Background to Dams and Levees - One of the issues resulting from civilization and urbanization is that most of the places humans chose to locate, for reasons of convenience, agriculture, transportation, and economic independence, have been near water. Dams provide hydroelectric power, help control floods, and make rivers navigable. Levees are quite similar to dams in their purpose, although they are primarily build to restrict water in times of high flow -- and for the majority of time are not under water. Per capita, floods are the most destructive and frequent of Mother Nature's natural disasters. In the last 50-60 years, in fact, the number and severity of flooding has worsened globally. Several reasons have contributed to this: global warming and worsening of storm activity; the deforestation and paving of natural watersheds; and more people living and working on known flood-plains. However, many scholars and engineers believe that it is not just the external conditions that are causing more flooding problems, but the very nature of the dam and levee method of flood control. When engineering projects reduce the capacity of river channels, block nature drainage, and increase the speed of floodwater, flood damage becomes greater. Additionally, the "hard path" flood control based on a working levee system often ruins the ecological health of rivers and estuaries (McCully, 2007).
Environmental Impact Issues- Despite the basic hydrological cycle, or the recirculation of moisture from ground to air and back, many areas of the world are overdeveloped and face an extreme shortage of potable water. The environmental impact of dams and reservoirs is increasingly receiving more attention as the global demand for water and energy increases, and the number and size of reservoir and damn projects increase. In general, the damming of a river creates some sort of a reservoir of water upstream from the dam. The dam project has four major environmental impact issues: upstream impacts, downstream impacts, effects beyond the reservoir, and then global or macro impacts.
Upstream Impacts- are due to the back flush of water that flows from the area of construction to the surrounding environment, flooding the natural habitats and creating a larger surface area than before the dam, and thus more evaporation. To date, scientists estimate some alarming statistics: over 400,000 km of the earth have been flooded dur to damming, and a loss of up to 2.1 meters of water to evaporation in some climates (Graham-Rowe, 2005). The initial filling of the reservoir floods the plant material already existing, leading to decomposition and rotting, which releases large amounts of carbon into the atmosphere. Because much of this rotted material exists at the lower or non-oxygenated bottom of the reservoir, there is little oxygen flowing, resulting in the over-production of dissolved methane (Marmulla, 2001).
Fragmentation of river ecosystems -- A dam also acts as a barrier between upstream and downstream movement of many migratory river animals and their spawning areas (e.g. trout and salmon). This threatens to reduce the species populations, and usually fish ladders are used to mitigate this. Even using this, or transporting fish via barge, wildlife often has difficulty migrating through or around a dam which, with the permanent altering of the wetlands surrounding the river, further disrupts eco systems. Dams may reduce the floodplains below, helping human populations, but the areas surrounding riverbanks are particularly rich in bio-diversity, which is also reduced. Many endemic species will not survive the environmental consequences, and new species are often likely to adopt the altered habitat, thus throwing off even more the indigenous eco-system (Maser, 2009).
Reservoir Sedimentation -- Rivers typically carry several different types of sediment down their riverbanks. This allows for a rich and diverse formation of river deltas, alluvial fans, braided rivers, riverbanks, oxbow lakes, and coastal shores. When a dam is constructed, it blocks the flow of sediment downstream, which leads to rather serious erosion of depositional environments, and increased sediment build-up in the reservoir. Eventually, most reservoirs develop a reduced water-storage capacity due to this build-up; which in turn results in a decrease in hydroelectric power, water for irrigation, and if left untouched, the eventual expiration of the dam and river (Morris and Fan, 1998).
Downstream Impacts - Placing a barrier where there was none creates havoc both upstream and downstream. As might be expected, the water that flows into the dam is not the same as the water that flows out.
