Science Energy Hydroelectricity: Environmental Costs, Benefits, and Outlook By David M. Kuchta David M. Kuchta Writer Wesleyan University, University of California, Berkeley David Kuchta, Ph.D. has 10 years of experience in gardening and has read widely in environmental history and the energy transition. An environmental activist since the 1970s, he is also a historian, author, gardener, and educator. Learn about our editorial process Updated November 30, 2021 Share Twitter Pinterest Email Matthew Micah Wright / Lonely Planet / Getty Images Energy Renewable Energy Fossil Fuels In This Article Expand How Hydroelectricity Works Costs Benefits The Future of Hydroelectricity Cleaner Alternatives Hydroelectricity is a significant source of power in many regions of the world, producing roughly 24% of the world's electricity. Brazil and Norway rely almost exclusively on hydropower. In Canada, 60% of electricity generation comes from hydropower. In the United States, 2,603 dams produce 7.3% of electricity, almost half of which is produced in Washington, California, and Oregon. The use of hydropower to generate electricity pits two environmental concerns against each other: while hydroelectricity is renewable and lower in greenhouse gas emissions than fossil fuels, its impact on the environment is destructive of native lands and wildlife habitats. Finding the right balance between these concerns is necessary to confront the twin crises of climate change and biodiversity loss. How Hydroelectricity Works Hydropower involves using water to activate moving parts, which in turn may operate a mill, an irrigation system, or a turbine to produce electricity. Most commonly, hydroelectricity is produced when water is held back by a dam, then channeled through a turbine that is coupled to an electricity-producing generator. The water is then released into a river below the dam. Rarer run-of-the-river hydroelectric plants also have dams, but no reservoir behind them. Instead, turbines are moved by river water flowing past them at a natural flow rate. Ultimately, the generation of hydroelectricity relies on the natural water cycle to refill reservoirs or replenish rivers, making hydropower a renewable process with little input of fossil fuels. Fossil fuel consumption is associated with a multitude of environmental problems: for example, the extraction of oil from tar sands produces air pollution; fracking for natural gas is associated with water pollution; the burning of fossil fuels produces climate change-inducing greenhouse gas emissions. Costs However, like all sources of energy, renewable or not, there are environmental costs associated with hydroelectricity. As the need to combat climate change makes hydroelectricity increasingly attractive, weighing the environmental costs and benefits is essential to determining hydro's future role in the electricity mix. Destruction of Indigenous Homelands Nothing could be more environmentally devastating than the loss of one's ancestral homeland. Looking at the issue from an environmental justice perspective, hydroelectric dams have long been seen among many indigenous people around the world as “a colonization of their land and their cultures,” since hydropower projects have often involved the involuntary displacement of indigenous people from their homelands. Protecting indigenous lands is not only a human rights concern, it's an environmental one, as indigenous people are caretakers of 80% of the world's biodiversity. As representatives to the COP26 summit in Glasgow, Scotland, testified, respecting the land rights of indigenous people is essential to preserving indigenous knowledge and indigenous practices of environmental management. Defending indigenous rights is central to, not separate from, environmental protection. Brazil's Belo Monte Dam, seen here still under construction, is the world's fourth-largest dam. It displaced up to 20,000 indigenous people and flooded 230 square miles of Amazonian rainforest. Mario Tama / Staff / Getty Images Barriers to Fish Many migratory fish species swim up and down rivers to complete their life cycle. Anadromous fish, like salmon, shad, or Atlantic sturgeon, go upriver to spawn, and young fish swim downriver to reach the sea. Catadromous fish, like the American eel, live in the rivers until they swim out to the ocean to breed, and the young eels (elvers) come back to freshwater after they hatch. Dams obviously block the passage of these fish. Some dams are equipped with fish ladders or other devices to let them pass unharmed. The effectiveness of these structures is quite variable. Changes in Flood Regime Dams can buffer large, sudden volumes of water following spring melt of heavy rains. That can be a good thing for downstream communities (see Benefits below), but it also starves the river of a periodic influx of sediment and natural high flows that renew habitats for aquatic life. To recreate these ecological processes, authorities