Hydropower is the production of electricity from the kinetic energy of moving water such as rivers, streams, or oceans. The earth’s water cycle virtually provides us with an endless supply of water. This makes it an ideal source of renewable energy.

There are many ways to obtain power from water. The most common methods include large hydroelectric dams, pumped storage facilities, small scale hydro for homes & small communities, and also ocean power technologies to harness tidal and wave power.

Today, hydro power accounts for about 8% of the total power production in the United States, and about 40% of total renewable power production. That number has been declining in recent years since most of the best sites for hydropower plants have already been developed. However, there are still many possibilities for small scale hydropower projects as well as the emerging ocean power industry.



Water has been used as a means of harnessing energy for centuries. Waterwheels were used by the Greeks thousands of years ago to catch falling water and make a large wheel spin, which could then be used for processing grains, pumping water, etc. In the early 1800s factories began using the waterwheel as a means of powering machinery.

It wasn’t until the late 1800s, the Renaissance era of electricity, that methods for using the power of water to create electricity started being developed. In 1880, a chair factory in Michigan successfully developed a water turbine generator to power electric lamps. Shortly afterwards, the very first hydroelectric power plant was developed at Niagara Falls.

At first, hydroelectric power plants could only be used and distributed near water sources. It wasn’t until long-distance electricity transmission was invented that hydro power became a more widely available energy source.

Since these discoveries, the United States has been a leading pioneer in the hydropower industry. Large-scale hydroelectric projects such as the Hoover dam have been developed since the 1930s all the way up until the 1980s.

How Hydropower Works

Using moving water to produce power

There are many ways to harness the power of moving water, but regardless of which method is being used, most hydropower is generated by using this general process:


1. Water is directed into a water turbine.

2. The force of the water makes the turbine spin.

3. The turbine is connected to a generator.

4. The generator produces electricity.


Hydropower is considered a renewable energy source because it’s supply is constantly recycled and replenished through a process called the water cycle.

Here is an overview of how the water cycle works

1. Solar energy heats up the oceans water surface.

2. The water evaporates and rises into the air.

3. The vapor condenses into clouds and turns into rain.

4. Rain falls back to the surface.

5. Surface runoff makes its way into rivers and streams.

6. Rivers flow back into the ocean due to the force of gravity.

7. The cycle starts all over again.


River Hydropower Systems

Power generation systems used on rivers include hydroelectric dams (impoundment systems), pumped storage systems, and run-of-the-river systems.

Hydroelectric Dam

This is the most common type of hydropower system, which accounts for the majority of all renewable energy production in the US. Some examples of these are the Hoover Dam and the Grand Coulee Dam.

There are about 80,000 dams throughout the US, but only 2,000 of them have hydropower power plants capable of producing electricity. Most of them were built for irrigation and flood control purposes, but The U.S. Department of Energy currently has plans to implement hydropower into many of them and increase total power output by around 10%.

A hydroelectric dam is essentially a mechanical gateway that can control how often, how much, and how fast the water is allowed to passed through.

Water above the dam is collected and stored into a reservoir. When power is needed, fast moving water is released through a spillway gate, flows through an enclosed pipe called a penstock, and then passed into a water turbine & generator.

The amount of power that can be produced is dependent on how much head (height that the water falls from) and flow rate (how much water there is). Put simply, the higher the dam is and the larger the body of water, the more power you can generate.


There are many advantages of using a hydroelectric dam. It can be used as an effective way to control floods and store water for communities. It’s also very cheap and the power output is flexible depending on how much power is needed.

There are also many recreational uses for the reservoir that a dam creates. This artificial lake provides opportunities for boating, camping, water sports, fishing, camping, hiking, etc.


There are also some disadvantages. A dam can wreak havoc on a rivers natural ecosystem. Submerged land can displace many animals in the area (and humans) and can negatively affect their food & water supplies. It can also affect migrating fish, such as salmon, by halting their ability to swim upstream and reach their spawning grounds to reproduce.

Various types of fish ladders (aka fishway, fish pass, fish elevator) have been developed on some dams to help fish bypass the dam, with varying degrees of success and failure.


Pumped Storage Hydroelectricity

This system is similar to the hydroelectric dam, but with an extra water recycling element. A lower reservoir pumps water into an upper reservoir, which then releases the water through a turbine back into the lower reservoir so electricity can be generated.

