Dams' Dark Side: Unveiling Their Pollution Secrets

Dams are constructed across rivers to store water and provide benefits to people. However, the construction and operation of dams can cause pollution and negatively impact the environment. This paragraph will discuss the ways in which dams can cause pollution and affect the surrounding ecosystem. From altering water temperatures and oxygen levels to disrupting natural river flow and fish migration, the presence of dams has significant ecological consequences.

Water quality degradation due to oxygen depletion and temperature changes

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

A study by DER-supported scientists from 2014 to 2016 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. The surface water of the dam impoundments was warmer than the water upstream of the impoundments. After spilling over the dams, the warm impoundment water elevated downstream temperatures at two-thirds of the sites, with average August temperatures up to 5.3°C higher downstream than upstream of the dams. These elevated temperatures persisted for up to 4.5 km downstream of the dams.

The highest stream warming was observed downstream of the dams with the widest impoundments (relative to the natural stream width) on the coldest streams. Two-thirds of the dam impoundments observed in this study had lower surface dissolved oxygen than upstream levels. Downstream dissolved oxygen levels were found to be minimally impacted, suggesting that the aeration that occurs when water spills over the dam may help alleviate the negative effects of lower dissolved oxygen levels in the impoundment. However, coldwater streams with a high gradient (steep slope and rapid flow of water) in small watersheds and dams with shallow impoundments had the greatest reduction in dissolved oxygen levels both within the impoundments and downstream of the dams.

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 coldwater species can be replaced by non-native generalist species, as has occurred in Massachusetts. This study found that the warming effects of dams were most pronounced in the naturally coldest streams, suggesting that temperature-sensitive, coldwater species such as eastern brook trout are especially vulnerable to warming impacts by small dams. All aquatic organisms require oxygen to survive. Hypoxic (low dissolved oxygen) and anoxic (no dissolved oxygen) zones that can form in dam impoundments are inhospitable to aquatic species.

Blocked fish migration, threatening aquatic life and ecosystems

The construction of dams can have a detrimental effect on fish migration, which in turn threatens aquatic life and ecosystems. Dams are often built across rivers to store water, disrupting the natural balance of the river and affecting the plant and animal life that depends on it. This includes blocking fish migration routes, which can have severe consequences for fish populations and the wider ecosystem.

Dams act as barriers, preventing fish from swimming upstream or downstream to reach their spawning grounds. This disruption to migration patterns can lead to a decline in fish populations, as fish are unable to complete their life cycles. For example, the wild salmon population in the Pacific Northwest's Columbia River basin has decreased by 85% since the introduction of large dams in the region. Similarly, Atlantic salmon populations have been significantly impacted by dams, with less than 0.5% of the historic population remaining in rivers north of the Hudson River.

Dams can also alter the water quality and temperature, creating inhospitable conditions for fish and other aquatic organisms. The water behind a dam is often warmer and has lower dissolved oxygen levels compared to the water upstream and downstream of the dam. These changes in temperature and oxygen levels can stress and even kill fish, mussels, stream insects, and other aquatic life. Additionally, organic materials built up behind dams consume oxygen as they decompose, sometimes triggering algae blooms and creating "dead zones" devoid of oxygen and incapable of supporting any river life.

The impact of dams on fish migration can further affect the entire ecosystem. Fish are an important source of prey for other animals, and a reduction in fish populations can have ripple effects throughout the food chain. Additionally, dams can alter the natural transport of sediment in a river, impacting the river's ability to maintain healthy organic riparian channels.

To mitigate these issues, some dams have been retrofitted with upstream passage structures, such as fish ladders, to facilitate fish migration. In some cases, dams have been completely removed to restore natural river processes and reopen rivers to migratory fish. These efforts aim to balance the benefits of dams for human communities with the need to protect and preserve aquatic life and ecosystems.

Flooding of land, negatively impacting people, wildlife, and heritage sites

The creation of reservoirs and the flooding of land can have a detrimental impact on people, wildlife, and heritage sites. Firstly, the flooding of land can lead to the displacement of communities, affecting people's homes, livelihoods, and cultural sites. This was the case for the town of Feng Du in China, which was inundated due to the Three Gorges Project. Similarly, heritage sites, including archaeological sites, historical buildings, sacred places, and monuments, can be lost forever when valleys are flooded to create reservoirs.

