Environmental Impacts of Dams
By
Tania M. Brown
Instructor: Timothy Butcher
Environmental Site Assessment
10/27/2008

Throughout the past few years, the negative impacts of dams have become so well known that most countries have stopped building them altogether and are now forced to invest their money into fixing the problems created by existing dams. The environmental impacts of dams have come under renewed examination in recent years. Dam advocates have historically understood that dams largely improve the status of water-related energy and environmental issues by, for example, producing hydroelectric power and increasing the water supply for irrigation. The construction of large dams completely change the relationship of water and land, destroying the existing ecosystem balance which, in many cases, has taken thousands of years to create. Currently there are around 40,000 large dams which obstruct the world's rivers, completely changing their circulation systems: this is not going to occur without dire environmental impacts.
. Dam advocates have historically understood that dams largely improve the status of water-related energy and environmental issues by, for example, producing hydroelectric power and increasing the water supply for irrigation. The damming of a river creates a reservoir upstream from the dam. The reservoir waters spill out into the surrounding environments, flooding the natural habitats that existed before the dam’s construction. To date, over 400,000 km2 of the earth have been flooded due to damming. The newly created reservoir has more surface area than the river would have had, and therefore more evaporation occurs than is normal. According to Science Encyclopedia, Volume 2, this can lead to an increased loss of up to 2.1 m3 of water per year. According to recent studies, reservoirs contribute to greenhouse gas emissions as well. The initial filling of a reservoir floods the existing plant material, leading to the death and decomposition of the carbon-rich plants and trees. The rotting organic matter releases large amounts of carbon into the atmosphere. The decaying plant matter itself settles to the non-oxygenated bottom of the stagnant reservoir, and the decomposition, unmitigated by a flow pattern that would oxygenate the water produces and eventually releases dissolved methane. (www.newscientist.com)
A dam also acts as a barrier between the upstream and downstream habitat of migratory river animals, such as Chinook and Steelhead salmon in the USA and Atlantic salmon in Europe. Dams block their migration upstream to spawn, threatening to decrease reproduction numbers and reduce the species population. In light of this consequence, efforts have been made to allow the fish a passage upstream, and newer dams often include artificial fish ladders. Some communities have also begun the practice of transporting migratory fish upstream to spawn by using a barge. Sometimes fish have difficulty migrating downstream through a dam, meaning that downstream populations are often reduced unless; the fish are able to swim safely through the dams’ spillways.
Permanent inundation caused by reservoir flooding also alters the wetlands, forests and other habitats surrounding the river. Further ecosystem disruption occurs along the banks of the river and downstream. The areas surrounding riverbanks are of a particularly rich bio-diversity, supported by the natural flooding of a dam-free river. Dammed rivers reduce flood rates, and this has negative consequences on the floodplains downstream that depend on seasonal waters for survival. The comparatively invariable ecosystem created by a reservoir-river supports a far-reduced range of wildlife. (Keller, Edward. Environmental Geology) Endemic (normal for that particular geographic area) species may or may not survive the environmental change, and new species are likely to adopt the altered habitat as a home. However, since dams change a key ecosystem to which all surrounding ecosystems have adapted dam construction nearly always reduces wildlife diversity, for better or for worse.
Rivers carry four different types of sediment down their riverbeds, allowing for the formation of riverbanks, deltas, alluvial fans, braided rivers, oxbow lakes, levees and coastal shores. The construction of a dam blocks the flow of sediment downstream, leading to downstream erosion of these depositional environments, and increased sediment build-up in the reservoir. While the rate of sedimentation varies for each dam and each river, eventually all reservoirs develop a reduced water-storage capacity due to the exchange of storage space for sediment. (Ecology and Politics of Large Dams) Diminished storage capacity results in decreased ability to produce hydroelectric power, reduced availability of water for irrigation, and if left unaddressed, may ultimately result in the expiration of the dam and river.
Dams also have a range of social impacts. Most importantly, dams have compelled the relocation of millions of people. For example, it has been estimated that since the independence of India, about 14 million people have been displaced by dams and related construction, such as irrigation canals. Perhaps another 10 million people have been displaced in China. This, at least, is the government's own figure. Others have estimated that somewhere between 40 and 60 million Chinese have been relocated. (www.idsnet.org)
Dams can also have range of other human impacts. Various diseases have become more prevalent, as a direct consequence of dams and related projects. One example is schistosomiasis, (an infectious blood disease that is usually caused by contaminated water) which now affects about 200 million people. Malaria is also becoming increasingly prevalent, and now affects about 300 million people. Dams create excellent habitat for water-borne disease parasites responsible for these diseases, i.e. large areas of standing water created by dam reservoirs or irrigation projects in tropical countries, which provide a good habitat for the snails that spread schistosomiasis, and the mosquitoes that spread malaria.
In recent years, dam removal has become a commonly used restoration tool. American Rivers has reported 465 successful dam removals across the county. In Wisconsin more than 70 dams have been removed. Their studies have shown that dam removal is one of the most cost-effective stream restoration techniques and is almost always cheaper than repair or rebuilding options. Many local residents who were initially opposed to dam removal found that restored rivers added significant value to their communities.
Management of sediment accumulated behind a dam is often the most critical issue. Release of large amounts of fine sediment is not desirable and could affect downstream areas. Extreme care must be taken if sediments are contaminated. Rivers routinely move large amounts of sediment during high flow periods; therefore, some release of dam sediments may be acceptable. Typically, the long term benefits of dam removal outweigh the short term impacts.
As an alternative to removal, other solutions have been used to address dam problems. Fish passage structures have been used with some success, but do not address the habitat and water quality problems upstream of the dam. Other alternatives such as bypass channels, roughened ramps, dam lowering, and/or combinations of these techniques have been used. These solutions are typically more expensive and less effective for restoration than removal, but allow maintenance of useful dams and may help retain accumulated sediment. (Dept. Of Natural Resources, Illinois)
The impacts of dams are both positive and negative, but with the negative outweighing the positive, it is very apparent that changes have to be made in a timely fashion before it is too late to reverse the negative effects and our wildlife is endangered even more and more of .our human population is forced to move to other regions.

Reference Page
Fred, Pearce. "The Dammed." Free The World (n.d.): 182-194.
Graham-Rowe, Duncan. "Hydro Power's Dirty Little Secret." NewsScientist (2005).
Keller, Edward. Environmental Geology. Prentiss Hall, Inc., 2000.
Wikipedia. Environmental Impacts of Dams. .