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Estuarine Oceanography AccessScience from McGraw-Hill Education Page 1 of 12 www.accessscience.com Estuarine oceanography Contributed by: Daniel G. MacDonald, K. R. Dyer Publication year: 2019 The study of the physical, chemical, biological, and geological characteristics of estuaries. An estuary is a semienclosed coastal body of water which has a free connection with the sea and within which the seawater is measurably diluted by freshwater derived from land drainage. Many characteristic features of estuaries extend into the coastal areas beyond their mouths; and because the techniques of measurement and analysis are similar, the field of estuarine oceanography is often considered to include the study of some coastal waters which, by the above definition, are not strictly estuaries. Also, semienclosed bays and lagoons exist in which evaporation is equal to or exceeds freshwater inflow, so that the salt content either is equal to that of the sea or exceeds it. Hypersaline lagoons have been termed negative estuaries, whereas those with precipitation and river inflow equaling evaporation have been called neutral estuaries. Positive estuaries, in which river inflow and precipitation exceed evaporation, form the majority. Topographic classification Embayments are the result of fairly recent changes in sea level. During the Pleistocene ice age, much of the seawater was locked up in continental ice sheets, and the sea surface stood about 100 m (330 ft) below its present level. In areas not covered with ice, the rivers incised their valleys to this base level. During the ensuing Flandrian Transgression, when the sea level rose at about 1 m (3.3 ft) per century, these valleys became inundated. Much of the variation in form of the resulting estuaries depends on the volumes of sediment that the river or the nearby coastal erosion has contributed to fill the valleys. Where river flow and sediment discharge were high, the valleys have become completely filled and even built out into deltas. Generally, deltas are best developed in areas where the tidal range is small and where the currents cannot easily redistribute the sediment the rivers introduce. They occur mainly in tropical and subtropical areas where river discharge is seasonally very high. The distributaries, or passes, of the delta are generally shallow, and often the shallowest part is a sediment bar at the mouths of the distributaries. The Mississippi and the Niger are examples of this type of delta. Where sediment discharge was less, the estuaries are unfilled, although possibly they are still being filled. These are drowned river valleys or coastal plain estuaries, and they still retain the topographic features of river valleys, having a branching, dendritic, though meandering, outline and a triangular cross section, and widening regularly toward the mouth, which is often restricted by spits. River discharge tends to be reasonably steady throughout the year, and sediment discharge is generally small. These estuaries occur in areas of high tidal range, where the AccessScience from McGraw-Hill Education Page 2 of 12 www.accessscience.com currents have helped to keep the estuaries clear of sediment. They are typical of temperate regions such as the east coast of North America and northwestern Europe, examples being the Chesapeake Bay system, the Thames, and the Gironde. See also: COASTAL LANDFORMS. In areas where glaciation was active, the river valleys were overdeepened by glaciers and fiords were created. A characteristic of these estuaries is the rock bar or sill at the mouth that can be as little as a few tens of meters deep. Inside the mouth, however, the estuaries can be at least 600 m (1800 ft) deep and can extend hundreds of kilometers inland. Fiords are typical of Norway and the Canadian Pacific coast. See also: FIORD. Another estuarine type is called the bar-built estuary. These are formed on low coastlines where extensive lagoons have narrow connecting passages or inlets to the sea. Within the shallow lagoons, the tidal currents are small, but the deep inlets have higher currents. Again, a sediment bar is generally present across the entrance. In tropical areas, the lagoons can be hypersaline during the hot season. They are typical of the southern United States and of parts of Australia. Estuaries are ephemeral features since great alterations can be wrought by small changes in sea level. If the present ice caps were to melt, the sea level would rise an estimated 30 m (99 ft), and the effect on the form and distribution of estuaries would be drastic. Physical structure and circulation Within estuaries, the river discharge interacts with the seawater, and river water and seawater are mixed by the action of tidal motion, by wind stress on the surface, and by the river discharge forcing its way toward the sea. The difference in salinity between river water and seawater—about 35 parts per thousand—creates a