Table of Contents
Introduction
Hydropower is often presented as a clean and reliable source of renewable electricity. It produces very low direct greenhouse gas emissions compared with fossil fuel plants and can support grid stability. At the same time, it is one of the most environmentally and socially disruptive forms of renewable energy. This chapter explores the main environmental and social issues that are specific to hydropower projects, especially those that involve dams and reservoirs, and introduces approaches that seek to reduce or manage these impacts.
River Transformation And Habitat Change
Most hydropower projects change rivers from free flowing systems into a series of reservoirs and regulated channels. This transformation affects the physical structure of the river, called its morphology, as well as its flow patterns, water levels, and the way sediment is transported.
Free flowing rivers usually have natural variations in discharge, with higher flows during rainy seasons or snowmelt and lower flows in dry periods. Hydropower regulation can flatten these seasonal cycles, or in some cases introduce artificial daily cycles linked to electricity demand. This modification alters habitats for aquatic plants and animals and can reduce the diversity of species that are adapted to natural flood pulses and changing water levels.
When a reservoir is created behind a dam, riverine habitats are replaced by lake-like conditions. Species that are adapted to fast flowing, oxygen-rich water may decline, while those that prefer stiller, deeper waters may increase. Downstream, reduced flow variability and changes in temperature and oxygen can alter habitat quality over long distances.
Flow Regimes And Ecological Consequences
The way water is released from a hydropower plant matters as much as the total volume that passes through it. In many projects, operators release water to match electricity prices and demand, not ecological needs. This can produce rapid, repeated rises and falls in downstream water level over a day or a week. These sudden changes, sometimes called hydropeaking, can strand fish and invertebrates on exposed banks and disturb spawning or feeding.
Long term reduction or elimination of seasonal floods can have profound impacts on floodplain ecosystems. Many fish species use floods to access floodplain forests or wetlands to feed and reproduce. Plants on riverbanks often depend on occasional flooding for seed dispersal and germination. Without these pulses, floodplain productivity can drop and the connection between the river and its surrounding landscape is weakened.
Some modern projects attempt to mimic aspects of natural flows through environmental flow releases. These are planned discharges designed to support key ecological processes, for example by creating an artificial flood pulse at a particular time of year. Designing effective environmental flows requires understanding the local river ecology and balancing water for power generation with water for ecosystems and other users.
Sediment Trapping And River Erosion
Sediment, including sand, gravel, and finer particles, is a critical part of river systems. It shapes channels, builds sandbars and deltas, and provides substrate for aquatic organisms. Dams trap sediment that would normally move downstream. Over time, this can fill parts of the reservoir, reduce storage capacity, and starve downstream reaches of material.
Without replenishment of sediment, downstream riverbeds can erode and deepen. This process may undermine bridges and riverbank structures, lower groundwater levels near the river, and increase the risk of bank collapse. In coastal areas, reduced sediment supply to deltas can contribute to coastal erosion and make low lying regions more vulnerable to storm surges and sea level rise.
Some hydropower projects use sediment management techniques such as flushing flows, bypass tunnels, or dredging to move sediment downstream. These measures can reduce but rarely eliminate the alteration of natural sediment regimes. Sediment management is especially challenging on large rivers with high sediment loads, where the cost and technical complexity can be significant.
Water Quality, Temperature, And Greenhouse Gases
Reservoirs can alter water quality both in the impounded area and downstream. Water that is stored for long periods can stratify into layers of different temperatures and oxygen levels. In deep reservoirs, the lower layer can become low in oxygen, which affects chemical reactions and the solubility of substances such as metals and nutrients. When low oxygen water is released through hydropower intakes, it can stress fish and other organisms downstream.
Temperature changes are also common. Releases from deep intakes may be much colder or warmer than the natural river would be at that time of year. Many aquatic species are sensitive to temperature, especially during reproduction and early life stages, so even a few degrees of change can affect survival and migration timing.
Although hydropower itself does not burn fuel, some reservoirs emit greenhouse gases. When land is flooded, vegetation and soil organic matter decompose under water, often under low oxygen conditions that favor methane production. Methane is a potent greenhouse gas. Emissions vary widely between reservoirs and depend on climate, reservoir depth and shape, pre existing land cover, and how the reservoir is operated. Tropical reservoirs with extensive flooded biomass and strong stratification tend to have higher emissions per unit of electricity than high latitude or deep, well oxygenated systems.
Key point: Hydropower has low direct combustion emissions, but some reservoirs, especially shallow tropical ones with large flooded areas, can emit significant methane from decomposing organic matter, which must be considered in climate impact assessments.
Fish Migration And Aquatic Biodiversity
Many river fish species migrate along rivers to reach spawning, feeding, or nursery grounds. Dams can act as physical barriers that block upstream and downstream movement. Species such as salmon, sturgeon, and many tropical migratory fishes depend on the ability to travel long distances. When migration routes are cut, populations can decline or collapse.
