a dam
A barrier constructed to hold back water and raise its level, the resulting reservoir being used in the generation of electricity
Typically, hydroelectric power station appears to be a dam holding back a reservoir of water. Inside are turbines that turn the water to steam to produce electricity.
It holds water behind it as potential energy. The difference in the pressure of the water between the reservoir and the turbine outlet make power. Some dams pump water back up behind the dam when power consumption is low.
Flowing water can be used to generate hydroelectric power. If you wish to go back farther, we can identify the ultimate source of energy as the sun, which heats water, causing it to evaporate, after which it can rain down in an elevated location, from whence gravity will pull it down to a lower location, which motion can then be harnessed for the purpose of generating power.
It's the wall that holds back the water in the reservoir. In a hydroelectric scheme it also maintains the level constant as far as possible to give a constant head at the turbines. A dam will also tend to smooth out flood surges in the river, to an extent anyway.
Dams are put in place to hold back water. This is generally used to create a water reservoir, but can also sometimes be used as a form of flood control. Hydroelectric dams, like the Hoover Dam, hold back water to create water pressure, to push turbines that generate electricity. In the case of the Hoover Dam, which holds 9.2 trillion gallons of water, extremely massive amounts of water pressure are obtained, and the 17 turbines combined make 2.8GW of power, which is about as much as 3 nuclear power plants.
A wall that holds back water is a dike, levee or dam
Hydroelectric power plants generate very little electric current for the cost of building, using, and maintaining the equipment used to produce it.
The Hoover Dam was built over five years from 1930 - 1935. Its reservoir holds 9.2 trillion gallons of water and produces 2.8GW of power, roughly as much as 3 nuclear power stations.
No, it was and is neither the largest or tallest dam in the US. However, it was and is the largest single hydroelectric power-generating dam in the US and North America. It is the largest power production plant (6800 mW) in the US, and the 7th largest in the world as of 2014.
The answer is a "dam."
levies
I'm assuming you mean hyroelectric power, like from dams? Well, the dam holds back water into an artificial lake. It then allows a controlled amount of that water to fall through tubes past a turbine. The water spins the turbine and continues out the bottom of the dam. The turbine is connected to a generator. Using magnets (I'm not sure exactly how it works) the generator creates an electric current that it passes on to be used as electricity.