Dams provide us with non-polluting hydroelectric power for our energy needs, but this comes at the expense of nature.
Many dams require the relocation of homes and farms, which can be devastating for residents living nearby.
Dams also disturb natural rainfall patterns and have an adverse impact on ecosystems in their vicinity. Evaporation from reservoirs alters water temperature and oxygen content levels affecting aquatic life in ways which alter aquatic habitat.
How Dams Affect the Environment
Dams are massive structures that dramatically change river environments, altering natural temperature fluctuations and ecosystems nearby, leading to flooding, chemical changes in water chemistry, trapping dissolved gases, and trapping natural resources like energy and minerals underground. Their impacts may not always be apparent or easily assessable; nonetheless they have an immense effect on nature.
Since the 1960s and 70s, dam construction has slowed worldwide; however, dams continue to fragment rivers and prevent their free-flowing status from returning. Only 37% of all rivers longer than 1,000 kilometers (620 miles) currently contain naturally flowing sections at their mouths today, of which only 23% flow uninterrupted to the sea.
Upstream of a dam, its impoundment alters air moisture concentration levels and can alter rain patterns to favor heavy precipitation, often more forcefully than anticipated by local communities and ecosystems. Furthermore, an increase in local air temperature may trigger drought conditions which pose serious threats to plants and wildlife alike.
At the point of reservoir drawdown (when water levels decline), exposed rock, wood, and other natural materials deteriorate, producing methane gas emissions up to 30 times stronger than carbon dioxide and contributing to climate change. Unfortunately, many governments and energy companies continue to consider dams an efficient source of power generation while neglecting to include reservoir-caused emissions into national greenhouse gas totals.
Downstream habitats also face reduced water quality as dam reservoirs build up organic materials that would normally wash downstream, creating algal blooms, eutrophication and oxygen-starved "dead zones" that marine life cannot survive in. Furthermore, temperature variations in dam reservoirs make surviving difficult for species that evolved to deal with natural temperature fluctuation patterns.
Dams can also pose a barrier for migrating fish species that require access to both upstream and downstream environments for reproduction and spawning, leading their populations to decrease dramatically and having ripple effects throughout their food chains, including birds and mammals.
Pros
Dams can offer an economical and sustainable source of power in areas where conventional methods are limited or unsustainable, providing energy in an environmentally-friendly way. Hydroelectricity is a renewable form of electricity production which does not produce greenhouse gases like fossil fuels (which come at great environmental costs). Dams also serve an invaluable function as water storage facilities as well as being useful tools against flooding.
But it is equally important to evaluate both sides of dams; their negative and positive impacts cannot be ignored separately. Dams can alter river flow patterns and have serious ramifications for downstream habitats; furthermore, dams create reservoirs which could host non-native and invasive species which threaten ecosystem stability.
Dams can alter river systems' nutrient levels, impacting aquatic life in numerous ways. Dams also change fish migratory routes that depend on moving for survival - and this change may have lasting consequences across an extended area.
As part of their construction, dams often uproot residents of the areas where they are built as well as communities with traditional ties to the land, leading them to seek compensation that doesn't match up to what was promised - often without success - which often escalates tension and even violent conflict in certain regions of the world; over 80 million people worldwide have been uprooted from their homes as a result of dams alone! This phenomenon has become widespread and devastating worldwide.
Dams can also reduce soil fertility, destroy bridges and infrastructure, and erode downstream banks and riverside woodlands, as well as having significant downstream effects that are usually limited to the downstream region from their location. Although such effects tend to be temporary in nature, their significance should not be overlooked.
Location is often the single greatest influencer of a dam's social and environmental impacts, making site selection critical. Careful consideration can help minimize or avoid many negative repercussions; such as those that would affect migration patterns of fish or sediment transport. Implementation of mitigation measures during design phase of a dam also can lessen its effects on both environment and communities.
Cons
Flood control, electricity generation and water supply are among the many benefits provided by dams; however, they come at the cost of ecological disruption, displacement and loss of biodiversity. Furthermore, reservoirs formed behind dams may alter river flows and soil structures significantly.
As river's waters are diverted away from their normal flow, sediment that would typically settle downstream builds up in the dam's reservoir instead. This can alter the shape and size of riverbanks, deltas, alluvial fans, braided rivers, oxbow lakes and levees while decreasing soil fertility and fertility overall. Nutrient-rich sediments normally carried by rivers tend to become trapped within an impoundment thus leading to algal blooms as well as lower oxygen levels resulting from trapped nutrients within its confines resulting from trapped nutrients being trapped inside impoundments as well.
At reservoirs, turbid waters from algae blooms can lead to high concentrations of bacteria and reduced oxygen content that threaten aquatic life in nearby reservoirs. Evaporation rate fluctuations caused by reservoirs also disrupt local weather patterns by disrupting natural precipitation rates and ecosystems; additionally, reservoirs often host non-native and invasive species that cause more environmental harm than good.
Dams can alter the migration patterns of fishes and aquatic animals, inhibiting their movements between breeding locations. Dams may block migration routes, leading them away from reaching mating locations altogether and ultimately leading to reduced population sizes; as evidenced in Asia with regards to Gilded Catfish and Sturgeon populations decreasing substantially due to dams built along the Mekong River.
Construction of dams also necessitates significant deforestation, leading to the loss of important habitats for animals and plants that has long-term negative consequences for biodiversity.
Dams can have an adverse impact on groundwater tables, which makes it more difficult for plant roots to find their way down into the soil and access nutrients they require for survival. A lower groundwater table also affects water quality by increasing salts and minerals found in it which could have negative implications for human health.
Conclusions
Dams serve many useful functions: providing clean carbon-free energy, irrigating agricultural land and communities, fighting drought conditions, controlling flood flows, aiding river navigation and supporting fisheries - to name but a few. Unfortunately, they also come at a significant environmental cost.
Dams alter the natural flow of rivers by changing temperatures, chemical composition, river sediment loads and physical characteristics. Reservoirs created as part of their operating regime may destroy important habitat areas or cover historic or archeological sites while forcing people to relocate.
Reservoirs capture sediment that would otherwise wash downstream, starving downstream reaches of essential material for healthy organic riparian channels and increasing velocity of the water flowing downstream, contributing to erosion. They may also block migration paths of migrating fish that need breeding grounds - cutting them off from breeding grounds and leading to population decrease. Wild salmon numbers in the Pacific Northwest have declined 76% since first dams went up fifty years ago; on Asia's Mekong River there are only 100 Irrawaddy dolphins left alive today.
Similar to dam reservoirs, evaporation from dam reservoirs disrupts surrounding environments by altering rainfall patterns and thus straining ecosystems and municipalities that depend on them. Dams also interfere with natural temperature cycling cycles of waters, disrupting marine life adapted to cope with fluctuations. Finally, dam releases oxygen-deprived, hot water into rivers often create dead zones incapable of supporting life below.
Dams used for hydropower generation can also create problems of their own in terms of reservoir storage: accumulations of cyanide and other forms of pollution as well as silting that alters river morphology and ecology, erosion issues caused by sediment blockages downstream, altering aquatic environments by undercutting riverbanks or washing away beaches are just a few consequences of dams that block sediment movement downstream.
Dams have long been seen as examples of projects with good intentions having unexpected repercussions, prompting many governments and financial institutions to withdraw support for dam construction in recent years and instead concentrate on supporting alternative energy sources or green technologies such as alternative solar cells or photovoltaic panels. Even the World Bank stopped funding large dams during most of the 1990s before returning with restricted financing conditions imposed to meet environmental guidelines.
