There are several methods of capturing and harnessing hydraulic energy to produce electricity:

Storage power stations and storage reservoirs 

Storage power stations have a dam at a higher altitude, from which water falls at high pressure to the plain via penstocks. This pressurised water drives turbines which are connected to a generator. The system can be started or stopped on demand; when the system is halted, the water remains in the storage reservoir.

Run-of-river power stations

Run-of-river power stations work by means of a dam placed across a river or stream. They operate continuously since they cannot hold water in a storage reservoir. There are run-of-river power stations with vertical-axis turbines (rivers with a steep gradient) and with horizontal-axis turbines (for rivers with a high flow rate and a low head).

Tidal power stations

Tidal power stations make use of the ebb and flow of the tides. Sea water operates turbines as it passes through the barrage. These schemes close off broad coastal inlets and require a large tidal range.

Pumped storage power stations

Pumped storage power stations have, in principle, an upper reservoir and a lower reservoir. During off-peak (low demand) periods, water from the lower level is pumped up to the upper reservoir, so that it can drive the turbines during periods of high demand.