Chesapeake Bay I

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The Chesapeake Bay as an Estuary An
introduction. Istvan A. Urcuyo Gettysburg
College
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The Chesapeake Bay
Algonquian Indians called it Tschiswapeki
great shellfish bay. -Native
Americans resided on its shores since 8000 B.C.
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The Chesapeake Bay
-Formed approximately 10,000 years ago during the
end of the last ice age . -180 miles long and
its watershed is fed by 19 principle rivers and
over 400 lesser creeks. -The Susquehanna,
Potomac and James Rivers contribute 80 of the
freshwater input . -On average, it holds over
15 trillion gallons of water!
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The Chesapeake Bay
-It has 11,700 miles of tidal shoreline . -It is
shallow average depth of 30 ft in the main
stem. -Varies in width from 3.4 miles near
Aberdeen MD to 35 miles at its widest point near
the mouth of the Potomac. -Semidiurnal tides
with a tidal range of 3ft at the mouth of the
Bay.
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The Chesapeake Watershed
-50 of the water comes from the Atlantic Ocean,
the rest comes from the 64,000 square-mile
drainage basin (watershed) which include parts of
New York, Pennsylvania, West Virginia, Delaware,
Maryland, Virginia and the entire District of
Columbia.
Image EPA 903-R-04-003
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The Chesapeake Watershed
-Approximately 16 million people live, work or
play in the watershed. -It is estimated that
by the year 2020 there will be 18 million people
in the watershed.
-The largest estuary in the USA.
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What is an Estuary?
An estuary is a body of water that is partially
surrounded by land and where freshwater mixes
with ocean water. -Estuaries are locations of
important biological productivity, diversity and
ecological significance. -Three important
physical characteristics of estuaries Salinity,
Temperature and Mixing.
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Salinity
-The total concentration of dissolved inorganic
solids in water is its salinity.
-Freshwater contains few salts (lt 0.5
ppt) while Ocean water averages 35 ppt.
-Salinity is measured by oceanographers as
parts-per-thousands (ppt).
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Salinity
-Water salinity in the Bay ranges from 30ppt near
the mouth to freshwater at the junctions with the
rivers. -Salinity increases from the surface to
the bottom with deeper waters having up to 3
times the salinity. -Most of the water in CB is
brackish water (defined as having a salinity gt
0.5ppt but lt 25ppt).
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Salinity
Saltier water is found along the eastern shore
due to the influence of the Coriolis Effect and
the larger input of freshwater from the western
shore.
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Salinity
Salinity in the bay also varies according to the
seasons due to differences in total rainfall
input.
Image EPA 903-R-04-003
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Temperature
-The ability of a substance to give up or absorb
a given amount of heat and undergo a change in
temperature is a measure of the substances heat
capacity (or specific heat). -The heat capacity
of water is among the highest of all known common
substances. -Water temperatures in the Bay
fluctuate dramatically due to its shallow depth.
It normally ranges from 34?F in the winter to
84?F in the summer.
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-Thermocline region where temperature changes
rapidly with depth. -Halocline region where
salinity changes rapidly with depth
-Pycnocline region where density changes
rapidly with depth
Image EPA 903-R-04-003
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Types of Estuaries
-Salt wedge estuary this is the simplest type of
estuary and it is characterized by the presence
of a saltwater wedge occurring near the mouth of
the river.
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Types of Estuaries
-Well-mixed estuaries these are shallow
estuaries with a strong tidal mixing and a
relatively low river flow. -This
combination creates a slow seaward flow of water
and a near complete mixing between freshwater and
saltwater.
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Types of Estuaries
-Partially mixed estuaries these estuaries have
a strong seaward flow of freshwater at the
surface and a strong inflow of seawater at depth.

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Types of Estuaries
-Fjord estuary these are deep estuaries with
little tidal mixing and high river input. -At
the bottom of the fjord estuary, particularly in
fjords with steep sills, a stagnant saltwater
layer can form.
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So, which one is it?
Fjord?
Partially mixed?
Salt-wedge?
Well mixed?