The third longest river in the world is also the longest river in Asia. The ancient Yangtze River begins its life high upon the Tibetan Plateau in Western China. The spark of its life is derived from glacial runoff. This runoff is joined by small streams and creeks as it descends from its lofty Tibetan perch. The river gains volume and ferocity as it tumbles and roils towards China’s east coast. As the river nears the coast, it splinters apart to form the capillaries of the Yangtze River Delta. For centuries, this unrelenting river etched its pathway unmolested across China, splitting the country in two. However, in 2006 the completion of the Three Gorges Dam forever changed the ecology, environment, and destiny of the river. The Yangtze is not alone. According to an article by David Biello (2009) called “Dam Building Boom: Path to Clean Energy,” the world has as many as 48,000 hydroelectric dams. These dams provide electricity to millions of people. According to Mara Hvistendahl’s article (2008) called “China’s Three Gorges Dam: An Environmental Catastrophe,” the Three Gorges Dam alone will generate 18,000 megawatts of electricity. That is eight times the amount of electricity generated by the America’s Hoover Dam. However, this green source of energy has a dark side. Although hydroelectric dams are believed to provide a relatively clean source of electricity, they cause an increase in greenhouse gases because flooding destroys carbon dioxide filtering vegetation and forests, decomposing organic material releases methane and stored carbon dioxide into the water, and the dam’s turbines release these gasses into the atmosphere.
Dams have a very simple yet extremely functional design. Simply build a wall across a river and effectively stop it in its tracks. Provide a pathway for the water to escape to the other side of the dam. This pathway is called a penstock. The penstock is filled from near the base of the dam thereby pulling water from near the bottom of the reservoir. At the end of the penstock the water enters a large chamber that houses the blades of the turbines. The water spins the blades of the turbines which generates the electricity. Once the water has passed the turbine blades it is spat out the back side of the dam where it is free to continue its journey downstream. When fish enter the turbine system the outcome is nearly always fatal for the fish (Carr 2012, Ferguson 2011, Keefer 2010). The amount of electricity generated depends on how much water is let through the penstock. On days where there is little electricity needed then only a little water is let through. On days where much more electricity is needed then more water can be let through. Dams will also open the penstocks fully to drain large amounts of water to accommodate floodwater during rainy seasons and during spring snow thawing.
However, by placing a dam in the river water begins to build up on the upriver side. Often a large lake will be formed. These lakes are called reservoirs. The reservoir can be massive. This flooding alone often has serious consequences for people. The flooding from the Three Gorges Dam created a lake that was so large it displaced nearly 1.3 million people. Deep beneath this lake rests the skeletal remains of a forest. This forest was once thriving, absorbing carbon dioxide to grow and releasing precious oxygen as a result. The lake that developed behind the Three Gorges Dam covers 468 square miles of land that once grew vegetation and forests. Imagine how much forested land has been destroyed by the flooding behind the world’s 48,000 hydroelectric dams. There are also thousands of acres of forested land that must be cleared to resettle the 1.3 million displaced residents. A dam in the Nile River drainage displaced nearly 50,000 residents (Bosshard 2009). Also, land must be cleared for the electrical transmission lines and other necessary dam projects.
Once the land behind a hydroelectric dam has been flooded, and the forests and vegetation are all dead, the biological material begins to decompose deep below the surface of the lake. The dead biological material such as the trees, plant and animal matter is attacked by microscopic bacteria. Some bacteria called aerobic bacteria need oxygen to live as they begin to break down the plant and animal matter (Demarty 2011). As these bacteria live, they release carbon dioxide. Other bacteria, called anaerobic bacteria, do not need oxygen to live. As these bacteria break down the animal and plant matter, they release a toxic combination of carbon dioxide and methane gas. This gas is released into the surrounding water by the bacteria where it then resides until it can be released into the atmosphere. The flooding caused by the dam is not the only source of this organic material. The dam also works like a net and water filter. Any plant or animal matter that is carried by the river from further upstream is stopped by the dam. Trees, branches, driftwood, and animal matter are all trapped behind the dam. This material sinks to the bottom of the lake where it will also decompose. Therefore, dams create a continual source of decomposing material which will release greenhouse gases. The production of greenhouse gases has been found to increase in dam reservoirs that are located in tropical climates.
