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Head of tide,tidal limit[2] ortidehead[3] is the furthest point upstream where ariver is affected bytidal fluctuations,[4] or where the fluctuations are less than a certain amount.[5] The river section influenced by tides and marine forces, but without salinity is atidal river. Downstream areas are brackish and termedestuaries.[6]
Although this point may vary due to storms,spring tides, and seasonal or annual differences in water flows, there is generally an average point that is accepted as the head of tide (inGreat Britain this is theNormal Tidal Limit, typically noted onOrdnance Survey maps as 'NTL').[1] The head of tide is important in surveying, navigation, and fisheries management, and thus many jurisdictions establish a legal head of tide. As the head of tide is useful for navigation, separate maps can be made of the tidal zones up to the head of tide, such as was done inNew Jersey.[7]
The head of tide may be many miles upstream from the river's mouth. For example, on theHudson River, it is located 140 miles (225 km) upstream, nearAlbany, New York.[8] On theSaint Lawrence River, tides affect the river up toLake St. Pierre.[9]
