What is hydropower?

Hydropower is one of the oldest forms of renewable energy, with Greeks using water wheels to grind wheat as long ago as 2,000 years ago. Fast-forward to 1936 and the Hoover Dam was built, one of the largest and most famous hydroelectric projects in history. From small to large scale projects, all hydroelectric projects capture the energy from flowing water to generate power.

Hydropower can be grouped into three methods of production: impoundment dams, diversion, and pumped storage. Combined, these account for 7% of total electricity generation in the United States , and 44% of the total renewable generation in 2017, making it the highest producing renewable energy. As solar capacity and wind power capacity increase, hydropower has declined in its percentage of the renewable generation but still maintains the largest capacity of renewable generation.

The purpose of an impoundment dam is to create a reservoir from which water can be released to turn a turbine and power a generator, producing electricity. This kind of energy generation is useful because it can store water power as long as it needs to, it is flexible, and it is available 24/7. However, only 2,400 out of 80,000 dams in the United States currently produce power (just 3%), the rest function as flood control, water supply, irrigation, and even for recreational purposes.

Though hydroelectric is the top producing renewable energy resource in the United States, it accounts for only 6% of total generation. Worldwide, other countries rely on hydroelectric for much higher percentages of their electricity such as Norway with 96%. Note that these countries produce the most hydro as a percentage of their total generation, the highest producers capacity-wise are China, Canada, Brazil, and the United States. Image: Climable.org Data Source: World Bank

Though hydropower is commonly associated with dams, it does not have to utilize a dam to capture power. Diversion, or run-of-river hydro, is when water is simply diverted from its natural path, run through turbines and then returned to the source. For pumped storage, during periods of low electricity demand, water is pumped uphill into a reservoir to store it for future energy needs. Then, in periods of high demand the water can be released, turning a turbine and generating electricity to meet the demand.


Environmental impacts of hydropower, especially impoundment, make it a contentious source of power. Diversion sometimes has fewer environmental impacts, but is far more variable since it relies on the natural flow of the river and therefore is much more sensitive to droughts and other natural changes. Pumped storage actually often has negative electricity generation, as it is usually only used to reduce peak demand and the power to pump it up to the reservoir can either be equal to or exceed that which is produced. Therefore, impoundment is the most commonly used since it is reliable and can store energy to be released when power is needed.

Ecological: Dams and reservoirs impact fish migration, water temperatures, and native plants and animals. The disruption to natural river flow and the changing current as water is held or released causes a constantly changing environment for local species both above and below the dam. Innovations such as fish ladders attempt to mitigate the harm by helping fish migrate upstream. In addition, the U.S. Department of Energy has sponsored research to help reduce fish deaths and injuries from turbines to 2% as opposed to 5-10%, but these turbines are not yet being used.

Land use: Reservoirs may cover important agricultural land or historic artifacts sites, making them disruptive to local communities. The construction of one of these massive power plants emits greenhouse gasses (in addition to methane gas that sometimes forms in reservoirs) and is expensive (as with any power plant), though the emissions can be offset by clean energy production over time.

Despite high upfront costs, hydropower continues to be the most widespread renewable source of energy. The flexibility and ability to react to varying demand gives it significant advantages. However, unless the impacts to the ecology and local communities are mitigated, the power source will continue to raise concerns and create resistance.