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The tidal freshwater
end of the Cape Fear River estuary. Across the
estuary you can see Wilmington, NC. |
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The brackish end of
the Cape Fear estuary. This picture was taken from Battery Buchanan,
near Fort Fisher State Recreation Area |
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A tugboat at the mouth of the Cape Fear River estuary
Photo by the National Ocean
and Atmospheric Administration (NOAA) |
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Estuaries are
places where fresh water mixes with salt water. They
are only partly open to the ocean. An example of an
estuary is the
mouth of a river where it empties into the sea.
Because they connect to the sea, the water in estuaries rises
and falls with the
tides.
Estuaries can be called bays, harbors, sounds, inlets,
bayous, fjords, and river deltas. But not all bodies of water
with these names are true estuaries. AN estuary must be a mix
of freshwater and saltwater. In freshwater, the amount of
salt, or salinity, is
close to zero. The salinity of
ocean water is about 35 parts per thousand (ppt). In
estuaries, salinity can be from 0.5 to 35 parts per thousand.
How salty a part of the estuary is depends on how
close it is to a river or to the ocean. Water
in the parts of the estuary close to the ocean can be almost as
salty as ocean water. Water in the parts of the estuary
closest to a river can be almost as fresh as river water. Salinity can also
change from one day to the next, because of changes in the tides
or the
weather.
The shape of the coastline and ocean floor, water
depth, and winds determine how tides move water in an estuary.
Tides may also be affected by any land, like a barrier island,
that blocks the flow of water into and out of the estuary.
Click on the bird at left to continue...
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