Estuaries

Classifying Estuaries - By Water Circulation

salt-wedge | fjord | slightly stratified | vertically mixed | freshwater

Salt-wedge estuaries are the most stratified, or least mixed, of all estuaries. Click the image for more details and to view a large animation.

In addition to classifying estuaries based on their geology, scientists also classify estuaries based on their water circulation. The five major types of estuaries classified according to their water circulation include salt-wedge, fjord, slightly stratified, vertically mixed, and freshwater (Levinson, 1995; USEPA, 1993).

Water movements in estuaries transport organisms, circulate nutrients and oxygen, and transport sediments and wastes. Once or twice a day, high tides create saltwater currents that move seawater up into the estuary. Low tides, also once or twice a day, reverse these currents. In some estuaries, the mixing of fresh water from rivers and saltwater from the sea is extensive; in others it is not. In the Hudson River in New York, for example, tidal currents carry saltwater over 200 km upstream.

Fjords are typically long, narrow valleys with steep sides that were created by advancing glaciers. Click the image for more details and to view a large animation.

The daily mixing of fresh water and saltwater in estuaries leads to variable and dynamic chemical conditions, especially salinity. When fresh water and saltwater meet in an estuary, they do not always mix very readily. Because fresh water flowing into the estuary is less salty and less dense than water from the ocean, it often floats on top of the heavier seawater. The amount of mixing between fresh water and seawater depends on the direction and speed of the wind, the tidal range (the difference between the average low tide and the average high tide), the estuary’s shape, and the volume and flow rate of river water entering the estuary. These factors are different in each estuary, and often change seasonally within the same estuary. For example, a heavy spring rain, or a sustained shift in local winds, can drastically affect the salinity in different parts of an estuary (Sumich, 1996).

The salinity of water in a vertically mixed estuary is the same from the water's surface to the bottom of the estuary. Click the image for more details and to view a large animation.

The degree to which fresh water and saltwater mix in an estuary is measured using isohalines. Isohalines are areas in the water that have equal salt concentrations, or salinities. The shape of the isohalines indicates the amount of mixing that is occurring, and may provide clues about the estuary’s geology (Sumich, 1996). To determine isohalines, scientists measure the water's salinity at various depths in different parts of the estuary. They record these salinity measurements as individual data points. Contour lines are drawn to connect data points that have the same salinity measurements. These contour lines show the bounderies of areas of equal salinity, or isohalines, and are then plotted onto a map of the estuary. The shape of the isohalines tells scientists about the type of water circulation in that estuary (Sumich, 1996).

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