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Classifying Estuaries by Water Circulation
Estuaries can be classified according to their water circulation as salt-wedge, fjord, slightly stratified, vertically mixed and freshwater estuaries.
Water circulation is important because it transports animals and plants, mixes nutrients, oxygen and sediments, and removes waste. Water carried by waves and the moving tides shape life in an estuary.
Estuaries are often classified based on their water circulation pattern.
- Salt-wedge – An estuary with a wedge-shaped layer of saltwater which lies below a fresh water layer. A salt-wedge occurs when a rapidly flowing river discharges into an estuary with weak ocean currents. A sharp boundary is created between the salt and freshwater layers. Fresh water floats on top and a wedge of saltwater lies on the bottom because the saltwater has a higher density than the fresh.
- Fjord – An estuary with restricted flow due to a series of sills and channels carved by glaciers. Glaciers carved deep channels and then melted and left shallow barriers, or sills, at the estuary floor near the ocean. These sills restrict estuary water from circulating between the estuary and the open ocean. Only the lighter freshwater near the surface flows over the sill and out toward the ocean.
- Slightly Stratified – An estuary where saltwater and freshwater mix at some, but not all depths. Lower layers of water near the estuary bottom typically remain saltier than the upper layers near the water surface. Higher salt content (salinity) is greatest at the mouth of an estuary near the ocean.
- Vertically Mixed – An estuary where the saltwater and freshwater mix at all depths. This occurs when a river’s fresh water flow is low and the ocean’s tidal currents are moderate to strong. The water salinity in a vertically mixed estuary is the same from the top of the water to the bottom of the estuary.
- Freshwater - Freshwater estuaries are semi-enclosed areas of the Great Lakes, where the lake waters become mixed with inflowing rivers or streams.
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Last Updated on: 05-26-2011
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