Head of tide

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Head of tide

Head of tide, also called the tidal limit or tidehead, is the furthest point upstream where a river is affected by the tides. Upstream of this point, the water is not influenced by the tide. The part of the river that is affected by tides but is not salty is called a tidal river, while downstream the water becomes brackish and forms an estuary.

This position can move because of storms, spring tides, and changes in water flow throughout the year. In Britain, there is an average point often used, known as the Normal Tidal Limit (NTL), which is shown on maps.

Why it matters: Knowing the head of tide helps with surveying, navigation, and fisheries management. Some places set a legal head of tide for regulatory purposes.

Examples: The Hudson River in the United States has a head of tide about 140 miles (225 km) upstream near Albany. On the Saint Lawrence River, tides reach up to Lake St. Pierre.

See also: Tidal bore, Tidewater region, Tideway, Mean high water.

Note: Cromwell Weir marks the tidal limit of the River Trent.