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Stephen Midzi, the biodiversity conservation manager of South Africa&#;s iconic Kruger National Park, wants you to know that he&#;s not an &#;anti-dam person.&#;
&#;Don&#;t get me wrong,&#; he says, adding &#;but, I also think it is a good thing to allow rivers to be what rivers should be.&#; Midzi is an advocate for free-flowing streams that offer unfettered connectivity for aquatic and terrestrial biodiversity.
But from as early as , Kruger, one of the largest protected areas in Africa, has not been managed that way. Over the decades, park supervisors tinkered with the preserve&#;s streams and aquifers, trying to improve on what nature had always done and cater to the park&#;s animals with a copious, assured water supply.
Over time, 97 concrete dams, weirs and earthen dams were constructed and borehole-fed catchments drilled. But this closely spaced and evenly spread water supply didn&#;t bring about an African Eden. Instead, it caused severe overgrazing, veld degradation and erosion.
Catchment basins and stagnant waters behind dams silted up and accumulated hippo dung, nurturing cyanobacteria and poisoning the animals that drank the water. Species that prefer habitat close to water, such as zebra and wildebeest, flourished. Those that prefer drier areas, away from water where there are fewer predators, less competition and trampling &#; animals like sable and roan antelope &#; floundered.
Over the past two decades, people and nature worked to undo the damming; 42 dams inside Kruger were breached and demolished mechanically or by floods.
Today, &#;Restoring river connectivity is a critical focus,&#; says Eddie Riddell, the park&#;s aquatic biodiversity manager. This turnaround in policy is in line with increasing global criticism of one of humankind&#;s oldest tools for securing water supplies.
Over the centuries, dams &#; and massive, well-funded, government-supported dam-building initiatives &#; have been used to manage floods and provide water for drinking, crop irrigation, industry and power generation. But, just as was learned in Kruger, damming benefits come at a price.
As the number and size of dams being built across the world exploded in the 20th century, and into the 21st, the ecological, social and economic costs rippled far beyond local dam sites, having regional and global ramifications.
Humanity&#;s large-scale effort to dam the world&#;s rivers has been described as the largest single anthropogenic alteration of the freshwater cycle, with dams now helping edge us closer to transgressing a number of critical planetary boundaries, with adverse impacts on biodiversity, the climate, land use, and freshwater. Dams, combined with all the many other human pressures on nature, are helping upset the balance of Earth&#;s critical operating systems and could endanger civilization, humanity, and even life on Earth as we know it.
The construction of large dams (defined as those storing more than 100 million cubic meters, or more than 3,500 million cubic feet, of water) peaked in the s, with the cumulative volume of water impounded peaking the decade after. Half the world&#;s large dams were built for agriculture and offer water for 30-40% of the 2.71 million square kilometers (1.05 million square miles) of irrigated crop and grazing land worldwide.
There are thought to be close to 60,000 large dams, storing about a sixth of the globe&#;s total annual river flow to the oceans. Over and above that, there are at least 16 million small dams and impoundments with reservoir surface areas larger than 100&#;square meters (1,076 square feet), totaling around 306,000 km2 (nearly 120,000 mi2), which increases Earth&#;s terrestrial freshwater surface by more than 7%.
South Africa is one country that exemplifies the benefits of dam construction. With low annual rainfall (just 55% of the global average) and high temperatures and evaporation rates, a mere 9% of precipitation that falls here ever reaches rivers &#; the country&#;s only large-scale freshwater source. Not surprisingly, the nation decided to rely on dams to store water and unlock South Africa&#;s development potential.
Today, an estimated 500 large dams store millions of liters of water and allow for activities otherwise near-impossible in the semi-arid climate. The major dams are able to store about 50% of the mean annual runoff, ensuring supply throughout the year. Major cities like Durban, Johannesburg and Cape Town all rely on dams for their water. Climate change recently sounded a warning, however, when drought took Cape Town to the brink of Day Zero in , forcing a much-needed reevaluation of the region&#;s water system toward becoming a water conservation role model for the world.
The construction of large dams and reservoirs has slowed globally since the s and &#;70s, but much larger rivers are now being dammed. Only 37% of rivers longer than 1,000 km (620 miles) remain free-flowing over their entire length today, and a mere 23% flow uninterrupted to the sea. Counting all hydropower dams planned or being built, natural hydrological flows will be altered for 93% of river volume worldwide by .
And with those dams will come massive aquatic biodiversity loss. &#;A major impact of the fragmentation of rivers [by dams] is the decline of freshwater species,&#; says Michele Thieme, WWF&#;s lead freshwater conservation scientist.
