Environmental impact of reservoirs
The environmental impact of reservoirs comes under ever-increasing scrutiny as the global demand for water and energy increases and the number and size of reservoirs increases.
Upstream impacts
Fragmentation of river ecosystems
A dam acts as a barrier between the upstream and downstream movement of migratory river animals, such as salmon and trout.[3]
Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge.[3]
Reservoir sedimentation
Rivers carry sediment down their riverbeds, allowing for the formation of depositional features such as
The trapping of sediment in reservoirs reduce sediment delivery downstream, which negatively impacts channel morphology, aquatic habitats and land elevation maintenance of deltas.[6] Apart from dam removal, there are other strategies to mitigate reservoir sedimentation.
Flushing flow method
The flushing flow method involves partially or completely emptying the reservoir behind a dam to erode the sediment stored on the bottom and transport it downstream.[7][6] Flushing flows aim to restore natural water and sediment fluxes in the river downstream of the dam, however the flushing flow method is less costly compared to removing dams or constructing bypass tunnels.
Flushing flows have been implemented in the
Sediment bypasses
Sediment bypass tunnels can partially restore sediment dynamics in rivers downstream of dams, and are primarily used in Japan and Switzerland.[11] Bypass tunnels divert part of the incoming water and sediments during floods into a tunnel around a reservoir and dam. The water and sediment thus never enter the reservoir but join the river again below the dam.[12] Bypass tunnels reduce riverbed erosion and increase morphological variability below the dam.[13]
Impact below dam
River line and coastal erosion
As all dams result in reduced sediment load downstream, a dammed river is greatly demanding for sediment as it will not have enough sediment. This is because the rate of deposition of sediment is greatly reduced since there is less to deposit but the rate of erosion remains nearly constant, the water flow erodes the river shores and riverbed, threatening shoreline ecosystems, deepening the riverbed, and narrowing the river over time. This leads to a compromised water table, reduced water levels, homogenization of the river flow and thus reduced ecosystem variability, reduced support for wildlife, and reduced amount of sediment reaching coastal plains and deltas.
In addition to coastal erosion impacts, reduced river flow may also alter ocean currents and ecosystems.[17]
Nutrients sequestration
Once a dam is put in place represents an obstacle to the flux of nutrients such as carbon (C), nitrogen (N), phosphorus (P), and silicon (Si) on downstream river, floodplains and delta. The increased
Since dammed rivers store nutrients during their lifespan, it can be expected that when a dam is removed, these legacy nutrients are remobilized causing downstream ecosystems' eutrophication and probable
Water temperature
The water of a deep reservoir in temperate climates typically stratifies with a large volume of cold, oxygen poor water in the hypolimnion. Analysis of temperature profiles from 11 large dams in the Murray Darling Basin (Australia) indicated differences between surface water and bottom water temperatures up to 16.7 degrees Celsius.[24] If this water is released to maintain river flow, it can cause adverse impacts on the downstream ecosystem including fish populations.[25] Under worse case conditions (such as when the reservoir is full or near full), the stored water is strongly stratified and large volumes of water are being released to the downstream river channel via bottom level outlets, depressed temperatures can be detected 250 - 350 kilometres downstream.[24] The operators of Burrendong Dam on the Macquarie River (eastern Australia) are attempting to address thermal suppression by hanging a geotextile curtain around the existing outlet tower to force the selective release of surface water.[26]
Natural ecosystems destroyed by agriculture
Many dams are built for irrigation and although there is an existing dry ecosystem downstream, it is deliberately destroyed in favor of irrigated farming. After the Aswan Dam was constructed in Egypt it protected Egypt from the droughts in 1972–73 and 1983–87 that devastated East and West Africa. The dam allowed Egypt to reclaim about 840,000 hectares in the Nile Delta and along the Nile Valley, increasing the country's irrigated area by a third. The increase was brought about both by irrigating what used to be desert and by bringing under cultivation 385,000 hectares that were natural flood retention basins. About half a million families were settled on these new lands. In 1983 the Franklin Dam project in Tasmania, Australia was cancelled following a campaign to protect surrounding forest from clearing and flooding.[27]
Effects on flood-dependent ecology and agriculture
In many[ 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 Lake Manatali reservoir formed by the depletion of grazing land, and is also drying the forests on the floodplain downstream of the dam.[29]
- After the construction of the Kainji Dam in Nigeria, 50 to 70 percent of the downstream area of flood-recession cropping stopped.[30]
Potential for disaster
Dams occasionally break causing catastrophic damage to communities downstream. Dams break due to engineering errors, attack or natural disaster. The greatest dam break disaster to date happened in China in 1975 killing 200,000 Chinese citizens. Other major failures during the 20th century were at Morbi, India (5,000 fatalities), at Vajont, Italy (2000 dead), while three other dam failures have each caused at least 1000 fatalities.
