An estuary is a transitional aquatic environment between a river and the sea. An estuary is influenced by the tides and has strong environmental gradients, from fresh waters near its head, brackish waters, and marine waters near its mouth. Intertidal zones are generally made up of ditches (mud) or oyster beds and other areas covered with marshes and swamps that are great areas for the development of aquatic species. Estuaries are areas of extraordinary productivity and biological diversity. The coasts of the United States contain approximately 15150 km² of estuarine waters. The Chesapeake Bay, San Francisco Bay and Puget Strait are all estuaries.
From the point of view of ecology and oceanography, an estuary is a semi-closed region of the ocean influenced by the discharge of fresh water from land, whether it is one or more rivers, or just the mainland drainage.
The word estuary is often used in contradistinction to delta, where the river mixes with the sea through various channels or arms of the delta. However, a delta can also be considered an estuarine region. On the other hand, an "inland sea" such as the Baltic Sea may have the characteristics of an estuary to its fullest extent.
There are several forms of estuaries, determined not only by the geomorphology of the coast, but also by the characteristics of the river(s) and the oceanic water bodies found there. One of these forms is the river.
A very important aspect is that, due to the nutrients that land waters carry, an estuary is generally a region with high biological productivity. However, as it is a semi-closed region, it suffers particularly from the effects of pollution and can become a biological desert.
Estuaries are classified into four types, depending on their origins:
- Outlet of drowned rivers: they are common all over the world, particularly along the Atlantic coast of the United States. It is important to remember that the sea level has risen approximately 125 meters since the end of the last longest glacial period, approximately 18,000 years ago, which resulted in the incursion of sea water into the mouths of the rivers. Examples of estuaries are York, James and Susquehanna and Chesapeake Bay are examples of this type of estuary.
- Fjords: they are steep, suffer glacial erosion and have a U-shaped channel. They are 300 to 400 meters deep, but typically end in a fringe or shallow sill formed by terminal glacial deposits. In fjords with shallow sills there is a small vertical mixing below the sill depth, and the bottom waters can become stagnant. In fjords with deeper sills, the bottom waters mix. There are fjords in Norway, Greenland New Zealand, Alaska and western Canada, but they are not common in the lower 48 states.
- Barred estuaries: formed when an island or barrier spur is built parallel to the coast, above sea level. As these estuaries are shallow and usually have only a small inlet that connects them to the ocean, tidal action is limited. The waters in barred estuaries are mostly mixed by the wind. Albemarle and the Pamlico Sounds in North Carolina and Chinconteage Bay in Maryland are barred estuaries.
- Tectonic: these are coastal indentations formed by local imperfection and subsidence. Both fresh and sea water flow into the depression and form an estuary. San Francisco Bay is, in part, a tectonic estuary.
Three factors determine the characteristics of the estuaries: the shape, the volume of the river flow at the head of the estuary and the reach of the tides at the mouth of the estuary. The mixture of water at different densities, the rise and fall of the tide, and variations in the river's flow - in addition to the action of wind and ge