difference in density of about 2%. Even though this difference is small, it is sufficient to cause horizontal pressure gradients within the water which affect the way it flows. Density differences caused by temperature variations are comparatively smaller. Salinity is consequently a good indicator of estuarine mixing and the patterns of water circulation. Obviously, there are likely to be differences in the circulation within estuaries of the same topographic type which are caused by differences in river discharge and tidal range. The action of wind on the water surface is an important mixing mechanism in shallow estuaries, particularly in lagoons; but generally its effect on estuarine circulation is only temporary, although it can produce considerable variability and thus make interpretation of field observations difficult. See also: SEAWATER. Salt-wedge estuaries. Freshwater, being less dense than seawater, tends to flow outward over the surface of seawater, which penetrates as a salt wedge along the bottom into the estuary (Fig. 1). This creates a vertical salinity stratification, with a narrow zone of sharp salinity change, called a halocline, between the two water masses, which can reach 30 parts per thousand in 0.5 m (1.5 ft). If the sea is tideless, the water in the salt wedge is almost motionless. However, if the surface layer flowing toward the sea has a sufficiently high velocity, turbulent mixing can occur through a mechanism known as Kelvin-Helmholtz instability, which is a process where the AccessScience from McGraw-Hill Education Page 3 of 12 www.accessscience.com WIDTH:BFig. 1 Diagrams of mixing in estuaries. (a) Salt-wedge type. (b) Partially mixed type. (c) Well-mixed type. (d) Fiord. denser salt water is drawn up into the overflowing freshwater in a coherent “rolled-up” pattern. Ultimately these instabilities break down completely, and the salt water is mixed entirely into the overlying fresh-water mass, increasing the salinity of the upper layer, before eventually being discharged to the ocean. This and similar processes are sometimes referred to as entrainment, which results in a net loss of fluid from the salt wedge. Consequently, for this loss to be replaced, there must be a compensatory flow of salt water toward the head, or landward portion, of the estuary within the salt wedge, but of a magnitude much less than that of the flow in the surface layer. There is a considerable velocity gradient near the halocline as a result of the friction between the two layers. Consequently, the position of the salt wedge will change according to the magnitude of the flow in the surface layer, that is, according to the river discharge. The Mississippi River is an example of a salt-wedge estuary. When the flow in the Mississippi is low, the salt wedge extends more than 160 km (100 mi) inland, but with high discharge the salt wedge extends only a mile or so above the river mouth. Some bar-built estuaries, in areas of restricted tidal range and at times of high river discharge, as well as deltas, are typical salt-wedge types. AccessScience from McGraw-Hill Education Page 4 of 12 www.accessscience.com Partially mixed estuaries. When tidal movements are appreciable, the whole mass of water in the estuary moves up and down with a tidal periodicity of about 12.5 h. Considerable friction occurs between the bed of the estuary and the tidal currents, and causes turbulence. The turbulence tends to mix the water column more thoroughly than occurs in salt-wedge estuaries, although little is known of the relationship of the exchanges to the salinity and velocity gradients. However, the turbulent mixing not only mixes the salt water into the fresher surface layer but also mixes the fresher water downward. This causes the salinity to decrease toward the head of the estuary in the lower layer and also to increase progressively toward the sea in the surface layer. As a consequence, the vertical salinity gradient is considerably less than that in salt-wedge estuaries. In the surface, seaward-flowing layer, the river discharge moves toward the sea; but because the salinity of the water has been increased by mixing during its passage down the estuary, the discharge at the mouth can be several times the river discharge. To provide this volume of additional water, the compensating inflow must also be much higher than that in the salt-wedge estuary. The velocities involved in these movements are only on the order of a few centimeters per second, but the tidal velocities can be on the order of 100 centimeters per second. Consequently, the only way to evaluate the effect of turbulent mixing on the circulation pattern is to average out the effect of the tidal oscillation, which requires considerable precision and care. The resulting residual or mean flow will be related to the river discharge, although the tidal response of the estuary can give additional contributions to the mean flow.
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