question: Which consequences of a dam being built lead to environmental change/damage?
comment: One problem is that the dam floods the valley behind it, turning it into a lake. This kills virtually everything that was previously growing in the valley.
passages: Impact is assessed in several ways: the benefits to human society arising from the dam (agriculture, water, damage prevention and power), harm or benefit to nature and wildlife, impact on the geology of an area (whether the change to water flow and levels will increase or decrease stability), and the disruption to human lives (relocation, loss of archeological or cultural matters underwater).
  
  Section::::Dam creation.:Impact assessment.:Environmental impact.
   A decrease in air pollution occurs when a dam is used in place of thermal power generation, since electricity produced from hydroelectric generation does not give rise to any flue gas emissions from fossil fuel combustion (including sulfur dioxide, nitric oxide and carbon monoxide from coal).
  
  Section::::Environmental impact.:Biology.
  
  Dams can produce a block for migrating fish, trapping them in one area, producing food and a habitat for various water-birds. They can also flood various ecosystems on land and may cause extinctions.
  
  Section::::Environmental impact.:Human impact.
   Section::::Impact below dam.
  
  Section::::Impact below dam.:Riverline and coastal erosion.
   All reservoirs will have a monetary cost/benefit assessment made before construction to see if the project is worth proceeding with. However, such analysis can often omit the environmental impacts of dams and the reservoirs that they contain. Some impacts, such as the greenhouse gas production associated with concrete manufacture, are relatively easy to estimate. Other impact on the natural environment and social and cultural effects can be more difficult to assess and to weigh in the balance but identification and quantification of these issues are now commonly required in major construction projects in the developed world
  
  Section::::Environmental impact.:Climate change.
   Section::::Impacts.:Social.
   Dams and the reservoirs can be used to supply drinking water, generate hydroelectric power, increasing the water supply for irrigation, provide recreational opportunities and flood control. However, adverse environmental and sociological impacts have also been identified during and after many reservoir constructions. Although the impact varies greatly between different dams and reservoirs, common criticisms include preventing sea-run fish from reaching their historical mating grounds, less access to water downstream, and a smaller catch for fishing communities in the area. Advances in technology have provided solutions to many negative impacts of dams but these advances are often not viewed as worth investing in if not required by law or under the threat of fines. Whether reservoir projects are ultimately beneficial or detrimental—to both the environment and surrounding human populations— has been debated since the 1960s and probably long before that. In 1960 the construction of Llyn Celyn and the flooding of Capel Celyn provoked political uproar which continues to this day. More recently, the construction of Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate.
   Dams occasionally break causing catastrophic damage to communities downstream. Dams break due to engineering errors, attack or natural disaster. The greatest dam break disaster happened in China killing 200,000 Chinese citizens. They have also happened in California killing 600 people, Germany during World War II and other countries.
  
  Section::::Impact below dam.:Flood control.
   Significant other engineering and engineering geology considerations when building a dam include:
  
  BULLET::::- Permeability of the surrounding rock or soil
  
  BULLET::::- Earthquake faults
  
  BULLET::::- Landslides and slope stability
  
  BULLET::::- Water table
  
  BULLET::::- Peak flood flows
  
  BULLET::::- Reservoir silting
  
  BULLET::::- Environmental impacts on river fisheries, forests and wildlife (see also fish ladder)
  
  BULLET::::- Impacts on human habitations
  
  BULLET::::- Compensation for land being flooded as well as population resettlement
  
  BULLET::::- Removal of toxic materials and buildings from the proposed reservoir area
  
  Section::::Dam creation.:Impact assessment.
   Dams are also found to have a role in the increase/decrease of global warming. The changing water levels in reservoirs are a source for greenhouse gases like methane. While dams and the water behind them cover only a small portion of earth's surface, they harbour biological activity that can produce large amounts of greenhouse gases.
  
  Section::::Dam creation.:Impact assessment.:Human social impact.
   In the case of the Three Gorges Dam in China the changes described above now appears to have arrived at a new balance of erosion and sedimentation over a 10-year period in the lower reaches of the river. The impacts on the tidal region have also been linked to the upstream effects of the dam.
  
