# Hydroelectricity
**Hydroelectricity** is electricity generated from [[Hydropower|hydropower]]: water falling through a turbine turns an electrical generator, converting gravitational potential [[Energy|energy]] into electric power. It is the largest renewable source of electricity in the world — 4,578 TWh in 2024, about 14.3% of global power[^iha2025] — and, because a reservoir can hold water back and release it on command, it is also the [[Electrical_grid|grid]]'s oldest and largest form of storage. A hydroelectric station with a dam can raise or cut its output in minutes, which makes it the natural partner of sources that cannot follow demand, from [[Nuclear_engineering|nuclear plants]] to wind farms.
[[Minnesota]] claims the first central hydroelectric station in the United States: on September 5, 1882, water-driven dynamos on Upton Island below St. Anthony Falls lit arc lamps along Washington Avenue in Minneapolis, twenty-five days before the Edison plant at Appleton, Wisconsin, came on.[^mnopedia1882]
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- [[Pumped-storage_hydroelectricity]] — a day at a plant sized like TVA's Raccoon Mountain: pump at night, generate at the peak, keep about 80%.
- [[Penstock]] — push more water and the power peaks where friction eats a third of the head.
- [[Capacity_factor]] — a year of hydro, wind and solar settles near 40%, 33% and 20%.
- [[Electric_generator]] — poles × rpm ÷ 120 = hertz: a slow hydro runner needs dozens of poles for 60 Hz.
- [[Grid_energy_storage]] — add storage to a sunny, windy grid; unmet demand and curtailment fall together.
- [[Run-of-the-river_hydroelectricity]] — choose the design flow on a snowmelt river: energy, spill and capacity factor trade off.
*Six stations built 2026-09-10 from the Portal Books; each child article carries its own player, sources and a 16:9 and 9:16 video.*
## History
Water had driven machines for two thousand years before it drove a generator, and the first hydroelectric plants were mills with a dynamo added. Once practical [[Electric_current|direct-current]] dynamos appeared in the 1870s, water wheels were coupled to them to power arc lamps, and in the early 1880s the first such plants were running in England and the United States. The Minneapolis plant grew out of the milling district. In 1881 the Pillsbury A Mill had installed sixteen electric lights; in July 1882 a company backed by Charles A. Pillsbury, William D. Washburn and other mill owners renamed itself the Minnesota Brush Electric Company, and on September 5 five Brush dynamos driven by a waterwheel on Upton Island sent current over wires to lamps on Washington Avenue.[^mnopedia1882] In 1883 the company raised a 257-foot mast carrying eight lamps over the street, and the last gas street lamp in Minneapolis was not turned off until 1924.[^mnopedia1882]
The next step was distance. Direct current could not be carried far, but [[Alternating_current|alternating current]] could be stepped up to high [[Voltage|voltage]], sent over long lines and stepped down again, and from the 1890s large hydroelectric plants were built to serve cities far from the falls. At St. Anthony Falls the installed turbine capacity rose from about 13,000 horsepower in the 1880s to 55,068 horsepower by 1908.[^nps-ch6] In the twentieth century hydroelectricity became the centerpiece of national development programs, and the dams grew with them: Hoover Dam at just over 2 GW, Grand Coulee on the Columbia at about 6.8 GW, and the Three Gorges Dam in China at 22.5 GW, the largest hydroelectric facility in the world.[^murphy-ch11]
## Future potential
The best sites were developed first, and the remaining potential is limited. Of the energy that sunlight puts into evaporating water, only about 0.1% remains as potential energy in rain falling on land; that sets a theoretical global potential of about 44 TW, of which about 2 TW is judged technically feasible and about 1 TW economic.[^murphy-ch11] Present generation averages about 0.52 TW,[^iha2025] so hydroelectricity might at most double; it cannot by itself meet global electricity demand. Growth is now concentrated in countries with undeveloped rivers and in pumped storage. In 2024 the world added 24.6 GW of hydropower capacity — 16.2 GW conventional and 8.4 GW pumped storage — and the development pipeline stood at more than 1,075 GW, about 600 GW of it pumped storage.[^iha2025]
### Modernization of existing infrastructure
Much of the world's hydroelectric fleet is decades old, and replacing runners, generators and controls can add output without building a new dam. Modern runners designed with computational [[Fluid_dynamics|fluid dynamics]] recover efficiency lost to worn or dated designs and reduce [[Cavitation|cavitation]] and the swirling [[Vortex|vortex]] that forms in the draft tube at part load, digital governors and excitation systems let old units respond faster to the grid, and new fish-friendly turbines and aerating runners address some of the environmental costs that were accepted when the plants were built. Adding generators to existing non-power dams — built for navigation, [[River_engineering|flood control]] or water supply — is another way to gain capacity without a new reservoir.