periodically release large volumes of water down the Colorado River, with positive effects on the native vegetation alongside the river. Downstream Impacts Depending on the design of the dam, water released downstream often comes from the deeper parts of the reservoir. That water is therefore much the same cold temperature throughout the year. This has negative impacts on aquatic life adapted to wide seasonal variations in water temperature. Similarly, dams trap nutrients coming from decomposing vegetation or nearby agricultural fields, reducing nutrient loads downstream and affecting both river and riparian ecosystems. Low oxygen levels in the released water can kill aquatic life downstream, but the problem can be mitigated by mixing air into the water at the outlet. Mercury Pollution Mercury is deposited on vegetation downwind from coal-burning power plants. When new reservoirs are created, the mercury found in the now submerged vegetation is released and converted by bacteria into methyl-mercury. This methyl-mercury becomes increasingly concentrated as it moves up the food chain (a process called biomagnification). Consumers of predatory fish, including humans, are then exposed to dangerous concentrations of the toxic compound. Downstream from massive Muskrat Falls dam in Labrador, for example, mercury levels are forcing indigenous Inuit communities to abandon traditional practices. Evaporation Reservoirs increase a river’s surface area, thus increasing the amount of water lost to evaporation. In hot, sunny regions, the losses are staggering: more water is lost from reservoir evaporation than is used for domestic consumption. When water evaporates, dissolved salts are left behind, increasing salinity levels downstream and harming aquatic life. Threats From Climate Change Increased evaporation also leaves reservoirs subject to dramatic climate change losses. Drought is a major factor in Earth's rising temperatures, as areas once blessed with rainfall adequate for hydroelectric power are increasingly faced with low dam levels and loss of electricity generation. In 2021, historic droughts across the Western United States dramatically lowered reservoir levels behind hydroelectric dams. In California, the Oroville Dam fell to just 24% of its normal capacity. Declining hydroelectricity has forced California utilities to increase natural gas generation, further exacerbating global warming. Low water levels on Lake Mead led to the Hoover Dam's electricity generation to just 35% of normal in 2021. 4kodiak / Getty Images Methane Emissions The nutrients trapped behind hydroelectric dams are consumed by algae and microorganisms, which in turn release large amounts of methane, a powerful greenhouse gas. This is especially the case in newly built hydroelectric projects, as methane emissions decrease over the lifespan of a dam. Benefits The main benefit of the massive amounts of relatively reliable electricity that hydroelectric dams provide is that the electricity is both renewable and low in carbon emissions. Clean(er) Renewable Electricity Hydroelectricity is renewable, supplying 37% of all renewable electricity generation in the United States. Examining the entire life-cycle of hydroelectricity from dam construction to electricity consumption, hydropower produces roughly one-fifth the greenhouse gas emissions of fossil fuels. Hydropower may be seasonably variable, but it is far less intermittent than solar and wind power, and it is projected to play a significant role as a reliable source of clean, renewable energy into the foreseeable future. Energy Independence As part of a portfolio of energy sources, using hydroelectricity means a greater reliance on domestic energy, as opposed to fossil fuels mined overseas, in locations with less stringent environmental regulations. Flood control Reservoir levels can be lowered in anticipation of heavy rain or snow melt, buffering the communities downstream from dangerous river levels. Recreation and Tourism Large reservoirs are often used for recreational activities like fishing and boating. The largest dams also generate income for local communities through tourism. The Future of Hydroelectricity While the heyday of building large-scale hydroelectric dams dates to the 1930s and 1940s, hydropower is expanding in the developing world. The future of hydroelectricity will involve new construction, dam removals, upgrades, and the declining costs of even cleaner alternatives. Dam Removal More than half of the dams built before the 1970s in the United States are reaching or beyond the end of their 50-year expected lifespan, part of the country's decaying infrastructure. Dam decommissioning and removals have increased as the economic benefits of older dams wane while their environmental costs mount. Dam removals, though infrequent, have been habitat success stories, with rapid renewals of migratory fish stocks. Re-purposing and Upgrading Existing Dams Increasing the efficiency of existing