Walls of Water: Hydroelectricity

The most powerful gale force winds can’t compete with the force of a little bit of flowing water. Anyone who’s had to haul their own water understands that water is dense, and that even small containers become quite heavy as they’re filled. A cubic meter of water weighs very close to 1 ton; nearly exactly because water was the original basis for the definition of mass in the metric system. It takes almost a thousand times more space to hold a ton of air.

There’s danger in the crushing pressures under the sea. If you dive underwater a distance of 10 meters, a depth that’s exceeded even in introductory scuba diving classes, the pressure from the water’s weight is equal to that from the entire weight of the Earth’s atmosphere, extending tens of thousands of kilometers above. At the bottom of the deepest trenches in the ocean, the pressure is over 1,000 times as large as typical atmospheric pressure.

Water is powerful and dangerous, but “water is life” — Mní Wičóni — as the Standing Rock pipeline resistance movement’s message goes. The health of freshwater is critical in every part of ecosystems, from the growth of plants in a large farm to microbial processes that keep soil and water clean. Megadam projects disrupt these substantially. The relative inexpensiveness of hydroelectricity, combined with its tremendous environmental disruption, had led to environmental justice struggles against dam construction all across the world.

Types of electricity delivery

Hydroelectricity is uniquely flexible in terms of its ability to deliver power when needed, ramping down output when demand decreases when needed for stability, and even restarting entire grids after a blackout. To understand its potential, we need a few definitions.

is the demand of an electrical grid, equaling all the power that is being requested in residences, industry, and other uses. The load of the grid needs to be matched nearly exactly by power generation by power plants at any given time, or blackouts can occur. This means that as intermittent sources like solar and wind experience a drop in generation, other power sources need to be ramped up to match the load.

are power plants that deliver a near-constant amount of power. This is true in day or night, summer and winter, regardless of the demand. Most coal and nuclear power plants are run as baseload plants, except for the times that they’re down for maintenance. This means their output is close to their nameplate capacity at all times. The Brunswick Nuclear Generating System, for instance, is designed to provide its 1.858 GW nameplate capacity to southeast North Carolina at nearly all times. A recent exception was when it was submerged during Hurricane Florence.

are facilities that can deliver a different amount of power at different times, depending on electricity demand. These typically ramp up more during the daytime, and decrease output at night, when loads are lower. As solar power increases to become a large fraction of generation in certain areas, load-following plants are increasingly generating more of their electricity in the late afternoon and evening, as the sun is lower in the sky but power is needed in residences.

are used to satisfy very high demand times, needed to prevent blackouts on days with very high loads, like on extremely hot days. Some peakers are only designed to be used a few times per year, and can use highly-polluting fuels to satisfy their demand. There are clearly major justice concerns with the use of peaker plants, which tend to be located in neighborhoods of color and low-income communities. For example, Ravenswood Generating Station between the Long Island City and Astoria neighborhoods in Queens, New York, produces peaking power and a large amount of harmful air pollution. It was originally only designed to be run a few days of the year, but it and other New York City peaker plants are used in increasing amounts as energy demand has risen. Most peaker plants are fossil gas or fuel oil, but some renewables can be used as well, and climate justice groups have plans to clean up peaker facilities.

Critical to whether a given power plant can be used in these modes is their ramp-up time. These vary substantially, as the table below shows. Energy storage options like batteries, flywheels, or gravity storage systems can essentially be started instantaneously, with fraction-of-a-second response times. Clearly, except for the storage options, hydroelectricity has unprecedented ramp-up capability.

Type of power plant Ramp-up time
Fossil gas combined cycle 30-60 minutes
Fossil gas open cycle 10-20 minutes
Coal 1-10 hours
Nuclear 2 hours-2 days
Concentrating solar power 1 hour
Hydroelectricity 15-30 seconds

Hydroelectricity worldwide

Dams are routinely built hundreds of meters in height. All that elevated water has a lot of potential energy, which can be used to turn a turbine and create electricity. The largest dams can have nameplate capacities of over 10 GW. These monstrous facilities back up vast quantities of water in reservoirs when the dam is built, flooding huge areas of land and disrupting the flow of the river and its ecosystems. Hydroelectricity can be generated in smaller scale facilities as well. Run-of-the-river plants turn turbines with the natural stream flow of a river. They are significantly less destructive to local ecosystems than are megadams, but cannot provide energy storage or run with load-following capability.

Why is water power renewable? It’s because of the power of the atmosphere, which continually soaks up moisture from the ocean, lakes and land ecosystems, and deposits it down effortlessly as rainfall. Moisture is transported and mixed all around by weather systems after it’s evaporated, and when moist air moves high enough upward, clouds are formed. Once the water particles within clouds are large enough to fall, precipitation occurs. Since it starts from high-elevation clouds, some of that precipitation falls well above sea level, over mountains or hills. It then flows downward into river basins, percolating into soils or drifting downriver towards the sea. Hydroelectricity is renewable because rain will continue to replenish river systems around the world.

Waterfall in Banff, Canada by Virginia Wright-Frierson

Rivers, though, have changed substantially as a result of human interactions, both from heat-trapping gas pollution and via direct interventions like irrigation and dams. Direct effects are discussed under environmental justice concerns below.

