The Ecological Effects of Dams on Tropical Watersheds in the Amazon Basin
Anne Brockman
BIOL 4503: Tropical Ecology
Northeastern State University


Hydroelectric power is an energy source millions across the world utilize. Today many neotropical countries are seeking ways to generate power for a more stable energy source and power grid, turning to hydroelectric dams and power as a renewable, inexpensive resource. Amazonian waterways have become the source of this energy, impacting the plants, animals and humans living in the Amazon basin.

Simply speaking, hydroelectric dams generate power by using falling water to turn turbines which turn a metal shaft in an electric generator to produce electricity. Hydroelectric dams are typically built in terrain where water flows from higher to lower elevation, with water stored in a reservoir, then flow through an intake to the turbines.


This video demonstrates how a hydroelectric dam works. It was provided by a Canadian company, but it does a good job of clearly explaining the mechanics of dams like these.

Hydroelectric dams are efficient in that they can provide power during peak times as well as store power when there is less demand (U.S. Geological Survey, 2012). Hydroelectric power is considered a relatively efficient power source for many reasons and far better for the environment than power plants driven by fossil fuels (U.S. Geological Survey, 2012). In America, peak dam construction was in the mid 20th century, but today dam construction in tropical countries is on the rise (Olivas, 2004, p. 1). Brazil, now the world’s fifth largest country with 192 million people, has seen rapid economic growth, staggering demand on its electrical resources, with a solution to this problem desperately needed.
Rio de Janeiro will host the 2016 Olympic Games. This, along with a booming economy and thriving population, are just some of the reasons the country is looking to hydroelectric power for an energy source. Photo credit: JorgeBRAZIL.

“To achieve Brazil’s goal to increase GDP by five percent a year over the next ten years, along with poverty eradication and improved income distribution, the country will need to install about 6,000 MW of additional capacity each year, according to the National 10-Year Energy Expansion Plan” (BRASIL Ministry of Mines and Energy, 2011, p. 1). Like many countries, Brazil uses a combination of energy sources to power its citizens’ needs, both traditional and more eco-friendly alternatives. In the Amazon, the primary reason for building dams comes from the need for electricity. Around the world, benefits for building dams also includes the formation of reservoirs for recreation and human consumption, irrigation needs for farmlands, flood control and mine waste retention. (Association of State Dam Safety Officials, 2012).

There are both positive and negative effects of dams, whether environmental, economical or social in each respect. A dam’s severity of effects relies strongly on the dam site, according to The World Bank, which says that some are “relatively benign” while others cause “major environmental damage.” Two examples on either side of the spectrum the World Bank gives are the Pehuenche Hydroelectric Project in Chile and the Brokopondo Dam in Suriname. The Chile project provides 500-megawatts of energy and only flooded approximately 400 hectares of land while minimizing damage to surrounding forest or wildlife. The Suriname project generates only about 30 megawatts, flooded 160,000 hectares of tropical rainforest and has been known for its serious water quality issues and aquatic weed problems (Ledec, Quintero, 2003, p. 1).

Dams are highly controversial and their effects have been recorded over time. “Nonetheless, many countries, in Latin America and worldwide, rely upon hydroelectric dams for a major portion of their electric power. Electricity remains a key ingredient for improving the lives of poor people almost everywhere” (Ledec, Quintero, 2003, p. v). As countries continue to grow in population with increasing demand on electricity, finding a suitable and reliable energy source is vital. “Development of the Amazon basin, managed correctly, could be a boon for the continent, lifting millions out of poverty and eventually bringing stability to a part of the world that has known too little of it. But in the short term it is creating new social and political tensions” (Risen, 2011).

Right now, dozens of dams are in progress – whether in design or construction – in South America, including the Amazon basin. One of the biggest projects Brazil is undertaking to meet the demand is the controversial Belo Monte Dam, to be constructed in the Para state of Brazil along the Xingu River in the Amazon basin that has an expected date of January 2015 for commercial generation. Belo Monte has the potential to generate 11,233 MW at full capacity during the rainy season (Brasil Ministry of Mines and Energy, 2011, p. 4) with a reservoir that will flood 160 square miles of forest. About 60 miles of the Xingu will be affected by the dam and potentially displace 20,000 people in the area (Barrionuevo, 2010).

