Do Surface and Groundwater Fluctuations Influence Sediment Surface Elevation
in the Coastal Everglades Wetlands?
Donald R. Cahoon & James C. Lynch
U.S. Geological Survey, Patuxent Wildlife Research Center, Laurel, MD
T. J. Smith III, Kevin R.T. Whelan & Gordon H. Anderson
U.S. Geological Survey, Center for Water & Restoration Studies, St. Petersburg &
Miami, FL
Christa Walker
DynCorp Systems & Solutions LLC, USGS, Everglades Field Station,
Homestead, FL
The Greater Everglades ecosystem stretches for over 360km, from Lake Kissimee
in the north to Florida Bay in the south. At its widest point it covers more than
100km from west to east. The vast flow way has been dissected by a network of
canals and levees over the recent past to control flooding, provide water storage
for human consumption and protect croplands. Natural surface water flows into
Everglades National Park have been seriously disrupted, in terms of quantity,
quality and timing of flow. Presently federal, state and local agencies have
established a $7.8 billion ecosystem restoration program for the Everglades,
known as the Comprehensive Everglades Restoration Plan (CERP). Many
science questions remain unanswered as the restoration begins. Major questions
exist concerning rates of soil formation in the various wetland plant communities
and about factors regulating surface elevation in these wetlands.
This report provides data from a network of Surface Elevation Tables (SETs) in
the southwest coastal wetlands of Everglades National Park. At each of eight
sites, three SETs were established. Three study locations were along each of the
Shark and Lostmans Rivers, the two major coastal drainages of the southwest
Everglades. On each river one site is in an upstream freshwater wetland, a second
site is in the middle reach of the river in a brackish marsh – mangrove forest
community, and the third site is downstream near the river’s mouth in a pure
stand of mangrove forest. The two freshwater sites are non-tidal and highly
seasonal with respect to surface water flows and depths (both highest in summer
and negligible in the winter dry season). Tides are measurable, but small, at the
brackish waters sites and there is a noticeable variation in surface water. Tidal
activity dominates the hydrological signature at the saline, downstream, mangrove
forest sites. The seventh and eighth sites are located at Big Sable Creek, a marine
dominated region on northwest Cape Sable. The cape was devastated by the 1935
Labor Day Hurricane and again by Hurricane Donna in 1960. At this location
three SETs are in mangrove forest and three are in open, unvegetated, intertidal
mudflat. Ground-water and surface-water sampling wells are present at all sites
and are instrumented to record elevation and conductivity at hourly intervals.
SETs have been sampled at 3-6 month intervals for more than three years (fig.1).
For each of the eight sites we calculated the rate of change in relative wetland
surface elevation (only one site is currently surveyed to MSL) between sampling
intervals. We also calculated the rate of change in daily ground and surface water
elevation for the same intervals. Average surface and groundwater stage was
determined for the day of SET sampling, and for the 15 and 30 days prior to
sampling. Simple linear regression was used to test for differences between
surface elevation change and the water level parameters and to calculate the slope
of that relationship for each site.
Sediment surface elevation at all sites showed apparent annual cycles that differed
between sites. Surface elevation appeared to be greatest during the dry season at
upstream freshwater locations (fig.1) and lowest during the dry season at
downstream saline locations. The two brackish water, middle river, sites showed
little variation in sediment surface elevation. Surface-water stage, over the 15
days prior to sediment sampling, was strongly related to the change in sediment
elevation between samplings (fig. 2). Most importantly, this relationship was the
opposite between sites. At freshwater sites, as average stage increased, sediment
change decreased, whereas at saltwater sites, increasing surface-water stages led
to increasing sediment elevation between sampling periods. The pattern at the
two brackish sites was intermediate.
Why these two wetland systems behave in an opposite manner is unclear at this
time. One hypothesis is that in the freshwater systems, a sedimentation – resuspension process is occurring whereby, as the dry season progresses, flocculent
material in the water column sediments out, and as the wet season sets in and
water levels (and current velocities) go up, this material becomes re-suspended
and lost from the site. This hypothesis will be tested in coming months.
Shark River SH1
Sediment Elevation
Ground Water Stage
Surface Water Stage
5
30.00
10.00
3
0.00
2
-10.00
-20.00
1
-30.00
-40.00
0
1998
1999
2000
2001
2002
Figure 1. Representative data showing the time series of surface & groundwater
stage and sediment surface elevation at the upstream freshwater site on the Shark
River (SH1).
Fresh (SH1) vs Salt (SH3) Water Wetland Comparison
Fresh-SH1
Salt-SH3
Linear (Fresh-SH1)
Linear (Salt-SH3)
40.0
y = -16.741x + 44.592
R2 = 0.3804
20.0
0.0
-20.0
-40.0
1.00
y = 20.041x - 25.43
R2 = 0.3703
1.50
2.00
2.50
3.00
3.50
Stage
Figure 2. Sediment elevation change versus average surface water stage for a
freshwater marsh (SH1) and a salt water mangrove forest (SH3).
Contact: Smith III, Thomas J. USGS, Center for Water & Restoration Studies,
600 Fourth Street, South, St. Petersburg, FL 33701. Voice: 727-803-8747, FAX:
727-803-2030;
Email:
Tom_J_Smith@usgs.gov
POSTER, Ecology &
Ecological Modeling
Stage
4
20.00
Sediment Elevation Change
Sediment Elevation - mm
40.00
.