Hydropower, in which the power of moving water – rivers, streams and ocean tides – generates electricity, is the provider of 16% of the world’s electricity and 7% of America’s. Until 2019, it was America’s largest source of clean, renewable electricity — when wind surpassed it — generating power in all but two states and accounting for 52% of the nation’s renewable electricity generation.

Most hydroelectricity is produced at large dams (called Impoundment Facilities) built by the federal government, and many of the largest hydropower dams are in the western United States, the majority in California, Washington and Oregon. The oldest dates back to 1882, when the world’s first hydroelectric power plant began operating in the United States along the Fox River in Appleton, Wisconsin. The biggest is the Grand Coulee Dam on the Columbia River in Washington, a state that gets about two-thirds of its electricity from hydropower. New York has more capacity than other states east of the Mississippi, followed by Alabama.

A typical hydroelectric plant is a system with three parts: a power plant where the electricity is produced, a dam that can be opened or closed to control water flow, and a reservoir where water is stored. The water behind the dam flows through an intake and pushes against blades in a turbine, causing them to turn. The turbine spins a generator to produce electricity (the amount of electricity that can be generated depends on how far the water drops and how much water moves through the system). It then gets transported through long-distance electric lines to homes, factories, and businesses.

There are other types of hydropower plants, which make use of the flow through a waterway without a dam (called Diversion Facilities) and Pumped Storage. Eighteen states have pumped-storage hydroelectric plants. These generate electric energy during peak load periods by using water previously pumped into an elevated storage reservoir during off-peak periods when excess generating capacity is available to do so. As additional generating capacity is needed, the water can be released from the reservoir through a conduit to turbine generators located in a power plant at a lower level.

Hydropower has significant advantages. Once a dam has been built and the equipment installed, the energy source — flowing water — is free and essentially clean, an energy source renewed by snow and rainfall. Hydropower plants can supply large amounts of electricity, and they are relatively easy to adjust for demand by controlling the flow of water through the turbines.

But there are some disadvantages, too. A dam that creates a reservoir, for example, may obstruct fish migration and affect the ecology of the river, having negative effects on native plants or animals, not to mention people who might have to be relocated. The manufacturing materials, particularly cement, needed also pose a problem because of the carbon emissions associated. And dam failure can be both disastrous and deadly.

Furthermore, the promise of carbon-free electricity from hydropower has been undermined by revelations that decaying organic material in reservoirs releases methane, a potent greenhouse gas that contributes to global warming.

As the controversy continues, global warming, the very condition hydropower wants to combat, has also become its adversary as drought dries up water sources.

Research continues on ways to make hydropower projects more friendly to the ecosystems around them. In some places, small hydro projects can take advantage of existing water flows or infrastructure. Special water intakes and turbines can help make sure the water released from the dam is better aerated to address the problem of low dissolved oxygen. Dams can be planned more strategically to allow fish passages, for example, while water flows at existing dams can be calibrated to give ecosystems more recovery time from flooding cycles.

Tidal and wave power are also being explored. Tidal turbines look similar to wind turbines. They can be placed on the sea floor where there is strong tidal flow. Because water is about 800 times denser than air, tidal turbines have to be much sturdier and heavier than wind turbines. Tidal turbines are more expensive to build than wind turbines but capture more energy with the same size blades. A demonstration tidal turbine project is planned for deployment into the East River of New York in the autumn of 2020.

Ocean waves also contain tremendous energy. It is estimated that harnessing just two one-thousandths of the oceans’ untapped energy could provide power equal to current worldwide demand.

Then there is ocean thermal energy (OTEC), a process or technology for producing energy by harnessing the temperature differences (thermal gradients) between ocean surface waters and deep ocean waters.

The United States became involved in OTEC research in 1974 with the establishment of the Natural Energy Laboratory of Hawaii Authority. The laboratory is one of the world’s leading test facilities for OTEC technology. The laboratory operated a 250-kilowatt (kW) demonstration OTEC plant for six years in the 1990s. The United States Navy supported the development of a 105 kW demonstration OTEC plant at the laboratory site. This facility became operational in 2015 and supplies electricity to the local electricity grid.

The Department of Energy estimates there are nearly 50 gigawatts of untapped hydropower potential, according to the 2020 Hydropower Status Report, and that the existing infrastructure provides reliable electricity amid intermittent renewables like solar and wind.