Hydroelectric: Hydroelectricity is the electricity generated by hydropower. The production of electricity uses the gravitational force of falling water and flows. It is the most widely-used source of renewable energy, producing approximately 20% of the world’s electricity and the equivalent of 88% of electricity from all forms of renewable energy.

Generation of electric power: Hydroelectricity is generated by force of falling water. The capacity of this energy depends on the height between the source and the water’s outflow, and the volume of the water in the reservoir. The difference of the height is called the head. Behind a high dam, water stores potential energy, which is proportional to the head. The energy is transformed into kinetic energy while the water rushes down the sluice. Then, the energy changed into mechanical energy, allowing the water to strike and rotate the blades of the turbines. This rotation spins the electromagnets which produce a current surrounded by stationary coils of wire. The current goes through a transformer where the voltage is increased for long distance transmission over power lines.

Advantages:

1. The major advantage of hydroelectric power is the elimination of the use of fossil fuels such as oil, natural gas, and coal. Hydroelectric power does not directly release greenhouse gases, i.e. carbon dioxide (C02), so there is minimal pollution. Although a small amount of C02 may be produced during the construction and operation of a dam, this is considered insignificant when compared to the emissions caused by the burning of fossil fuels. Hydroelectric power also conserves fossil fuels, which are considered non-renewable resources.
2. Hydroelectricity is a renewable energy source. It uses the energy of running water, without reducing its quantity. Rainfall renews the water in the reservoir, naturally maintaining the storage volume. Hence, water provided to run the power plant is free from nature.
3. Hydroelectric plants are less costly and more cost stable than traditional fuel generation. The spending for the actual construction and maintenance is relatively low. The considerable revenue stream is enough to offset the operating cost. It has been calculated that the sale of electricity from the Three Gorges Dam will cover the construction costs after 5 to 8 years of full generation. The cost of ongoing labor is also small, as operation is mostly automatic and has few personnel demands. It avoids the fluctuations of the price of oil and reduces the amount of energy imported, protecting the domestic economy.

4. Hydroelectric power is a long-term investment. The average working life of a hydroelectric plant is 50 – 100 years. This is a long-term investment that can benefit multiple generations.

Disadvantages:

1. Environmental damage: Hydroelectric projects can be disruptive to the local ecosystems, such as inundation of land, and fish and wildlife habitats. Dams may reduce fish population by preventing access to spawning grounds upstream, though some dams are installed with fish ladders.
   
   Additionally, water exiting the turbine usually contains very little suspended sediment, which scours the river beds and causes the loss or riverbanks. The post-dams water also has higher temperature than pre-dam water. This can affect local fish population and could also prevent freezing during winters.
   
   
2. Greenhouse gases emission: Due to the anaerobic decay of plant materials under the flooded area, the reservoirs and dams generate a significant amount of methane and carbon dioxide. Some plants may produce a much higher amount of greenhouse gases than conventional oil-fired thermal generation plant.
3. Displacement of human populations: In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction (Briefing of World Commission on Dams). Historical and cultural sites can also be flooded and sacrificed. For example, the Three Gorges Dam flooded some 1,300 archaeological sites and altered the appearance of the Three Gorges as the water level rises over one hundred meters at various locations (Regine Debatty (2007-12-09). "Flotsam, Jetsam and the Three Gorges Dam". World Changing. http://www.worldchanging.com/archives/007682.html. Retrieved on 2008-01-20. ).
   
   Cultural and historical relics were moved to higher ground as they were discovered but the flooding of the Gorge inevitable covered some undiscovered relics. Some other sites could not be moved because of their location, size or design. For example the hanging coffins site high in the Sheng Nong Gorge was inherently part of the sheer cliffs themselves (C.Michael Hogan. "Shen Nong Gorge Hanging Coffins". The Megalithic Portal. http://www.megalithic.co.uk/article.php?sid=17946. Retrieved on 2008-01-20).
4. Dam collapse: Dam failures are rare, but the potential damage is serious. The BanQiao Dam in China resulted in the deaths of 171,000 people and left millions of others homeless.