Editing Estuary (section)

==Classification based on geomorphology==

===Drowned river valleys===
{{main|Ria}}
Drowned river valleys are also known as coastal plain estuaries. In places where the sea level is rising relative to the land, sea water progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley. This is the most common type of estuary in temperate climates. Well-studied estuaries include the [[Severn Estuary]] in the [[United Kingdom]] and the [[Ems Dollart Region|Ems Dollard]] along the Dutch-German border.

The width-to-depth ratio of these estuaries is typically large, appearing wedge-shaped (in cross-section) in the inner part and broadening and deepening seaward.  Water depths rarely exceed {{convert|30|m|ft|-2|abbr=on}}. [[Ria#Locations|Examples of this type of estuary]] in the U.S. are the [[Hudson River]], [[Chesapeake Bay]], and [[Delaware Bay]] along the [[Mid-Atlantic states|Mid-Atlantic]] coast, and [[Galveston Bay]] and [[Tampa Bay]] along the [[Gulf Coast]].<ref>{{cite journal |last1=Kunneke |first1=J. T. |first2=T. F. |last2=Palik |year=1984 |url=http://www.nwrc.usgs.gov/wdb/pub/others/85_15.pdf |title=Tampa Bay environmental atlas |journal=U.S. Fish Wildl. Serv. Biol. Rep. |volume=85 |issue=15 |pages=3 |access-date=January 12, 2010 }}</ref>

===Lagoon-type or bar-built===
{{see also|Mouth bar}}
Bar-built estuaries are found in a place where the deposition of sediment has kept pace with rising sea levels so that the estuaries are shallow and separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations.

These estuaries are semi-isolated from ocean waters by barrier beaches ([[barrier island]]s and barrier [[Spit (landform)|spits]]). Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than {{convert|4|m|ft|abbr=on}}. The barrier beaches that enclose bar-built estuaries have been developed in several ways:
* building up of offshore bars by wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline,
* reworking of sediment discharge from rivers by a wave, current, and wind action into beaches, overwash flats, and dunes,
* engulfment of mainland beach ridges (ridges developed from the erosion of coastal plain sediments around 5000 years ago) due to [[sea level rise]] and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons,
* elongation of barrier spits from the erosion of headlands due to the action of [[longshore current]]s, with the spits growing in the direction of the littoral drift. {{citation needed|date=December 2020}}

===Fjord-type===
Fjords were formed where Pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or [[Aquatic sill|sills]] of [[Moraine|glacial deposits]], which have the effects of modifying the estuarine circulation.

[[Fjord]]-type estuaries are formed in deeply eroded valleys formed by [[glacier]]s. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial [[moraine]]s or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed {{convert|300|m|ft|-2|abbr=on}}. The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted, and there is a slow but steady exchange of water between the estuary and the ocean. Fjord-type estuaries can be found along the coasts of [[Alaska]], the [[Puget Sound]] region of western [[Washington (state)|Washington state]], [[British Columbia]], eastern Canada, [[Greenland]], [[Iceland]], New Zealand, [[Fjords and channels of Chile|Chile]], and Norway.

===Tectonically produced===
These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with [[Fault (geology)|faulting]], [[volcano]]es, and [[landslide]]s. [[Inundation]] from eustatic sea-level rise during the [[Holocene]] [[Epoch (geology)|Epoch]] has also contributed to the formation of these estuaries. There are only a small number of [[Tectonics|tectonically]] produced estuaries; one example is the [[San Francisco Bay]], which was formed by the crustal movements of the [[San Andreas Fault]] system causing the inundation of the lower reaches of the [[Sacramento River|Sacramento]] and [[San Joaquin River|San Joaquin rivers]].<ref name="Kennish">{{cite book |last=Kennish |first=M. J. |year=1986 |title=Ecology of Estuaries. Volume I: Physical and Chemical Aspects |location=Boca Raton, FL |publisher=CRC Press |isbn=978-0-8493-5892-0 }}</ref>