Hydropower, which utilizes the energy of moving water—such as rivers, streams, and ocean tides—to generate electricity, currently accounts for about 23% of total U.S. utility-scale renewable electricity generation and roughly 6% of total U.S. utility-scale electricity generation. Until 2019, it stood as America's largest source of clean, renewable electricity—accounting for over half of the nation's renewable power—before being surpassed by wind. Today, conventional hydroelectric facilities operate in all but two states (Delaware and Mississippi), though its relative share of the green energy mix has gradually declined as utility-scale solar and wind generation experience rapid, record-breaking expansion across the national grid.
Most hydroelectricity is produced at large dams (called Impoundment Facilities) built by the federal government. Many of the largest hydropower dams are in the western United States, with the vast majority of capacity concentrated in Washington, California, and Oregon.
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—known as the head—and how much water moves through the system). It then gets transported through long-distance electric lines to homes, factories, and businesses.
The U.S. history of water power dates back to 1882, when the world’s first commercial hydroelectric power plant began operating along the Fox River in Appleton, Wisconsin.
Today, the largest facility by capacity is the Grand Coulee Dam on the Columbia River in Washington, a state that gets about two-thirds of its electricity from hydropower. East of the Mississippi River, New York holds more hydroelectric capacity than any other state (largely due to massive installations near Niagara Falls), followed closely by Alabama.
There are other types of hydropower plants that make use of waterways without requiring massive reservoir dams, such as Diversion Facilities (often called run-of-the-river plants). Additionally, 18 states utilize Pumped-Storage hydroelectric plants. These facilities act as giant mechanical grid batteries. They generate electric energy during peak load periods by releasing water from an elevated storage reservoir through a conduit to turbine generators at a lower level. During off-peak periods, when cheap excess generating capacity (such as midday solar or nighttime wind) is available, the system reverses to pump the water back up into the elevated reservoir.
Hydropower has significant advantages. Once a dam has been built and the equipment installed, the energy source—flowing water—is free, clean, and continuously renewed by snow and rainfall. Hydropower plants can supply large amounts of electricity, and unlike weather-dependent wind or solar, they are highly dispatchable and easy to adjust for grid demand simply by controlling the flow of water through the turbines. [1, 2]
However, there are notable disadvantages. A dam that creates a reservoir obstructs fish migration and permanently alters river ecology, which impacts native plants, wildlife, and can displace local human communities. Furthermore, the construction materials—particularly cement—carry a heavy carbon footprint. The promise of completely carbon-free electricity has also been challenged by scientific evidence showing that trapped, decaying organic material at the bottom of reservoirs releases methane, a potent greenhouse gas. Finally, dam failures pose rare but catastrophic and deadly risks.
As environmental debates continue, global warming—the very condition hydropower aims to combat—has also become its adversary. Severe, climate-driven droughts directly threaten water sources. This vulnerability was heavily highlighted when Western U.S. hydropower generation plunged to a 22-year low due to severe drought conditions across the Pacific Northwest.
HOW CAN HYDROPLANTS BECOME MORE SUSTAINABLE?
Research continues on ways to make hydropower projects more sustainable for surrounding ecosystems. In many places, developers are avoiding new dams by installing small hydro modules on existing water infrastructure, such as non-powered agricultural canals or flood-control dams. To protect water quality, modern facilities utilize special water intakes and aerating turbines to ensure the water discharged downstream maintains healthy dissolved oxygen levels. Additionally, dams are being mapped more strategically to integrate advanced fish passages, while water release cycles are being carefully calibrated to mimic natural seasonal flooding and give river ecosystems time to recover.
Energy researchers are looking beyond rivers to harness marine energy:
CREDIT: Student Energy