## Generating methods
### Conventional (dams)
In a conventional plant a dam raises the water level and forms a reservoir. Water is drawn through an intake into a penstock, a large pipe that carries it down to the turbine; the pressure at the bottom is set by the height of the water surface above it, the head, exactly as hydrostatics prescribes.[^murphy-ch11] After the turbine, the water leaves through a draft tube and tailrace into the river below. The reservoir is what gives a conventional plant its flexibility: water stored in wet months can be used in dry ones, and water held overnight can be spent at the evening peak.
### Pumped-storage
A pumped-storage plant moves water between two reservoirs at different heights. When power is cheap or plentiful, [[Electric_motor|motor]]-generators pump water uphill; when demand is high, the water runs back down through the same machines acting as turbines. The plant is a net consumer of energy, because pumping and generating each lose some, but it shifts energy from hours when it is worth little to hours when it is worth much. It is by far the largest form of [[Electrical_grid|grid]] energy storage; world pumped-storage capacity reached 189 GW at the end of 2024.[^iha2025] Good sites need two large water bodies close together with a large height difference, and they are few.
### Run-of-the-river
A run-of-the-river plant has little or no storage: it uses the water the river brings that day, and any surplus spills. Its output follows the river's hydrograph — high in spring melt, low in late summer — so it cannot be dispatched like a reservoir plant, but it floods little land.
### Tide
A tidal barrage traps water behind a dam at high [[Tide|tide]] and releases it through turbines as the tide falls, or in both directions. The tides are perfectly predictable, but suitable estuaries with a large tidal range are rare, and tidal power remains a small niche.
### Conduit
Conduit hydroelectricity recovers energy from water already flowing through pipes and canals built for other purposes — municipal water mains, irrigation canals, aqueducts. Where a water system would otherwise burn off excess pressure through a valve, a turbine can take that head and return it as electricity.
## Sizes, types and capacities of hydroelectric facilities
Hydroelectric plants span more than seven orders of magnitude, from household turbines of a few hundred watts to stations of more than 20 GW. Classification by size varies by country; a plant above about 50 MW is treated as large everywhere, while the line for small hydro is drawn anywhere from 10 MW to 30 MW.
### Large facilities
Large stations of hundreds to thousands of megawatts are almost always dam-and-reservoir plants on major rivers, and they are among the largest power stations of any kind. Three Gorges (22.5 GW), Grand Coulee (about 6.8 GW) and Hoover (about 2 GW) are representative of the class.[^murphy-ch11] Their size is also their liability: the land they flood, the people they move and the consequences of failure all scale with the reservoir.
### Small
Small hydro, conventionally up to about 10 MW, serves a town, a mill or an industrial site, and usually runs off the river with little storage. It can feed the grid or stand alone where no grid exists. Its environmental footprint is smaller than a large dam's but not negligible, since it still diverts water and interrupts a stream.
### Micro
Micro hydro plants, up to about 100 kW, power single farms, villages or remote lodges. They are especially useful in hilly regions with year-round streams, and they complement solar panels where stream flow peaks in the season when [[Sun|sunlight]] is weakest.
### Pico
Pico hydro, under about 5 kW, can run a few lights and a radio from a drop of only a meter or two. A pipe diverts part of a stream through a small turbine and returns it downstream, with no dam at all.
### Underground
An underground station places the turbine hall in a cavern near the bottom of a large natural drop, such as a waterfall or a mountain lake above a valley. Tunnels bring water down to the machines and carry it away to the lower waterway. Underground construction protects the plant and the landscape but costs more to build.
### Calculating available power
The electrical output is the product of efficiency, water [[Density|density]], the acceleration of [[Gravity|gravity]], flow rate and head: `P = η ρ g Q H`. With ρ = 1,000 kg per cubic meter and g ≈ 9.8 m/s², a flow of 2,000 cubic meters per second through a 50 m head carries about 1 GW, and a plant at 90% efficiency delivers about 900 MW.[^murphy-ch11] Annual energy is set by how much water actually arrives, so a plant's capacity factor — average output divided by rated output — is well below 100%; United States hydroelectric plants run at about 40% on average.[^murphy-ch11] The derivation of the formula is given in [[Hydropower]].
