Question

How electricity produced in hydro power plants?

Answer 1

Water from rivers on high mountains falls into valley below (like 100 meters).  A large rotor with huge fan wings is placed below the stream of water.  Then the gravitational potential energy of water is converted into kinetic energy and that rotates the blades of the fan.

The rotation of rotor causes an electrical conductor (armature windings) inside the generator in the presence of a magnetic field around it.  That generates current in the windings .  

This is based on the Faraday's law of Induction.