Making this kind of system cost effective is a time-dependent operation. Water is pumped to the upper reservoir during off-peak hours when it is cheapest to run, then released into the turbines during peak hours to produce power that can be sold at higher prices.

New technologies are currently being developed which incorporate solar energy and wind energy to run power to the water pump, making it an even more cost effective operation.

Run-of-the-river Hydroelectricity

This type of hydropower uses a much smaller, less intrusive dam, and relies more upon the river’s natural flow to capture kinetic energy. A run-of-the-river system is considered much better for the environment since it does not require a land submerging reservoir.

Although this type of system is much better for the environment, it hasn’t proven to be very efficient in producing electricity. This is mainly due to the lack of water pressure and limited ability to store water, although some projects have incorporated small storage “pondage” stations to divert the water flow and store it for later use when demand is high.

The exception to this is when a run of the river system is built next to a natural waterfall, such as Niagara Falls. In this case, the potential power generation can be huge. Niagara Falls, by itself, is the largest producer of electricity in the state of New York.

Small-Scale Hydropower

Small-scale Hydro projects are usually used for a single home, community, or industrial plant. The power capacity is generally less than 30 megawatts (compare that to the Hoover dam, which has a capacity of 2,074 megawatts). This is pretty similar to the output expectations of solar and wind power systems, although overall the cost is much cheaper per kilowatt hour.

If you are lucky enough to live next to a flowing body of water, you can even set up your own micro hydro system to power your house. Setting it up can be tricky and maybe even costly, but once it’s installed, it can provide a constant supply of power for a very long time with very little maintenance required.


Ocean Power Technologies

About 70% of the Earth’s surface is covered by oceans. They contain an enormous amount of energy. If you’ve ever tried to swim in the ocean when the waves are large and strong, you’ll understand just how powerful a force it can be.

Ocean power is considered to be a renewable energy source since it’s dependent on the Earth’s relationship to the moon, and will always provide energy (so long as the moon continues to grace us with its presence).

So how can we take advantage of this vast potential of untapped energy?

Ocean energy technology has come up against a few roadblocks in the past, and is still not very widely implemented. Currently, a lot of money is being spent on research and development. There are many experimental projects, but not many workable solutions yet. That being said, the technology is still in its infancy and is steadily improving.

The three primary categories of Ocean power technologies are tidal power systems, wave power systems, and ocean thermal energy conversion.

Tidal Power

The most common application uses a tidal barrage system to take advantage of the oceans natural tidal forces. The water level naturally rises and falls twice per day with the tide. Water is captured when the tide comes in, and fed through the turbine when the tide goes back out.

It’s very similar to a pumped storage dam (see above) but without the need for a pump. It’s a very predictable source of energy, more so than wind and solar energy.

Wave Power

Although not as consistent as tidal power, there is an enormous amount of energy available from the ocean’s waves.

The most promising method of generating power from waves is with oscillating water columns. These systems harness the force and motion of waves to create air pressure from trapped air. This compressed air is used to spin a turbine to produce electricity.

Other methods use floating mechanisms or tethered underwater tubes to pump turbines with the rising and falling of the waves.

Ocean thermal energy conversion (OTEC)

This process takes the differences in temperature between deep cool water and shallow warm water to operate a heat engine that generates electricity. The temperature difference generally needs to be at least 20°C (36°F) to be effective.

The warm water is boiled to create steam vapor which then expands and spins a turbine. Cold water is then used to condense the vapor back into a liquid to be reused again.

It is still in the experimental stages, but many experts are claiming that it can someday become competitive with standard available power sources.

Currently, ocean power plays a pretty small role in the world of renewable energies, but as research and development continues, it will become much more important for providing us with much needed clean energy in the future.


The Future of Hydroelectric Power

Although hydropower currently holds the most production out of all of the renewable energy sources, water power programs have been steadily declining throughout the years.

There are several reasons for this:

  • Increased concern about the environmental impact of dams.
  • Lack of new sites to build hydropower plants.
  • More promising and sustainable renewable options like solar & wind power.

With the exception of ocean power and small scale hydropower, you can probably expect to see hydropower production continue to decline and eventually be surpassed by other forms of renewable energy.