Fish can also be injured or killed as they pass through turbines, especially large species and those that are not adapted to rapid pressure changes. Juvenile fish and eggs may be particularly vulnerable. In addition, changes in flow, water level, and temperature can disturb cues that fish use to initiate migration, disrupt spawning habitats, and alter food webs.
Many hydropower developments include fish passes, ladders, elevators, or bypass channels to restore some connectivity. Their effectiveness varies. Successful designs are often species specific and require careful attention to swimming ability, behavior, and attraction flows that guide fish to the entrance. Multi dam river systems pose additional challenges, as fish may pass through many barriers on a single journey.
Terrestrial Biodiversity And Land Use Change
When reservoirs are created, large areas of land can be permanently flooded. Forests, wetlands, farmland, and human settlements may all be lost. This leads to immediate habitat destruction and fragmentation. In tropical forests, reservoirs can inundate areas that are rich in endemic species as well as habitats used by terrestrial mammals, birds, reptiles, and insects. The abrupt shift from land to open water can create long term ecological changes that extend beyond the reservoir shoreline.
Construction of hydropower infrastructure brings access roads, power lines, construction camps, and ancillary facilities. These can open previously remote areas to logging, mining, agricultural expansion, and hunting. Even after construction ends, the new access routes may remain, intensifying human pressure on surrounding ecosystems.
Some projects attempt to offset habitat loss by protecting or restoring other areas. However, created or conserved habitats may not fully replace unique ecosystems lost under the reservoir. Once flooded, original habitats are effectively irrecoverable on human timescales, so project planning often emphasizes avoiding the most sensitive areas rather than relying only on compensation.
Downstream Floodplains, Wetlands, And Delta Systems
Hydropower operations have strong effects on floodplain and wetland ecosystems that depend on seasonal inundation. Regular floods bring nutrients and sediments that support fertile agricultural soils and high biological productivity. They also create temporary habitats that support fish spawning, bird nesting, and grazing for herbivores.
If dams greatly reduce or eliminate seasonal floods, former floodplains can dry out or be converted to permanently cultivated lands with irrigation. While this can bring short term economic benefits, it can reduce biodiversity, alter soil processes, and make landscapes more vulnerable to prolonged drought. Wetland areas that rely on flood pulses can shrink or disappear, affecting water birds and other specialized species.
At river mouths, reduced floods and sediment loads alter estuaries and deltas. Salt water can intrude further inland, affecting freshwater ecosystems and agricultural lands. Combined with sea level rise, this can increase coastal flood risk and undermine the livelihoods of people who depend on delta fisheries and farming.
Human Displacement And Social Disruption
One of the most visible social issues of hydropower is the displacement of people from areas that are to be flooded. Large reservoirs often require relocation of entire villages or towns. In some projects, tens or hundreds of thousands of people have been resettled. Displacement involves loss of homes, farmland, cultural sites, and social networks.
Compensation and resettlement programs vary in quality and fairness. Inadequate compensation, delays, and lack of participation in decision making can leave affected communities worse off than before. People may be moved to land that is less fertile, more crowded, or in unfamiliar environments, leading to economic hardship. Social cohesion can be damaged if communities are split or if host communities feel burdened by incoming populations.
Beyond physical displacement, there may be loss of cultural and spiritual values. Sacred sites, burial grounds, or landscapes with deep cultural meaning can be submerged. For Indigenous peoples whose identities and traditional practices are closely linked to rivers and specific places, this can be especially harmful.
Livelihoods, Fisheries, And Food Security
Hydropower projects can transform livelihoods that depend on rivers and floodplains. Inland fisheries are a major source of protein and income in many regions. Changes in flow, habitat, and fish migration can reduce fish catches upstream and downstream. While reservoirs sometimes create new fisheries, these are usually dominated by different species and may require new skills and equipment.
Farmers who rely on floodplain agriculture may see changes in soil fertility and water availability. Traditional systems that use flood recession agriculture, where crops are planted in moist soils as waters recede, may no longer function if floods no longer occur or are mistimed. Pastoralists may lose grazing areas when floodplains are flooded permanently or no longer receive seasonal water.
In some cases, hydropower reservoirs are used for irrigation, aquaculture, tourism, or transport, which can provide new opportunities. However, access to these benefits is often uneven. Those who bear the costs of change are not always the same as those who receive new benefits, which can deepen local inequalities.
Health Impacts And Local Environments
Health impacts around hydropower reservoirs can be both positive and negative. Improved access to electricity can support better health services, refrigeration of medicines, and reduced indoor air pollution if people shift from traditional biomass fuels. On the other hand, new reservoirs can create conditions favorable for disease vectors.
Standing or slow moving water with fluctuating shorelines can promote the breeding of mosquitoes and snails that transmit diseases such as malaria and schistosomiasis. Changes in water quality, such as higher nutrient levels, can increase algal growth and reduce suitability for drinking or bathing without treatment. Increased traffic and in migration during construction can also bring public health challenges, such as higher rates of accidents, sexually transmitted infections, or substance abuse.
