Worldwide, hydropower has been part of the energy transition for several decades. Often presented as a clean and renewable source of energy, it generates electricity without directly burning coal, oil or gas. Yet behind this positive image lies a more complex environmental reality. Some dams fragment rivers, alter habitats, disrupt sediment transport, influence greenhouse gas emissions and profoundly transform the landscapes in which they are built. Between the need to decarbonise electricity production and the protection of ecosystems, their overall impact deserves a nuanced examination.
Rivers Cut in Two
The first impact of a dam is immediately visible in the landscape: the structure interrupts the natural continuity of a river. Yet a river is far more than a volume of moving water. It forms an ecological corridor used by fish, invertebrates and many other species to feed, reproduce or move between different habitats throughout their life cycles.
As a result, migratory fish such as salmon, eels and sturgeon can face major difficulties when a dam blocks their route to breeding grounds. Operators sometimes install fish passes or bypass systems. However, their effectiveness varies depending on the species, the design of the structure and the characteristics of the river.
In addition, a dam changes the very nature of the river. Upstream, the current slows and forms an artificial lake. The water becomes deeper, calmer and sometimes warmer. Species adapted to fast-flowing water may then decline, while species better suited to slow-moving or stagnant water become more common. Gradually, the entire food web can be affected.
Sediment: An Invisible but Essential Resource
A river carries much more than water. It also transports sand, gravel, silt and organic matter. These materials feed floodplains, deltas, beaches and certain coastal ecosystems. However, dams interrupt part of this natural transport. Sediment accumulates in reservoirs, gradually reducing the storage capacity of some of them. At the same time, downstream areas receive less material and, in some cases, fewer associated nutrients.
This disruption can have several consequences. It changes river morphology, can increase bank erosion and reduces the amount of sediment reaching deltas and coastlines. In some regions, this decline weakens coastal areas already affected by erosion and rising sea levels. Sediment management has therefore become a major issue, both for extending the lifespan of dams and for limiting their effects on downstream environments.
Did you know?
A dam can retain a significant proportion of the sediment naturally transported by a river. Over several decades, some reservoirs can accumulate metres of deposits. This sedimentation gradually reduces their storage capacity and can also alter ecosystems far downstream, including coastal areas.
Renewable Energy That Is Not Always Carbon-Neutral
Hydropower does not directly burn fossil fuels to generate electricity. In many situations, it therefore remains a relatively low-carbon source of power. However, this does not mean that every dam has a zero-carbon footprint. When a reservoir is created, water may cover forests, grasslands, soils and vegetation. This organic matter gradually decomposes. In oxygen-poor environments, particularly at the bottom of reservoirs, this process can release methane as well as carbon dioxide.
These emissions vary considerably depending on the climate, reservoir depth, quantity of submerged organic matter, age of the dam and the way the reservoir is managed. Some reservoirs in tropical regions can, in particular, produce more methane than those in temperate climates. For this reason, each project needs to be assessed individually. In some specific cases, emissions relative to the amount of electricity generated can become significant and considerably reduce the expected climate benefit.
When Dams Also Alter the Water Cycle
Large reservoirs greatly increase the surface area of water exposed to the atmosphere. This change encourages evaporation, particularly in hot, dry or highly sunny regions. As a result, part of the stored water returns directly to the atmosphere before reaching downstream areas. In river basins where water resources are already limited, these losses can intensify competition between electricity generation, irrigation, human consumption and ecosystem needs.
At the same time, dams also change the natural rhythm of rivers. Operators adjust water releases according to electricity demand, agricultural needs or safety constraints. Yet aquatic ecosystems are often adapted to very specific seasonal variations. When flows change abruptly or no longer follow natural cycles, riverbank habitats, wetlands and certain species may face additional pressure.
Did you know?
One of hydropower’s main technical advantages is its flexibility. Unlike solar or wind power, whose output depends directly on weather conditions, some reservoir-based hydropower plants can increase electricity generation very quickly. They can therefore help balance power grids when demand changes or when other renewable energy sources produce less electricity.
A Sometimes Considerable Human Cost
The consequences of large dams extend beyond biodiversity. Since the twentieth century, the construction of major reservoirs has also displaced millions of people around the world. By flooding entire valleys, some projects have submerged villages, farmland, places of worship, forests and historic sites. Displaced communities must then rebuild their livelihoods, sometimes in areas with which they are less familiar.
The impacts can become particularly significant when communities depend directly on the river for fishing, agriculture or cultural practices. Today, environmental and social impact assessments increasingly seek to include these dimensions. Nevertheless, conflicts surrounding dam projects show that energy is also a question of territory, social justice and governance.
Producing Electricity Without Forgetting Rivers
Despite these impacts, hydropower continues to play an important role in many electricity systems. It provides a controllable source of renewable energy and can help stabilise grids that rely increasingly on solar and wind power. However, approaches are changing. In some regions, authorities are now removing old or unused dams to restore ecological continuity. Elsewhere, operators are improving fish passages, restoring flows that more closely resemble natural cycles or developing new sediment-management methods.
Small-scale hydropower projects and run-of-river installations can also reduce some impacts, although they are not automatically free from environmental consequences. The issue, therefore, is not simply to oppose dams and nature conservation. Rather, it is to determine where hydropower remains appropriate, how its impacts can be reduced and which ecosystems should remain free from new development. The energy transition requires a rapid reduction in greenhouse gas emissions. It must also preserve the functioning of rivers, wetlands and ecosystems on which millions of people depend.
FAQ
A hydroelectric dam retains or diverts part of a river’s water in order to drive turbines that generate electricity. Some facilities include large reservoirs, while others operate more as run-of-river systems.
Yes. It relies on the natural water cycle, powered by rainfall and river flow. However, the fact that an energy source is renewable does not mean that it has no environmental impact.
They interrupt river continuity and can prevent certain species from reaching their breeding or feeding grounds. Fish passes can sometimes reduce this impact, but their effectiveness depends heavily on their design and on the species concerned.
When water slows down inside a reservoir, sand, gravel and silt gradually settle to the bottom. Downstream areas then receive less sediment, which can alter rivers, deltas and some coastal environments.
Yes. Operators can restore environmental flows, improve fish passage, manage sediment more effectively or remove structures that are no longer needed. However, every river has its own ecological dynamics and therefore requires an approach adapted to local conditions.
Sources
IPCC – Climate Change 2022: Mitigation of Climate Change
The IPCC highlights that the carbon footprint of reservoir-based hydropower can vary considerably depending on local conditions. Flooding vegetation and soils can notably lead to methane emissions caused by the anaerobic decomposition of organic matter.
Read the IPCC report
Nature Reviews Earth & Environment – Hydropower impacts on riverine biodiversity (2024)
This publication examines the effects of hydropower on river biodiversity, including river fragmentation, altered flow regimes, temperature changes, habitat loss and disruption to species movement.
Read the study
International Energy Agency – Hydropower Special Market Report
The IEA highlights the important role of hydropower in electricity systems, particularly because of its ability to provide flexibility and support the integration of variable renewable sources such as solar and wind power.
Read the IEA report
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