Some of the gas is oxidized in the water to form carbon dioxide which is released at the surface of the lake by the gentle agitating motion of the waves and wind (Roland 2010). However, most of the methane gas remains suspended in the deep water. As the dam releases water through the penstock the water becomes highly agitated by the movement. The spinning of the turbines, and eventually the turmoil caused by spilling from the down river side of the dam will cause most of the greenhouse gases to be released. The rest of the greenhouse gases will be released from the water as it continues its journey downstream. According to a report by International Rivers Network (2012) called “Frequently Asked Questions: Greenhouse Gas Emissions from Dams,” as much as 104 million metric tons of methane gas will annually be released by the large dams of the world. This release of methane gas is responsible for as much as 4% of humankind’s warming impact on the planet. Methane gas in the atmosphere traps more heat than carbon dioxide. The International Rivers Network (2012) report states that dams are responsible for 23% of the world’s methane gas production. According to International Rivers Network (2012), hydroelectric dams actually are less environmentally-friendly than fossil fuel burning facilities when generating the same amount of electricity:
“Large hydropower reservoirs in the tropics can have a higher global warming impact per kilowatt hour generated than fossil fuels, including coal.” (International River Network 2012)
There are some methods to curb the production of greenhouse gases in dam reservoirs. Prior to the flooding of a proposed dam reservoir area it is possible to log off forested areas to cut down on the volume of organic material that will be covered with water. This is by no means a cure all. It would be impossible to fully remove all material from a proposed flood zone. The sheer volume would be incomprehensible. However, the removal of large timber would cut down on a large proportion of organic material. When the Three Gorges Dam reservoir flooded it covered numerous villages, cities and towns. This flooding added inorganic trash and material to the reservoir’s collection of decomposing sediment. Additional problems arise when dam building is proposed in tropical climates such as Brazil where a large scale dam on the Amazon River is proposed. The dam, named The Bella Monte, would be the third largest hydroelectric dam in the world. The flooding that would ensue from the construction of this dam could flood large portions of pristine Amazon rainforest. This flooding would release a catastrophic volume of methane gas into the atmosphere.
In addition to releasing vast amounts of toxic greenhouse gases into the atmosphere, dams have other effects on the environment (Grahm-Rowe 2005, Marriot 2010, Mourad 2011) around them. Up river flooding can cause serious seasonal flooding of agricultural and natural wetlands. The flooding of agricultural lands not only affects those that depend on agriculture but this flooding also releases toxic pesticides and poisons into the river water. Flooding natural wetland ecosystems with many times the normal amount of water can destroy these delicate ecosystems. Many natural wetlands are destroyed and cannot ever recover from the flooding. Continuious flooding of a seasonally flooded region upsets breeding, spawning, and migratory schedules of numerous species of aquatic wetland indigenous creatures. An unknown number of delicate and endangered species can be wiped out by this flooding (Lopez-Pujol and Ming-Xun 2009). Dams also inhibit migrations of keystone fish species such as salmon. The loss of salmon and salmon habitat can impact hundreds of species of birds, fish and mammals that rely on salmon as a food source (Welch 2008). The Three Gorges Dam also threatens endangered species such as the Yangtze Alligator and the Yangtze Freshwater Dolphin. These species are found nowhere else in the world. Hydroelectric dams also increase the chances of landslides upstream of the dam where the reservoir erodes riverbanks. Large scale land slides into reservoirs have often caused large waves that flood river side villages leaving death and destruction in their wake. Cities that were destroyed by the Three Gorges Dam reservoir, released and unknown amount of human sewage, toxic waste, petroleum products and trash into the water system. These underwater ghost cities will undoubtedly pollute the Yangtze for generations to come.
Although hydroelectric dams were once thought to be the best source of electricity, new studies have proven that hydroelectric dams are no better, and in some cases a worse method of generating electricity. Hydroelectric dams can produce stunning amounts electricity. However, the release of methane and other greenhouse gases from hydroelectric dam reservoirs, especially those in tropical climates can in fact make hydroelectric dams less efficient than fossil fuel electric facilities that produce similar amounts of electricity. Methane production in reservoirs can be cut down by the removal of plant and animal matter from proposed dam reservoir areas prior to flooding however it cannot be eliminated completely. Dam reservoirs will always continue to produce greenhouse gases. As the human race continues to grow and expand into previously untouched wilderness the need for more and more electricity generating facilities will also grow. The damming and building of hydroelectric dams will continue and the subsequent production of greenhouse gases will also increase. Not only do humans increase greenhouse gases by building hydroelectric dams, but humans also inadvertently decrease the planet’s ability to filter carbon dioxide form the atmosphere by killing the vegetation in a proposed reservoir flood area. With today’s technology it is no obvious better or worse method for producing electricity. Each need must be analyzed and examined to determine the best method of electrical production whether it be a fossil fuel burning facility or a hydroelectric dam or a nuclear power plant. It is up to humankind to determine the appropriate method of generating electricity for a certain area. There may never be a single environmentally sound method of electricity generation. Every method of electrical generation has downsides. The safest choice is for every single person to attempt to cut down on unnecessary use of electricity. Every person can lead by example and cut down on unnecessary use of electricity. Turning off lights after leaving a room, unplugging unused electrical appliances when not in use because many appliances contain clocks which are continuously drawing current. It is cutting down on the small uses of electricity that can add up to save large volumes of power. By cutting down on the need for electricity, the need for large scale hydroelectric dams can be cut down. This cutback on electricity is the best method currently available to humans to keep planet Earth green.
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