Touted by many renewable energy and sustainable development proponents as an important solution to the interconnected challenges of climate change and poverty, the reality of hydropower is far more complex. 

Not all hydropower is created equal. The efficiency of energy generation, as well as the environmental and social impacts of hydropower, depend upon the topography, climate, ecosystem, size, design, technology, maintenance, watershed management and whether the construction is hybridized with other energy-generating technologies such as wind or solar power. Additionally, proper environmental and social impact assessments and consultations with local communities and those downstream, should always be carried out to determine short and long-term temporal, regional and cultural benefits and costs.

For example, “Run-of-the-River” (ROR) “Micro” and “Mini” hydropower systems built on rivers along steep mountainous slopes, have far fewer negative externalities than massive hydroelectric dams built within lowland tropical river basins. The former has potentially minimal negative impacts while providing relatively clean energy to rural and urban communities alike, while the latter can flood hundreds of square kilometers of forest, fragment rivers, and displace entire communities. Despite this wide case-by-case spectrum of positive-to-harmful impacts of hydropower, the reality is that many past, present and future proposed constructions have had, and will continue to have, profound consequences for local ecosystems, biodiversity and communities. Additionally, a growing body of evidence is demonstrating that the damming of rivers for hydropower and other purposes such as flood control and irrigation, is often far from being clean and sustainable: Reservoirs emit substantial amounts of greenhouse gasses, especially methane, and also have high evaporation rates

The Current and Projected State of Global Hydropower

Approximately 16% of the world’s electricity (around 1.3 terawatts of installed capacity) is currently generated by hydroelectric dams. As of 2018, China is the world leader in installed hydropower capacity, producing around 352 gigawatts (GW) of electricity from hydropower, followed by Brazil at 104 GW, the United States (103 GW), Canada (81 GW), Japan and India (50 GW each), Russia (49 GW) and Norway (32 GW), but more than 150 different countries have some form of hydroelectric generation. Several countries get the majority of their electricity from hydropower, including Brazil and Colombia. In fact more than 70% of Colombia’s domestic energy consumption comes from a current installed hydropower capacity of 12 GW, and this capacity is due to expand with several large hydroelectric dams currently in the planning or construction phase of development. 

In so-called “developed” countries, like the United States and much of Europe, the “ideal” sites for hydroelectric dams have already been mostly exploited, leaving limited “potential” for further construction. In the developing world, however, where people are focused on improving their quality of life through better access to services and electricity, hydropower remains only partially exploited. It is here in the developing world where the current and future “boom” of hydropower development is taking place, often with unrealistic expectations as to the benefits, and with little consideration for both the short-term and long-term consequences. 

Dam construction on the Teesta River, Sikkim, India. 2018.

Hydropower projects, both large and small (for simplicity, above 100MW and below 100MW, respectively, though definitions of micro, mini, medium, large and mega constructions of hydroelectric dams vary from country to country) are in various stages of construction and planning, around the world. This current rapid expansion of developing world hydropower has been taking place since around 2013, and according to a study published in 2015 and their ongoing associated database, more than 3,700 hydroelectric dams are in the planning or construction stage. 

State-backed Chinese companies, such as Three Gorges Project Corporation (the builders of the world’s largest dam along the Yangtze river, which has an installed capacity of 22,500 megawatts), and Sinohydro Corporation, are at the forefront of the expansion of large hydroelectric dams in the developing world. China’s influence on infrastructure development, including dam construction, is strong throughout Southeast Asia, Africa and parts of Central and South America.

Large regional and national construction and energy companies, some with well-documented corruption and suspect environmental track records, are also pushing large hydroelectric projects forward. In Latin America for example, Brazilian construction conglomerate Odebrecht has been rocked by scandal throughout the region.

In 2018-2019 there may be some signs of a slowdown to the global hydropower boom. Less new projects are currently being proposed, and some megaprojects have been abandoned. Despite this present reduction in growth rate, there are still an enormous number of new projects in the works for large hydroelectric dams. The majority of these new dams are in the developing world, many in diverse and fragile ecosystems, and some of these projects are massive in scale and multiple along single rivers and throughout interconnected river basins. A significant number of these new dams are also being constructed along some of the last remaining free-flowing rivers of the world. According to a recent study in the journal Nature, more than 60% of the world’s longest rivers (more than 1,000 kilometers, or 620 miles) have already been dammed and fragmented by approximately 60,000 existing dams.

The Impacts of Hydroelectric Dams

Vannessa Circe – “Hydro Myth” – Oil on Canvas – 36″ x 48″ – 2019

Extensive Social and Environmental Impacts of Hydroelectric Dams in Lowland Tropical Ecosystems

Hydroelectric dams impact local ecosystems regardless of their size or geographic location, but lowland tropical ecosystems can suffer the most wide-ranging negative impacts from large hydropower constructions. It is in these rich tropical forests and floodplains that so much of the world’s biodiversity and native populations reside, and also where carbon is stored in large quantities in the soil and vegetation. In mighty river basins like the Amazon in South America, the Congo in Africa, and the Mekong in Southeast Asia, current and planned construction of large dams in tropical ecosystems are due to cause extensive damage to aquatic and terrestrial species, local communities and both global and regional climate patterns. Even in less expansive river basins, communities can be displaced, and already vulnerable specialized endemic species can be brought to extinction by dams; this may happen to the recently discovered Tapanuli Orangutan in Sumatra, Indonesia if a proposed hydroelectric project is allowed to be completed along the Batung Toru.

