Tidal energy is produced by harnessing the rising and falling tides caused by the gravitational pull of the moon and sun on Earth's oceans. Dams can be built across coastal bays or estuaries with a large difference between high and low tides. Gates and turbines installed along the dam generate electricity as the rushing tidal water flows through. Tidal power stations can only produce electricity for around 10 hours per day since output changes with the daily ebb and flow of the tides. While a renewable source, tidal energy schemes can impact local habitats and populations of birds and fish by altering tidal currents.