Dams And Pollution: A Troubling Relationship?

Dams are large structures that block the flow of water across a river, creating a reservoir. While they can provide benefits to people, dams also have a significant impact on the environment. This impact includes altering water temperatures and oxygen levels, affecting the health of the river and the wildlife that depends on it. The construction and operation of dams can also emit greenhouse gases and threaten entire hydrologic systems. In this paragraph, we will explore the question: do dams cause pollution?

Dams can cause higher water temperatures and lower oxygen levels, harming wildlife

Dams are often constructed across rivers to store water that would naturally flow into the lower reaches of the river and the sea. While reservoirs are created to benefit people, they can have an unfavourable impact on the wildlife and environment in and around the river valley, as well as downstream of the dam. The presence of a dam upsets the natural balance of the river, affecting the animal and plant life in and around it.

Dams can cause higher water temperatures and lower oxygen levels, which can be harmful to wildlife. Small, surface-release dams are commonplace and tend to slow water flow and decrease canopy cover. This can increase stream temperatures. A study of 30 dam sites across Massachusetts found that most dams (67%) warmed downstream waters, with August mean temperatures 0.20–5.25 °C higher than upstream. Downstream temperatures cooled with increased distance from the dam at 68% of sites, but the warmest temperatures were observed closest to the dam. A separate study of select dams in Massachusetts from 2014 to 2016 found that two-thirds of sites studied had warmer stream temperatures downstream of dams.

Dams can also decrease the dissolved oxygen concentration of water downstream. In fast-moving streams, rushing water is aerated by bubbles as it churns over rocks, saturating these streams with oxygen. In slow, stagnant waters, oxygen only enters the top layer of water, and deeper water is often low in dissolved oxygen concentration due to the decomposition of organic matter by bacteria. This decomposition requires oxygen, and so when there is excess organic waste or algae, excess bacteria grow, depleting the dissolved oxygen levels required by other aquatic species. A study of dam impoundments found that two-thirds had lower surface dissolved oxygen than upstream levels.

The elevated stream temperatures and low dissolved oxygen levels associated with small dams can result in increased stress and death among fishes, mussels, stream insects, and other aquatic organisms.

They can prevent fish migration, impacting spawning and food sources

Dams are often constructed across rivers to store water that would naturally flow downstream. This upsets the natural balance of the river, affecting the wildlife and plant life in and around it. The flooding of land and the management of reservoir water can have an unfavourable effect on people, wildlife, and the environment.

The building of a dam can have a significant impact on fish populations. Fish migrations and movements can be stopped or delayed, and the quality, quantity, and accessibility of their habitat can be affected. For example, the construction of the Gulam Mahommed Dam on the Indus River deprived the migratory Hillsa ilisha of 60% of their previous spawning areas. Similarly, the construction of dams on the Columbia River and its main tributary, the Snake River, resulted in the flooding of most spawning habitats.

Dams can block fish from moving along their natural pathways between feeding and spawning grounds, interrupting their life cycles and limiting their ability to reproduce. This can lead to changes in species composition, with the loss of certain fish species. For example, the Chinese paddlefish, a source of food and cultural reverence for communities along the Yangtze River, went extinct due to the construction of dams.

Hydropower dams, in particular, have been found to threaten fish habitats worldwide. They can prevent fish from accessing both ocean and freshwater habitats, which is necessary for completing their life cycles. This can lead to the decline of migratory fish populations, such as salmonids in North America, which relied on annual migration for their livelihoods.

NOAA Fisheries works to improve fish passage at non-federal hydropower dams to recover threatened and endangered migratory fish and support the sustainability of economically important fisheries. They provide design and operation guidance for fish passage structures to help dam owners create safe and effective solutions.

Dams can cause flooding, destroying heritage sites and displacing communities

Dams are human-made structures designed to collect and control water. While they are often constructed to benefit people, dams can also cause flooding, leading to the destruction of heritage sites and the displacement of communities.

Dams are built across rivers to store water and generate electricity. However, their presence can upset the natural balance of the river, affecting the animal and plant life that depends on it. When a dam is built, the river upstream of it is flooded and becomes a reservoir. This flooding can have detrimental effects on the surrounding areas, including communities, wildlife, and the environment.

The construction of the Three Gorges Dam in China, for example, caused water levels in the Yangtze River to rise, inundating entire cities and historical locations along the river. Similarly, the Grand Ethiopian Renaissance Dam, a massive hydropower plant, has the potential to cause massive flooding if it overflows or breaks. In 2019, a dam collapse in Brazil led to a toxic mudslide that killed 270 people, demonstrating the catastrophic consequences that can occur when dams fail.

