Do large dams benefit the environment?

big dams are good or bad for the environment zeray yihdego michael simon debate water reservoir
Numéro 1

Learn the ropes

What are dams are what they are used for?
Dams are barriers which are built to stop or restrict the flow of water. They create water reservoirs, which can be used as a ressource for irrigation, industrial use and consumption. They generally serve to retain water and to distribute it between different actors.

Dams have been created since antiquity. The oldest known dam was built around 3,000 BC in Jordan. Historically, they were built in Mesopotamia to regulate the flow of the Tigris and Euphrates rivers and for flood control in Ancient Egypt.

How do dams impact the environment?
Dams are used to provide water for consumption, irrigation and industrial use. Therefore, they restrict the water flow of rivers, which also limits the flow of sediment naturally present in the river, leading to erosion of coasts and beaches. Besides that, dams can stop or fragment the movement of fish: by periods of river animals’ migration, a system of transportation is implemented via barges, fish ladders or fish lifts.

Also, dams can have a more direct impact on ecosystems. Researches have studied the difference of water temperature between surface water and bottom water in large dams, which can go up to 17 degrees Celsius. Large tropical reservoirs are also held responsible for increased production of greenhouse gases, such as methane.

Why do we talk about it today?
In the light of worldwide demonstrations for more climate protection, the use of large dams and their environmental impact is questioned. More and more researchers study this question to assess whether the negative impact of dams is more important that their positive impact on the environment and on human populations.
Numéro 2

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The weighty benefits of dams

Zeray Yihdego large dams are a benefot to the environment

Zeray Yihdego

Chair in International Law, School of Law, University of Aberdeen

The more than 260 international river basins are very important source of freshwater to billions of people in our planet and they provide a wide-range of benefits to society, countries and to the natural environment.

One of the usual ways of making use of river freshwater is constructing dams and creating reservoirs for various purposes. Small scale dams are, in fact, largely seen positively for socio-economic development and conservation of natural resources.

They are constructed with proper study, environmental impact assessment

Mega dams, by contrast, are proved to be very controversial among scholars and activists of environmental protection. The International Commission of Dams considers mega dams as economically, environmentally highly beneficial to society provided that they are constructed with proper study, environmental impact assessment, and carefully filled, operated and managed.

This is why international and/or national water law and other widely accepted standards, such as the principle of equitable utilisation, the duty to prevent significant harm to riparians and the obligation to protect ecosystems, ought to be followed, to ensure that the construction, filling and operation of mega dams promote inclusive and environmentally sustainable development.

Mega dams certainly come with their financial and safety risks and changes to hydrological flow of water. They could also affect the livelihood of humans, plants and wild animals. However, subject to the safety and legal requirements, the benefit of mega dams outweigh their disadvantages:

Comparatively less, preventable and manageable risks

First, mega dams bring tremendous socio-economic growth for countries and their populations. They enable them to secure: a) food through large-scale irrigated farming or expansion and modernisation of fisheries, b) renewable energy through hydropower generation, c) significant income from tourism and recreation, d) access to clean water to cities and e) river navigation. The mega dams in the Americas, Asia, Europe and Africa are clear evidences of such benefits, although most of these dams might, or might not, satisfy the internationally recognised standards of construction, operation and environmental protection.

Secondly, any mega project comes with its own risks. When compared with the other alternatives of energy projects, in particular nuclear power projects, however, mega dams not only provide renewable energy but also come with comparatively less, preventable and manageable risks for generations to come.

Thirdly, dams could positively contribute to a better natural and man-made environment through regulating flooding, water flow and slit, thereby help encounter the potential effects of climate change.

This assessment proves two important findings: that the benefits of dams, compared to their negative impacts, are weighty, as confirmed in the role of mega dams in the economic advancement of developed countries, and that each mega dam must be judged on its merits, either to acclaim or denounce it as environmentally decent or not so decent.


Large dams do not benefit the environment

Michael Simon international rivers large big dams environment ong

Michael Simon

Director of programs, International Rivers

Large dams are catapulting a range of species toward extinction. They are flooding critical carbon sinks and depriving still others of their sustenance. And dams are the greatest single source of manmade emissions of methane: the most potent of greenhouse gases. Needless to say, it’s hard to argue dams benefit the environment. That is, unless, you stand to profit from the greenwash.

Dams are the greatest single source of manmade emissions of methane: the most potent of greenhouse gases

On May 14-16, the World Hydropower Congress will convene in Paris with the supposed commitment to deliver on the Paris Climate Agreement and Sustainable Development Goals. It’s a convenient posture given the money now available to mitigate climate change through mechanisms like the Green Climate Fund (GCF). In addition to drawing money (some $130 million in the case of but one project in Tajikistan alone) from newer, greener technologies, large hydro is, as close to 300 NGOs have noted in a letter to the GCF: inefficient, ineffective, and disrupts the function of rivers as regulators of the carbon cycle.

So why, despite the preponderance of evidence that large hydro is bad for the environment is it maintaining its green luster? For this, you need look no further than the co-host of the upcoming World Hydropower Congress: the United Nations Educational, Scientific and Cultural Organization (UNESCO). UNESCO is at once the international body responsible for selecting World Heritage Sites for legal protection, while at the same time legitimizing an industry that threatens close to 20 of these sites.

A further aspect of the greenwash is the argument that hydropower is the lesser of two evils. It is less caustic to the environment than conventional power sources so it is thus beneficial to the environment. Needless to say, there are fallacies in this premise. First, studies are finding that in tropical environments and high-sediment areas, dams can release more greenhouse gases than coal-fired power plants. Second, even if large dams were less caustic, their carbon footprint is by no means beneficial nor comparable to alternatives like wind and solar.

Even if large dams were less caustic, their carbon footprint is by no means beneficial nor comparable to alternatives

It’s time for those institutions giving credence to the myth of “dams as eco-savior” to shift course, before this supposed “solution” to climate change accelerates us even faster to toward the Intergovernmental Panel on Climate Change’s point of no return.

What is your opinion now?

3 thoughts on “Do large dams benefit the environment?

  1. I’d take issue with Yidhego’s assessments on dam benefits. Sounds nice in theory, but what evidence is he premising his take on? This piece from Yale Environment 360 provides some useful perspective: Scientific journals now frequently publish articles undercutting key assumptions about dams. Probably the best-known research, a 2014 Oxford University study of 245 large dams built between 1934 and 2007, concluded that without even taking into account dams’ vast social and environmental costs, they are too expensive “to yield a positive return”— that is, the dams aren’t cost-effective. The study found that on average dams’ actual costs were 96 percent higher than their estimated costs⁠, and the average project took 44 percent longer to build than predicted. According to data⁠ published last year by Bent Flyvbjerg, one of the authors of the Oxford study, compared to other energy technologies, only nuclear power has a worse record for cost and schedule overruns than dams; solar and wind projects are at the top of the list.

  2. Dear Nick, thanks for your comments.
    I am aware of the reports and published work you have referred to. All mega projects, whether water related or not, have financial and other risks. The question should therefore be are the negative impacts ought weigh their benefits? As per the independent Dams Commission (which is constituted of diverse scientists and not activists!), a carefully designed and utilised mega dams server communities for decades with minimal environmental and other effects. I encourage you to have a look at the response of the Dams Commission to the Oxford Said business School report of 2014. I also encourage you to read some of the technical articles published @
    All best wishes

  3. Dams should have been constructed to harvest rain water without blocking the rivers.

    Marine life have been badly affected due to dams blocking the rivers. River basins have contributed much benefits in the past. But due to heavy storm/rain some basins have been badly damage and surprisingly elected governments have failed to restore them. Natural river basins where Rocky path of the rivers are intact.

    Trust human species will wake up and restore the water sheds, river basins, ponds, river banks for the future generations to make use of it.

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