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Head of tide is the farthest point upstream where a river is affected by tidal fluctuations. This applies to rivers which flow into tidal bodies such as oceans, bays and sloughs. 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. A river's tidal data is 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 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.