Notes

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. The power extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. A large pipe (the "penstock") delivers water from the reservoir to the turbine.

Once a dam is constructed, electricity can be produced at a constant rate.
If electricity is not needed, the sluice gates can be shut, stopping electricity generation. The water can be saved for use another time when electricity demand is high.
Dams are designed to last many decades and so can contribute to the generation of electricity for many years / decades.
The lake that forms behind the dam can be used for water sports and leisure / pleasure activities. Often large dams become tourist attractions in their own right.
The lake's water can be used for irrigation purposes.
The build up of water in the lake means that energy can be stored until needed, when the water is released to produce electricity.
When in use, electricity produced by dam systems do not produce green house gases. They do not pollute the atmosphere.