## Properties
### Advantages
Hydroelectric units can start from standstill and reach full load in minutes, far faster than coal or nuclear plants, so they are used to follow demand, cover peaks and hold grid frequency steady. A reservoir stores energy cheaply, and the power it returns is worth more than baseload because it can be timed. Once built, a plant burns no fuel and emits almost no carbon dioxide, sulfur dioxide or [[Air_pollution|particulates]]; operating crews are small, and dams and turbines last fifty to a hundred years. Where a dam already serves navigation, irrigation or flood control, adding generation is comparatively cheap. And the conversion itself is efficient — close to 90% of the water's potential energy leaves the generator as electricity.[^murphy-ch11] Cheap, steady hydroelectric power has long attracted electricity-hungry industries, above all [[Aluminium|aluminum]] smelting, whose [[Electrolysis|electrolytic]] cells run around the clock.
### Disadvantages
The costs fall on the river and its people. Reservoirs flood land and displace communities; dams fragment [[Ecosystem|habitat]], block fish migration and trap sediment; releases change downstream temperature, oxygen and flow. Reservoirs in tropical forest can emit large amounts of methane from decaying vegetation, and flooding soils can release [[Mercury_(element)|mercury]], which accumulates in reservoir fish. Output depends on rainfall and snowmelt, and droughts cut generation just as heat raises demand. Reservoirs slowly fill with sediment and lose storage. A dam failure can release a flood far larger than any natural one. Hydroelectricity is also a low-density resource: even the most developed state in the United States produces only about 0.05 W per square meter of land area.[^murphy-ch11]
### Comparison and interactions with other methods of power generation
Hydroelectricity is valuable less for how much energy it makes than for when it can make it. [[Nuclear_engineering|Nuclear plants]] and most thermal plants are expensive to run below full output, so hydro is used to supply the swings of daily demand around them. With wind and solar, the relationship runs the other way: a reservoir can hold water back while the wind blows and release it when it drops, so the reservoir acts as a battery for the wind farm. The Manitoba–Minnesota interconnection energized in June 2020 was built on exactly this logic — Manitoba Hydro's stations "backstop" Minnesota Power's large wind investment, with 250 MW of firm hydroelectric power sold south across a [[Voltage|500-kilovolt]] line.[^mb-hydro] Where no suitable river exists, pumped storage or batteries must do the same job at higher cost.
## Hydro power by country
Hydroelectricity follows rain and relief. Countries with large rivers falling from mountains or plateaus — Norway, Brazil, Canada, China, Paraguay — draw a large part of their electricity from water, while flat or arid countries draw little. Within the United States the resource is concentrated in the Pacific Northwest: Washington, Oregon, California and New York together produce more than half of the country's hydroelectricity, with Washington alone averaging about 8.9 GW, largely from the Columbia River.[^murphy-ch11] [[Minnesota]], with low relief, produces comparatively little of its own and imports hydroelectric power from Manitoba.[^mb-hydro]
## Economics
A hydroelectric plant is expensive to build and cheap to run. The dam, tunnels, penstocks and powerhouse are [[Civil_engineering|civil works]] that take years and carry large [[Geological_engineering|geological]] and [[Hydrology|hydrological]] risks, and most of the lifetime cost is paid before the first kilowatt-hour. After that there is no fuel bill, operating staff is small, and the plant can run for most of a century. The result is that hydroelectricity's economics depend heavily on the cost of capital and on the value placed on its flexibility: a plant that can sell power at peak hours, provide reserve and absorb surpluses earns more than its energy output alone would suggest. Deciding when to release water is itself an economic problem — spending water today forgoes power tomorrow, under uncertain inflows — and reservoir scheduling was one of the first applications of dynamic programming and [[Mathematical_optimization|mathematical optimization]] in [[Power_engineering|power systems]].
## Minnesota
*This section is specific to Wikitube.*
Hydroelectricity in [[Minnesota]] began where the state's industry began, at the falls of the [[Mississippi_River|Mississippi]] in Minneapolis, and it has stayed tied to the falls and rapids of its rivers. Minnesota Power, which started in 1906 on the St. Louis River near Duluth, runs eleven hydroelectric stations with more than 120 MW of capacity on the Mississippi, Kawishiwi, Crow Wing, Prairie and St. Louis rivers, under eight federal licenses whose renewals run through 2044.[^mnpower] The state Department of Natural Resources reviews those licenses for minimum flows and fish passage.[^dnr-hydro]
The larger hydroelectric presence is imported. Rivers that leave Minnesota northward — the [[Red_River_of_the_North|Red River of the North]], the Rainy River and the waters of the Boundary Waters — join the Hudson Bay drainage that Manitoba has developed for power, and since June 2020 a 500 kV line has brought 250 MW of that power back south.[^mb-hydro] A raindrop in northern Minnesota can therefore reach a lamp in Duluth by either of two routes: through a St. Louis River turbine if it falls south of the Laurentian Divide, or through a Manitoba turbine if it falls north of it.
