Answer:
By taking advantage of gravity and the water cycle, we have tapped into one of nature's engines to create a useful form of energy. In fact, humans have been capturing the energy of moving water for thousands of years. Today, harnessing the power of moving water to generate electricity, known as hydroelectric power, is the largest source of emissions-free, renewable electricity in the United States and worldwide.
Although the generation of hydropower does not emit air pollution or greenhouse gas emissions, it can have negative environmental and social consequences. Blocking rivers with dams can degrade water quality, damage aquatic and riparian habitat, block migratory fish passage, and displace local communities. The benefits and drawbacks of any proposed hydropower development must be weighed before moving forward with any project. Still, if it's done right, hydropower can be a sustainable and nonpolluting source of electricity that can help decrease our dependence on fossil fuels and reduce the threat of global warming.
On Earth, water is constantly moved around in various states, a process known as the hydrologic cycle. Water evaporates from the oceans, forming into clouds, falling out as rain and snow, gathering into streams and rivers, and flowing back to the sea. All this movement provides an enormous opportunity to harness useful energy.
In 2011, hydropower provided 16 percent of the world’s electricity, second only to fossil fuels. Worldwide capacity in 2011 was 950 gigawatts (GW), with 24 percent in the China, eight percent in the United States, and nine percent in Brazil [1]. Globally, hydroelectric capacity has more than doubled since 1970.
In the United States, hydropower has grown steadily, from 56 GW of installed capacity in 1970 to more than 78 GW in 2011 [2]. However, as a percentage of total U.S. electricity generation, it has fallen from 12 percent in 1980 to 7 percent in 2012, largely as a result of the rapid growth in natural gas power plants and other renewable energy technologies such as wind and solar [3].
Since hydropower depends on rivers and streams for generation, the potential to use hydropower as a source of electricity varies across the country. For example, the Pacific Northwest (Oregon and Washington) generates more than two-thirds of its electricity from hydroelectric dams [4]. The Grand Coulee dam on the Columbia River in Washington is one of the largest dams in the world, with a capacity of more than 6,750 megawatts (MW).
In addition to very large plants in the western states, the United States has many smaller hydropower plants. In 1940 there were 3,100 hydropower plants across the country, though by 1980 that number had fallen to 1,425. Since then, a number of these small plants have been restored; as of 2013, there were 1,672 hydro plants (not including pumped storage) in operation [5]. These plants account for only a tiny fraction of the dams that block and divert our rivers.
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