What is A Hydro Dam?

They are dams that hold back water but also generate hydro electricity by using the force of gravity pushing the water down and through large generators in order to pass through the dam. Hence why...

Very carefully :) More seriously, there are two main types of hydrolectric installations: Impoundment, and run-of-river (there's also electricity generation from the tides, waves, etc, but we'll...

So long as there is enough water ruining into them they can supply power without having to burn fuel. So the expense after the initial construction of the dam and generating equipment is down to...

You put things together

Dams store up a large volume of water in a very sharp gravitational gradient. This is used to drive the turbines and electromagnets that produce the electricity.