A tide mill is a specialist type of water mill driven by tidal rise and fall.
For a tide mill to work the mill requires a dam or mill pond with a sluice gate to allow the water onto the wheel. Normally a section of a river estuary is made into the reservoir to ensure the mill has an adequate supply of water.
The way it works is; as the tide comes in it fills the mill pond through the only sluice gate. The sluice will automatically closed as the tide starts to drop. When the tide has dropped low enough the water stored in the mill pond can be released to turn the water wheel. From this stage the whole process works in the same fashion as a water mill.
One problem for tide mills can be heavy tides and large waves. To combat these natural forces tide mills are often situated in estuaries, away from the extreme sea waves but near enough to have a suitable tidal range. Mills such as these have existed since the Middle Ages and some have even been dated back to the Roman period.
There aren’t many tide mills left now but you can visit the Eling Tide mill in Southampton which is a great example of a working tide mill or visit their website.