Fishing along the Mekong River. Phnom Penh, Cambodia.

Due to the relatively flat topography of these tropical lowlands, in order to generate sufficient power to create electricity, massive reservoirs must be filled that cover hundreds to tens of thousands of hectares of forest (1 hectare is approximately 2.5 acres). Local communities are displaced, terrestrial habitats are lost, and migratory aquatic species are fragmented and face extinction. Sediment, flow and flood patterns, as well as temperature and oxygen levels of rivers are disrupted, which impacts communities and ecosystems downstream, and large volumes of precious fresh water evaporate. 

Greenhouse gasses, especially methane (which on a 20-year timescale may be more than 80 times as powerful in warming the planet as carbon dioxide), are also emitted through bubbles coming up from these massive reservoirs as inundated vegetation decomposes at the bottom of the reservoir in anaerobic conditions, enhancing climate change. Despite mounting evidence of the substantial emissions of greenhouse gasses coming from the reservoirs of dams, these emissions are still not factored into international carbon emissions agreements, or the International Panel on Climate Change (IPCC) reports that guide policy discussions around the world. Furthermore, trapped sediment builds up over time in the reservoirs, both of small and large dam constructions in different topographies, decreasing the efficiency of electricity production, and also emitting greenhouse gasses

Are Hydroelectric Dams Built in Steep Topographies Less Destructive?

Dams built on steep mountainous slopes or within canyons, for example, do not require large destructive reservoirs to produce sufficient power to generate electricity, which means they typically cause smaller impacts on local habitats and communities compared to dams built in lowland river basins. But even if a hydroelectric dam is built in a topography that is considered ideal for efficient electricity generation, it can still have significant consequences downstream.

Free-flowing rivers often connect distinct ecosystems and transport sediments that are critical for the health of natural habitats and agriculture over long ranges. For example, many rivers that end up as part of the Amazon river basin are born on the slopes of the Andes, and the sediments from these mountains provide distinct nutrients that lowland plants and animals in the tropical forests downstream depend upon. This so-called “Andes-to-Amazon connectivity” is critical for certain migratory fish species as well. For example, Goliath catfish, an emblematic migratory species of the Amazon, have their spawning grounds at higher elevations on the Andean slopes. So these highland hydroelectric dams can still fragment species that are critical for tropical lowland ecosystems, as well as unique species of the local highland ecosystem.

Free-Flowing Caquetá-Japurá river, the ninth largest tributary of the Amazon river basin. Solano, Caqueta, Colombia.

Indirect and Integrated Impacts of Hydroelectric Dams

To fully consider the potential impacts of hydroelectric dams, both in the present and in the future, it is necessary to think in a more integrated manner with some openness to speculation. To date, most scientific studies as well as social and environmental impact assessments, consider only the impacts of individual dams on surrounding ecosystems and communities, mostly neglecting downstream impacts and the compounded impacts of having multiple dams built along a single river or river basin. A recent report stressed the importance of “system-scale planning” in the construction of hydroelectric dams, in order to preserve ecosystem services and biodiversity, but this kind of approach remains mostly ignored by national and international policy makers, financial backers and construction companies. 

While beyond the scope of this article to fully address the potential consequences of hydropower, it is worth mentioning a few of the less considered wide-scale impacts that hydroelectric dams can at least contribute to:

  • The infrastructure required for building large hydroelectric dams, especially roads, can open the door to land-use changes, such as illegal logging, mining, and eventually extensive deforestation. These activities are carried out both by displaced communities and new opportunistic migrants. 
  • Communities displaced (or euphemistically described as “involuntarily resettled”) by dams often end-up living in poverty
  • For traditional indigenous communities, profound cultural losses are experienced with the flooding of their ancestral lands, in addition to evident environmental and territorial loss.
  • Poorly planned and constructed dams can suffer from catastrophic failures, as has happened recently in Laos, Brazil and Colombia
  • Basin-scale impacts of “Mega-dams” and multiple dams along single river systems, may have profound impacts on hydrologic cycles, which will impact local, regional and global weather patterns as well as flood cycles.
  • The blocking of sediments by dams may impact not only the local and downstream ecosystems along the riverbanks, but also the health of coastal mangrove forests.
  • As climate change advances and water scarcity becomes pervasive, the power to manipulate river flows along transnational rivers has the potential to trigger conflict between countries. For example, in the always volatile Indian sub-continent, Pakistan is projected to suffer from dwindling water supplies as temperatures rise, while India has dammed many of the rivers that flow into Pakistan.


