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Hydropower or water power (from ὕδωρ, "water") is a process that involves forms of energy transformation in which kinetic energy of falling or fast-running water is converted to electrical or mechanical energy to be utilized for human benefits.

Since ancient times, hydropower from many kinds of watermills has been used a number of times as a renewable energy source for irrigation and the operation of different  mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance.

Earliest evidence of the utilization of hydropower for human benefits using irrigation machines dates back to ancient civilizations such as Sumer and Babylonia in the region of Mesopotamia. Further more, Historical evidence suggest that the water wheel was the initial form of water power, however, the water wheel was driven by either humans or animals. In the 19th century the Fourneyron turbine was developed by Benoit Fourneyron which was the first device using hydropower turbine. This device was implemented in the commercial plant of Niagara Falls and it is still operating today. In the early 20th century specifically 1978, the English Engineer William Armstrong made a significant achievement in the development of hydropower when he built and operated the first private electrical power station which was located in his house in Cragside in Northumberland, England.

The use water power has several benefits such as the generation of electricity, and powering machines. However, by extension hydropower has other environmental benefits such as the reduction of fossil fuels, and improve climate conditions. On the other hand, there are multiple economic, sociological, and even some environmental downside.

International institutions such as the World Bank view hydropower as a means for economic development without adding substantial amounts of carbon to the atmosphere,

 Hydroelectricity  Hydropower is currently the world's first renewable energy source of electricity since it generates about 15% of the global electricity. Furthermore, hydroelectricity is the number one application of hydropower. The generation of electricity through hydropower starts with the process of converting either the potential energy of water that is present due to the site's elevation or the kinetic energy of falling water into electrical energy.

There are a few types of Hydroelectric power plants in terms of the way the water is harvested and used to generate energy. One of these types is one involving building a dam and a reservoir that collects a certain amount of water. In this case the water in the reservoir will be available upon demand and it will be used to generate electricity by passing through specialty channels that connect the dam to the reservoir. This is then passed to turbines that derive a generator which as the name implies generates electricity. The other type of hydroelectric power plants is called the run-of-river project. In this case, to control the flow of water a barrage is built and instead of collecting water in a reservoir. Also, the kinetic energy of flowing water is the main source of energy that is converted to electrical energy.

Both of these types of hydroelectricity plants designs have some limitations. For example, the construction of dams can be very loud and will result in discomfort to the people living around it, also the presence of the dam and reservoirs will occupy a relatively large amount of space which is not usually favoured by communities living in the area. Moreover, the reduction of the amount of water of the downstream due to presence of a reservoir can potentially have major environmental consequences such as harming habitats living in the downstream of the site. On the other hand, the limitation of the run-of-river project is the decreased efficiency of electricity generation because the process depends on the speed of the river flow which is seasonal. This means that when the rainy season begins in the area, electricity generation will be maximized while it will decrease during the dry season.

The size of hydroelectricity water plants can vary small community sized plants called micro hydro, to very large plants supplying power to a whole country. As of 2019, the five largest power stations in the world are conventional hydroelectric power stations with dams.

Hydroelectricity can also be used to store energy in the form of potential energy between two reservoirs at different heights with pumped-storage hydroelectricity. Water is pumped uphill into reservoirs during periods of low demand to be released for generation when demand is high or system generation is low. Other forms of electricity generation with hydropower include tidal stream generators using energy from tidal power generated from oceans, rivers, and human-made canal systems to generating electricity.

Disadvantages and Limitations of Hydropower:
Large dams can major negative impacts on river ecosystems. Also, as mentioned before the dam and reservoir plant cover large areas of land which causes greenhouse gas emissions from underwater rotting vegetation. Furthermore, although at lower levels than other renewable energy sources, it was found that hydropower produces methane gas which is a greenhouse gas. This occurs when organic matters accumulate at the bottom of the reservoir because of the deoxygenation of water which triggers anaerobic digestion. Furthermore, the presence of large dams in an area poses as a major hazard for people living in the communities surrounding it. There are several reason that correspond to this hazard, one of which being floods during rainy seasons due to dams overflow.