Just the global heating-induced changes, though, are worth categorizing. First, because warmer air can hold more moisture, weather systems are transporting an ever-increasing amount of the wet stuff, resulting in stronger and stronger downpours. A stronger hydrologic cycle can create larger streamflow rates at certain times of the year, which dams or other human interventions were not built to handle. Second, evaporation losses are larger into warm air, again because hotter air can hold more moisture. You can think of a warmer atmosphere as a big sponge above the earth, sucking up more moisture from the ocean and land, and occasionally wringing the vapor out in huge downpours. Finally, climate disruption from fossil fuels has been causing weather systems to shift in location. Many mid-latitude weather systems are shifting poleward with global heating, a phenomenon that was predicted back in the 1980s before such changes were observed. The southward shift of weather systems in the Southern Hemisphere has been particularly strong, and has already led to such scares as the Day Zero drought in Cape Town, South Africa, and prolonged drought in southern Australia. There have been northward shifts of weather systems in the Northern Hemisphere as well, associated with the same tropical expansion caused by global heating. The Mediterranean has been particularly effected. Drought in the Middle East has been linked to conflict in Syria, as another stressor in tenuous situation that led to one of the largest refugee crises in recent history. Shifts in the location of weather systems have occurred in the tropics as well, affecting locations like Lake Chad, which nearly dried up in the 1980s but has since recovered. Rainfall shifts in the tropics are generally less predictable, but are no less important than elsewhere. Dislocations of weather systems are expected to continue until fossil fuel emissions cease.

Hydropower worldwide

Hydroelectricity is generated by flowing water downward past a turbine, rotating it and creating electricity. Some megadam projects have giant nameplate capacities, including the Three Gorges Dam, a 22.5 GW facility in China, and the Itaipu Dam, a 14 GW dam along the border of Brazil and Paraguay. The United States has 84,000 dams but only about 3% of them generate electricity, with the rest used exclusively as reservoirs. Nameplate capacities of the 2500 US American dams range from a few kilowatts to 6.8 GW for the Grand Coulee Dam in Washington. Washington has the largest capacity and generates more than one-fourth of the nation’s hydropower. The US Department of Energy has suggested that 12 GW of hydropower capacity could be added by simply adding generators to existing non-powered dams, with most of the capacity coming from 100 non-powered dams. Existing power-producing dams can be uprated, meaning their nameplate capacity can be increased by adding additional generators to a plant, even outside the dam, if proper equipment is installed.

Worldwide, the nameplate capacity of hydroelectric facilities totals 1.3 TW, up from 800 GW in 2001. China has 366 GW of nameplate capacity and is the source of over half of the expansion in the new millennium. Capacity is 240 GW in Europe, 110 GW in Brazil, 100 GW in the US, and 80 GW in Canada. Some of these facilities are limited by rainfall, so have lower capacity factors by necessity. Those plants that are run in load-following rather than baseline mode will also have lower capacity factor. The capacity factor of hydroelectric facilities worldwide averaged 37% in 2020, but this varies significantly from location to location. Some recent additions have averaged as high as 80% capacity factors, while others are as low as 20%.

The price of hydroelectricity projects varies considerably as well. Projects are often stricken with large cost overruns, which nearly double the expected price, and delays, leading to around 50% longer than expected construction times. Some projects have had levelized costs of energy as low as $0.02/kWh, while others have been more than ten times this amount. Projects in 2019 averaged around $0.05/kWh, placing them near the other inexpensive electricity sources like solar and wind. The installation costs for hydro average anywhere between $1/W to $5/W. Uprating of hydroelectricity is significantly less expensive than new installations, and is one of the most inexpensive forms of new power generation.

All in all, IRENA states that “its low cost, its growing importance – where storage reservoirs exist – in facilitating the high penetration of variable renewables and unrivalled ability to provide grid flexibility make hydropower an increasingly valuable component of the energy transition.” Over 3000 projects over 1 MW are being planned worldwide. It remains to be seen whether these will be built in a way that brings procedural, distributive, or restorative justice to the communities that are most affected.

Environmental justice concerns

Dam construction has resulted in large displacements of people, between 40 and 80 million people worldwide, and has impacted nearly half a billion living downstream. Often constructions have little benefit to the local people. There are locations around the world where despite the closeness to a large dams, communities have no power supplied from the project because no electrical connections were built. These projects are often designed to provide power for industrial or mining operations far from the dam itself.

Dam construction in Brazil alone has resulted in eviction of one million people, 70% of whom have not received any compensation for their displacement. The arrival of so-called “man camps” to construct these projects leads to a sharp increase in crimes against women. Dam failures routinely occur after their construction, and have resulted in disastrous consequences for local communities.

Dams affect fish migration, and increase the temperature of water, which has further effects on ecosystems. When vegetation is submerged as the reservoir behind dams is created, microbes create methane, often leading to quite substantial pulses of emissions. Carbon dioxide is also released. The heat-trapping gas emission varies substantially from project to project, but the methane release in particular can lead to a large decade-scale warming effect.


The Movimento dos Atingidos por Barragens (MAB, the Movement of People Affected by Dams), is a movement in Brazil founded in 1991 to resist megadam projects, and seek justice and reparations for affected communities.