Reservoirs store water so that it can be used in times of drought. Another purpose is to store water in times of heavy rain and so prevent a river swelling with rainwater and flooding. Yet another purpose is to take water through turbines to generate electricity. Yet another is to provide a place for anglers, boaters and others.
It is hard to meet all of these needs at the same time. Suppose, for instance, there is heavy rain and the reservoir needs to hold back as much water as it can. At the same time water needs to be released so that electricity can be made.
