Hydro systems use the power of flowing water to drive a generator that produces electricity. Ideal hydro sites either have a high head (several metres or more) or a high flow rate (1000s litres per second) or ideally both. Hydro systems can be more expensive that other technologies to install but their higher capacity factor and long working life easily offset this.
The UK has around 20,000 disused water mills, many of which can be renovated as it is much easier and cheaper than building new water works. Even renovations of existing water mills will typically require consent from the Environment Agency in addition to the normal planning consents and building regulations.