Freshwater species are in rapid decline planetwide. In its &#;Living Planet Report ,&#; WWF monitored 3,741 freshwater populations worldwide (representing 944 species of aquatic mammals, birds, amphibians, reptiles and fish), and found an 84% average decline in freshwater population size since . Freshwater amphibians, reptiles and fish are the worst impacted, with about a third of all freshwater fish species threatened with extinction, while 80 species have already vanished.
There&#;s plenty of blame to spread around: Freshwater river systems are where we base our civilizations, where we build cities, roads, industry, and grow our food. &#;There are multiple levels of interactive impacts on freshwater systems that make it difficult to point to just one [cause of harm] alone,&#; Thieme says. Habitat modification, invasive species, overfishing, pollution, poor forestry practices, disease and climate change all play a part. However, she adds, the impact of dams on rivers, and the loss of connectivity, are a huge known contributor.
Most directly, dams block the migration of fish and other aquatic species, separating them from breeding grounds and reducing population sizes. Migratory fish populations &#; including sturgeon, salmon, hilsa and gilded catfish &#; have fallen by 76% since . In Brazil, dams and other causes are endangering the Amazon&#;s giant catfish; on Asia&#;s Mekong River, fewer than 100 Irrawaddy dolphins may remain as proposed dams loom.
Iconic fish, like the beluga sturgeon and the Mekong giant catfish, are also in danger. The world&#;s largest freshwater predatory fish, the Chinese paddlefish, is already gone. Officially declared extinct in , a paper by Chinese scientists concluded that the &#;Panda of the Yangtze&#; disappeared after 200 million years due to the combined effects of overfishing and the disruption of migration routes by both small and large dams.
People, too, are paying a very high price due to lost connectivity. The hilsa fishery once made up the majority of catches in India&#;s Lower Ganges, where many people rely on freshwater fish as their primary source of protein. Since the s, following the construction of the Farakka Barrage that likely prevented fish from reaching their spawning grounds, catches declined by 94%. Upstream of the barrage, the annual catch dropped from 19 metric tons to 1 metric ton after construction.
Beyond the physical barrier, dams result in vast changes to ecosystems and the life-forms that depend on them. Downstream water temperatures change as water is released, and the natural ebb and flow of the hydrological cycle is altered.
Downstream sites, including lakes and estuaries, are also impacted by the reduced flow of phosphorous, nitrogen and silicon trapped behind dams. Nitrogen and phosphorous nutrients trapped in stagnant reservoirs can trigger algal blooms, eutrophication and massive fish kills.
Another important harm that needs the world&#;s attention is the disruption by dams of sediment flow, says Thieme. &#;The cascading effects of this are not always considered, but it has real, global implications.&#;
According to some estimates, 25-30% of Earth&#;s land-to-ocean sediment flux is trapped behind dams. Though the science behind the numbers is complex, it&#;s easy to see the impact of reduced sediment flow on the livelihoods of people living in Earth&#;s deltas today. Deltas are landforms created by the deposition of sediment carried downstream by rivers as they enter an estuary or ocean.
While ancient civilizations like those of Egypt and Sumeria flourished in these resource-rich environments for thousands of years, a recent study found that, today, at least 25 million people live in sediment-starved deltas. Dams upstream prevent nutrient-rich sediment from ever getting to the deltas, resulting in the loss of large tracts of fertile land to subsidence, erosion, flooding and sea-level rise.
An example is the biodiversity-rich Mekong Delta in Vietnam, the world&#;s third-largest delta, home to nearly 20 million people and key to Southeast Asia&#;s food security. Large dams have already been constructed on the Mekong, and more are in the pipeline.
Sediment flow to the Mekong Delta will be reduced by an estimated 97% by , with expected major damage to river&#;s productivity, geomorphology and persistence of the delta landform itself. Though sand mining contributes, the bulk of Mekong sediment loss is attributed to dams.
Free-flowing rivers transport carbon in the form of organic matter and sediment from upland headwaters, through watersheds to the sea, moving as much as 200 million metric tons every year. But dam disruptions could decrease the export of organic carbon to the oceans by an estimated 19% by , with major potential repercussions for freshwater and marine ecosystems.
The construction of new hydroelectric dams, access roads, and transmission networks in remote areas can initially cause significant deforestation. But that&#;s just the beginning: Cheap, government-subsidized hydropower attracts energy-intensive, ecologically destructive industries, such as bauxite mining and aluminum smelting and industrial gold mining. This widespread, often dam-triggered, industrial infrastructure expansion has severe impacts on ecosystems and species &#; exacerbated in developing nations where environmental regulations are weak.
