River Damming: Water Pollution And Its Environmental Impact

Damming is the process of blocking a river to create a reservoir, and it has been used for economic development and to meet societal needs. While it has benefits, damming can also cause water pollution and negatively impact the environment. This is because the damming of rivers can alter the natural flow of water, disrupt the migratory routes of fish, and lead to the accumulation of sediments and nutrients in the reservoirs, which can affect the river's ecosystem and water quality. The release of water from dams can also be unpredictable, potentially stranding fish and causing sudden changes in river levels. With an estimated 16 million dams worldwide, the environmental impact of damming is a significant issue that requires attention and sustainable solutions.

Characteristics of Damming of Rivers and Water Pollution

Dams can cause soil erosion, species extinction, and the spread of diseases

Damming of rivers, a common practice for water control and hydropower generation, can have unintended consequences on the environment and human health. One of the adverse effects of damming is its contribution to water pollution and the degradation of aquatic ecosystems.

Dams can indeed cause soil erosion, species extinction, and the spread of diseases, which poses significant ecological and public health risks. Firstly, soil erosion can occur due to the accumulation of sediments in reservoirs. Over time, the build-up of sediments can increase the water's turbidity, reducing light penetration and disrupting the aquatic ecosystem. This process can also release nutrients, such as phosphorus, from the sediments, leading to excessive algal growth and further degrading water quality.

The construction of dams can also lead to species extinction and a loss of biodiversity. Free-flowing rivers are essential for various freshwater species, providing them with food and water and enabling them to complete their life cycles. By dividing rivers into upstream and downstream habitats, dams can disrupt migration patterns and reduce the diversity and abundance of fish. This can have far-reaching impacts on ecosystems and human communities that depend on these fish as a source of food and income.

Furthermore, dams have been linked to the spread of diseases, particularly in the Lower Mekong River Basin. The creation of reservoirs and changes in water flow can provide new habitats for disease vectors, such as infected snails and fish intermediate hosts. The confined environments and altered water release patterns can increase the probability of native fish hosts coming into contact with pathogens. For example, there are concerns about the spread of Opisthorchis viverrini, a parasite that can cause opisthorciasis, in the Lower Mekong Basin due to the development of small aquaculture ponds facilitated by dam construction.

The potential threat to public health via disease ecology and food safety is a significant concern, especially in regions with a diverse range of water- and food-associated pathogens. The reduction in wild-captured fish due to dam construction can also lead to an increase in aquaculture, further elevating the risk of disease transmission. Additionally, the disruption of natural river flows can impact the purification of water, affecting both human and animal health.

To mitigate these issues, it is crucial to carefully consider the potential environmental and societal impacts of dam construction. Strategies such as sustainable dam projects, proper location selection, and implementing measures to minimize ecological disruption can help reduce the negative consequences on soil erosion, species extinction, and the spread of diseases.

They can alter the natural flow of rivers, negatively impacting fish migration and reproduction

River damming has played a crucial role in human civilization, offering benefits such as flood management, energy production, and water supply. However, it has also significantly altered the natural flow of rivers, negatively impacting fish migration and reproduction.

Firstly, dam construction changes the sediment regime and river morphology, disrupting fish habitats. The altered flow velocity can hinder fish spawning, leading to declines in reproduction. For fish that fertilize in vitro, the reduced flow velocity can decrease the success rate of fertilization and cause issues with egg drifting or reaching hatching destinations.

Secondly, reservoir operations can distort hydrological and water temperature regimes, affecting fish reproduction, foraging, and wintering. These changes can impact the entire life cycle of fish, including migration timing, reproductive performance, embryo health, and growth rate. Stagnant reservoir pools can disorient migrating fish and prolong their migration duration.

Additionally, damming can obstruct or delay upstream fish migration, contributing to the decline or extinction of species that rely on longitudinal movements during their life cycles. The construction of dams can cut off fish species from their spawning grounds, negatively impacting their survival.

To mitigate these negative effects, various conservation measures have been implemented. These include installing fish passage facilities, conducting reservoir ecological operations, removing small dams and barriers from tributaries, and artificial breeding and release programs for endangered and commercial fish species.

Dams can increase water temperature, affecting the health of fish species

Dams are artificial barriers that slow water flow and decrease canopy cover. This can lead to an increase in water temperature, which can have significant impacts on the health and ecology of fish species.

Most dams tend to warm downstream temperatures, with the magnitude of warming depending on various factors such as the dam's construction, climate, and the characteristics of the landscape and waterbody where they are built. For example, small, surface-release dams that slow water flow and reduce canopy cover are likely to increase water temperatures. The Xiangjiaba, Xiluodu, Baihetan, and Wudongde dams in China, for instance, have been shown to increase water temperatures by several degrees Celsius.