A map of the proposed Xingu River dams in Brazil. Photo credit: International Rivers

As dams flood waterways, what was originally above the waterline now resides under a blanket of fresh water. In tropical ecosystems miles of forest are flooded to create reservoirs. These forests are areas of high plant biomass (International Rivers) and when flooded the plant matter decomposes producing methane gas (Smith, 2012) which remains in the atmosphere for about 9-15 years and is “20 times more effective in trapping heat in the atmosphere than carbon dioxide over a 100-year period” (U.S. Environmental Protection Agency, 2011). The decomposition also produces “changes in conductivity, dissolved oxygen, nutrient and phaeophytin concentration, and pH values (Brandimarte, Anaya, Shimizu, Meirelles, Caneppele, 2008, p. 29). Many sources say today, dam preparation includes clearing surrounding forest to decrease the greenhouse gas emissions, but that still eradicates and destroys the original forest habitat. Depending on a reservoir’s size, the dam may not have as many detrimental effects as ones with larger surface areas. A reservoir set in narrow gorges, smaller rivers, or a deep reservoir that silts up very slowly is considered an environmentally good hydro project site (Ledec, Quintero, 2003, p. 13).

Dams affect water up and downstream, changing the environment for the animals living in the water, as well as changing the riparian areas where aquatic life, land animals and plants inhabit. Freshwater ecosystems are driven by water flow which is responsible for the system’s structure, nutrients, distribution and more. “The loss of natural flow regimes has far-reaching impacts, including alteration in pristine hydrological dynamics, historical patterns of biological production, distribution of biodiversity in space and time, and changes in functions and services provided by aquatic ecosystems” (Agostinho, Pelicice, Gomes, 2008, p. 1121). Freshwater ecosystems are known for their high species diversity with more per unit area than other kinds of environments, with “10 percent more than land and 150 percent more than the oceans,” (McAllister, Craig, Davidson, Delany, & Seddon, 2001, p. iii) and are affected when a system is dammed. “Freshwater biodiversity is unevenly distributed. High numbers of species or endemics are found, for example, in the Amazon, Congo, Nile and Mekong basins. Such species-rich areas are called ‘hotspots’ and dominate other patterns or trends” (McAllister et al., 2001, p. iii). There are several impacts on freshwater diversity that dams create. Dams inhibit the movement of migratory species throughout the water system which can mean the extirpation or extinction of genetically distinct stocks or species (McAllister et al., 2001, p. 53). A new dam can fragment populations, making “gene flow, hitherto biodirectional, becomes unidirectional, downstream, reducing genetic diversity.” (McAllister et al., 2001, p. 44). The reservoirs create new habitats for wintering waterfowl or produce adequate habitat in arid regions that can increase the animal’s population. Exotic species can flourish leading to the displacement of indigenous species, while in reservoirs species that are “vectors” of animal and human diseases can colonize the body of water. When altering the habitat with a dam, woody debris that typically creates habitat and food chain sustainability is eradicated. A dam changes the natural conditions of a river – moving water becomes still in the reservoir, deepwater zones are created, temperature and oxygen conditions are modified that may not be suitable for species living in the water or area. One source claims that “one of the biggest cumulative impacts may be that a greater proportion of running water is converted to still reservoirs habitat” (McAllister et al., 2001, p. 44). This is also noted in the changed turbidity and sediment levels that present species were adapted to, as well as changing the overall water quality and flow patterns. Dams also trap silt, depleting downstream ecosystems like deltas and estatuaries with needed nutrients and materials. Dams inhibit flooding, diminishing flood plains with species that use that habitat during those periods. Multiple dams along a waterway can have cumulative effects as well. (McAllister et al., 2001, p. iii).

Ecological processes are also altered as changes in water flow affect the transport of sediment, nutrients and energy. Whether the new body acts more like a river or more like a lake depends a lot on water retention time (Brandimarte et al., 2008, p. 23). Rainfall and snowmelt will always be a significant element to any dammed river, but especially in the Amazon watershed, where many sources come from the Andean range in the western portion of South America (Vaidyanathan, 2012). “When an artificial lake is created, it is expected that the same environmental changes that occurred in the central body of the reservoir also will be observed in the upper reach, although with less intensity” (Brandimarte et al., 2008, p. 28). Immediately after a reservoir is filled it is expected that increasing turbidity, as well as decreased water transparency will occur. This happens because of the large amounts of organic materials that are suspended in the water. With rainfall and annual snowmelt, flooding is expected to occur. Dams and other flood-control projects can greatly affect flooding and drought cycles that plants and animals grow accustomed to. “Flow regime alteration, especially by large dam projects that eliminate flood and droughts, has facilitated invasions by non-native organisms that might not otherwise survive extreme flows” (Lytle, Poff, 2004, p. 99). Of course, fish and other aquatic life are greatly affected by flooding, droughts and damming. Regulated rivers can alter populations in that they can manipulate water quantity to favor certain species spawning.

