What
is an Estuary?
There have been many proposed definitions for estuaries but there is not one that is accepted by all. To give one general overall definition, scientists would have to take many characteristics into consideration. Perhaps the best definition comes from a paper by Perillo, 1995, discussing different definitions of estuaries: "An estuary is a semi-enclosed coastal body of water that extends to the effective limit of tidal influence, within which sea water entering from one or more free connections with the open sea, or any other saline coastal body of water, is significantly diluted with fresh water derived from land drainage, and can sustain euryhaline biological species from either part or the whole of their life cycle."(Perillo, 1995). Although estuaries can be categorized in two ways, one by origin and the other by circulation, perhaps the best way to categorize an estuary is by circulation. The reason for this judgment is because the origin of the estuary does not reveal the dynamics of the system. There are many factors that determine the circulation of an estuary including tidal flux, river input, bathymetry, and depth of the estuary. In systems as dynamic as estuaries more than one type of circulation can occur. This has become evident from the data collected by the MARE group in Winyah Bay, SC. The different types of estuaries are illustrated in the figures at the end of this section.
The majority of today's estuaries originated from the rise in the sea levels from the melting of the last ice age. When estuaries are categorized by origin or the mode of formation, there are the coastal plain, fjords, bar-built, and tectonic estuaries.
Coastal plain estuaries were formed by rising sea levels in already existing river basins. They are also known as drowned river valleys. The movement of the glaciers that caused deep gouging in the earth's surface formed fjord estuaries. There is typically a glacial deposit that forms a sill at the entrance of the fjord. Fjord estuaries are typically very deep with steep mountainous walls. A bar-built estuary is formed from sand being deposited along the coast that separates portions of the estuary from the ocean. An example of this type would be barrier islands. Tectonic estuaries are formed from faulting or folding that causes rivers to flow into them and then into the ocean. An example of this would be San Francisco Bay.
Estuaries are used by many different organisms (see inset drawing). Some use the estuary in some point of their life stage and others spend their entire life there. Some animals that frequent these areas are the Atlantic blue crab, white shrimp, grass shrimp, mussels, oysters, clams, fiddler crabs, and wading birds.
Estuaries are considered safe havens for many different organisms. Many species of fish use the estuary to breed and leave their young to grow and mature before moving off to the open sea. The dynamic nature of estuaries forms the basis of a very complex food chain. Estuaries also provide a way to separate juvenile fish and adult fish to reduce competition.
Different estuaries exhibit an array of plant life. These plants have stabilizing root systems that trap sediment and form mud flats and salt marshes. Some of these plants include Spartina, Glass Wort, Cat Tails, and Black Needle Rush.
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* The salt wedge estuary. The high flow rate of the river holds back the lesser flow of salt water. The slat water is drawn upward into the fast-moving river flow. Salinity is given in parts per thousand. |
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* The well mixed estuary. Strong tidal currents distribute and mix the seawater throughout the shallow estuary. The net flow is weak and seaward at all depths. |
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* The partially mixed estuary. Seawater enters below the mixed water that is flowing seaward at the surface. Seaward surface net flow is larger than river flow alone. |
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* The fjord-type estuary. River water flows seaward over the surface of the deeper seawater and gains salt slowly. The deeper layers may become stagnant due to the slow inflow rate of salt water. |
The salt wedge estuary is a type of estuary that is defined by the inflow of ocean water and river outflow and how the two interact. For this particular type of estuary the influx of ocean water pushes a body of water that is of higher salinity and more dense than that of the river outflow. Therefore, strong density stratification is established and causes a salt wedge that can be easily seen by taking salinity measurements throughout the water column.
A partially mixed estuary is a mix between the well mixed estuary and the salt wedge estuary. The salt content varies both horizontally and vertically with moderate density stratification. Vertical mixing and flow combine and form a well-developed seaward surface flow. At times when the the river outflow is high, a high-density stratification occurs near the surface. Net flows insure that there is a good exchange of water with the ocean.
The fjord type estuary is much different than the other types of estuaries. Fjord estuaries have a sill that separates a large body of water from the sea as shown above. The water in the deeper portion of the estuary is generally uniform in salinity. The surface layer is very dilute with strong changes in salt content and density at sill depth. Strong stratification at sill depth acts as a false bottom to the seaward-moving surface layer. There is a strong net seaward flow in the surface layer. Water at depth may be stagnant.
Ten Ways to Protect Esutaries
taken from the National Esturay Program
1. Learn more about estuaries.
2. Keep septic systems working properly. Pump your system every three years.
3. Pave less. Hard surfaces hasten runoff and increase pollution and erosion.
4. Adhere to "no-wake" zones when on your boat. Waves destroy shorelines and increase erosion.
5. Think before you pour. Many hazardous products flow from household drains through sewage treatment plants and into coastal bodies of water.
6. Fish respectfully. Follow "catch and release" practices and keep more fish alive.
7. Create non-toxic pesticides. A bit of soap and water does the job and keeps harmful chemicals from ending up in nearby waterways.
8. Use native plants. Garden and landscpae with plants native to your area to reduce the need for watering and fertilizing.
9. Respect habitat. Treat the homes of vital marine life with care. Habitat and survivial go hand-in-hand. When habitat disappears, so do many plants and animals.
10. Take action! Organize a stream or beach cleanup. Encourage your local newspaper to write a story, or ask an expert to speak at your community organization or local school.
*
Fundamentals of Oceanography, 1996 Duxbury, A.B, and Duxbury,
A.C., second edition
with permission from Bonnie Beacher, Director of Contracts, Copyrights
and Permissions, McGraw-Hill Education
References:
Perillo, G.M.E. 1995. Geomorphology and Sedimentology of Estuaries. Definitions and Geomorphologic Classifications of Estuaries, Development in Sedimentology 53.
Duxbury, A.B. and A.C. Duxbury. 1996. Fundamentals of Oceanography second edition. McGraw-Hill, pp. 196-197.
Brown, J., Colling, A., Park, D., Phillips, J., Rothery, D., and Wright, J. 1995. Waves, Tides and Shallow-Water Processes. Open University, Walton Hall, Milton Keynes, MK7 6AA, and Butterworth-Heinemann, Linacre House, Jordan Hill, Oxford OX2 8DP, pp. 116-126.
Ballantine, T. 1991. Tideland Treasure. University of South Carolina Press, Columbia SC, pp. 33.
For more information on estuaries try these sites: