1. Hydroelectric stations can act as a store of energy. Excess power from conventional or nuclear power stations can be used to pump water into a reservoir at height. When there is a shortage of power, the water can be released to drive a turbine and generate electricity.
2. Hydroelectric power can be generated at very short notice, since all that is required is for a valve to be opened. The power output can also be precisely tuned to requirements, unlike for example, wind or solar power.
The basic principle behind hydroelectric power is that water is allowed to fall through some height, acquiring kinetic energy which turns a turbine and generates electricity.
Most of the energy is produced indirectly by the Sun, which drives weather cycles to produce rain, which collects into rivers and lakes.