Secondly, flooding can negatively impact wildlife and ecosystems. The increased volume of water can wash out fish and their fry, leaving them stranded or unable to spawn. It can also displace wetland mammals, such as otters, and small mammals, including hedgehogs, mice, and voles, which may be forced to relocate or face drowning. In addition, flooding can introduce contaminants such as agricultural pesticides, industrial chemicals, debris, and sewage into water bodies, further harming aquatic life.

Moreover, the altered river flow caused by dams can affect the natural balance of the river ecosystem, impacting plant and animal life. The dam's obstruction can lead to the build-up of sediment and organic materials, resulting in reduced water quality and the creation of oxygen-depleted "dead zones" that are incapable of supporting aquatic life. The change in water temperature caused by dams can also negatively impact species adapted to specific temperature ranges, such as coldwater fish like trout.

Finally, while some ecosystems rely on seasonal flooding, the intensity and frequency of floods are increasing due to global warming and climate change. Intense rainfall and changing landscapes contribute to more disastrous floods, which can have catastrophic consequences for both human populations and the natural world.

Increased sediment build-up, altering river flow and ecology

Dams are often constructed across rivers to store water that would naturally flow downstream. This upsets the natural balance of the river, affecting the animal and plant life in and around it. The river flow downstream changes, and the dam can hold back sediment that would normally flow downstream.

Sediment build-up behind dams has negative effects downstream. Dams trap sediment, causing a reduction in the river's storage capacity. This sediment build-up can have substantial impacts on the river ecology. Ecologically important pools can fill with sediment, gravel, and cobble riffles can be buried in finer sediment, and fine sediment can clog the riverbed, thereby eliminating surface-groundwater exchanges, smothering eggs, and clogging the void spaces between stones used as habitat by aquatic invertebrates and larval fish. The accumulation of sand and finer sediment on the riverbed can also impact the river's capacity to convey floodwaters.

The flow of the river and the sediment it carries create the form of the river and how it will migrate over hundreds to thousands of years. Rivers also provide ecological niches for a wide range of creatures, and the deposition of sediment and its sculpting create those habitats. The nutrients and organic materials it carries create biodiversity in many of the world's rivers.

A 2022 study found that 20th-century dam building had caused the rivers of North America, Europe/Eurasia, and Asia to deliver 49% less suspended sediment to the oceans. Dams and their impoundments have been shown to have the most negative impact on the water quality of high-gradient, cold-water streams in small watersheds. Elevated stream temperatures and low dissolved oxygen levels associated with small dams can result in increased stress and death for fishes, mussels, stream insects, and other aquatic organisms. As streams warm, native cold-water species can be replaced by non-native generalist species.

Greenhouse gas formation in reservoirs due to biomass decomposition

Dams are often constructed across rivers to store water that would naturally flow downstream. This causes the upstream river to flood and become a reservoir. The creation of a reservoir can have a negative impact on the environment, wildlife, and people.

Reservoirs are a known source of greenhouse gas emissions. The flooding of land can cause the decomposition of biomass, which releases greenhouse gases such as carbon dioxide (CO2) and methane (CH4). This process can occur over a long period, with one source stating that each square kilometre of Amazonian forest converted to a reservoir will emit over 140 Gg of CO2-eq over 100 years.

The decomposition of biomass in reservoirs is influenced by factors such as temperature, oxygen concentration, and moisture content. Higher temperatures and low oxygen levels can accelerate decomposition and increase greenhouse gas emissions. For example, methane concentrations were found to be significantly higher at 60°C than at 20°C or 40°C.

The impact of greenhouse gas emissions from reservoirs is particularly significant in lowland Amazonian reservoirs with low power density. These reservoirs could emit similar or even higher amounts of greenhouse gases compared to gas power plants. The planned construction of hydroelectric dams in the Amazon basin, which involves flooding tropical forests with high organic carbon stores, has raised concerns about the potential environmental impact.

Additionally, the creation of reservoirs can lead to the build-up of organic materials, which consume oxygen as they decompose. This can trigger algae blooms and create "dead zones" incapable of supporting aquatic life. The altered water temperatures and oxygen levels in reservoirs can also impact the survival of marine life and affect downstream environments.

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