What Does A Dam Do In Hydropower?


A hydropower dam is a large, man-made barrier across a waterway like a river or stream used for generating hydroelectricity via the flow of water. Hydropower provides around 16% of the world’s electricity and dams are an important component of most hydropower systems (Wikipedia). Dams vary greatly in size and purpose but generally function to control water flow, manage floods, facilitate navigation, and generate electricity.

This article provides an overview of hydropower dams, including how they are constructed, their key components, main functions, how they generate electricity, their environmental impact, and dam removal efforts. It aims to explain in detail what dams do in hydroelectric systems and how they operate to produce renewable energy.

Dam Construction

There are several types of dams used in hydropower projects, the most common being gravity, arch, and buttress dams. Dams are generally constructed with concrete, earthfill, masonry, or a combination of materials (Types of Hydropower Plants). The type and size of dam is determined by factors like the topography, geology, hydrology, and purpose of the dam.

The dam construction process begins with extensive site investigations and planning. This includes geological surveys, hydrological studies, environmental assessments, materials testing, and architectural design. Once the dam design is finalized, the reservoir area is cleared of vegetation and structures. The river may be temporarily diverted while the dam foundation is prepared. Materials like concrete, steel, and aggregate are transported to the site and onsite batch plants are set up for concrete production. The dam structure is built up layer-by-layer, with technicians monitoring quality control and concrete curing conditions. Spillway gates, penstocks, and powerhouse structures are also constructed. Finally, the reservoir is allowed to fill up and testing is conducted before the dam becomes fully operational (What types of dams are there?).

Dam Components

The main components of a hydropower dam that enable it to generate electricity are the reservoir, dam wall, spillway, intake, penstock, turbine, and generator 1. The reservoir is the body of water held back by the dam. The dam wall blocks and controls water flow, while the spillway releases excess water pressure. Water enters through the intake and travels down the penstock towards the turbine. The force of the moving water turns the turbine, which spins a shaft connected to the generator to produce electricity.

Some key components of hydropower dams include:

  • Reservoir – Stores river water behind the dam
  • Dam Wall – Made of concrete, earthfill, rockfill, masonry, or other material to hold back water
  • Spillway – Allows controlled release of excess water to avoid overtopping the dam
  • Intake – Gate structure that controls water flow into the penstock
  • Penstock – Pipe or tunnel that delivers water to the turbine
  • Turbine – spins from the force of moving water to drive the generator
  • Generator – Converts mechanical energy from the turbine into electrical energy

Understanding the purpose of each component provides insight into how dams are designed and operated to generate renewable hydropower.

Dam Functions

Dams serve several important functions related to water management. One of the main purposes of a dam is water storage. By blocking and impounding a river, a dam creates a reservoir where water can be stored. This stored water can then be used for things like electricity generation, irrigation, and municipal water supplies. Dams provide a consistent and reliable source of water even during dry seasons or droughts.

Another key function of dams is flood control. By impounding excess water behind the dam during periods of high rainfall or snowmelt, dams help prevent downstream flooding. Reservoirs act as flood buffers by absorbing and storing flood waters. Dams also allow for controlled release of water downstream, preventing the river from overflowing.

Dams are also used to regulate downstream flows. By storing water during wet periods and releasing it during dry periods, dams help maintain stable water levels downstream year-round. This flow regulation ensures adequate water supply for human uses as well as ecosystems that depend on stable water conditions.

Electricity Generation

Hydroelectric power plants convert the potential energy of falling water into electricity using turbines and generators (Source). The process begins as water is held back by the dam, creating a reservoir and potential energy from the elevated water. The water travels through penstocks and pushes against turbine blades to spin them. The mechanical power from the spinning turbine transfers to the generator shaft, spinning electromagnets inside the generator. As the electromagnets spin past copper wire coils, it generates electricity through electromagnetic induction (Source). The electricity is then fed into transformers to increase the voltage for efficient transmission over long distances.

By harnessing the potential energy in water as it flows downstream, hydroelectric plants efficiently convert this renewable energy into electricity with minimal pollution. The amount of electricity generated depends on both the volume and height of water held back by the dam.

Environmental Impacts

While hydropower is a renewable energy source, dams and reservoirs can have negative environmental impacts (US Energy Information Administration). The flooding of land to create a reservoir results in the loss of forests, wildlife habitat, and agricultural land (Union of Concerned Scientists). This flooding disrupts terrestrial and aquatic ecosystems.

