Did you know that hydroelectricity is the world’s biggest source of renewable energy? According to recent figures from the International Renewable Energy Agency (IRENA), it represents 40% of total capacity, ahead of solar (28%) and wind (27%).

This type of energy is generated by hydroelectric power stations, which are essentially large dams that use the water flow to spin a turbine. They can also serve secondary functions such as flow monitoring and flood control.

To help you learn more about hydropower, Visual Capitalist's Marcus Lu has visualized the five largest hydroelectric dams in the world, ranked by their maximum output.

The following table lists key information about the five dams shown in this graphic, as of 2021. Installed capacity is the maximum amount of power that a plant can generate under full load.

At the top of the list is China’s Three Gorges Dam, which opened in 2003. It has an installed capacity of 22.5 gigawatts (GW), which is close to double the second-place Itaipu Dam.

In terms of annual output, the Itaipu Dam actually produces about the same amount of electricity. This is because the Parana River has a low seasonal variance, meaning the flow rate changes very little throughout the year. On the other hand, the Yangtze River has a significant drop in flow for several months of the year.

For a point of comparison, here is the installed capacity of the world’s three largest solar power plants, also as of 2021:

Compared to our largest dams, solar plants have a much lower installed capacity. However, in terms of cost (cents per kilowatt-hour), the two are actually quite even.

The Three Gorges Dam is an engineering marvel, costing over $32 billion to construct. To wrap your head around its massive scale, consider the following facts:

Of course, any man-made structure this large is bound to have a profound impact on the environment. In a 2010 study, it was found that the dam has triggered over 3,000 earthquakes and landslides since 2003.

While hydropower can be cost-effective, there are some legitimate concerns about its long-term sustainability.

For starters, hydroelectric dams require large upstream reservoirs to ensure a consistent supply of water. Flooding new areas of land can disrupt wildlife, degrade water quality, and even cause natural disasters like earthquakes.

Dams can also disrupt the natural flow of rivers. Other studies have found that millions of people living downstream from large dams suffer from food insecurity and flooding.

Whereas the benefits have generally been delivered to urban centers or industrial-scale agricultural developments, river-dependent populations located downstream of dams have experienced a difficult upheaval of their livelihoods.

– RICHTER, B.D. ET AL. (2010)

Perhaps the greatest risk to hydropower is climate change itself. For example, due to the rising frequency of droughts, hydroelectric dams in places like California are becoming significantly less economical.