A tide mill is a water mill driven by tidal rise and fall. A dam with a sluice is created across a suitable tidal inlet, or a section of river estuary is made into a reservoir. As the tide comes in, it enters the mill pond through a one way gate, and this gate closes automatically when the tide begins to fall. When the tide is low enough, the stored water can be released to turn a water wheel.
This tide mill is situated in Sado river estuary, close to Setúbal, away from the effects of waves but close enough for the sea to have a reasonable tidal range. These mills have existed in Portugal since the Middle Ages, and some may go back to the Roman period.