In the Brazilian Amazon, every kilometer of legal road built through wild areas is typically accompanied by 3 kilometers of illegal roads, resulting in significant forest fragmentation, giving access to wildlife traffickers and illegal loggers, causing roadkill, and allowing more traffic into sensitive areas and attracting settlers. Dams, and the roads that accompany them, significantly diminish diversity.
Though commonly promoted by governments, construction companies, big banks and international investment firms as a clean source of green energy, hydroelectric dams located in tropical regions can accentuate climate change significantly.
Tropical hydroelectric plants and their reservoirs can emit two to three times more greenhouse gases than natural gas, oil, or coal plants, due to deforestation and potent methane emissions.
Rapid, ongoing rot of submerged vegetation in equatorial heat turns reservoirs into major emitters of methane &#; a greenhouse gas many times more powerful than CO2. Despite that scientific fact, the U.N. still considers dams a clean source of energy, fails to count reservoir-caused emissions or deforestation in national greenhouse gas totals, and offers carbon credits for new dam construction.
In recent years, researchers have begun overthrowing past assumptions about dams, concluding that they impact Earth&#;s climate in complex ways through the modification of the carbon cycle and accompanying greenhouse gas exchanges.
&#;The general opinion was that [dams] store more carbon than they emit,&#; explains Matthias Koschorreck, a biologist in the Department of Lake Research at the Helmholtz Centre for Environmental Research, Germany. Koschorreck was part of a research team that recently published a paper turning the green status of dams on its head. Their work analyzed the influence of drawdown areas &#; the edges of reservoirs exposed to air when water levels drop and what looks like a bathtub ring appears, extending around the entire body of water.
&#;Our study shows that [dam carbon] emissions are much higher [than previously thought],&#; Koschorreck says. &#;On a global scale, reservoirs emit more carbon to the atmosphere than they bury in the sediments.&#; Adding drawdown areas into the emissions equation, dams release twice as much carbon globally on average than they store.
But here the science gets complicated and murky: On the flip side, Koschorrek says, drawdown areas also seem to emit less methane. &#;If the water level goes down in reservoirs and we have these dry areas, then we increase the CO2 emission from the whole system, but at the same time, we reduce the methane emission from the water surface.&#;
More research will be needed to determine how these emissions, or lack thereof, sort themselves out, weighing a multitude of factors, such as tropical vs. temperate location, types of vegetation involved, and more.
Humanity&#;s future relationship with dams will likely remain an uncomfortable, ambivalent partnership.
&#;We&#;re not an anti-dam organization; we recognize the value and benefits those dams bring to society,&#; says WWF&#;s Thieme. &#;The holistic view in the long run, I think, is one that will allow for a greater ability of the [freshwater] system to be resilient in the face of a changing climate. In practice, we will have some parts of our rivers that are more working rivers and some parts we keep free flowing. That&#;s the ideal because, to survive and to flourish, we also need to use water in ways that are sustainable.&#;
Put simply, we need to learn to live with some dams, while scientifically balancing their benefits against their harms.
An example of this attempted balance can be found in the Penobscot River Restoration Project. This effort to revive New England&#;s second-largest river system entailed the removal of two dams and construction of a stream-like bypass channel around a third. Hydropower generation was increased at six nearby dams to compensate for the removed dams. The project has given locally endangered Atlantic and shortnose sturgeon and striped bass unobstructed river access to their historical habitats, opening 3,200 km (2,000 mi) of river and tributary habitat for sea-run fish.
Back in Kruger, Midzi reckons there&#;s enough natural water in the landscape for animals to survive and maintain ecological function. &#;People do not need to panic when a dam is being removed. It doesn&#;t mean there is any less water for the wildlife.&#; Even in dry seasons, natural surface water is available in pans, springs and pools, or below the sand, which digging animals such as elephants can access. During severe drought years, most wild animals are affected more by lack of food rather than water, Midzi says. But by removing artificial infrastructure, he says, park managers are reversing past mistakes.
The reality is that Kruger, like other parts of the world, will probably remain at least somewhat dependent on dams into the future, even as officials continue removing dams inside the national park. Riddell notes that water releases from South African dams located upstream, outside the park, maintain water flows that benefit the environment inside the preserve while also aiding various industries and farmers.
