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What is the environmental impact of hydroelectric dams?

Discover 4 impacts of hydroelectricity on the environment

Hydroelectric dams have impacts on the environment – nature, the river, fauna and flora – and on local communities. Indeed, although hydroelectricity is a renewable energy, it is not without consequences.

Upheaval of aquatic ecosystems

Hydroelectric dams have impacts on aquatic ecosystems. A study carried out in 2017 by a team of researchers from the Institute of Temperate Forest Sciences of the University of Quebec en Outaouais (UQO) notes that reservoirs in the boreal region, such as in Quebec, have little effect on the number of species found there. However, they affect the structures of species. In other words, the presence of these reservoirs does not lead to the appearance or disappearance of species, but their presence modifies the quantity of individuals within the same species. The reservoirs thus prove advantageous for some, including coregonids and pike, but harmful for others, including suckers and a salmonid.

The increase in water temperature due to stagnant water as well as the decomposition of organic matter following soil flooding could explain this phenomenon. Also, the fluctuation in water levels caused by the operation of the dam could have a negative effect on the macroinvertebrates on which fish feed.

Meunier Noir crédit photo Rappel perca31-Licence-CC-BY-NC - impact des barrages hydroélectriques
Black Miller. Credit: REMINDER

As for run-of-river dams, that is to say dams without a large reservoir, they represent an obstacle to the migration of aquatic species. This is why several of these dams are equipped with fish passes, and sometimes, for eels. Hydro-Québec puts a lot of effort into their maintenance, but the results are often uncertain. These passes therefore do not guarantee the survival of all the species that pass through them. Faced with climate change and rising water temperatures, it is important to facilitate the transition of species to waters that are suitable for them.

The consequences of dams on the environment could be more significant than we believe: there is no independent study on their cumulative effects, both on the water cycle and on biodiversity in Quebec.

Disturbance of woodland caribou habitat

The woodland caribou is a vulnerable species in Quebec, and threatened in Canada. Since it is a species that chooses to live in wild environments, with rich plant cover and as little disturbance as possible by any form of human activity, it is very sensitive to the latter, including hydroelectric dams. Precisely, it is the energy transmission lines as well as the construction of access roads to power stations that disrupt its habitat.

These changes to the territory have the effect of opening the way for predators, including wolves. Transportation lines would generate cumulative impacts with those of logging and roads on the behavior of woodland caribou, according to a impact assessment report conducted in 2013 by the University of Quebec at Rimouski (UQAR), the University of Quebec at Montreal (UQAM) and the Ministry of Sustainable Development, Environment, Wildlife and Parks of Quebec.

Caribou_Environnement et Changement climatique Canada
Caribou. Credit Environment and Climate Change Canada

For its part, Hydro-Québec is studying the effects of the construction and operation of the Romaine complex. The study, which began in 2009, is expected to continue until 2025, but already notes a decay of 21% of the population in the study area. On the other hand, flooding an area to create a reservoir causes the displacement of populations of other terrestrial species such as bears and beavers.

Have you seen Chasing Current?

Nicolas Boisclair and Alexis de Gheldere, accompanied by Roy Dupuis, present in their documentary the beauty of the Romaine River before Hydro-Québec built four hydroelectric dams there.

Access the full documentary →

Greenhouse gas emissions

Dams with large reservoirs emit greenhouse gases (GHGs) including carbon dioxide (CO2) and methane (CH4). The CO2 emitted comes from organic matter that decomposes in water after being submerged by the creation of a reservoir. Emissions decline over time over a period of about a hundred years. These emissions are particularly significant during the first 20 years of operation.

Large dams also generate methane. This powerful greenhouse gas also forms from submerged organic matter in sediments at the bottom of the water. However, methane emissions do not decrease as quickly as CO2 emissions over time, since methane is formed in the absence of oxygen. As oxygen is consumed at the bottom of the tank, CO2 emissions are therefore gradually replaced by methane emissions.


Some reservoirs emit much more GHGs than others. Several factors come into play, such as the depth of the reservoir, its configuration or the climate of the place where it is located. For example, in Quebec, methane emissions are lower than in a tropical climate. Indeed, since the water here is colder, it is less conducive to bacteria responsible for the formation of methane (methanogenesis). Nevertheless, from a global point of view, the large reservoirs of hydroelectric power stations represented 5.2 % of methane emissions from human activities in 2020, a significant contribution, underlines a study supervised by professor from the Department of Biological Sciences at UQAM, Yves Prairie.

730,000,000 to 2,410,000,000 tonnes of GHGs are produced in the first year of operation of a reservoir hydroelectric plant

Increase in the level of methylmercury in the food chain

Mercury is a chemical element that occurs naturally in soils and vegetation. It can also be emitted in gaseous form by certain industries and be transported by the wind, sometimes thousands of kilometers, before being deposited on the ground. In this form, mercury does not present a significant risk.

On the other hand, filling a hydroelectric reservoir creates favorable conditions for the transformation of this mercury into methylmercury, a form of mercury which can be harmful to human health if ingested in very large quantities. This methylmercury is absorbed by bacteria which serve as food for plankton, which will in turn be consumed by fish.

At each level of the food chain, methylmercury is therefore more concentrated: this is what we call biomagnification. The construction of dams can therefore have an impact on the food practices of First Nations, in particular by forcing them to reduce their fish consumption or by switching to other lakes or rivers whose fish are less contaminated.

Succeed in the energy transition without new dams

According to Hydro-Québec forecasts, Quebec must add 150 to 200 terawatt hours (TWh) of energy to its annual production by 2050. Three quarters will be used for decarbonization; the rest to economic growth. Currently, 94 % of 200 TWh produced annually are from hydroelectric sources.

The solution does not lie in building new hydroelectric dams. Their environmental and social impacts, better documented today than at the time of the construction of the large dams, are too significant. Alternatives exist, for the better of the population and natural spaces of Quebec.

Above all, fundamental questions must be addressed.

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