Reliability
Unlike solar or wind power, where the resource may suddenly disappear, hydroelectric energy can be controlled.
When we dam a river, we form a lake which we let through the dam in a controlled way. Since the water stored is vast, we can let it through whenever we want, ensuring a continuous supply of electricity.
Speed
The flow of water through the turbines of a dam can be started in a matter of moments. This is extremely useful, as there are times when demand for electricity surges, for example during the break of a popular television programme.
One application of hydroelectric energy is called pumped storage, designed precisely for this purpose. Two lakes, one higher than another are connected by pipes and generators.
When energy is needed, water is released from the top lake to the bottom one. When there is a surplus, the water is pumped back up again for use next time.
This is the kind of technology which would be required to compensate for variability in wind and solar energy if they were widely adopted.
Clean
A staple of renewable energy, hydroelectricity is not a carbon emitter. There would be carbon emissions associated with the concrete and construction of dams, however.
Free
In terms of fuel, the only thing needed is water. Assuming the natural processes running the Earth's water cycle continue, lakes will be refilled by rain and water and the dam will continue to operate.
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