Sediment Happens!
It’s cleaning it up
that’s difficult
ESI
What if we
piled up the
estimated 5
million tons of
sediment/year
carried into the
Bay and its
tidal
tributaries ?
Bay Journal
July/August
2002
Colonial ports
• Joppatown at the mouth of the
Gunpowder River (today 2 miles inland
from mouth!)
• Bladensburg on the Anacostia River
• Port Tobacco on the Potomac River
• Charles Town at Jug Bay on the
Patuxent River
What was the purpose of the ports?
Siltation
Massive sedimentation in river channels
What is the source of all the sediment?
New channel is cut as marsh develops in shallow waters
Present channel
Ancient channel
How does sediment pile up?
particles settle from
water column
Where is the
oldest sediment?
Youngest
Oldest
Does particle size make a difference?
sand
silt
(0.063 – 2 mm) (0.0039 - 0.063 mm)
clay
(<0.0039 mm)
The larger the particle,
the faster it settles.
Which particle will get
transported the
greatest distance?
Fine grain material is the cause of siltation.
Source of sediments
Increased soil erosion
by deforestation
From
CBP
Baltimore Harbor
• Very large commercial port today
• Dredged to maintain channel for ships
• Larger ships need deeper channel
• What do you do with the contaminated
dredged material (dredge spoil)?
Overboard disposal is banned!
Dredge Spoil Disposal
•Hart-Miller Island (State Park, today)
1985 view MGS
1999 view NOAA
•Poplar Island
The next area
Corps of Engineers
Stream Flow
How does the velocity vary in the channel?
Faster on the
outside of a
meander
a
c
d
Slower on
the inside of
a meander
e
b
f
Same velocity in straight sections of channel
Channel cross-sections
a
b
Straight
c
Meander bend
d
erosion
deposition
inside
outside
How does the water velocity vary from c to d?
Draw the cross-section for e-f.
The geology at a meander in a stream
Outside of bend
EROSION
undercutting of stream bank
Inside of bend
DEPOSITION
young or no vegetation
How does the channel migrate with time?
…another source of sediment
to the Chesapeake Bay
Wave
erosion
This will worsen as sea level rises!
From NOAA
Here is a lake/reservoir formed by
damming a stream valley.
Former stream channels before flooding
stream
What do you notice
about the shape of
the man-made lake?
lake
What is the fate
of the lake?
As the fast flowing streams flow into the quieter lake
water, what happens to sediment?
Natural vs. Man-Made Lakes
• How do you tell the difference?
• Natural lakes are bowl, rounded in
shape- formed by subsidence or glacial
carving.
• Man-make lakes are irregular, typically
following topography of stream
drainage. Plus look for the dam!
DEEP CREEK LAKE, MARYLAND
Not part
of Bay
watershed,
goes to
Miss. River
dam
N
Maryland’s
largest
man-made
lake.