Introduction to Hydropower: How Falling Water Generates Electricity

What hydropower is

Hydropower is electricity generated from the energy of moving or falling water. The underlying physics is the same as any other form of generation that uses a rotating turbine: a force is applied to a turbine’s blades, the turbine spins a shaft, the shaft turns a generator, and the generator produces electric current. What makes a power plant a hydroelectric plant is the source of that force — gravity acting on water that has been collected at a higher elevation than the turbine.

The energy in a hydroelectric plant is largely the gravitational potential energy of water held above the generator. When water is released and falls toward the turbine, that potential energy converts to kinetic energy — the moving water’s motion. The turbine intercepts that kinetic energy and converts it into rotational mechanical energy. The generator then converts the rotational mechanical energy into electrical energy. Two physical quantities largely determine how much electricity a hydropower plant can produce: the head, or vertical drop the water travels, and the flow, or volume of water moving through the turbines per unit time. Greater head and greater flow produce more electricity.

How a hydroelectric plant works

The U.S. Geological Survey’s description of dam-based hydroelectric operation walks through a fixed sequence of components. Following the water from start to finish:

The dam blocks a river and holds water back in a reservoir, which raises the water surface and creates the head the plant runs on. Near the bottom of the dam wall, an intake opens into a large pipe called a penstock. When the operator opens the intake, gravity pulls water down the penstock at high speed. At the bottom of the penstock the water reaches a turbine, where it strikes the blades and spins the turbine’s rotating assembly. A vertical shaft connects the turbine to a generator sitting above it. Inside the generator, magnets attached to the rotating shaft pass through stationary coils of copper wire; that motion induces an electric current in the wire. The current leaves the generator as alternating-current electricity, runs through a step-up transformer to grid voltage, and feeds out onto the transmission lines. After passing through the turbine, the water exits the plant through a channel called a tailrace and rejoins the river below the dam.

That sequence — reservoir, intake, penstock, turbine, generator, transformer, transmission line, tailrace — is the recognizable shape of a conventional hydroelectric plant. Run-of-river plants and pumped-storage plants vary the upstream geometry, but the turbine-and-generator assembly is the same.

The three main types of hydropower

The U.S. Department of Energy categorizes hydropower facilities into three working types.

Impoundment plants

Impoundment plants are the dam-and-reservoir form most people picture when they hear “hydroelectric.” A large dam holds back a substantial reservoir behind it; operators release water through the turbines as electricity demand requires. The reservoir functions as a buffer between water supply (rainfall, snowmelt, river inflow) and electricity demand, which lets the operator adjust generation up or down without needing the river to cooperate hour by hour. Most U.S. hydropower capacity is impoundment.

Diversion (run-of-river) plants

A diversion plant, also called a run-of-river plant, channels a portion of a river through a canal or pipe to a turbine and then returns the water to the river downstream. Run-of-river plants store little or no water; their output rises and falls with the natural flow of the river. They have lower environmental footprint than large impoundments because they do not flood a reservoir, but they are also less flexible — you cannot dispatch them as freely against demand peaks.

Pumped-storage plants

Pumped-storage plants use two reservoirs at different elevations connected by a turbine and a reversible pump. When electricity is cheap and abundant — typically overnight — the system uses electricity from the grid to pump water from the lower reservoir up into the upper reservoir. When electricity is expensive and demand is high — typically late afternoon and evening — the system releases water from the upper reservoir back through the turbine to generate electricity. Pumped-storage is effectively a very large rechargeable battery: it does not produce net new energy, but it shifts when energy is available, which is increasingly valuable as more variable wind and solar generation comes onto the grid. The U.S. has roughly forty pumped-storage facilities, far fewer than the conventional impoundment fleet.

The Department of Energy also distinguishes between conventional, small, and micro hydropower based on installed capacity. Small hydropower plants are defined as projects with capacity between 100 kilowatts and 10 megawatts. Micro hydropower plants have capacity of up to 100 kilowatts — small enough to power a single farm, ranch, or off-grid home with a suitable stream and head.

U.S. hydropower at scale

According to the U.S. Energy Information Administration, the United States operates approximately 1,450 conventional hydroelectric plants and 40 pumped-storage facilities. Total conventional hydroelectric net summer generation capacity was about 80,090 megawatts as of 2023. Roughly half of that installed capacity is concentrated in three states — Washington, California, and Oregon — for the obvious geographic reason that the Pacific Northwest and the Sierra Nevada have the largest rivers and the steepest drops in the country.

The two largest federal hydropower operators are the U.S. Army Corps of Engineers and the Bureau of Reclamation. The Bureau of Reclamation, the second-largest U.S. hydropower producer, owns 77 hydropower facilities and directly operates and maintains 53 of them, totaling more than 14,750 megawatts of installed capacity. The 53 directly operated facilities generate, on average, about 40 million megawatt-hours of electricity per year — the equivalent annual demand of more than 3.8 million U.S. homes. The Bureau’s portfolio includes some of the most recognizable dams in U.S. history, including Hoover Dam on the Colorado River and Glen Canyon Dam upstream. The remaining 24 Reclamation-owned facilities are operated and maintained by non-federal entities under formal transfer contracts. The remaining U.S. hydropower capacity outside the Bureau is split among the U.S. Army Corps of Engineers, the Tennessee Valley Authority, large private utilities, and a long tail of smaller municipal and cooperative operators.

Environmental and operational considerations

Hydropower generators do not directly emit air pollutants while running, which is why hydropower is grouped with wind and solar as a renewable, low-carbon source of electricity. The fuel — water moving downhill — is replenished naturally by the water cycle. And hydropower assets last a long time: the EIA notes that many U.S. hydropower plants have been in operation for fifty years or more, and major facilities are typically modernized in place rather than retired.

The environmental considerations are not in the generator itself; they are in the dam and the reservoir. As the EIA documents, dams obstruct fish migration; large reservoirs change natural water temperatures and water chemistry, which affects the species that evolved to live in the original river; impounded water alters when and how much water moves through the river system downstream; and trapped sediment that would otherwise reach the ocean accumulates behind the dam. Modern hydropower projects address each of those effects with engineered fish ladders and bypass channels, scheduled environmental flow releases, sediment-management programs, and operating rules that limit ramp rates and protect water quality.

Operationally, the most significant non-environmental factor for U.S. hydropower is hydrology. Drought reduces the water available to generate, which is why hydropower-heavy states like California and Washington see year-to-year variation in hydroelectric output that thermal-generation states do not. Climate trends that shift snowpack timing and reservoir refill in the western U.S. interact directly with hydropower scheduling. None of those factors change the basic physics of the plant; they change the volume of fuel the plant has to work with in a given year.