{{Short description|Furthest point upstream where a river is affected by tidal fluctuations}} [[File:Cromwell Weir - Nottinghamshire (Geograph 2561867 by Jonathan Thacker).jpg|thumb|{{center|Cromwell Weir marks the tidal limit of the River Trent, a major river in England<ref name=OSmap121/>}}]]
'''Head of tide''', '''tidal limit'''<ref>{{cite book|url=http://www.bom.gov.au/water/awid/id-535.shtml|title=Australian Water Information Dictionary|chapter=tidal limit|publisher=Commonwealth of Australia Bureau of Meteorology|date=2015|access-date=22 February 2015}}</ref> or '''tidehead'''<ref>{{cite book|chapter=Tidehead.|title=Merriam-Webster.com Dictionary|publisher=Merriam-Webster|url=https://www.merriam-webster.com/dictionary/tidehead|access-date=19 September 2020}}</ref> is the furthest point upstream where a river is affected by tidal fluctuations,<ref name=Davis2011>{{cite book|author1=Richard A. Davis (Jr.)|author2=Richard A. Davis, Jr.|author3=Robert W. Dalrymple| title=Principles of Tidal Sedimentology|url=https://books.google.com/books?id=hyuIYTTjFL4C&pg=PA82|date=20 October 2011|publisher=Springer Science & Business Media|isbn=978-94-007-0123-6|pages=82}}</ref> or where the fluctuations are less than a certain amount.<ref name=ConnerDoyle2007>{{cite book|author1=William H. Conner|author2=Thomas W. Doyle|author3=Ken W. Krauss|title=Ecology of Tidal Freshwater Forested Wetlands of the Southeastern United States|url=https://books.google.com/books?id=lYgLLGaSAjYC&pg=PA36|date=24 June 2007|publisher=Springer Science & Business Media|isbn=978-1-4020-5095-4|pages=36}}</ref> The river section influenced by tides and marine forces, but without salinity is a tidal river. Downstream areas are brackish and termed estuaries.<ref name="A. J. F. Hoitink, D. A. Jay">{{cite journal | title=Tidal river dynamics: Implications for deltas | date=2016 | doi=10.1002/2015RG000507 | last1=Hoitink | first1=A. J. F. | last2=Jay | first2=D. A. | journal=Reviews of Geophysics | volume=54 | issue=1 | pages=240–272 | bibcode=2016RvGeo..54..240H | s2cid=130986687 | doi-access=free }}</ref>
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 (in Great Britain this is the '''Normal Tidal Limit''', typically noted on Ordnance Survey maps as 'NTL').<ref name=OSmap121>{{Cite map |publisher=Ordnance Survey |title=121 Lincoln & Newark-on-Trent |scale=1 : 50,000 |year=2004 |series=OS Landranger Map Series| isbn=9780319227213 }}</ref> 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 in New Jersey.<ref>[http://www.state.nj.us/dep/gis/mapcrit.html New Jersey] {{webarchive|url=https://web.archive.org/web/20130213162227/http://www.state.nj.us/dep/gis/mapcrit.html |date=2013-02-13 }}</ref>
The head of tide may be many miles upstream from the river's mouth. For example, on the Hudson River, it is located {{convert|140|mi|km|0}} upstream, near Albany, New York.<ref name="caryinstitute">{{cite web | url=https://www.caryinstitute.org/sites/default/files/public/downloads/curriculum-project/tides.pdf |title=Tides in the Hudson River}}</ref> On the Saint Lawrence River, tides affect the river up to Lake St. Pierre.<ref name="baleinesendirect">{{cite web | url=https://baleinesendirect.org/en/discover/the-ecosystems-of-the-whales/stlawrence/ | title=The St. Lawrence }}</ref>
==See also== * Tidal bore * Tidewater (region) * The Tideway * Mean high water
==References== {{Reflist}}
{{physical oceanography}}
Category:Tides Category:Estuaries