How Do Hydroelectric Dams Work: Parts of a Dam Examined & Explained

Three Gorges Dam on the Yangtze River, China generates up to 22,500 MW.

Three Gorges Dam on the Yangtze River, China generates up to 22,500 MW. Image by Le Grand Portage. License: CC BY 2.0

How do dams work? Hydroelectric dams work by converting the movement or falling of water into useful forms of energy such as electricity. Many forms of hydro power exist and have been used by humans throughout history. Some examples of this are watermills which powered machinery such as sawmills, irrigation which used gravity, and dams which use falling water to power turbines. Check out the environmental impact of hydro power as well. This page will focus on hydroelectric dams as it’s the most commonly used hydro power today. Above is a picture of the Three Gorges Dam which is currently the largest hydroelectric dam in the world on the third largest river in the world, the Yangtze River.

Hydroelectric dams use a reservoir to create potential energy from the dammed water. This water flows through an intake and into a large pipe called a penstock. The penstock feeds water into a turbine which powers a generator.

Diagram showing how hydroelectric dams generate electrcitiy. Image by Tomia. License: GNU FDL

Diagram showing how hydroelectric dams generate electrcitiy. Image by Tomia. License: GNU FDL

Many dams will have multiple reservoir tanks which are held at the top of the dam. During low electricity usage the turbines will power pumps to fill these tanks. Then during peak electricity demand the tanks will be dumped onto the turbines to increase their production potential.

Hydroelectric turbine diagram showing scale and operation.

Hydroelectric turbine diagram showing scale and operation.

The dam generator consists of a turbine powered by water flowing through the wicket gate. This gate can be opened more or less to determine the rate of flow through the turbine and thus the amount of power generated. The turbine is attached by a shaft to an electric generator. Hydroelectric dam generators are quite large as you can see by the human scale example in the lower left of the image on the left.

According to Faraday’s law of induction, “The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.” Essentially this means if you have a closed circuit (such as a coil of copper wire connected at both ends) and you move magnets through it (changing the magnetic flux) you can produce electromotive force (electricity).

Looking at the image above you’ll notice two parts at the top of a hydroelectric turbine, the stator and rotor. The stator provides the closed circuit by looping wire which leaves the generator (this loop is still connected but we place a load on the circuit, the electrical grid) and the rotor has magnets attached or evenly distributed metal with magnets outside the system (as shown in the image). When the rotor spins due to the water turning the blades of the turbine it begins to produce magnetic flux as the magnets move or system moves relative to the magnets. This creates electromotive force in the closed circuit of wire which is connected to the electrical grid and thus electricity has been generated from flowing water.

Parts of a Dam

There are certain dam parts that are common to hydroelectric dams. It’s important to remember though that there are many types of dams and only a few of these are present in all dams.

  • Reservoir: One of the main components of any dam is that they hold back a body of water known as the reservoir. Usually this will be a manmade lake that was flooded when the dam was created to form a reservoir.
  • Dam: Another primary component is the actual dam. Usually constructed from concrete but other material such as dirt or rocks can be used. Must have enough gravitational pull (gravity dams) to overcome the water pushing behind it, sometimes combined with an arch shape to allow for less material (gravity arch dams).
  • Intake: Water needs to enter the dam so it can be allowed to the other side. Most intakes will have screens and control mechanisms to prevent anything from entering the penstock which could damage the turbine.
  • Penstock: This is simply an open shaft through the dam that allows the water to flow from the intake to the outflow.
  • Wicket Gate: Wicket gates surround the turbine which can be opened or closed by varying degrees to control the flow rate and thus power generation of the turbine.
  • Turbine: All hydroelectric dams have at least one turbine but most have multiple turbines all fed by independent penstocks. Water flowing through the turbine causes the blades of the turbine to spin which turns a shaft connected to the generator.
  • Generator: Where electricity is produced by creating a changing magnetic flux through a closed circuit using the mechanical energy of the spinning turbine shaft.
  • Transformer: Power generated by the generator needs to be converted to match the electrical grid for distribution which is the function of the transformer.
  • Outflow: Where water leaves the dam and rejoins the river or other source of water that was dammed.

There are a lot of different types of hydro power and this is only one way hydro power work can be harnessed to produce useful energy. You might want to look up tidal power or ocean power as well. Also, there are many different types of hydroelectric dams but they all operate based on these principles.



  1. Freya says:

    Cool, ty tis very useful =D

  2. betty says:

    this was really useful thanks a ton

  3. catherine says:

    Hi I am catherine kim from kristin school in auckland
    Year 4′s are making a project or somthing.
    I really like your website


    From Catherine Kim

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