Tidal power. Diagram of a tidal barrage used to generate electricity. A barrage is a large dam built across a river estuary. Channels through the barrage allow incoming tidal water to turn the blades of a turbine, generating electricity. When the tide is at its height, doors close to prevent the water returning. When the tide has fallen again, the doors are opened and the penned-back water rushes out, again generating electricity. This kind of system is relatively simple and the tides are a reliable source of energy. However, such schemes can be expensive and can impact on estuary wildlife.