Hydropower, also known as hydro-electric power, is generated by hydro-electric dams placed on many of the world's major rivers. These structures divert the flow of the river and harness the force of the water to turn massive turbines that power electricity generators.
According to the National Hydropower Association, modern hydro-electric dams provide one of the most reliable sources of clean energy available. Unlike wind, solar or tidal energy, hydro-electric generators are capable of a sustained output with few factors affecting their long-term performance. Many of these structures have been in operation for several decades or longer.
However, there are some drawbacks to the use of hydro-electric power plants. Many of these drawbacks, according to the U.S. Geological Survey (USGS), directly affect wildlife and fish habitats. Dams that are constructed without fish-ladders prevent migratory species from returning to their native habitat to spawn, resulting in reduced populations. Further, the initial investment required for a new dam is substantial and may take a decade or more to see completion.
Alternately, the benefits of hydro-electric power, according to the USGS, include a free source of fuel, minimal or zero pollution in most cases and very little maintenance necessary to keep the powerhouse online. As of April 2014, the USGS states that United States is the fourth largest producer of hydro-electric power in the world, behind China, Canada and Brazil respectively.