What are hydroelectric dams?
They are structure used to change the Potential Energy of Water into Electric Energy.
[AEP’s Smith Mountain Hydro Project on the Roanoke River southeast of Roanoke, Virginia]
Have a look at the diagram:
In (A) you have a water reservoir (a lake of other water basin) where water is stored ready to be used. The water is secured by the actual dam (B) that is a kind of big wall that keeps the water from flowing down. The water in the reservoir has Potential Energy because it is at a higher level compared to, say, the valley below.
We use a pipe (C) to let the water flow down towards a structure (D) where is housed a big turbine connected to a dynamo.
The turbine (a kind of propeller) is moved by the water and acting on the dynamo produces electrical energy that, after transformation to high voltage, will be transmitted through a line (E).
The dynamo is normally called a Generator: