An estuary is a semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.
Estuaries are often associated with high rates of biological productivity.
The key feature of an estuary is that it is a mixing place for seawater and fresh water to supply fresh water.
A tide is a necessary force to maintain a dynamic relationship at the meeting between the two waters.
In non-tidal seas, rivers naturally form deltas or liman.
In eastern Canada, the Saint Lawrence River widens into the Gulf of Saint Lawrence, the world's largest estuary.
Estuaries are often associated with high rates of biological productivity.
The key feature of an estuary is that it is a mixing place for seawater and fresh water to supply fresh water.
A tide is a necessary force to maintain a dynamic relationship at the meeting between the two waters.
In non-tidal seas, rivers naturally form deltas or liman.
In eastern Canada, the Saint Lawrence River widens into the Gulf of Saint Lawrence, the world's largest estuary.
- 1967, a somewhat restrictive definition of “estuary” was offered: An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater is measurably diluted with freshwater derived from land drainage.1 This definition is still used today, sometimes slightly modified, and it is close to the “river meets the sea” definition because it highlights the dilution of seawater.
Not all estuaries are salty.
Many, but not all, estuaries have salinities (i.e., concentration of salt in water) intermediate between seawater and freshwater. This dilution process may be fostered by tides. Tidal “waves” move water in and out of the estuary from the ocean and may help mix the fresh and salt waters. When the tidal energy is not strong enough, the less dense, fresh, water from rivers may ride above the denser salt water from the sea, forming what is called a salt wedge or stratification. Each estuary has its own pattern of dilution and stratification based on the intensities of river flow and tides and connection to the sea. Some estuaries, such as the Laguna Madre, even concentrate salt through excess evaporation compared to those estuaries where dilution is the pattern.
One of the broadest definitions of an estuary, which removes the interaction of fresh and seawater is: An estuarine system is a coastal indentation that has a restricted connection to the ocean and remains open at least intermittently.2 This definition still does not include coastal systems of the Great Lakes, such as Old Woman Creek, that have no neighboring ocean. These systems are sometimes called “lacustrine” or “lake” estuaries.3
Wetlands are an important area in estuaries.
To further complicate the definition, estuaries often have considerable wetlands associated with them:
- Some wetlands exist within the body of the estuary at depths below the low-tide mark. These are dominated by a variety of plant species, collectively called submerged aquatic vegetation; and many are considered sea grasses.
- Other wetlands are flooded with estuarine water—at least periodically.
- In temperate areas, most of these wetlands are marshes—although in fresh and low salinity regions, wetland forests may be found.
- In subtropical and tropical regions, mangrove trees may dominate the wetlands.
Although the definition of estuary may not be established universally, most researchers agree that estuaries are an important interface where the activities of land and large water bodies interact. Further, they are the sites where human activities interface with high levels of biological productivity and ecological activity.