Hydroelectricity is a type of hydropower that releases energy and in result of the water pouring due to gravity it creates electricity. Specifically, the energy of the flowing water gets changed to electrical energy by a type of generator. These generators are usually located where water falls from a hieght. This is a renewable energy resource since the water flowing from the rivers (which ends up falling over the hydropower) is made from precipitation.
Water has been used as a supply of power ever since the age of the Roman Empire. Hydroelectricity was originally used in the 16th century. Similar to solar and wind power, hydroelectric is both natural plus reusable. Like wind power hydroelectric stations use fast flowing water which turn turbines like the wind mills use the wind to roll their turbines. Which produces the electricity.
Early hydroelectric systems didn't use a dam. There would be a big amount of energy produced by breaking part of the water flowing overhead the Niagara Falls.
The power station here was the first examples of alternating-current electric power generation for commercial stock.
A normal hydroelectricity scheme contains of a dam where the water is grounded. Then the electricity is in demand, the valves are released letting the water fall downwards to a power station where turbines turn because of the force of the water flowing. So if it is connected to generators the electricity can be created. Then the used water can return to the original river.
There are some good reasons for hydroelectricity such as their capacity to handle daily peek loads. A not so good reason would be water replenishing rates might not keep up with usage rates.