Dams' Dark Side: Unveiling Thermal Pollution's Impact

Thermal pollution is the degradation of water quality by any process that changes the ambient water temperature. It is caused by human influence and affects the physical properties of water. The construction of dams can cause thermal pollution by changing the flow of downstream river channels, and through the release of cold water from the base of reservoirs into warmer rivers. This can have a significant impact on the water temperature of the river, which in turn affects the aquatic biota.

Water temperature changes

Dams are often constructed to release water from the bottom of the reservoir, which is colder than the surface water. This can significantly reduce the temperature of the receiving water body, which may be a river or another reservoir downstream. The impact of these changes can be mitigated by designing dams to release warmer surface water instead. The natural river channels downstream of a dam will experience changes in flow and may also undergo thermal stratification, where the water at the bottom is much colder.

The discharge water temperature from reservoirs may be significantly different from the water temperature before dam construction. The downstream water temperature will vary, and the impact of dam construction on the thermal conditions can be analysed through numerical simulation. The construction of multiple dams on the same river, such as the Xiangjiaba, Xiluodu, Baihetan, and Wudongde dams on the Yangtze River, can have a cumulative effect on water temperature.

The release of unnaturally cold water from a reservoir can dramatically change the fish and macroinvertebrate fauna of rivers and reduce river productivity. This can lead to the elimination of indigenous fish species and the alteration of macroinvertebrate fauna populations. Conversely, water warming effects can also be detrimental, as an increase in water temperature can cause an increase in the growth rate of aquatic organisms up to a certain point, after which damage occurs.

In addition to the direct effects of water temperature changes, thermal pollution can also have indirect effects. For example, when water used as a coolant by power plants is returned to the natural environment at a higher temperature, the sudden change in temperature decreases the oxygen supply and affects the composition of the aquatic ecosystem. This can be mitigated by converting facilities from once-through cooling to closed-loop systems, which release water at a temperature more comparable to the natural environment.

Impact on aquatic life

The construction of dams can have a significant impact on aquatic life by causing thermal pollution, which is the degradation of water quality due to changes in the ambient water temperature. This can be caused by the release of cold water from the base of reservoirs into warmer rivers or the increase in water temperature due to the use of water as a coolant by power plants and industrial manufacturers.

The release of unnaturally cold water from reservoirs can have a dramatic effect on the fish and macroinvertebrate fauna of rivers. It can lead to the elimination of indigenous fish species and the alteration of macroinvertebrate fauna populations. For example, in Australia, the release of cold water from dams for irrigation during summer and autumn has resulted in water temperatures that are more than 10°C cooler than the receiving water bodies. This can have deleterious effects on aquatic organisms, causing their physiology to be altered and their communities to be disrupted, and even leading to their death.

Dams can also cause thermal stratification in reservoirs, where the temperature at the bottom drops significantly. If a dam is designed to release this cold water from the bottom into natural systems, it can further contribute to the thermal pollution downstream. This effect can be mitigated by designing dams to release warmer surface waters instead of colder bottom water.

The change in water temperature due to thermal pollution can interfere with temperature cues for spawning in fishes and facilitate the establishment of exotic species. It can also alter the growth and development of aquatic organisms, as their metabolic rates are controlled by temperature. Freshwater organisms are particularly sensitive to temperature fluctuations as they evolved in relatively thermally buffered environments. While there are optimal temperature ranges for growth and development, increases in temperature beyond a certain threshold can cause damage.

Additionally, the presence of dams can alter the natural flow of downstream river channels, resulting in changes to the thermal regime. This includes variations in annual average water temperature, extreme water temperatures, and the occurrence time of the highest and lowest water temperatures. The construction of multiple dams can have a cumulative effect on water temperature, and the impact on downstream thermal conditions can be influenced by the number and capacity of the dams.

Thermal stratification

When water is stored in a reservoir, it can become thermally stratified, meaning that the water at the bottom is colder than the water at the top. This is because the surface water is heated by the sun, while the deeper water remains cold. In natural water bodies, such as lakes, this stratification is usually disrupted by winds and currents that mix the water and distribute heat more evenly. However, in reservoirs, especially those created by dams, the water may be relatively still, allowing stratification to persist.

