The idea of getting electricity from water is not a modern day idea.  This method has been used since the Greeks and Romans used water mills to grind up corn.  A modern day hydroelectric plants are more complex but has the same concept of supporting human needs.  Humans use these hydroelectric plants to produce electricity for the cities or towns nearby.  To produce this electricity by water we need to build a dam so we can create a reservoir which power can be harvested from.  Then the water behind the dam flows through a canal called the penstocks.  The penstocks control the flow of water to maintain the correct amount of electricity that is generated. They also build a sluice gate to maintain how much water flows into the penstocks. The water then is lead to into the turbine.  This step is important because the turbine converts the energy of flowing water into mechanical energy.  Then the water is released into the tailrace which lets the water out.  The swift currents spin the turbines at a high speed.  Then the spinning generators generate the electricity that is harvested from the moving water.

Hydroelectric power plants have many advantages to humans, but at the same time, there are disadvantages.  One advantage of having hydroelectric power plants is that these plants are not powered by fuel.  There is no need to burn oil and cause pollution.  These plants use the electricity that is produced by the hydroelectric power plants.  Another advantage is that this is a renewable energy.  The power plants will never run out of fuel because water is their fuel.  Water is a natural source and the reservoir is always renewed with rain so we never run out.

One disadvantage of having a hydroelectric power plant is that it may the streams water quality for living organisms.  After the water has been through the turbine and such, the water may have been contaminated with some kind of metal or chemical the plants used.  Another disadvantage is that it takes a large amount of  land away from the wildlife.  The land that is used for Hydroelectric power plants could of been a home for a school of fish or even a drinking place for several different animals.

I think hydroelectric power plants are very useful because we need to stop burning fuel that creates pollution.
Power plants that use coal or gas has to dig into the earth to get their energy and create a lot of pollution. They don't have a renewable resource so one day they won't be able to do anything.  The resource that the power plants use like coal  or gas using very valuable resource that can be used for smaller machines.  Also building a hydroelectric power plant will be cheaper.  The largest nuclear power plant the Palo Verde cost $5.9 billion to construct.  But the largest dam which is the Grande Coulee only cost $169 million.  You can build three hydroelectric power plants with $5.9 billion.  Also hydroelectric power plants aren't as dangerous as any other power plants, especially the nuclear power plants.  On March 11, 2011, a 8.9 magnitude earthquake struck Japan and caused a tsunami that was incredible.  It destroyed  three reactors that released radiation through out the area.  This radiation was bad for the citizens in Japan because it caused cancer and death.  There are still cancer developing inside the citizens of Japan and it might even develop few year after.  About 15,000 people died from the this and some of them even died instantly.  I would prefer a power plant that isn't built near a city and doesn't use fuels or chemicals that can be harmful.

http://www.statesman.com/opinion/insight/future-cancers-from-japan-nuclear-disaster-might-be-2010697.html