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Estuaries - Classification by Water Mixing

Water Mix Estuary
(Click for larger view). The basic flow pattern in an estuary is a surface flow of less dense freshwater toward the ocean and an opposite flow in the subsurface of salty seawater into the estuary. Numbers represent salinity in 0/00; arrows indicate flow directions. (a) Vertically mixed estuary. (b) Slightly stratified estuary. (c) Highly stratified estuary. (d) Salt wedge estuary.

In all estuaries, less dense freshwater from rivers flow over higher-density seawater and the water masses will mix at their boundaries. The shape of the estuary, the volume of the river flow and the range of tides at the estuary mouth create unique conditions for water mixing and circulation. The classification of estuaries is discussed in detail in your text; here we provide a brief overview. The links provided on this page are for your own interest, they are not required readings.

Salt wedge estuaries form at the mouth of high volume rivers where freshwater moves as a thick layer over a “wedge” of salt water. A steep halocline and very little mixing between the fresh and salt water characterize these estuaries. A narrow estuary, low tidal ranges, and limited wave activity also promote salt-wedge conditions. A good example of a salt-wedge estuary is the mouth of the Columbia River at the Washington-Oregon border. Here, the high-volume Columbia River flows out and over a small wedge of salty Pacific Ocean water.

Slightly stratified estuaries form where tidal activity is strong and river volume is moderate. In these estuaries the halocline is still present, but less pronounced than in salt wedge estuaries. Seawater moves landward along the bottom of partially mixed estuaries and is diluted progressively landward with freshwater moving out towards the estuary mouth. Daily tidal currents in and out of the estuary enhance mixing of the two layers. As seawater moves landward and river water moves seaward they are influenced by the Coriolis Effect. For example, in the Chesapeake Bay, salty ocean water tends to follow the eastern side of the bay floor and freshwater tends to follow the western side of the bay surface.

Vertically mixed estuaries are characterized by very strong mixing between the upper freshwater layer moving toward the sea and lower seawater layer moving toward the estuary head. Also called well-mixed estuaries, these bodies of water are usually wide, shallow, and have limited freshwater inputs and strong tides. In these estuaries no halocline exists as water mixes completely and salinity increases from the head to the mouth of the estuary. Because of the Coriolis Effect, in the Northern Hemisphere, higher salinity water is found on the right side of the estuary and lower salinity water is found on the left side. The lower reaches of the San Francisco Bay are vertically-mixed.

Fjord estuaries are deep, narrow estuaries created in glaciated valleys. (Note: Your text calls these highly stratified estuaries) These estuaries have a well-developed halocline separating deep saline water from surface shallow water. Circulation in fjord estuaries is often limited because of the presences of a sediment ledge (called a sill) at the mouth of the estuary. Generally, circulation only exists in the upper layers of the water above the level of the sill. Often the bottom waters become anoxic (oxygen-deprived) due to the lack of circulation at depth.

Reverse estuaries are a rare type of estuary found in very arid regions where precipitation is low and evaporation rates are high. In reverse estuaries there is little to no freshwater input, and flow is inverted from usual conditions. The bottom, higher density layer flows towards the sea while the surface, lower-salinity water flows toward the head of the estuary. Some examples of reverse estuaries include Laguna Madre in Texas and the Red Sea.

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