The Chesapeake Bay as an Estuary: An introduction. Istvan A. Urcuyo Gettysburg College
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The Chesapeake Bay as an Estuary:
An introduction.
Istvan A. Urcuyo
Gettysburg College
The Chesapeake Bay-Native Americans resided on its shores since 8000 B.C.
The Chesapeake Bay
-Original Native American group around the
CB were the Powhatans. They left no
written records and spoke Algonquian.
-Natives called it “Chesepiooc”
[Great Water].
-Native population when first Europeans
settlers: 13000-14000
-1st European to enter CB region: Spanish
explorer Lucas Vásquez de Ayllón (1524).
Source: http://www.mariner.org/chesapeakebay/native/nam002.html & http://www.baydreaming.com/history.htm
The Towne of Pomeiooc
The Chesapeake Bay
-In 1605, the French started a colony at Port Royal (Annapolis).
-In 1607, Jamestown settlement was established on the James River.
-In 1608, Captain John Smith was the first European to thoroughly
explore and map the Chesapeake Bay.
-In 1634, Lord Baltimore established the first English colony in
Maryland, known as St. Mary's City.
Source: http://www.mariner.org/chesapeakebay/native/nam002.html & http://www.baydreaming.com/history.htm
The Chesapeake Bay
-Formed approximately 10,000 years ago during the end of the
last ice age . It is a “Drowned river valley”.
-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!
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.
-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.
The Chesapeake Watershed
Image: EPA 903-R-04-003
-Pennsylvania makes up over 1/3 of the entire watershed, more
than any other state.
-52% of Pennsylvania is found within the Chesapeake watershed.
-The Susquehanna River alone drains 92% of the watershed and
another 7% is drained by the Potomac River.
-The Susquehanna River has an average flow of 19 million gallons
of water per minute and it supplies 50% of the freshwater entering
the Bay.
Pennsylvania and the Chesapeake Bay
Source: The Pennsylvania Chesapeake Bay Program
-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 Chesapeake Watershed
-The Chesapeake Bay is the LARGEST ESTUARY in the country.
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.
-Important physical characteristics of estuaries: Salinity, Light
penetration, Temperature and Mixing.
-The total concentration of dissolved inorganic solids in water
is its salinity.
-Freshwater contains few salts (< 0.5 ppt) while Ocean water
averages 35 ppt.
Salinity
-Salinity is measured
by oceanographers as
parts-per-thousands
(ppt).
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 (salinity > 0.5ppt
but < 25ppt).
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.
Salinity
Salinity in the bay also varies according to the seasons due to
differences in total rainfall input. Image: EPA 903-R-04-003
Light
Light is a form of electromagnetic radiation from the Sun.
-Visible light is ONLY a portion of the total range.
-About 50% of the entering light is absorbed in the first meter of
depth and ~80% is absorbed at a depth of 10 meters (33 ft).
-Only ~1% of the total light remains in the clearest waters at a depth
of ~150 meters.
Light
The wavelengths of visible
light are not transmitted
equally.
-The long wavelengths
(reds) are absorbed rapidly
within the first 10 meters.
The shorter green-blue
wavelengths are transmitted
to greater depths.
-Open-ocean water is
often clear and deep blue.
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
substance’s 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 34F in the winter to
84F in the summer.
-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
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.
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.
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.
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.
So, which one is it?
Fjord? Partially mixed?
Well mixed? Salt-wedge?
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