Head of tide is the farthest point upstream where a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 is affected by tidal

Tide

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

 fluctuations. This applies to rivers which flow into tidal bodies such as oceans, bays and slough

River delta

A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...

s. 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

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

.

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

Hudson River

The Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York...

, it is located 140 miles (225 km) upstream, near Albany, New York

Albany, New York

Albany is the capital city of the U.S. state of New York, the seat of Albany County, and the central city of New York's Capital District. Roughly north of New York City, Albany sits on the west bank of the Hudson River, about south of its confluence with the Mohawk River...

. On the Saint Lawrence River

Saint Lawrence River

The Saint Lawrence is a large river flowing approximately from southwest to northeast in the middle latitudes of North America, connecting the Great Lakes with the Atlantic Ocean. It is the primary drainage conveyor of the Great Lakes Basin...

, tides affect shipping upstream past Quebec City

Quebec City

Quebec , also Québec, Quebec City or Québec City is the capital of the Canadian province of Quebec and is located within the Capitale-Nationale region. It is the second most populous city in Quebec after Montreal, which is about to the southwest...

, which is located several hundred miles inland from the Gulf of Saint Lawrence

Gulf of Saint Lawrence

The Gulf of Saint Lawrence , the world's largest estuary, is the outlet of North America's Great Lakes via the Saint Lawrence River into the Atlantic Ocean...

 and the Atlantic Ocean

Atlantic Ocean

The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

.