Effective management can reduce some of these risks. Shoreline management, targeted health campaigns, and coordination between the energy and health sectors can help control disease vectors and respond to emerging health issues.
Cultural Heritage And Landscape Values
Rivers often play central roles in cultural and spiritual life. They may be associated with myths, rituals, festivals, and collective identities. Hydropower development can alter or erase these cultural landscapes. Temples, shrines, historic settlements, and archaeological sites may be flooded or isolated by reservoirs. Even if moved or documented, their original context is lost.
For many Indigenous and local communities, particular stretches of river, waterfalls, or confluences have deep significance that cannot be reproduced elsewhere. Changing a river into a reservoir or heavily regulated channel can therefore be experienced as a loss of heritage and identity, not only of physical assets.
Recognition of intangible cultural values is increasingly important in project planning. Tools such as cultural impact assessments, along with meaningful consultation and consent processes, aim to identify and respect cultural priorities. However, there can still be tensions between national development goals and the desire to preserve landscapes that communities see as irreplaceable.
Equity, Participation, And Rights
Hydropower decisions often involve trade offs between national or regional benefits and local costs. This raises questions of equity and justice. Communities that are directly affected may have limited influence over project design, location, or operation, especially when decisions are made far away by government agencies or private developers.
In many countries, dams have been built in areas inhabited by marginalized groups, including ethnic minorities and Indigenous peoples. Where land rights are weak or unrecognized, people may receive little or no compensation for lost resources. Lack of transparent information, consultation, and grievance mechanisms can intensify conflict and mistrust.
International standards and guidelines increasingly emphasize principles such as free, prior, and informed consent for Indigenous peoples, fair benefit sharing, and inclusive stakeholder participation. Implementing these principles in practice requires time, resources, and strong institutions. It also implies that some projects may need to be altered or not built when social and cultural costs are too high.
Cumulative Impacts And River Basin Perspectives
A single hydropower project may have significant local impacts, but many rivers now host cascades of multiple dams. The combined or cumulative effects of several projects can be much greater than the sum of their individual impacts. Migratory routes may be blocked over long distances, sediment reduction may affect entire deltas, and multiple reservoirs may alter climate and hydrology across a large region.
Assessing cumulative impacts requires looking beyond the site scale to the level of the river basin. Decisions about where to place hydropower projects, which tributaries to leave free flowing, and how to coordinate operations among different plants become crucial for overall sustainability. This basin scale approach also needs to integrate other water uses such as irrigation, drinking water supply, navigation, and ecosystem needs.
Planning at the basin scale can help avoid building dams in the most ecologically or socially sensitive locations and can concentrate development where impacts are more manageable. However, it often requires cooperation between different agencies, sectors, and sometimes countries, which can be challenging.
Risk, Safety, And Extreme Events
Hydropower dams introduce new types of risk into river systems. Structural failure or uncontrolled releases due to extreme rainfall, earthquakes, or poor maintenance can cause catastrophic flooding downstream. Even when dams do not fail, rapid operational changes in water release can pose safety risks for people living or working along the river.
Climate change adds uncertainty to future hydrology. Changes in precipitation patterns, glacier melt, and extreme events can alter river flows compared to historical patterns that were used in design and planning. This affects not only the reliability of hydropower production but also flood risk and drought risk management. There is a growing need to design and operate projects with climate resilience in mind, including consideration of dam safety and emergency preparedness.
Communities living downstream often rely on clear communication and warning systems related to reservoir operations. Without these, sudden releases for hydropower or flood management can endanger lives, livestock, and crops.
Mitigation, Better Practices, And Alternatives
Awareness of environmental and social issues has led to the development of guidelines and practices that seek to improve hydropower performance. These include screening and avoiding the most sensitive sites, such as key biodiversity areas and territories of Indigenous peoples that oppose development, and improving design and operation where projects proceed.
Examples include incorporating environmental flow regimes, designing more effective fish passages, applying sediment management strategies, and planning resettlement programs that genuinely improve living conditions for affected people. Independent environmental and social impact assessments, along with ongoing monitoring and adaptive management, are central tools.
In some cases, alternatives to large dams may be preferable. Options can include smaller run of river projects with limited storage, energy efficiency measures, or development of other renewable sources such as wind and solar, depending on local conditions. Comparing hydropower with these alternatives requires attention to both energy benefits and the full range of environmental and social costs.
Conclusion
Hydropower sits at a complex intersection of renewable energy benefits and substantial environmental and social challenges. It reshapes rivers and landscapes, affects biodiversity and water quality, and can profoundly alter the lives and cultures of people who depend on river systems. Understanding these issues is essential for evaluating hydropower projects, improving their design and operation, and making informed choices about where and how hydropower fits within broader sustainable energy and water strategies.