  Section::::Impact below dam.:Water temperature.
   Lower positive impacts are found in the tropical regions, as it has been noted that the reservoirs of power plants in tropical regions produce substantial amounts of methane. This is due to plant material in flooded areas decaying in an anaerobic environment and forming methane, a greenhouse gas. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant.
   Section::::Impact below dam.:Natural ecosystems destroyed by agriculture.
   BULLET::::- The Lake Manatali reservoir formed by the Manantali dam in Mali, West Africa intersects the migration routes of nomadic pastoralists and withholds water from the downstream savanna. The absence of the seasonal flood cycle causes depletion of grazing land, and is also drying the forests on the floodplain downstream of the dam.
  
  BULLET::::- After the construction of the Kainji Dam in Nigeria, 50 to 70 percent of the downstream area of flood-recession cropping stopped.
  
  Section::::Impact below dam.:Potential for disaster.
   Section::::Biological aspects.:Ecological effects.
   Dams and reservoirs can be used to supply drinking water, generate hydroelectric power, increase the water supply for irrigation, provide recreational opportunities, and flood control. In 1960 the construction of Llyn Celyn and the flooding of Capel Celyn provoked political uproar which continues to this day. More recently, the construction of Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate.
  
  Section::::Upstream impacts.
  
  Section::::Upstream impacts.:Fragmentation of river ecosystems.
   Section::::Impact below dam.:Effects on flood-dependent ecology and agriculture.
  
  In many low lying developing countries the savanna and forest ecology adjacent to floodplains and river deltas are irrigated by wet season annual floods. Farmers annually plant flood recession crops, where the land is cultivated after floods recede to take advantage of the moist soil. Dams generally discourage this cultivation and prevent annual flooding, creating a dryer downstream ecology while providing a constant water supply for irrigation.
  
  "Case studies"
   The environmental impact of reservoirs is coming under ever increasing scrutiny as the world demand for water and energy increases and the number and size of reservoirs increases. Dams and the reservoirs can be used to supply drinking water, generate hydroelectric power, increasing the water supply for irrigation, provide recreational opportunities and for flood control. However, adverse environmental and sociological impacts have also been identified during and after many reservoir constructions. Whether reservoir projects are ultimately beneficial or detrimental—to both the environment and surrounding human populations— has been debated since the 1960s and probably long before that. In 1960 the construction of Llyn Celyn and the flooding of Capel Celyn provoked political uproar which continues to this day. More recently, the construction of Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate.
   Dams can severely reduce the amount of water reaching countries downstream of them, causing water stress between the countries, e.g. the Sudan and Egypt, which damages farming businesses in the downstream countries, and reduces drinking water.
  
  Farms and villages, e.g. Ashopton can be flooded by the creation of reservoirs, ruining many livelihoods. For this very reason, worldwide 80 million people (figure is as of 2009, from the Edexcel GCSE Geography textbook) have had to be forcibly relocated due to dam construction.
  
  Section::::Environmental impact.:Limnology.
   Section::::Environmental and social impact.:Environmental impact.
   However, in addition to the advantages above, there are several disadvantages to dams that create large reservoirs. These may include: dislocation of people living where the reservoirs are planned, release of significant amounts of carbon dioxide at construction and flooding of the reservoir, disruption of aquatic ecosystems and bird life, adverse impacts on the river environment, potential risks of sabotage and terrorism, and in rare cases catastrophic failure of the dam wall.
   Section::::Properties.:Disadvantages.:Siltation and flow shortage.
  
  When water flows it has the ability to transport particles heavier than itself downstream. This has a negative effect on dams and subsequently their power stations, particularly those on rivers or within catchment areas with high siltation. Siltation can fill a reservoir and reduce its capacity to control floods along with causing additional horizontal pressure on the upstream portion of the dam. Eventually, some reservoirs can become full of sediment and useless or over-top during a flood and fail.
   Section::::Environmental and social impact.
   Section::::Environmental impacts.
   The impact on human society is also significant. Nick Cullather argues in "Hungry World: America's Cold War Battle Against Poverty in Asia" that dam construction requires the state to displace individual people in the name of the common good, and that it often leads to abuses of the masses by planners. He cites Morarji Desai, Interior Minister of India, in 1960 speaking to villagers upset about the Pong Dam, who threatened to "release the waters" and drown the villagers if they did not cooperate.
   Hydroelectricity's main cause of water usage is both evaporation and seepage into the water table.
  
  Reference: Nuclear Energy Institute factsheet using EPRI data and other sources.
  