Specifically, the creation of dams and reservoirs negatively affects wildlife by altering natural water temperatures, flow rates, and flooding patterns. Fish migration routes can be hindered, preventing access to spawning grounds. Reservoir stratification and reduced flow velocities favor lentic species over lotic species. Rare species with limited habitat ranges are especially vulnerable to extinction (US Geological Survey).

Greenhouse gas emissions, especially methane, can also occur from the decomposition of flooded biomass in the reservoir. However, hydroelectricity has lower lifecycle emissions compared to fossil fuel electricity generation.

Dam Safety

Dams must be properly maintained and monitored to ensure their structural integrity and prevent failures. According to the Federal Emergency Management Agency’s Federal Guidelines for Dam Safety, dam owners are responsible for developing emergency action plans, conducting dam safety inspections, and performing maintenance (FEMA).

Regular inspections and maintenance are critical to identify potential issues before they become larger problems. Monitoring often includes visual inspections and instrumentation to track the dam’s performance. Instrumentation can measure water levels, pore water pressure, seepage, and settlements (Dam Safety Manual). Maintenance activities ensure critical dam components like the spillways and gates are functioning properly.

Spillways must be regulated to safely release excess water and prevent the reservoir from overtopping the dam. Spillway gates may need to be opened during heavy rainfalls or spring snowmelt to control the water level. Proper spillway operation requires regulating flows while considering downstream impacts.

Dam Removal

The removal of dams, known as dam decommissioning, has become an important river restoration strategy in recent decades. Growing concerns about aging infrastructure, declining fish populations, and a renewed recognition of the environmental impacts of dams have led to an increase in dam removal projects.

One of the largest dam removal efforts in history is currently underway on the Klamath River along the Oregon-California border. The project involves demolishing four hydroelectric dams – Iron Gate, Copco No. 1, Copco No. 2, and J.C. Boyle dams – which have disrupted salmon migration routes for over a century (Source). Removing these dams is expected to reopen over 400 miles of steelhead and salmon habitat and restore the natural flow regime of the river.

There are several benefits associated with dam removal. It can help restore river connectivity and allow fish migration again. River ecosystems recover as natural sediment transport resumes and downstream erosion is reduced. Rivers regain their natural hydrology, with natural seasonal high and low flows. Dam removal also eliminates safety risks associated with aging infrastructure. However, there are also costs, as electricity generation and reservoir storage capabilities are lost.

Each dam removal project requires extensive planning and assessment of environmental impacts. Sediment built up behind the dam must be managed to avoid downstream flooding when the dam comes out. Overall, dam removal aims to restore the natural free-flowing nature of rivers and recover river ecosystem functions.

Notable Dams

Some of the most notable and largest hydropower dams in the world include:

The Three Gorges Dam in China is one of the world’s largest power stations. Located on the Yangtze River, it has a generating capacity of 22,500 MW, making it the biggest hydroelectric power station in the world (List of largest hydroelectric power stations).

The Itaipu Dam on the Paraná River bordering Brazil and Paraguay is the second largest hydroelectric power station in the world with a generating capacity of 14,000 MW (These are the world’s largest hydroelectric dams).

The Xiluodu Dam in China, operational since 2014, generates 13,860 MW of power from the Jinsha River, ranking it as the third largest in the world (The 10 biggest hydroelectric power plants in the world).

Some other major dams include the Grand Coulee Dam in the United States, Tucuruí Dam in Brazil, and Sayano–Shushenskaya Dam in Russia.


Hydropower dams are complex infrastructure projects that have the capacity to provide renewable electricity to millions of people. Dams are made up of key components like an embankment, spillway, reservoir, and hydropower plant that enable them to block and control rivers. By blocking and controlling river flow, dams create reservoirs that store water and provide a controlled release that can be used to generate electricity.

While dams provide clean electricity, they also have downsides. Dam construction often displaces communities and harms wildlife habitats. There are also concerns around dam safety and their vulnerability to natural disasters. Many dams also lose efficiency over time due to sediment build-up in reservoirs. There is debate around removing outdated and unsafe dams while also building new sustainable hydropower projects.

Overall, hydropower dams look to have a strong future as countries seek to expand renewable energy. New dam projects are incorporating more environmental protections. There are also efforts to increase hydropower generation at existing dams through turbine upgrades. With climate change threatening reliability of many energy sources, the controlled electricity provided by dams will continue to play a vital role in the global energy transition.

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