Geological History and Sediments
Origin of the Estuary
20,000 years ago to last ice age
end of Pleistocene
cooler climate
sea level = 325 feet lower than today
Mammoths roamed
Receding Atlantic
water locked up in ice
temperatures 20 o F degrees colder on average
evaporation still occurred in northern latitudes
Glacial sheet grows
Arctic Circle - Canadian Shield - New York - Pennsylvania
presently just before the Bay
Mother river of the Chesapeake
Tongue of glacier feeds the headwaters of the Susquehanna River
carved a deep valley through PA., MD., VA, 100 miles across continental shelf
18,000 years ago - ice sheet began to melt
warmer climate
Susquehanna raged
power surpassed the Mississippi of today
To the Atlantic Ocean
Atlantic overflows due to Susquehanna
Atlantic marched across continental shelf
50 feet per year
brackish sea
10,000 years ago
brackish water reaches Virginia Beach, Ocean City and current mouth of the Bay
floods valley
7,500 years ago Bay front reaches mouth of Potomac - Md - VA line
another 2,500 years Bay passes Annapolis and current site of Bay Bridge.
3,000 years ago - Bay head reaches its present location
rising sea stops
equilibrium reached
The Bay as we know it was born.
Life and Death of the Estuary
Other coastal Bays share similar history
East Coast topography changes
inland lake disappears
now Long Island Sound
Ancient Susquehanna loses 1/3 of length to encroaching seas
termed "drowned river valleys"
Three more ways to form a estuary
2. Coastal Embayment or lagoon
Pamlico Sound
Outer Banks
dune covered sandbars
lagoon brackish
riptides
others: OC, MD., Assateague, Florida and Gulf Coasts
3. Fjords
High latitudes
northern hemisphere: Newfoundland, Alaska, Norway
southern hemisphere: Chile, New Zealand
Glaciers carved coastal valleys
steep rock walls
deep waters prevent growth
aquatic rooted plants
no marshes are present
oxygen depletion
poor circulation
4. Tectonic estuary
Mirror image of the other three
land subsides below seas level
earthquakes
plate shifting
eg. San Francisco Bay
Common characteristics
Fresh and salt waters mix
Share ephemeral nature - estuaries are short-lived
30 foot drop in sea level
another ice age
climate altered by man
At present - Holocene Interglacial
a temporary time-out
usually brief-lasting 10,000 years
presently lasting nearly 20,000 years
Ice age lasts 100,000 years
Erosion and Sedimentation
Natural pattern
Carry silt from Appalachian and Piedmont provinces into the coastal plain.
Rivers lose speed and drop sediment.
" deposition exceeds erosion "
net accumulation of silt
Sediments
Mud settles on the bottom and at the mouths of major tributaries.
Sandbars and mud flats seen at river junctions.
Deltaslike alluvial exposed at low tides.
Marsh grasses
begin succession
Joppatowne, MD. once on the Bay
today 2 miles from the Bay
deforestation and agricultural expansion in last 350 years
Sediments continued
Suspended sediments = fine-grained silts and light clays (remain suspended)
Tidal currents resuspend sediments
sediment traps - Baltimore Harbor
Channel mud becomes coarser as you go south
result from shore erosion of Bay proper
Salinity Zones
ECOSYSTEM ZONE VENICE SYSTEM SALINITY
Riverine Nontidal Fresh 0 ppt
freshwater
Estuarine
Tidal limit Tidal freshwater Fresh 0 - 0.5 ppt
Upper Bay Low brackishand
upper tidal rivers (slightly brackish) Oligohaline 0.5 - 5 ppt
Mid-Bay Brackish Mesohaline 5 - 18 ppt
and lower tidal rivers (moderately brackish)
Lower Bay High brackish Polyhaline 18 - 30 ppt (highly brackish)
Marine Marine Euhaline above 30 ppt
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