The flooding caused by dams can also lead to the destruction of heritage sites. Throughout history, many communities have settled in river valleys, which are often home to archaeological sites, historical buildings, and sacred places of cultural significance. When a valley is flooded to create a reservoir, these heritage sites can be lost forever. For instance, China's aggressive development has resulted in the destruction of over 30,000 items listed by the state administration of cultural heritage, including historical neighbourhoods in Beijing and Nanjing.

In addition to causing flooding and destroying heritage sites, dams have also been responsible for the displacement of communities. The filling of reservoirs can lead to forced resettlement, destabilizing communities and causing social disruption. A conservative estimate suggests that 50-60 million people have been displaced by dams worldwide in the past fifty years, with a disproportionate number belonging to indigenous and politically marginalised communities. The development afforded by dams for some has often come at the cost of the impoverishment of those who are displaced.

They can emit methane, a potent greenhouse gas, contributing to global warming

Dams are massive structures that retain water for various purposes, including domestic use, irrigation, and industrial processes. While they serve several benefits to people, they also have unfavorable effects on the environment. Notably, dams can emit methane, a potent greenhouse gas, contributing to global warming.

The accumulation of organic matter and sediments in reservoirs and impoundments, the bodies of water created behind dams, provide an ideal environment for underwater microbes that produce methane. This methane emission significantly contributes to global warming. The greenhouse gas effect of methane is 20 to 40 times more potent than that of carbon dioxide (CO2). Tropical regions are especially susceptible to methane emissions from decaying organic matter in flooded reservoirs.

The disruption of natural river processes by dams leads to increased water temperatures and decreased dissolved oxygen levels. This phenomenon is further exacerbated by the elevated temperatures of the impounded water, which then spills over the dams and raises downstream temperatures. The warming effect is most pronounced in naturally cold streams, threatening temperature-sensitive species such as eastern brook trout.

Additionally, dams can prevent fish migration, limiting their access to spawning habitats, food sources, and ability to escape predators. The reduction in dissolved oxygen levels can also increase stress and mortality rates among aquatic organisms, including fishes, mussels, stream insects, and other species. The negative impacts on aquatic ecosystems and fisheries further contribute to the overall ecological consequences of methane emissions from dams.

The construction and operation of large dams have been associated with environmental damages, including the degradation of aquatic ecosystems, loss of biodiversity, and adverse effects on human communities. The release of large amounts of mining sludge, as seen in the case of the Mount Polley mine dam failure in Canada, highlights the potential for catastrophic environmental impacts when dams fail or breach. Overall, the methane emissions from dams and their contribution to global warming are significant factors in assessing the environmental impact of these structures.

Dams can alter water quality, impacting downstream areas and aquatic life

Dams are typically constructed across rivers to store water that would naturally flow downstream. While they are created to benefit people, dams can have unfavourable effects on the environment, wildlife, and people living in the reservoir area.

Dams and their impoundments (the body of water created behind a dam) can negatively impact the water quality of the streams on which they are located. Good water quality is essential for a healthy stream, and dams can cause temperature and dissolved oxygen levels to fall outside of the optimal range for aquatic life. From 2014 to 2016, DER-supported scientists conducted a study in Massachusetts to understand the implications of small dams on water quality. They found that dam impoundments consistently had lower water quality compared to their upstream and downstream waters. Two-thirds of the sites studied had warmer stream temperatures downstream of the dams, and one-third had lower dissolved oxygen levels. Elevated stream temperatures and low dissolved oxygen levels associated with small dams can result in increased stress and death among fishes, mussels, stream insects, and other aquatic organisms. As streams warm, native cold-water species can be replaced by non-native generalist species, as has been observed across Massachusetts.

Dams can also prevent fish migration, limiting their ability to access spawning habitat, seek out food resources, and escape predation. Fish passage structures can enable some fish to pass around a dam, but their effectiveness decreases depending on the species of fish and the number of dams in the waterway. Additionally, gravel, logs, and other important food and habitat features can become trapped behind dams, negatively affecting the creation and maintenance of more complex habitats downstream.

The construction and operation of large dams can also emit CO2 and methane, a potent greenhouse gas. This is due to the large amounts of decaying organic matter retained in flooded reservoirs, which produce methane as microbes feed on the organic matter. Dams also lead to the fragmentation of rivers and the destruction of surrounding forests, which eliminates valuable carbon sinks and contributes to global warming.

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