Flood control
The controversial
Mercury cycling and methylmercury production
The creation of reservoirs can alter the natural biogeochemical cycle of mercury. Studies conducted on the formation of an experimental reservoir by the flooding of a boreal wetland showed a 39-fold increase in the production of toxic methylmercury (MeHg) following the flooding.[31] The increase in MeHg production only lasted about 2–3 years before returning to near normal levels. However, MeHg concentration in lower food chain organisms remained high and showed no signs of returning to pre-flood levels. The fate of MeHg during this time period is important when considering its potential to bioaccumulate in predatory fish.[32]
Effects beyond the reservoir
Effects on humans
Diseases
Whilst reservoirs are helpful to humans, they can also be harmful as well. One negative effect is that the reservoirs can become breeding grounds for disease vectors. This holds true especially in tropical areas where
Resettlement
Dams and the creation of reservoirs also require relocation of potentially large human populations if they are constructed close to residential areas. The record for the largest population relocated belongs to the
Greenhouse gases
Reservoirs may contribute to changes in the Earth's climate. Warm climate reservoirs generate
Research conducted at the Experimental Lakes Area indicates that creating reservoirs through the flooding of boreal wetlands, which are sinks for CO2, converts the wetlands into sources of atmospheric carbon.[31] In these ecosystems, variation in organic carbon content has been found to have little effect on the rates of greenhouse gas emission. This means that other factors such as the lability of carbon compounds and temperature of the flooded soil are important to consider.[39]
The following table indicates reservoir emissions in milligrams per square meter per day for different bodies of water. [40]
Location | Carbon Dioxide | Methane |
---|---|---|
Lakes | 700 | 9 |
Temperate reservoirs | 1500 | 20 |
Tropical reservoirs | 3000 | 100 |
See also
- Akosombo Dam Impact
- Alta controversy
- Environmental impact of irrigation
- Environmental racism
- Fish barrier
- Fish ladder
- Renewable energy debate – Hydroelectricity
References
- ISBN 978-3-319-73249-7
- ^ a b A comparative survey of dam-induced resettlement in 50 cases by Thayer Scudder and John Gray
- ^ S2CID 129268573.
- ISBN 1-85649-902-2
- ^ a b Reservoir Sedimentation Handbook; Morris, Gregory & Fan, Jiahua; McGraw-Hill Publishers; 1998.
- ^ ISSN 2328-4277.
- ^ S2CID 97748305.
- ^ S2CID 129530817.
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- ^ Kondolf, G. M.; Annandale, G.; Rubin, Z. (2015). "Sediment starvation from dams in the lower Mekong river basin: Magnitude of effect and potential mitigation opportunities". 36th IAHR World Congress.
- ISBN 978-3-85125-118-0.
- ^ Boes, R. M.; Auel, C.; Müller-Hagmann, M.; Albayrak, I. (2014). "Sediment bypass tunnels to mitigate reservoir sedimentation and restore sediment continuity". Reservoir sedimentation. CRC Press, Taylor and Francis Group. pp. 221–228.
- PMID 31562373.
- ^ Sedimentation Engineering; American Society of Civil Engineers Committee; American Society of Civil Engineers Headquarters; 1975.
- .
- ^ Gies, Erica (3 May 2023). "The Oceans Are Missing Their Rivers". Nautilus Quarterly. Retrieved 5 May 2023.
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- S2CID 95558971.
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- PMID 9789038.
- S2CID 84575211.
- ^ .
- ISSN 0343-6993.
- ^ "Burrendong temperature control structure". StateWater.com.au. WaterNSW. Archived from the original on 2015-09-23. Retrieved 2015-09-22.
- ^ Stobbe Reimer, Asha Miriam (2021-09-30). "Tasmanian Wilderness Society blocks dam construction (Franklin River Campaign) 1981-83". The Commons Social Change Library. Retrieved 2023-07-07.
- ^ ILRI, 1982. Modern interferences in traditional water resources in Baluchistan. In: Annual Report 1982, pp. 23-34. ILRI, Wageningen, The Netherlands. Reprinted in Water International 9 (1984), pp. 106- 111. Elsevier Sequoia, Amsterdam. Also reprinted in Water Research Journal (1983) 139, pp. 53-60.
- ^ A. deGeorges and B.K. Reilly, 2006. Dams and large scale irrigation on the Senegal river: impacts on man and the environment. UNDP Human Development Report.
- ^ C.A.Drijver and M.Marchand, 1985. Taming the floods. Environmental aspects of the floodplain developments of Africa. Centre of Environmental Studies, University of Leiden, The Netherlands.
- ^ S2CID 129247176.
- PMID 15046335.
- ISBN 0-419-22360-6
- ^ a b Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams.
- ^ Graham-Rowe, Duncan (2005). "Hydroelectric Power's Dirty Secret Revealed", NewScientist.com.
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- ^ Grossman, Daniel (18 September 2019). "Deliberate drowning of Brazil's rainforest is worsening climate change". New Scientist. Retrieved 30 September 2020.
- S2CID 30088541.
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External links
- Rivers No More: The Environmental Effects of Large Dams at International Rivers (an excerpt for Rivers No More: The Environmental Effects of Large Dams)
- World Commission on Dams