The environmental impacts of dams have come under renewed examination in recent years.
Dam proponents 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. Recently however, the negative impacts of dams have come increasingly into focus. Discussion over whether dam projects are ultimately beneficial or detrimental--to both the environment and surrounding human populations--has emerged with a particular gravity following various public debates about the construction of Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America. In evaluating the effects of dam projects, experts tend to discuss the advantages and disadvantages of river development in terms of upstream and downstream impacts. Since dams are essentially walls built across rivers, the upstream/downstream vocabulary facilitates organized discussion of the environmental and localized effects of dams, although all effects are intertwined and not necessarily easily divided.
Impact above dam
Creation of a reservoir
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. 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.
Fragmentation of river ecosystems
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 “fishways” or “fish ladders." Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge. Fish sometimes have difficulty migrating downstream through a dam, meaning that downstream populations are often reduced unless, as can occur, 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. Endemic 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.
Sedimentation behind the dam
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. 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.
Impact below dam
Riverine and coastal erosion
As all dams result in reduced sediment load downstream, a dammed river is said to be “hungry” for sediment. 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 eats away at 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. This prompts coastal erosion, as beaches are unable to replenish what waves erode without the sediment deposition of supporting river systems. Channel erosion of rivers has its own set of consequences. The eroded channel could create a lower water table level in the affected area, impacting bottomland crops such as alfalfa or corn, and resulting in a smaller supply.
The water of a reservoir is usually warmer in the winter and cooler in the summer than it would be without a dam. As this water flows into its river, the altered temperature also affects the temperature of the river in question. This impacts the plant and animal life present in both the reservoir and the river, often creating environments that are unnatural to the endemic species. A substantial decrease in fishing success for Salmon and Sewin on the River Towy has been shown to be related to the reduced water temperature caused by the bottom water discharge from Llyn Brianne a dam created in the 1960s. Recent shifts in the populations of fish in the Snake River and Klamath region have, for example, prompted the renewal of research and restoration projects aimed at relieving the stress of temperature change.