Whereas wave and tidal power are still in development, a kind of water power known as hydroelectricity is already widespread. Hydroelectricity refers to power generated by hydroelectric dams. These dams are placed across rivers, creating huge lakes or reservoirs of water behind the dams. The water can then be released as needed, pouring over turbines that generate power.
Hydroelectricity already generates massive amounts of electricity. Hydroelectric dams accounted for about 20 percent of total electricity worldwide in 2005, according to Renewables Global Status Report2006 Update.9 In the United States, "about 7 percent of total power is produced by hydroelectric plants." While "most of the good spots to locate hydro plants have already been taken" in the U.S., as much as two-thirds of potential hydropower remains to be developed worldwide in Latin America, Africa, India, and China.10
Hydroelectric power is a useful form of energy for several reasons. First of all, it is renewable. Hydroelectricity is essentially powered by the sun, which evaporates water. That evaporated water then falls as rain, filling the rivers that supply the reservoirs behind the dam. As long as water evaporates and falls as rain, the water behind the dam will be renewed, which means that there will always be more water to flow downstream and turn the turbines. In addition, dams "have a low operating cost once installed and can be highly automated."11
UNITED STATES ELECTRICAL PRODUCTION IN 2007 BY SOURCE OF ENERGY
Source: "Net Generation by Energy Source by Type of Producer," Energy Information Administration, January 21, 2009. www.eia.doe.gov.
Another advantage of hydropower is that hydroelectric dams can vary the release of electricity easily, simply by reducing water flow during slow times and increasing it at peak hours. "By comparison, thermal power plants take much longer to start up when they are 'cold,'" according to Catherine Gautier in her book Oil, Water, and Climate.12 Furthermore, damming rivers has many other advantages besides generating electricity. Dams are an important source of water for irrigation and for drinking water. Indeed, according to Gautier, "about half of the world's largest dams were built solely or primarily for irrigation."13
There are numerous disadvantages to the use of hydroelec-tricity as well. Dams can have large negative ecological effects. For example, dams often interfere with the migration patterns of fish. Thus, on the Mekong River in Southeast Asia, where "many commercial species [of fish] have highly developed migratory patterns . . . dams act as a barrier to fish migrating upstream; and returning fish migrating downstream . . . generally must go through turbines____As a result, spawning is greatly reduced and replenishment of fish stocks is diminished," according to Chris Barlow, the manager of the Mekong River Commission's fisheries program.14
Falling fish stocks may cut into human food supplies. But dam construction can affect people directly as well. When a river is dammed, it creates a large lake. This lake may flood human settlements, forcing massive migration. For example, one proposed dam in northwestern Burma on the Chindwin River would generate more electricity than all of Burma's current generating capacity combined—but it would also flood 17,000 acres (6.880 hectares) and force "30,000 inhabitants to move."15
Dams also prevent rivers from flooding, which can have serious consequences on wetlands and on groundwater tables around the river. Vegetation in a river's floodplain may dry out, and species that depend on regular floodwaters may be threatened. Furthermore, according to Catherine Gautier, dams cause "river flows to fluctuate unnaturally" as water is released and contained in accordance with electrical needs. These fluctuations can wreak havoc with the stable water temperatures and constant oxygen levels that fish species need to survive.16
There has also been concern that dam building may cause earthquakes. Some scientists have suggested that the massive Chinese earthquake of May 2008, which killed 80,000 people, may have been triggered in part by water collected behind the massive Zipingpu Dam. "The dam was built 500 meters from the earthquake's fault line. A research paper by a group of Chinese scientists concluded that the weight of collected water clearly affected seismic activity," according to a 2009 Wall Street Journal article.17
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