In the end, dams are a mixed blessing, Riddell says. &#;From an ecological point of view, you want to oppose that dam, but you also realize you kind of need it.&#;
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Banner image: Detroit Dam in Oregon, U.S. Image by Dan Meyers via Unsplash.
Citations:
Grill, G., Lehner, B., Thieme, M. L., Geenen, B., Tickner, D., Antonelli, F., &#; Zarfl, C. (). Mapping the world&#;s free-flowing rivers. Nature, 569(), 215&#;221. doi:10./s-019--9
Thieme, M. L., Khrystenko, D., Qin, S., Golden Kroner, R. E., Lehner, B., Pack, S., &#; Mascia, M. B. (). Dams and protected areas: Quantifying the spatial and temporal extent of global dam construction within protected areas. Conservation Letters, 13(4), e. doi:10./conl.
Almond, R. E. A., Grooten, M., & Petersen, T. (Eds). (). Living Planet Report &#; Bending the curve of biodiversity loss. Retrieved from WWF website: https://f.hubspotusercontent20.net/hubfs//LPR/PDFs/ENGLISH-FULL.pdf
Damme, P. A., Córdova-Clavijo, L., Baigún, C., Hauser, M., Doria, C. R., & Duponchelle, F. (). Upstream dam impacts on gilded catfish Brachyplatystoma rousseauxii (Siluriformes: Pimelodidae) in the Bolivian Amazon. Neotropical Ichthyology, 17(4). doi:10./--
Mulligan, M., Van Soesbergen, A., & Sáenz, L. (). GOODD, a global dataset of more than 38,000 georeferenced dams. Scientific Data, 7(1). doi:10./s-020--5
Lehner, B., Liermann, C. R., Revenga, C., Vörösmarty, C., Fekete, B., Crouzet, P., &#; Wisser, D. (). High&#;resolution mapping of the world&#;s reservoirs and dams for sustainable river&#;flow management. Frontiers in Ecology and the Environment, 9(9), 494-502. doi:10./
Zhang, H., Jarić, I., Roberts, D. L., He, Y., Du, H., Wu, J., &#; Wei, Q. (). Extinction of one of the world&#;s largest freshwater fishes: Lessons for conserving the endangered Yangtze fauna. Science of The Total Environment, 710, . doi:10./j.scitotenv..
Anthony, E. J., Brunier, G., Besset, M., Goichot, M., Dussouillez, P., & Nguyen, V. L. (). Linking rapid erosion of the Mekong River Delta to human activities. Scientific Reports, 5(1). doi:10./srep
Walling, D. E. (). The role of dams in the global sediment budget. IAHS-AISH publication, 3-11. Retrieved from https://iahs.info/uploads/dms/.05-3-11-356-07-ICCE_Des_Walling&#;34&#;1&#;corr.pdf
Maavara, T., Lauerwald, R., Regnier, P., & Van Cappellen, P. (). Global perturbation of organic carbon cycling by river damming. Nature Communications, 8(1). doi:10./ncomms
Freitag-Ronaldson, S., McGeoch, M., Joubert, M., Viljoen, J., & Du Toit, J. (). Biodiversity: Conservation in times of change. Retrieved from SANParks Scientific Services website: http://www.sanparks.org/assets/docs/conservation/scientific/biodiversity-brochure.pdf
Van Vuren, L. (). In the footsteps of giants &#; Exploring the history of South Africa&#;s large dams (SP 31/12). Retrieved from Water Research Commission of South Africa website: http://www.wrc.org.za/wp-content/uploads/mdocs/Footsteps%20of%20giants_web.pdf
Erosion and Sediment Yields in the Changing Environment (Proceedings of a symposium held at the Institute of Mountain Hazards and Environment, CAS-Chengdu, China, 11&#;15 October ) (IAHS Publ. 356, ). Copyright &#; IAHS Press 3 The role of dams in the global sediment budget DES E. WALLING Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK
Maavara T., R. Lauerwald, P. Regnier, and P. Van Cappellen (). Global perturbation of organic carbon cycling by river damming. Nature Communications 8, .
Freitag-Ronaldson, S, McGeoch, M., Joubert, M., () Biodiversity Conservation in Times of Change, published by SANParks Scientific Services
Van Vuuren, L. (). In the Footsteps of Giants &#; Exploring the history of South Africa&#;s large dams, Water Research Commission of South Africa
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Reservoirs are created to provide a benefit to people. However, the flooding, or inundation, of land and the management of the reservoir water can have an unfavourable effect on people, the wildlife and the environment, not only in and around the valley, but also downstream of the dam. The disadvantages of creating a new reservoir should be considered during the planning stages. Suitable methods should be decided upon to eliminate or reduce the disadvantages, so that the reservoir provides an overall benefit to people.
