Geology Reference
In-Depth Information
(a)
N
(b)
Florida Bay
inset:B
East
Side Creek
Station 2
Station 3
Ebb in ECC
Ebb in ECC
Florida Bay
(c)
(d)
N
Fig. 14.
'Fly-over' cartoons visualizing
the movement of sediment though a
tidal cycle (12 hours). Cartoons show
a birds-eye view above East Cape
Canal (ECC), fi rst moving away from
Lake Ingraham eastward (a and b),
then southward (c), turning around
to north (d) and then moving west-
ward back to Lake Ingraham (e and f).
Waters draining from Lake Ingraham
through ECC on an ebbing tide (a) join
with organic-rich density plumes from
Homestead Canal (HSC) South (b).
Large sediment peaks are measured
at Station 1 at the end of the ebb (c),
but as the tide reverses, Florida Bay
water carries organic-rich waters that
are still draining from East Side Creek
(c) and Homestead Canal South
(d) towards Lake Ingraham (e), where
fl ow diverges, and mixed carbonate-
organic sediment settles to form a
rapidly accreting delta (f).
HSC
South
Station 1
East
Side Creek
Station 3
Flood in ECC
Flood in ECC
N
(e)
(f)
Gulf of Mexico
Marl
Ridge
Station 5
Collapsed
marsh
Station 4
N
Daily tidal currents are the most important
agent responsible for the high sedimentation
rates, as revealed by
in situ
sedimentation and
time-series measurements made during 2004
and 2005. Past hurricanes have surely been the
most important contributor to large, sudden
morphological changes such as stepwise shore-
line erosion and breaching of canals. However,
since 1960 there has not been a category 4 or 5
hurricane affecting the study area directly; smaller
hurricanes passing in 2005 had minimal effect on
the sediment patterns and rates. Thus, the rapid
sedimentation is interpreted to be a response to
accelerated rate of RSL rise, responsible for the
large sedimentological and ecological changes
set into motion behind the ridge, probably given
a hand by small human modifi cations (canals)
acting as temporal catalysts.
Field observations, geochemical and salinity
data suggest that the large volumes of sediment
fi lling Lake Ingraham and the Southern Lakes are
derived from three sediment sources that feed
suspended sediment into ECC and the lakes.
Source 1 is a carbonate-rich component that comes
from fl ood tidal waters entering from Florida
Bay through ECC and Hidden Creek; Source 2 is
a mixed carbonate/organic component, derived
from erosional widening of creeks and canals; and
Source 3 is a dominantly organic component, of
material discharging from the collapsed interior
marsh out through HSC South and West and ESC.
Because fl ood tide waters are washing across the
marl ridge into the interior wetlands, prolonged
discharge occurs from HSC South and West and
ESC, persisting well after the fl ood tide has come
in through ECC. This organic-rich, sediment-laden
water from the interior (Source 3) thus largely
moves with the fl ood tide into Lake Ingraham and
the Southern Lakes rather than out to sea. It is
well established that offshore sediment (Source 1)
commonly moves landward into coastal bays
(Postma, 1961), and it is expected that erosive
widening of tidal channels will provide sediment
(Source 2) to interior bays. One important result