hydroelectric dams and re-purposing existing non-hydro dams are two ways to expand hydroelectricity generation without increasing its environmental impact (though not decreasing it either). In a pilot program, the U.S. Department of Energy's Water Power Program increased the efficiency of three hydroelectric plants, adding more than 3,000 megawatt-hours per year to local electricity grids. Of the dams in the world today, no more than 10% are used for electricity generation. Re-purposing them to produce electricity could provide an additional estimated 9% of current global hydroelectric power. Cleaner Alternatives Evaluating the environmental impact of hydroelectricity involves not just comparing it to fossil fuels, but also to less impactful clean-energy alternatives to fossil fuels. No form of electricity production is without negative impacts, yet the greenhouse gas emissions from hydroelectricity are roughly ten times that of nuclear, solar, and wind power. One recent study estimated that solar photovoltaic (PV) panels could produce the same amount of electricity as all 2,603 hydroelectric dams in the United States using roughly one-eighth of the existing reservoir area. Replace those dams with solar PV and 87% of the land would return to wildlife, while the remaining 13% could support solar electricity. View Article Sources Mulligan, Mark, Arnout van Soesbergen, and Leonardo Sáenz. “GOODD, a global dataset of more than 38,000 georeferenced dams.” Scientific Data 7:1 (January 21, 2020), 31. DOI:10.1038/s41597-020-0362-5; International Hydropower Association. 2021 Hydropower Status Report: sector trends and insights. IHA Central Office, Chancery House, London, 2018. Cooke, Fadzilah Majid, et al. “The Limits of Social Protection: The Case of Hydropower Dams and Indigenous Peoples' Land.” Asia & the Paciﬁc Policy Studies 4:3 (September 2017), 437–450. https://doi.org/10.1002/app5.187; VanCleef, Ali. “Hydropower Development and Involuntary Displacement: Toward a Global Solution.” Indiana Journal of Global Legal Studies 23:1 (Winter 2016), 349-376. Brito‑Santos, Jhonnes Luciano, et al. “Fishway in hydropower dams: a scientometric analysis.” Environmental Monitoring and Assessment 193:752 (October 2021). https://doi.org/10.1007/s10661-021-09360-z. Kunz, Manuel J., et al. “Optimizing turbine withdrawal from a tropical reservoir for improved water quality in downstream wetlands.” Water Resources Research 49:9 (September 2013), 5570–5584, https://doi.org/10.1002/wrcr.20358. Cebalho, Elaine C., et al. “Effects of small hydropower plants on mercury concentrations in fish.” Environmental Science and Pollution Research 24 (October 2017), 22709–22716. https://doi-org.une.idm.oclc.org/10.1007/s11356-017-9747-1. Lehner, Bernhard, et al. “High-resolution mapping of the world’s reservoirs and dams for sustainable river-flow management.” Frontiers in Ecology and the Environment 9:9 (November 2011), 494–502. https://doi-org.une.idm.oclc.org/10.1890/100125. Dorber, Martin, et al. “Controlling biodiversity impacts of future global hydropower reservoirs by strategic site selection.” Scientific Reports 10:1 (2020), 21777. https://doi.org/10.1038/s41598-020-78444-6. The average emissions from fossil fuels are estimated at 504 gCO2 kWh−1, while hydropower produced emissions ranging from range from 78 to 109 gCO2 eq kWh−1. See Pehl, Michaja, et al. “Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling.” Nature Energy 2 (December 2017), 939–945. https://doi.org/10.1038/s41560-017-0032-9. Zarfl, Christiane, et al. “A global boom in hydropower dam construction.” Aquatic Sciences 77 (October 2015), 161–170. https://doi-org.une.idm.oclc.org/10.1007/s00027-014-0377-0. Hansen, Henry H. et al. “Exit here: strategies for dealing with aging dams and reservoirs.” Aquatic Sciences 82:2 (2020). https://doi.org/10.1007/s00027-019-0679-3. O'Connor, J.E., J.J. Duda, and G. E. Grant. "1000 dams down and counting.” Science 348:6234 (1 May 2015), 496–497. https://doi.org/10.1126/science.aaa9204. Mulligan, Mark, op. cit. Garrett, Kayla, Ryan A. McManamay, and Jida Wang. “Global hydropower expansion without building new dams.” Environmental Research Letters 16:11 (1 November 2021), 114029. http://dx.doi.org/10.1088/1748-9326/ac2f18. Nuclear, wind, and solar PV produce an estimated 3.5–11.5 gCO2 eq kWh−1. See Pehl, op. cit. From a cost perspective, solar and wind power have been on consistent dramatic cost declines since the 1980s, while the cost of nuclear energy has only increased, with nuclear now more expensive on a per-kilowatt basis than solar and wind. Way, Rupert, et al. “Empirically grounded technology forecasts and the energy transition.” Oxford, U.K.: Institute for New Economic Thinking, INET Oxford Working Paper No. 2021-01, September 14, 2021. Waldman, John, et al. “Solar-power replacement as a solution for hydropower foregone in US dam removals.” Nature Sustainability 2 (2019), 872–878. https://doi.org/10.1038/s41893-019-0362-7.