Hydropower: Dams, Rivers and Water Years

A hydro plant is a machine for cashing in gravity. Water sitting high behind a dam holds energy because of its height. Let it fall through a turbine and that energy turns a shaft, the shaft spins a generator, and the generator pushes electrons onto the grid. No fuel bill. No smokestack. The catch is that the fuel arrives as snow on a mountain, and nobody controls the weather.

Hydropower made about 6 percent of all utility-scale electricity in the United States in 2025, according to the Energy Information Administration, roughly 247 billion kilowatt-hours from about 80,000 megawatts of conventional hydro capacity. That national number hides how lopsided the map is. Washington alone produces about 26 percent of the country's hydropower. Oregon adds 12 percent. In the Northwest, hydro is the main event.

The one formula you need

Every hydro plant obeys the same physics. Power depends on how much water moves through the turbine each second (the flow), how far it falls (the head), and how well the machinery converts that fall into electricity (the efficiency).

Power (watts) = density x gravity x flow x head x efficiency
P = 1,000 kg/m3 x 9.81 m/s2 x Q (m3/s) x H (m) x efficiency

1,000 x 9.81 x 200 x 100 x 0.90 = 176,580,000 watts = about 177 MW

1,000 x 9.81 x 50 x 10 x 0.90 = 4,414,500 watts = about 4.4 MW

Modern turbines are very good at their job, often 85 to 90 percent efficient or better at their best operating point. Most of the losses in a hydro system happen elsewhere: water spilled over the dam because the turbines are full or the power is not needed, water released for fish or irrigation instead of generation, and water that simply never arrived because the snowpack was thin.

Three kinds of hydro plant

Water years: the fuel you cannot order

A gas plant's output depends on whether someone buys gas. A hydro plant's output depends on last winter. The Northwest Power and Conservation Council says hydropower supplies about half of the region's annual energy and 54 percent of its flexible capacity. The Bonneville Power Administration markets power from 31 federal hydro projects and supplies about a third of the electricity generated in the Northwest.

Idaho shows how much a dry stretch moves the numbers. Here is what EIA's state tables report for in-state generation:

Hydro output fell about 24 percent from 2018 to 2023, while natural gas, wind and solar grew at the same time. In 2024 Idaho's generation mix was roughly 46 percent hydro, 31 percent natural gas, 15 percent wind, 5 percent solar and half a percent geothermal. Idaho also brought in a lot of power from other states: EIA lists net interstate imports of 8.35 million MWh in 2024, against 26.4 million MWh of retail sales. Roughly a third of what Idahoans used came from outside the state's borders.

Capacity factors tell the same story. EIA lists Idaho's hydro fleet at 39 percent in 2024, 43 percent in 2019 and 36 percent in 2022. The national hydro fleet ran at about 35 percent in 2024 and 2025. Those figures look low next to a nuclear plant at 90 percent, but they reflect a choice as much as a limit: operators save water for the hours and months when power is worth the most.

Maximum possible: 1,222.3 MW x 8,760 hours = 10,707,348 MWh
Capacity factor: 5,571,005 / 10,707,348 = 0.52, or 52 percent

Pumped storage: hydro as a battery

Pumped storage plants make no new energy. They move it in time. When power is cheap (a windy night, a sunny spring afternoon) they pump water uphill. When power is expensive they let it run back down through turbines. EIA counts about 23,000 MW of pumped storage across 18 states, with California and Virginia holding the most. In 2019 EIA measured the average round-trip efficiency at 79 percent, meaning about 21 percent of the energy used for pumping never comes back.

Pump 1,000 MWh uphill at $20/MWh   = $20,000 spent
Get back 790 MWh at $90/MWh        = $71,100 earned
Gross margin before costs          = $51,100

The business is arbitrage, buying low and selling high. DOE's water power office, citing its 2024 Hydropower Market Report, says pumped storage held 88 percent of U.S. utility-scale storage. That share is falling fast as batteries arrive by the gigawatt, covered in the storage lesson of this module.

What hydro costs the river

Hydro has the lowest operating costs on the grid and some of the longest lives; plenty of dams have run for 80 years. The bill comes in other forms. Dams block salmon and steelhead migrations. Reservoirs warm water and trap sediment. Some reservoirs, especially shallow tropical ones, release methane from rotting vegetation, which is why lifecycle emission estimates for hydro range so widely (see the lifecycle lesson). In the Hells Canyon case, the FERC record runs back to a final environmental impact statement in 2007, Clean Water Act certifications from Idaho and Oregon in 2019, and a draft supplemental statement in January 2026. The original license was issued in 1955 and expired in 2005; the project has run on annual licenses since. FERC's schedule targeted September 2026 for the final supplemental statement; check eLibrary under P-1971 to see whether it came out on time.

For a benchmark on what Northwest hydro costs at wholesale: the Bonneville Power Administration's BP-26 rate case set the average Priority Firm Tier 1 power rate at $37.77 per MWh, about 3.8 cents per kWh, for October 2025 through September 2028, an 8.9 percent average increase. That is the generation rate. Transmission is priced separately and rose 19.9 percent in the same case.