Head of tide
Though this point may vary due to storms and seasonal or annual differences in water flows, there is generally an average point which is accepted as the head of tide (in Great Britain this is the Normal Tidal Limit, typically noted on Ordnance Survey maps as 'NTL'). A river's tidal data are recorded at various locations downstream of this point. A river's head of tide may be considered the upper boundary of its estuary.
The head of tide is important in surveying, navigation, and fisheries management, and thus many jurisdictions establish a legal head of tide.
The head of tide may be many miles upstream from the river's mouth. For example, on the Hudson River, it is located 140 miles (225 km) upstream, near Albany, New York. On the Saint Lawrence River, tides affect shipping upstream past Quebec City, which is located several hundred miles inland from the Gulf of Saint Lawrence and the Atlantic Ocean.
- "tidal limit". Australian Water Information Dictionary. Commonwealth of Australia Bureau of Meteorology. 2015. Retrieved 22 February 2015.