![]() ![]() ![]() The force of the river pushing fresh water out to sea rather than tidal currents transporting seawater upstream determines the water circulation in these estuaries. Salt-wedge estuaries occur when a rapidly flowing river discharges into the ocean where tidal currents are weak. They are also called highly stratified estuaries. Salt-wedge estuaries are the most stratified, or least mixed, of all estuaries (Molles, 2002 Ross, 1995). The shape of the isohalines tells scientists about the type of water circulation in that estuary. These contour lines show the boundaries of areas of equal salinity, or isohalines, and are then plotted onto a map of the estuary. Contour lines are drawn to connect data points that have the same salinity measurements. They record these salinity measurements as individual data points. To determine isohalines, scientists measure the water's salinity at various depths in different parts of the estuary. The shape of the isohalines indicates the amount of mixing that is occurring, and may provide clues about the estuary’s geology. Isohalines are areas in the water that have equal salt concentrations, or salinities. The degree to which fresh water and saltwater mix in an estuary is measured using isohalines. For example, a heavy spring rain, or a sustained shift in local winds, can drastically affect the salinity in different parts of an estuary. These factors are different in each estuary, and often change seasonally within the same estuary. 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. 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. When fresh water and saltwater meet in an estuary, they do not always mix very readily. The daily mixing of fresh water and saltwater in estuaries leads to variable and dynamic chemical conditions, especially salinity. Barrier beaches and islands are formed by the accumulation of sand or sediments deposited by ocean waves. Bar-built estuaries are characterized by barrier beaches or islands that form parallel to the coastline and separate the estuary from the ocean. In the Hudson River in New York, for example, tidal currents carry saltwater over 200 km upstream.Ĭoastal plain estuaries, or drowned river valleys, are formed when rising sea levels flood existing river valleys. In some estuaries, the mixing of fresh water from rivers and saltwater from the sea is extensive in others it is not. Low tides, also once or twice a day, reverse these currents. Once or twice a day, high tides create saltwater currents that move seawater up into the estuary. Water movements in estuaries transport organisms, circulate nutrients and oxygen, and transport sediments and wastes. The five major types of estuaries classified according to their water circulation include salt-wedge, fjord, slightly stratified, vertically mixed, and freshwater. In addition to classifying estuaries based on their geology, scientists also classify estuaries based on their water circulation. ![]()
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