Editing Dam (section)

==== Environmental impact ====
{{main|Environmental impacts of reservoirs}}
[[File:Dam-pollution.JPG|thumb|Wood and garbage accumulation due to a dam]]

Reservoirs held behind dams affect many ecological aspects of a river. Rivers topography and dynamics depend on a wide range of flows, whilst rivers below dams often experience long periods of very stable flow conditions or sawtooth flow patterns caused by releases followed by no releases. Water releases from a reservoir including that exiting a turbine usually contain very little suspended sediment, and this, in turn, can lead to scouring of river beds and loss of riverbanks; for example, the daily cyclic flow variation caused by the [[Glen Canyon Dam]] was a contributor to [[sand bar]] [[erosion]].

Older dams often lack a [[fish ladder]], which keeps many fish from moving upstream to their natural breeding grounds, causing failure of breeding cycles or blocking of migration paths.<ref>Silva, S., Vieira-Lanero, R., Barca, S., & Cobo, F. (2017). Densities and biomass of larval sea lamprey populations (Petromyzon marinus Linnaeus, 1758) in north-western Spain and data comparisons with other European regions. ''Marine and Freshwater Research'', 68(1), 116–122.</ref> Even fish ladders do not prevent a reduction in fish reaching the [[spawn (biology)|spawning]] grounds upstream.<ref>Tummers, J. S., Winter, E., Silva, S., O'Brien, P., Jang, M. H., & Lucas, M. C. (2016). Evaluating the effectiveness of a Larinier super active baffle fish pass for European river lamprey Lampetra fluviatilis before and after modification with wall-mounted studded tiles. ''Ecological Engineering'', 91, 183–194.</ref> In some areas, young fish ("smolt") are transported downstream by [[barge]] during parts of the year. Turbine and power-plant designs that have a lower impact upon aquatic life are an active area of research.

At the same time, however, some particular dams may contribute to the establishment of better conditions for some kinds of fish and other aquatic organisms. Studies have demonstrated the key role played by tributaries in the downstream direction from the main river impoundment, which influenced local environmental conditions and beta diversity patterns of each biological group.<ref name="Lansac-Tôha, Fernando Miranda 2019">Lansac-Tôha, Fernando Miranda (2019).</ref> Both replacement and richness differences contributed to high values of total beta diversity for fish (average = 0.77) and phytoplankton (average = 0.79), but their relative importance was more associated with the replacement component for both biological groups (average = 0.45 and 0.52, respectively).<ref name="Lansac-Tôha, Fernando Miranda 2019"/> A study conducted by de Almeida, R. A., Steiner, M.T.A and others found that, while some species declined in population by more than 30% after the building of the dam, others increased their population by 28%.<ref name="Almeida, Ricardo 2018">Almeida, Ricardo (2018).</ref> Such changes may be explained by the fact that the fish obtained "different feeding habits, with almost all species being found in more than one group.<ref name="Almeida, Ricardo 2018"/>

A large dam can cause the loss of entire [[ecology|ecospheres]], including [[endangered species|endangered]] and undiscovered species in the area, and the replacement of the original environment by a new inland lake.  As a result, the construction of dams have been opposed in various countries with some, such as Tasmania's Franklin Dam project, being cancelled following environmentalist campaigns.<ref>{{Cite web |last=Stobbe Reimer |first=Asha Miriam |date=2021-09-30 |title=Tasmanian Wilderness Society blocks dam construction (Franklin River Campaign) 1981-83 |url=https://commonslibrary.org/tasmanian-wilderness-society-blocks-dam-construction-franklin-river-campaign-1981-83/ |access-date=2023-07-07 |website=The Commons Social Change Library |language=en-AU}}</ref>

Large reservoirs formed behind dams have been indicated in the contribution of [[earthquake|seismic activity]], due to changes in water load and/or the height of the water table. However, this is a mistaken assumption, because the relatively marginal stress attributed to the water load is orders of magnitude lesser than the force of an earthquake. The increased stress from the water load is insufficient to fracture the Earth's crust, and thus does not increase the severity of an earthquake.<ref>{{cite book |last1=Jain |first1=Sharad K. |last2=Singh |first2=V. P. |title=Water Resources Systems Planning and Management |date=12 September 2003 |publisher=Elsevier |isbn=978-0-08-054369-7 |page=408 |url=https://books.google.com/books?id=dR1sFWlUpPUC&pg=PA408 |language=en}} "However, a reservoir, at worst, can only advance an earthquake which would have occurred otherwise too. The magnitude of forces associated with an earthquake is several orders bigger compared to the additional load of water in the reservoir. The change in stresses due to water load is too small to cause fracture in the Earth's crust (Srivastava, 1993). Therefore, the presence of a reservoir does not increase the severity of an earthquake."</ref>

Dams are also found to influence [[global warming]].<ref>{{cite news|ssrn=1108425|title=The Potential of Water Power in the Fight Against Global Warming|first=Lea-Rachel|last=Kosnik|date=1 March 2008}}</ref> The changing water levels in reservoirs are a [[Greenhouse gas#Sources|source for greenhouse gases]] like [[methane]].<ref>{{cite web |url=http://frenchtribune.com/teneur/1212763-water-reservoirs-behind-rising-greenhouse-gases |title=Water Reservoirs behind Rising Greenhouse Gases |work=[[French Tribune]] |date=9 August 2012 |access-date=9 August 2012}}</ref> While dams and the water behind them cover only a small portion of earth's surface, they harbour biological activity that can produce large quantities of greenhouse gases.<ref>{{cite web|url=http://timesofindia.indiatimes.com/home/environment/global-warming/Dams-the-latest-culprit-in-global-warming/articleshow/15403985.cms |title=Dams the latest culprit in global warming |work=[[The Times of India]] |date=8 August 2012 |access-date=9 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120809003651/http://timesofindia.indiatimes.com/home/environment/global-warming/Dams-the-latest-culprit-in-global-warming/articleshow/15403985.cms |archive-date=9 August 2012 }}</ref>