the latest reservoir storage levels and historical comparisons for many U.S. hydropower plants, you can use this interactive tool. If you’re one of millions of people who rely on the shrinking Colorado River for potable water or electricity, this tool is very informative.

At the very least, it could prepare you for upcoming water rationing or water bans in your area.

The future of hydroelectric power in the United States

According to the American Society of Civil Engineers, there are approximately 91,000 dams in the U.S. Only 3% actually produce electricity today. The majority of dams were built for commercial navigation, irrigation, water storage, or flood control.

After extensive review and analysis, experts estimate that suitable non-powered dams could generate only 4,800 MW of additional hydroelectric capacity after renovations and repairs. (1 MW = 1,000 kilowatts.) This is the equivalent of only 0.0048 terawatts or a mere 0.00012% of all the electricity used in the U.S. in 2021.

So the question of whether it makes economic sense to invest in hydroelectric power development by converting non-powered dams, especially when solar, wind, and geothermal energies are available as electricity sources, is very pertinent.

How does hydroelectric power work?

Have you ever seen a historic grist mill on the back roads of America, like the one in the photo below? 

old watermill

That water wheel works on the same principle as today’s hydroelectric plants.

FERC’s Hydropower Primer provides a clear and concise description of how hydroelectric power works. Here is a brief summary.

Turbines in hydroelectric power

Since the early 1820s, three major types of “water wheels” have been invented. They were dubbed  “turbines” from the Latin word turbo meaning whirlwind or whirl by Benoît Fourneyron who invented the earliest type.

There are the three main types of turbines used in hydroelectric projects today:

  1. Pelton
  2. Francis
  3. Kaplan

Here are schematic drawings detailing their differences:

types of water turbine
The three main types of water turbines: (A) the Pelton turbine (or wheel); (B) the Francis turbine; (C) the Kaplan turbine
Source: David Darling

To understand the different ways each turbine type operates, it’s helpful to know the basic principles of how water flows in hydropower setups.

Hydraulic head

In order to have hydropower at all, there must be a difference in elevation through which water falls. 

As it’s moving downward, the water possesses kinetic energy.

But even when the water is perfectly still in an upper reservoir behind a dam, it possesses that same energy in a different form called potential energy.   

The vertical change in elevation between the reservoir water level (head) and the downstream water (tailwater) is known as the hydraulic head

schematic view of hydraulic head

As you can probably guess, the world’s largest hydroelectric plants (30+ MW) have huge hydraulic heads of hundreds of feet. 

By contrast, those producing only a couple megawatts of electricity have considerably smaller hydraulic heads, even under 100 ft.


There is another factor involved with hydropower that’s important in understanding how hydroelectric power works. It’s called flow.

In simplest terms, flow is the volume of water passing a certain point in a specified amount of time. 

As an example of flow, imagine filling up a bathtub. When the flow rate is high, the tub will fill up fast. But when it’s slow, you could spend 20 minutes or more and still not fill even half of it up.

Using both concepts, here are some generalities about how hydroelectric power works:

  • Water possesses more potential energy (which can be converted to more electricity) when both the hydraulic head and the rate of water flow are high.
  • If a hydroelectric plant has a small rate of flow, it must have a huge hydraulic head to compensate in order to generate significant electricity. 
  • If a hydroelectric plant doesn’t have great vertical elevation (meaning its hydraulic head is low), a very high flow rate can still generate substantial amounts of electricity.

Keeping this background in mind, here are the hydraulic situations for each major type of turbine that will produce the most electricity.

Type of TurbineOptimal Hydraulic Situation
PeltonHigh head, low flow
FrancisLower head, higher flows
KaplanMatches a wider range of head and flow conditions (adjustable turbine)

Hydroelectric power dams

There are many types of dams used in hydroelectric power. They serve to hold back water in reservoirs. Here are the major types of dams used today:

Type of DamPhoto of Dam
GravityGravity dam
ArchArch dam
Roller Compacted Concrete (RCC)Roller Compacted Concrete dam
Slab and ButtressSlab and buttress dam
EmbankmentEmbankment dam
RockfillRockfill dam

What are the 4 main types of hydroelectric power?

The U.S. Federal Energy Regulatory Commission (FERC) categorizes hydroelectric power into four general types:

  1. Conventional Impoundment (dam with reservoir)
  2. Conventional Diversion (run-of-river)
  3. Pumped Storage 
  4. Marine and Hydrokinetic (MHK)

In this article, we focus on the first three types of hydroelectric power. For more information on the fourth type, see this article.

1. Conventional Impoundment (dam with reservoir)

The most common type of hydroelectric dam has a wide and deep reservoir of water behind it. These may be used as recreational facilities for picnicking, boating, or fishing.