River and Coastline Erosion -- Dams reduce the sediment load downstream just by their construction; and a dammed river is often said to be "hungry" for sediment. Ironically, because the rate of deposition of sediment is drastically reduced since there is less to deposit but the rate of erosion remains fairly constant, the continual water flow eats away at the river shores and riverbed, continuing to threaten shoreline ecosystems, deepening the riverbed, and, over time, narrowing the river. Eventual consequences to this are a compromised water table, a reduction in water levels, and a continual homogenization of the river flow and a reduction in the viability of its ecosystem (Berga, et.al., eds., 2006).
Water Temperature -- Without a dam, water temperature is seasonally variable, but relatively constant over time. The water in a reservoir is typically warmer in the winter and colder in the summer than it would be without a dam. As this water flows downstream, it affects the temperature of the river, impacting both flora and fauna downstream, and creating unnatural environments. Many fish will either not return to the river, or experience a drastic reduction in breeding (Wieland, Ren and Tan, eds., 2004, 321).
Effects Beyond the Reservoir (Regional, National, Global) -- We know we live in an interdependent world, one in which events in one part of the world have a profound and measurable effect not only on the local environment, but on the regional, continental, and global eco-system as well.
Effects on humans -- there are two primary effects on humans that tend to be harmful. First, while reservoirs are helpful to humans in agriculture and energy, reservoirs can become breeding grounds for serious disease vectors, particularly in warm or tropical environments where mosquitos are endemic (Jobin, 1999). In much of the developing world, creation of dams necessitates the relocation of human populations which often cause negative economic impacts, human trauma, and social catastrophe (Scudder and Gay, 2001).
Effects on flood-dependent ecology/agriculture -- Typically, river floodplains are located near savannah and forest ecosystems, and the ecology depends on seasonal flooding from rivers. This takes advantage of the residual soil and nutrients left after the floods recede. Dams prevent this, and both agriculture and ecology suffer. For example, the Kainji Dam in Nigeria caused a 70 per cent reduction of downstream agriculture and endemic famine (Drijver, 1986).
Global effects -- Reservoirs, particularly at the quantity and size of current projects, contribute to changes in global climate. Warm climate reservoirs create methane and a degradation of biomass. Methane is a greenhouse gas, and is thought to contribute to global warming. To put this in perspective, a combination of greater evaporation and methane in floodplain areas results in pollution that is almost four times more than an oil-fired power plant would be for the same generation capacity (Graham-Rowe).
Potential Solutions - As with many ecological issues, solutions for wildlife are neither simple, nor inexpensive. Moving fish from one place in the system to the other may help that species, but fails to take into account the rest of the flora and fauna. Clearly, the larger solution is to find alternative means of energy production that do not involve damming rivers. However, until that is done there are a few suggested solutions that can help mitigate the seriousness of the problem:
Changing the operation of the dam -- water can be released to spill over dams at critical migration times; screens can be installed to allow fish to bypass turbines; water levels can be lowered to provide run-of-river flow in non-flood periods; refraining from producing power during periods of migration.
Providing access to habitat -- fishways can pass above and below dams; building fishways species dependent; work to move and restock fauna as much as possible.
Monitoring temperature and controlling pollution -- install temperature control systems to mimic natural temperatures; filter water and monitor for pollutants.
Mimicking natural effects -- as much as possible, mimic natural temperature, flow rate, and control of flooded land; this may involve artificially reoxygenating deep reservoirs, helping to re-establish flora and fauna colonies; monitor and protect indigenous life, and reduce environmental impact by reconsidering where certain dam projects are built (Dams Solution, 2010; Gerritsen and Young, 2008; Chiras, 2010).
Dams Solution. (2010). U.S. Fish and Wildlife Service. Retrieved from: http://www.fws.gov/r5crc/Habitat/damsolutions.html
Berga, L. (2006). Dams and Reservoirs, Societies and Environment in the 21st Century, Volume 1. New York: Taylor and Francis.
Chiras, D. (2010). Environmental Science. Sudbury, MA: Jones and Bartlett.
Drijiver, C. (1986). Taming the Floods: Environmental Aspects of Floodplain Development in Africa. Nature…