The points mentioned above, both the relatively well studied direct impacts of hydroelectric dams, and the more speculative and indirect impacts, are extremely important and deserve their own individual treatments. They represent some of the most critical impacts that are due to happen on a massive scale if projected constructions of large hydroelectric dams are allowed to go forward as planned around the world. It is also essential to better understand the alternatives that are available, both non-hydroelectric “clean energy” generated from nuclear reactors, wind turbines and certain properly placed and manufactured solar panels, as well as less destructive hydroelectric alternatives, like off-the-grid Microhydro technologies or other run-of-the-river designs in ideal topographies.

The impacts of hydroelectric dams are not an issue to sleep on. Mekong River, Cambodia.

Inevitably, hydropower will continue to play a significant role in the growing global consumption of energy, so it is essential to understand the impacts and differences of the varying incarnations of hydropower; on ecosystems and biodiversity, global and local economies, greenhouse gas emissions, weather patterns, and indigenous and local communities.

Resources and Further Reading

Anderson, D., Shucksmith, J., et al.. “The impacts of ‘run‐of‐river’ hydropower on the physical and ecological condition of rivers.” Water and Environment Journal. June 2015.

Anderson, E.P., Tedesco, P.A., et al.. “Fragmentation of Andes-to-Amazon connectivity by hydropower dams.” Science Advances. January 2018. 

Angarita, H., Delgado, J., Purkey, D., et al.. “Basin-scale impacts of hydropower development on the Mompós Depression wetlands, Colombia. Hydrology and Earth System Sciences. May 2018.

Barthem, R.B., Mercado, A., et al.. “Goliath catfish spawning in the far western Amazon confirmed by the distribution of mature adults, drifting larvae and migrating juveniles.” Nature. February 2017. 

Borunda, A. “Methane, Explained.” National Geographic. January 2019.

Deemer, B.R., Arie Vonk, J., et al.. “Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis.” Bioscience. October 2016.

Darlington, S., Andreoni, M., et al.. “A Tidal Wave of Mud.” New York Times. February 2019.

Degu, A.M., Chronis, T., et al.. “The influence of large dams on surrounding climateand precipitation patterns.” Geophysical Research Letters. February 2011.

Ezcurra, E., Aburto-Oropeza, O., et al.. “A natural experiment reveals the impact of hydroelectric dams on the estuaries of tropical rivers.” Science Advances. March 2019.

Fearnside, P.M. “How a Dam Building Boom Is Transforming the Brazilian Amazon.” Yale E360. September 2017.

Fearnside, P.M. “Brazil’s São Luiz do Tapajós Dam: The Art of Cosmetic Environmental Impact Assessments.” Water Alternatives. 2015.

Frenken, K., Kohli, A. “Evaporation from artificial lakes and reservoirs.” FAO Aquastat. July 2015.

Gallas, D. “Brazil’s Odebrecht corruption scandal explained. BBC News. April 2019.

Gill, S. “Colombia’s Hidroituango dam floods: hundreds left homeless. Colombia Reports. May 2018.

Global Freshwater Biodiversity Atlas.

Humphrey, C. “Devastating Laos dam collapse leads to deforestation of protected forests.” Mongabay. December 2018.

Jones, I.L., Dent, D.H,, et al.. “Instability of insular tree communities in an Amazonian mega-dam is driven by impaired recruitment and altered species composition.” Journal of Applied Ecology. April 2019.

Kaufman, Alexander. “The World’s First Power Plant To Combine Hydro And Solar Opens In Portugal.” Huffington Post. October 2017.

International Rivers. “Chinese Dam Builders.”

Latrubesse, E.M., Stevaux, J.C., et al.. “Damming the rivers of the Amazon basin.” Nature. June 2017.

Laurence, W. “As Roads Spread in Rainforests, The Environmental Toll Grows.” Yale E360. January 2012.

Leahy, S. “Hydroelectric dam threatens to wipe out world’s rarest ape.” National Geographic. March 2019.

Leslie, Jacques. “After a Long Boom, an Uncertain Future for Big Dam Projects.” E360. November 2018.

Maeke, A., Lorke, A., et al.. “Sediment Trapping by Dams Creates Methane Emission Hot Spots.” Environmental Science and Technology. June 2013.

Moran, E.F., Hyndman, D.W., et al.. “Sustainable hydropower in the 21st century.” PNAS. November 2018.

Opperman, J., Hartmann, J., et al.. “The Power of Rivers A Business Case: How system-scale planning and management of hydropower can yield economic, financial and environmental benefits.” The Nature Conservancy. September 2017.

Renewables First. “What is the difference between micro, mini and small hydro?”

Winemiller, K.O., Saenz, L., et al.. “Balancing hydropower and biodiversity in the Amazon, Congo, and Mekong.” Science. January 2016.

Zarfl, C., Tockner, K., et al.. “A global boom in hydropower dam construction.”Aquatic Sciences. January 2015.

Zarfl, C., Grill, G., et al.. “Mapping the world’s free-flowing rivers.” Nature. May 2019.