The construction of dams can alter the flow of downstream river channels, leading to thermal stratification in the reservoirs. The discharge water temperature from the reservoirs can be significantly different from the water temperature before the dam construction, impacting the downstream water temperature. This change in water temperature, known as thermal pollution, can have various ecological effects.

To mitigate the negative effects of thermal stratification and cold-water releases, dam designers can incorporate strategies such as releasing warmer surface waters instead of colder bottom waters. This approach can help reduce the thermal shock experienced by aquatic organisms and maintain a more stable ecosystem. Additionally, implementing eflows (environmental flows) in dam management has been shown to improve downstream dissolved oxygen levels and enhance macroinvertebrate richness. However, eflows alone may not be sufficient in all contexts, and additional strategies, such as sediment flushing, may be required to address water quality issues.

Power plants and industrial manufacturers

Power plants are not the only contributors to thermal pollution, as industrial manufacturers also play a significant role. Some important industrial sources of thermal pollution include nuclear power and electric power plants, crude oil refineries, steel melting factories, coal fire power plants, and boilers. These industries release large amounts of heat into water bodies, leading to changes in the physical, chemical, and biological characteristics of the receiving waters. The water used for cooling in these industries can be controlled through various methods, such as cooling ponds, cooling towers, and cogeneration.

The impact of thermal pollution on freshwater communities can be significant. As the majority of freshwater organisms are ectothermic, their metabolic rates are controlled by temperature. Alterations to normal water temperature regimes can interfere with temperature cues for spawning fishes, facilitate the establishment of exotic species, and alter the growth and development of aquatic organisms. The release of unnaturally cold water from reservoirs can also have detrimental effects, changing the fish and macroinvertebrate fauna of rivers and reducing river productivity. This can lead to the elimination of indigenous fish species and the alteration of macroinvertebrate fauna populations.

To mitigate the impact of thermal pollution, some measures can be taken. For example, dams can be designed to release warmer surface waters instead of colder water from the bottom of the reservoir. Additionally, converting facilities from once-through cooling to closed-loop systems can significantly reduce the amount of thermal pollution emitted. These systems release water at a temperature more comparable to the natural environment, minimizing the impact on aquatic ecosystems.

Climate change

Dams also contribute to climate change through the emission of greenhouse gases. When vegetation is flooded to create a reservoir, organic matter begins to decompose underwater, leading to the production and release of significant amounts of methane, a potent greenhouse gas. Methane has a much higher global warming potential than carbon dioxide, and its release contributes to the greenhouse effect, resulting in global warming. The decomposition process can continue for decades, especially in tropical regions, leading to sustained methane emissions. In addition to methane, carbon dioxide is also released during the initial flooding and through the ongoing decomposition of aquatic plants and algae in the reservoir.

The altered water flow caused by dams can also impact regional and global climate patterns. By regulating water release, dams can influence downstream temperatures and humidity, affecting local weather patterns. Changes in water flow can impact the natural cooling or warming processes of a region, leading to temperature variations. Additionally, the altered flow can affect the ocean currents, which play a crucial role in distributing heat around the globe and regulating global climate patterns. Changes in ocean currents can have far-reaching consequences, influencing weather systems and climate in distant regions.

The impact of dams on water temperature can also have indirect effects on climate change. As mentioned earlier, dams create stratified water bodies, with deeper layers deprived of oxygen. This condition favors the production of phosphine, a potent greenhouse gas, through the anaerobic decomposition of organic matter. While phosphine has a relatively short atmospheric lifetime, its release contributes to the overall greenhouse effect. Furthermore, the altered thermal regime caused by dams can impact aquatic ecosystems, affecting the composition of plant and animal species. These ecological changes can have feedback effects on climate, as different species have varying impacts on carbon and nutrient cycles, influencing the exchange of greenhouse gases between the water body and the atmosphere.

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