Hydroelectricity is generally considered to be a reliable method of generating electricity. Operators of hydroelectricity facilities which have been designed with flexibility in mind can exert a degree of control over their electrical output by choosing how much water to allow through the intakes. This means that as long as there is plenty of water in the reservoir, operators can plan in advance how much electricity the dam will generate at any given moment.
An extended drought could affect the reliability of a hydroelectric dam by reducing the amount of water in the reservoir. Droughts and floods can also affect the reliability of smaller, run-of-river installations, as their electrical output depends on the available volume of water and the rate at which it flows past the turbine.
These are the reliability challenges facing hydro. Read about the possible solutions.