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Geological History and Sediments

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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


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


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



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


Riverine Nontidal Fresh 0 ppt



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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