The major similarity among all types of impoundment hydropower projects is that the powerhouse is close to the reservoir.

Here is a diagram of what a conventional impoundment hydroelectric plant looks like:

Conventional impoundment hydroelectric project
Source:  FERC

2. Conventional Diversion (run-of-river)

Hydroelectric power plants located on rivers where salmon and other fish species return from the sea for spawning going upstream frequently have fish ladders to allow returning fish to get through.

The fish ladders are situated close to the main water flow in a diversion channel. They are not literal ladders. They are bypass channels for fish to get to their ancestral spawning areas. 

Often in diversion dams, the powerhouse is located far from the reservoir, maybe even a few miles away.

Here is a diagram of a conventional diversion hydroelectric project:

Conventional diversion hydroelectric project
Source:  FERC

3. Pumped Storage

Built mostly in the 1960s-1980s, pumped storage facilities are used to supply energy and/or water on demand to nuclear or fossil fuel power plants adjacent to them.

During off-peak periods when electric rates may be cheaper, water is pumped from a lower reservoir to a higher one. This is an energy-intensive process.

When electricity is needed (high demand), water is released to generate it. 

In 2021 in the U.S., there were only 23 TW hours of electricity created from pumped storage projects in 18 states out of 3,930 TW hours used. (1 TW = 1 trillion watts.)

Pumped storage systems use more electricity to pump water upwards than they produce. So, these facilities have net negative electricity generation.

Here is a diagram of a typical pumped storage hydroelectric setup:

Pumped storage hydroelectric project
Source:  FERC

4. Marine and Hydrokinetic (MHK)

Also referred to as tidal energy, marine and hydrokinetic energy use ocean waves, currents, tides, or inland waterways to generate energy.

So, no powerhouse or dam is necessary. An energy-generating apparatus (a turbine or similar) is placed directly in the water. 

The ocean’s kinetic energy or sub-surface pressure difference is transformed into electricity.

As of yet, MHK energy is still in its R&D phase.

Here is a schematic of one example of an MHK project:

Schematic of tidal current hydrokinetic project

FAQs about hydroelectric power

Here are a few frequently asked questions and answers about hydroelectric power. 

1. What are the top countries producing hydroelectric power in 2020?

The top four producers of hydroelectric power (in terawatt hours, TWh) in 2020: 

  1. China (1,355)
  2. Brazil (391)
  3. Canada (382)
  4. United States (286)

Note: One TWh = 1 billion kilowatt hours = 1 trillion watt-hours.

2. What is a microhydropower plant?

Green homesteaders, preppers, and small communities or businesses with a knack for DIY projects and a readily accessible stream or river on their property may consider building a DIY microhydro power plant as a source of off-grid renewable energy.

“Microhydropower can be one of the most simple and consistent forms or renewable energy on your property.”

United States Department of Energy (DOE)

In fact, the DOE has an entire page devoted to planning your very own DIY microhydro power plant!

A microhydro power plant usually generates 100 kW of electricity. Since the average household needs 5-6 kW of energy to run comfortably, a microhydro setup + battery storage could supply a small neighborhood, farm, or ranch easily. 

In conjunction with solar, wind, or geothermal energy, a microhydro system with battery storage would guarantee complete energy independence from the public utility grid. 

The type of hydroelectric power that works most sustainably and cheaply in micro form is a run-of-river system. Below is a diagram of the basic setup:

For more details on the nuts and bolts of such a DIY home energy project from start to finish, Manfred Mornhinweg in Chile documents his DIY adventures here.

Note: Even though the DOE didn’t mention it on their pages, be sure to get any necessary permits from your local jurisdiction before undertaking a DIY microhydro power project.

3. Can non-powered dams be converted to produce hydroelectric power?

It is possible to convert non-powered dams to produce hydroelectric power in some cases. Because these dams are older, they will require significant and costly renovation and retrofit. Whether it is economically feasible or not is questionable, especially in a climate crisis.

A group of researchers published a 2019 article asserting that due to environmental, safety, and cost concerns, there is a growing tendency in the U.S. to decommission aging dams or allow licenses to expire, including those with hydroelectric capacity.

The researchers hypothesized that photovoltaic (PV) panels could produce the equivalent amount of electricity generated today from hydroelectric power on only 13% of the land taken up by the reservoirs slated to be torn down. 

Furthermore, they calculate that if all the dams were removed and only 50% of the freed-up land was used by PV, the solar panels could produce over three times as much electricity.

This research suggests the heyday of hydroelectric power is over and solar power — the cheapest form of electricity in history — is here to take its place.

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