A percentage of the water is taken from the river or stream, through a carefully designed weir.
Water is diverted from the watercourse to the powerhouse either over, or undergound through the penstock; large bore piping.
The head is the distance that a given water source has to fall before the point where power is generated. In short, for a given water flow, a larger head will be converted into greater energy.
The pressurised water is excites a turbine which drives the alternator. The power is then synchronised to the grid via a computerised control box.
The powerhouse is connected to the national grid, unless the system is to be used off grid.
Once the energy has been harvested from the water, it is returned to the watercourse.