Dams can block fish migrations and separate spawning and rearing habitats (International Rivers).This can cause extinction of species on either side of the dam. “Fish migration is affected, since many neotropical fish migrate long distances, often in complex patterns involving flood plains” (Schochet, 2012). Some species of fish have become locally extinct upstream of dams as a result, including the dorada, picuda, bagre and patalo above Colombia’s Betania Dam. In the upper Parana River in Brazil and Paraguay, migratory fish have become extinct. Catfish and other fish populations have declined in the Lower Plata basin (Schochet, 2012). For upriver areas, if the forest is cut down prior to the flooding, then habitat loss is drastically radical in its timing. Below the dam, less flooding and less flows create a drastically different life for animals living in that area. Less fish in a river with less water affects all, including people. Indigenous tribes in the Amazon are threatened by the increasing number of proposed dams, not only in their water, food and sometimes economical livelihood with the river, but also in dam’s canals, irrigation schemes, roads, power lines and industrial developments (International Rivers). Again, referring to the Belo Monte Dam, which when completed will be the third largest hydroelectric dam on the planet, (Lyons, 2012), the Brazilian government claims that none of the 10 indigenous communities located in the Belo Monte project area will be flooded as a result of the dam and that those depending on the river for food, transportation and such will not be effected because a minimum flow condition of 700 m3/sec (greater than the 400 m3/sec rate for the previous 80 years) will be maintained (Brasil Ministry of Mines and Energy, 2011, p. 5). Non-governmental organizations are refuting the displacement claim, saying that more than 20,000 people including those in native tribes will be displaced (Barrionuevo, 2010). While this is still a large amount of people – about the population of Sapulpa, Okla. (U.S. Census) - The World Bank has recognized a number of dam projects in southeast Asia, Africa and other Latin American countries that have displaced 50,000, 100,000 and up to 250,000 people. (Ledec, Quintero, 2003, p. 12).

Besides the immediate waterway, newly built reservoir and lower river area, further downstream ecosystems can feel the effects of dams. “The alteration of a river’s flow and sediment transport downstream of a dam often causes the greatest sustained environmental impacts” (International Rivers). The Amazon is a sediment-rich waterway, carrying mineral-rich soil from the Andes downstream to be deposited on riverbanks and floodplains (Kricher, 1997, p. 17). Dams hold back sediments that would naturally flow downstream, eventually to the ocean. Rivers often seek to “recapture” that lost sediment by eroding the downstream riverbanks and beds. “Riverbeds downstream of dams are typically eroded by several meters within the decade of first closing a dam; the damage can extend for tens or even hundreds of kilometers below a dam” (International Rivers). This can also cause water tables to be lowered as riverbeds are deepened.


While America has moved past hydroelectric dams as a large power source for electricity, many tropical countries have looked to rivers to construct and create dams for hydroelectric power that will provide energy to growing economies. This rings true in the Amazon basin where dozens of dam projects are in the works for the world’s largest river basin. While dams do offer a significant amount of energy in a relatively cleaner fashion compared to power plants using fossil fuels, the environmental effects, including effects on land, plants, humans and other animals can be substantial.

Works Cited
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Barrionuevo, Alexei. (2010, April 17). Amazon Dam Project Pits Economic Benefit Against Protection of Indigenous Lands. New York Times. Retrieved from

Brandimarte, A.L., Anaya, M., Shimizu, G.Y., Meirelles, S.T., Caneppele, D. (2008, March). Impact of damming the Mogi-Guacu River (Sao Paulo State, Brazil) on reservoir limnological variables. Lakes & Reservoirs: Research & Management, Vol. 13, Issue 1, pp 23-25. DOI: 10.1111/j.1440-1770.2007.00359.x.

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McAllister, Don E.; Craig, John F.; Davidson, Nick; Delany, Simon; Seddon, Mary. (2001). Biodiversity Impacts of Large Dams, Background Paper Nr. 1. Prepared for International Union for Conservation of Nature and Natural Resources, United National Environmental Programme.

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