The warming effect of dams is more pronounced during lower flow periods and in cool headwaters with wide impoundments. The warmest temperatures are typically observed closest to the dam, with a gradual decrease in temperature further downstream. This warming can reduce the habitat available for coldwater fish and invertebrates, as their preferred environments shift from coldwater/coolwater to warmwater.

Fish are ectothermic, meaning their body temperature is influenced by their external environment. As a result, changes in water temperature can have profound effects on their physiology, metabolism, and behavior. Warmer temperatures, for example, increase the metabolic rate and oxygen consumption of fish, while also decreasing the oxygen content in the water. This can lead to increased competition for limited oxygen resources, impacting the health and growth of fish populations.

Additionally, temperature changes can affect the muscle mechanics and locomotion of fish. Higher temperatures cause muscles to contract and relax faster, influencing swimming performance. Fish may exhibit frantic activity in response to high temperatures, which can assist them in fleeing potentially harmful situations. Conversely, low temperatures can induce lethargy and impact their ability to escape dangerous conditions.

They can trap sediments, burying rock riverbeds and fish habitats

Dams are constructed for various purposes, including hydropower, irrigation, flood control, and water storage. While they can be beneficial, dams also cause significant harm to rivers and aquatic ecosystems. One of the adverse effects of dam construction is its impact on fish habitats and river morphology.

Dams can trap sediments, which have negative consequences for the river and its inhabitants. The sediment settles in the reservoirs behind the dams, and this buildup can bury rock riverbeds, disrupting the natural processes that shape the river over time. The trapped sediment can include coarse material, which forms the "architecture" of riverbeds, as well as fine-grained sediment important for floodplains and mudflats. This sediment buildup can also lead to a 26% loss of water storage capacity in dams worldwide by the middle of this century.

The sediment buildup behind dams can have far-reaching effects, including the disruption of fish habitats and spawning grounds. Fish species like salmon are particularly affected, as they are adapted to living in rivers with specific flow patterns and rely on steady flows for migration and spawning. The trapped sediment can bury the rocky riverbeds where fish like salmon spawn, and the altered river morphology can negatively impact the creation and maintenance of complex habitats downstream, such as riffles and pools.

Additionally, the sediment trapped behind dams can contain toxic substances such as pesticides and heavy metals. If these sediments are disturbed, they can release these toxic substances into the water, causing serious environmental problems. Furthermore, the reduced flow rates caused by dams can inhibit the formation of important fish habitats, and the sudden release of water from dams can strand fish, impacting their survival.

The removal of dams has been suggested as a solution to mitigate these issues. Proper dam removal can lead to the ecological restoration of rivers, as seen in the case of the Elwha River in Olympic National Park, where the removal of the Elwha Dam resulted in the recovery of fish habitats and the extension of the shoreline, providing protection from sea level rise.

Dam removal can expose and transport large amounts of accumulated pollutants downstream

Damming a river involves blocking it with a large barrier, which can serve several purposes, including hydropower generation, irrigation, flood control, and water storage. While these structures have their benefits, they can also cause significant harm to rivers and aquatic life.

One of the adverse effects of damming is the accumulation of pollutants in the reservoirs. These pollutants can include nutrients such as phosphorus (P) and nitrogen (N), as well as toxic substances. When a dam is removed, these accumulated pollutants are exposed and can be transported downstream in large quantities. This process can cause an increase in N, P, and toxic pollution in the water bodies located downstream.

For example, the demolition of the Fort Edward Dam in 1973 resulted in the release of sediments containing polychlorinated biphenyls (PCBs) into the water bodies downstream in New York. Similarly, the removal of two large reservoirs on the Elwha River in Washington state in 2011 mobilized 7.1 million cubic meters of sediment that had built up, leading to substantial sediment accumulation and changes in the river's course.

The surge in dam construction has led to a significant increase in P retention in rivers globally. As reservoirs age and operate over time, P accumulates in the sediments, and during flood conditions, these sediments can be easily eroded, releasing P into the water and causing environmental issues. Additionally, dam removal can lead to a decrease in groundwater levels due to reservoir runoff, resulting in permanent changes to downstream streams, such as stream bed degradation and erosion by nutrient-rich sediments.

The removal of dams offers the opportunity for the ecological restoration of rivers. However, the potential impact on downstream areas due to the release of accumulated pollutants needs to be carefully considered and managed to prevent further environmental damage.

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