**On the spine:** [[Hydropower]] · [[Hydrology]] · [[Fluid_dynamics]] · [[Electrolysis]] · [[Mississippi_River]] · [[WT!Thury_Hydrodynamics_Compendium]].
## See also
- [[Hydropower]]
- [[Hydrology]]
- [[Electrical_grid]]
- [[Electric_power_transmission]]
- [[Power_engineering]]
- `Pumped-storage_hydroelectricity` · `Run-of-the-river_hydroelectricity` · `Tidal_power` · `Small_hydro` · `Micro_hydro` — not yet on Wikitube
## References
[^iha2025]: International Hydropower Association (June 25, 2025). "Global hydropower generation rebounds in 2024 and pumped storage development surges — flagship 2025 World Hydropower Outlook out now." https://www.hydropower.org/news/flagship-2025-world-hydropower-outlook-out-now
[^mnopedia1882]: Huber, Molly. "Hydroelectricity in Minneapolis, September 5, 1882." *MNopedia*, Minnesota Historical Society. https://www.mnhs.org/mnopedia/search/index/event/hydroelectricity-minneapolis-september-5-1882
[^nps-ch6]: National Park Service, Mississippi National River and Recreation Area. "River of History — Chapter 6." https://home.nps.gov/miss/learn/historyculture/river-of-hisory-chapter-6.htm
[^murphy-ch11]: Murphy, Tom W., Jr. (2021). *Energy and Human Ambitions on a Finite Planet*, chapter 11, "Hydroelectric Energy," pp. 173–181. eScholarship, University of California. CC BY-NC 4.0. https://escholarship.org/uc/item/9js5291m
[^mb-hydro]: Manitoba Hydro (July 2020). "New transmission line makes grid more reliable and benefits Manitobans." Company release. https://www.hydro.mb.ca/articles/2020/07/new_transmission_line_makes_grid_more_reliable_and_benefits_manitobans/
[^mnpower]: Minnesota Power. "Hydro." Company page. https://www.mnpower.com/Environment/Hydro
[^dnr-hydro]: Minnesota Department of Natural Resources. "Hydropower." https://www.dnr.state.mn.us/waters/surfacewater_section/stream_hydro/hydropower.html
## Sources
- Murphy, Tom W., Jr. (2021). *Energy and Human Ambitions on a Finite Planet*. eScholarship, University of California. — on the [[PORTAL_Energy]] book shelf (`Portal Books/PORTAL_Energy/`); chapter 11 is the source for this article's physics and U.S. figures.
- *Future Energy: Opportunities & Challenges* (2013). — on the [[PORTAL_Thury_Hydrodynamics_Apex_Spine]] and [[PORTAL_Energy]] book shelves.
## External links
- [International Hydropower Association](https://www.hydropower.org/)
- [Water Power Technologies Office](https://www.energy.gov/eere/water/water-power-technologies-office), U.S. Department of Energy
- [Hydroelectricity in Minneapolis, September 5, 1882](https://www.mnhs.org/mnopedia/search/index/event/hydroelectricity-minneapolis-september-5-1882), MNopedia
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**Part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]]** — main article for section 16, *Hydroelectricity*. Related sections: [[Electrolysis_of_water]] · [[Hydropower]] · [[Mississippi_River]] · [[Saint_Anthony_Falls]].
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**Microsim — three.js (Wikitube framework):** *Hydroelectricity*
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*Built from `MICROSIM_GUIDE/specs/sims/Hydroelectricity.json`; part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]] set.*
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## Wikipedia : Wikitube
**Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Hydroelectricity) : [Wikitube](https://en.wikitube.io/wiki/Hydroelectricity) · pinned revision [1371047992](https://en.wikipedia.org/w/index.php?oldid=1371047992) · 2026-09-10
## Previous hub tags
Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Energy]].
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*Thury main articles · 2026-09-10 · drafted · microsim layer pending (THY-065).*