These are some of the issues that are considered:
When the Three Gorges Project, in china,
is completed this town called Feng Du will
be innundated by the reservoir.
People and their livelihoods are affected when the areas where they live and work are inundated by a reservoir. For some large reservoirs, tens of thousands of people have had to leave their homes and set up elsewhere. In the past, many of these people have not been given adequate compensation for their losses, and some have not even been given new places to live. Also, existing communities have been broken up and moved to different areas.
Some people made their living from farming the land or fishing from the river. Many of them suffered when they were relocated, as they were not given new land to work and were too far from a river to fish. They needed different skills to get another job and training was not always provided.
These days, authorities are becoming more aware of these issues. Resettlement plans have been developed to minimise the disruption and suffering caused to people in the reservoir area. Good plans make sure that fair compensation and employment opportunities are provided for all people, and that the law protects their rights. In some cases, efforts have been made to resettle people in their communities.
Dams are often constructed across rivers to store water that would naturally find its way to the lower reaches of the river and into the sea. The presence of the dam upsets the natural balance of the river, affecting the animal and plant life in and around it. These are some of the reasons.
When the river valley is inundated with water, animals are forced to leave the area and plants and trees are killed. Sometimes, rare species can be affected.
The fish ladder at Pitlochry
Dam in Scotland
For some large reservoir projects, nature reserves have been created. Plants and trees have been replanted in them and some of the affected animals have been moved there. However, the reserves can only be really successful when careful thought has been given to the way that the plants and animals depend on each other and their environment.
A dam across a river can form a barrier to fish that migrate, such as salmon. Fish passes can be included in the design of a dam to allow adult fish to swim upstream to spawn, and back downstream later with their young. Fish passes usually take the form of a fish ladder or a fish lock. These fish passes have to be designed very carefully to make sure that the conditions are right for the fish to use them.
Rivers carry sediment. When a river enters a reservoir, the speed of the flowing water slows down and sediment can be deposited on the reservoir bed. Over a number of years, the sediment in the reservoir can build up, and reduce the space available for storing water. Some of the sediment held back in the reservoir would normally be carried downstream. If too much sediment is stored, the natural balance of the river downstream can be changed, affecting people, wildlife and plants as far away as the river estuary.
A build up of black peaty silt behind Bottoms Dam, near Manchester, NW England
Farming land, used for growing crops, can be deprived of silt and its nutrients that are normally deposited when the river floods. Nutrients are important for fertilising the soil.
When designing a dam, the quantity of sediment that will flow into the reservoir has to be considered. The reservoir is designed to reduce the amount of sediment deposited, and to maximise the sediment flow downstream.
The flow of water carrying the sediment can be controlled by carefully positioning spillways and outlet pipes and tunnels. Sometimes sediment is allowed to build up in the reservoir. Then periodically, it is removed. This can be done by letting water out of the reservoir through outlets at the bottom of the dam, so that the sediment gets flushed out. Sometimes dredging is used for small reservoirs, but this is an expensive operation.
The quality of water can deteriorate when it is stored in a reservoir.
River water contains dissolved oxygen. Sufficient dissolved oxygen is needed to maintain aquatic animal and plant life, and to prevent some types of chemical reactions that form unwanted pollution in the water.
There are many factors that can reduce oxygen levels in a reservoir, for example, organic material in the water can use up oxygen as it decomposes (or rots). The depth of the water, its temperature and its flow can also affect the oxygen levels.
The type of land that is inundated by a reservoir may affect the water quality. Pesticides from farmland and toxic materials from industrial land can pollute the water. Also, the streams and rivers flowing into the reservoir may be carrying pollutants.
The designers of a reservoir have to consider whether any of these factors will have a significant effect on the quality of the water and whether they have to include special measures to offset any problems that could occur.
This old temple in Sri Lanka was innundated
when a reservoir was created
Throughout history, people have settled and built their communities in river valleys. This means that many of the world's archaeological sites and historical buildings and monuments can be found in these areas. Often they include sacred buildings such as churches and temples and their burial sites, which can be of great value to the local inhabitants. Such heritage can be lost forever when a valley is inundated with water to create a reservoir.
In the past, there have been dam projects where no efforts have been made to explore or save any of the local heritages. More recently, special measures have been taken on some projects. They have included:
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