Estuaries are semienclosed indentations along the coast where salt water from the sea mixes with fresh water from the land. At any given place in an estuary, the amount of mixing depends upon the amount of runoff from the land, the tidal range, and the wind.
Salinity varies from that of normal sea-water (35 parts per thousand) at the mouth to completely fresh, well upstream. The intermediate brackish water, mixed fresh and salt, occupies a large segment of many estuaries. During times of heavy rainfall the freshwater extends farther downstream, and during times of low rainfall saltwater moves farther upstream. Tidal action is also an important fac
tor in determining how far the salt water will go up the estuary. As a result of these factors, oceanographers classify estuaries into several types, based on the amount of mixing.
In the Hudson River estuary, a 5-foot tide occurs in salt, brackish, and fresh water for 210 km, until a dam upstream blocks it. The Native Americans used to call the Hudson the river that flows two ways: up and down, back and forth. Often estuaries are stratified as a result of the density difference, although mixing takes place during storms. During cold winters the upper freshwater layer can freeze over, and ice can be found at times at the mouth of the estuary. Net flow is to the sea, although floating organisms and other objects remain in the estuary for long periods, carried back and forth by the tide. Some estuaries are very long—such as Chesapeake Bay, where the tide takes a long time to reach the interior. As a result there are two high tides along its length.
Estuaries are young features on the earth, formed during the recent rise in sea level, a rise that began when glaciers started their retreat at the end of the Ice Age, some 15,000 years ago. Estuaries can also form as a result of tectonic activity causing blocks of the earth's crust to drop downward; glacial erosion's converting valleys entering the sea into fjords by deepening them; and the development of barrier islands. As sea level rises, estuaries within low-lying areas increase their life span as they widen laterally and extend upstream. Often this rise also enhances erosion, which destroys the low cliffs along the edge of the estuary. In some parts of Chesapeake Bay, for example, recent sea level rise has eroded the shoreline up to 3 m per year.
Brackish water charged with nutrients from freshwater inflow provides a rich basis for life, making opportunities for commercial and recreational fishing for finfish, molluscs, and crustaceans. In addition, many species inhabit the estuary for part of their life cycles. However, some large cities, such as New York and San Francisco, have been built up adjacent to estuaries, where pollution, diking, and filling along the estuaries' edges have caused a marked decrease in the abundance of many species; recent pollution controls in estuaries, however, have seen the return of a number of them, such as Teredo, the shipworm, in the Hudson estuary. This has had the negative consequence of destroying unprotected submerged wooden structures.
Where rivers discharge abundant sediment, estuaries may fill completely or partially, because the coastal processes, tides, long-shore currents, and wave action cannot remove the material fast enough, resulting in the growth of a delta at the mouth of the river.
Fjords are former river valleys that have been deepened by valley glaciers to below sea level, which, upon its rising, floods the trough. Fjords may be extremely long, extending hundreds of kilometers inland, as well as deep, in many instances exceeding 800 m; but generally they are very shallow, 10 to 20 meters, at their entrance. As a result, the flow of water in many fjords occurs at the top of the water column, while the rest of the water in the deep parts of the fiord has restricted circulation. As a result they are susceptible to pollution becoming trapped within the fjord.
See also: Freshwater; Oceans; Rivers and Streams
Bertnmess, M. D. 1992. "The Ecology of a New England Salt Marsh." American Scientist 80: 260-268;
Dyer, Keith. 1998. Estuaries: A Physical Introduction.
New York: John Wiley and Sons; Ketchum, Bostwick
H. 1983. Estuaries and Enclosed Seas. New York: Elsevier Science; Perillo, G. M. E. 1996. Geomorphology and Sedimentology of Estuaries. New York: Elsevier Science.
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