“Greening” Dams – Is It Possible?


  • The majority of rivers longer than 1,000 km are interrupted, diverted or completed dried up
  • Environmental flows and watershed management need to be incorporated into discussions on building dams
  • The threat to freshwater biodiversity from large dams is high, but there are ways to mitigate it

Hydropower plants have great benefits in terms of green energy production: clean, renewable, and it is possible to adjust the electricity production based on the actual demand. But large dams also carry potentially significant environmental and social issues including: land clearing and inundations, resettlement of entire villages, slopes erosion and river sedimentation, methane emissions from the reservoir, and interruption of ecological connectivity of rivers. There is also a tendency to underestimate the far-reaching effects on biodiversity such as the impact on fish.

Do we still need large dams?

When the ancient Greece philosopher Heraclitus created the famous aphorism “Panta rei” (“everything flows”), he could not imagine that about 15 centuries later, only 37% of the world rivers, longer than 1,000 km, freely flow over their entire length. All the other rivers are interrupted, diverted, or even completely dried up by large infrastructures like hydropower plants, irrigation and flood control dams. Roughly 2.8 million dams exist worldwide (with 60,000 considered “large dams”). Many of them are already obsolete, not working properly or heavily sedimented. In Europe, thousands of dams are to be removed in the next few years. But at the same time, not less than 3,700 major hydroelectric dams are either planned or under construction, primarily in countries with emerging economies. Some of them are threatening biodiversity and inland fisheries worth billions of dollars and encountering strong protests from the affected communities.

Protests of affected communities in from of the World Hydropower Congress (Paris, 2019)

It is possible to solve the long-standing conflict between hydropower, local communities’ livelihood and river ecosystems connectivity through integrated strategic planning considering the energy needs and the different energy options available in the market. The problem is that many large dams are still prepared without considering the regional influence on the whole watershed. Instead of a single energy source, installing a variety of renewable energy sources, including smaller-scale hydropower dams on tributaries, could produce power reliably and cheaply while protecting ecosystems and local communities. Solar and wind energy can, in some scenarios, already offer electricity prices as cheap as or cheaper than electricity prices from hydropower, and that is not even mentioning the additional environmental externalities of a big dam project.

Let the water flow

Large dams normally create an alteration of the natural river flow, both in terms of volume of water and sediment transport. To partially mitigate the negative effects of these changes, national legislations often refer to rivers’ “environmental flow” that needs to be maintained in the dewatered section of the river to allow the ecological components to survive, and for downstream human communities to have enough water for their necessities. This value is often established as a fraction of the annual natural flow, and unfortunately it often fails to achieve both requirements. International funding agencies are moving towards a more comprehensive approach to determine the minimum flow, considering multiple parameters, including river hydrology, freshwater biodiversity and socioeconomic values.

A river downstream to a dam where no environmental flow is released

Ideally, the environmental flow should be calculated for the whole river – not just for the consideration of a dam project. Doing so would provide a strong basis for valuing hydropower developments going forward and make it possible to ‘green’ dams in that environmental impacts are properly accounted for in the decision-making process. If the minimum environment flow is not enough to ensure the financial feasibility of the project, the proponents (either governments or private companies) should look at other options in terms of location, or other sources of renewable energy, as mentioned above. Some rivers should be left flowing without interruptions so that they can continue to give freshwater to communities, guarantee thriving ecosystems, and fertilize estuaries with the load of sediments coming from the watershed. 

It is not only about fishes…

Continuing to build large dams without considering their impact on freshwater biodiversity could have monumental consequences. The International Union for Conservation of Nature (IUCN) estimates that approximately one third of freshwater species are in danger of extinction, threatened by habitat destruction, interruption of river continuity, chemical pollution, spread of invasive alien species and climate change related issues – all problems that large dams can cause or exacerbate.

Freshwater biodiversity threatened by dam construction includes migratory prawns, crabs, and even mammals and amphibians: it is not only about fishes. Some invertebrates have important but neglected roles, like controlling the population of vectors of human diseases. Moreover, many local communities rely on river resources for food and their livelihood. Substantial capture-fishery declines occur when barriers to fish movement block access to spawning, nursery and growth habitats. These barriers also create artificial fish aggregations, which are extremely vulnerable to overexploitation and exposed to infectious diseases. These fish either spawn at the wrong time in the wrong place, or cannot even spawn at all if the riverbed is also intensively modified. Dams can seriously affect fisheries (and biodiversity).  Yet, management agencies or project developers often consider that mitigating the environmental impacts of dams are unnecessary (or excessive) expenses, and consequently many projects proceed without biodiversity-related considerations. 

Mitigating the impacts on migratory species and on river connectivity is possible, given that enough resources are allocated for it. Some structural fish-friendly measures are being implemented in watershed management projects in the Lao People’s Democratic Republic, one of the regions with the highest freshwater biodiversity. Even the choice of the right turbines is critical, as some of them are more “fish-friendly” than others. A holistic approach to the downstream and upstream passage of fish and other wildlife should be a component of the environmental assessment for any new dam project. A shift in the narrative of how large dams are discussed to include environmental flows and freshwater biodiversity could help alleviate the social and environmental issues than can and often do accompany the building of large dams.

Francesco is an Environment Specialist at the Asian Development Bank, focusing his efforts on water quality and biodiversity related issues. He holds a PhD in Environmental Sciences, with research focused on the effects of water pollution on water ecosystems. He is also a former EU Marie-Curie fellow, where he worked on the implementation of the EU Water Framework Directive.

Leave a Reply

You have to agree to the comment policy.