U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3117

October 2006

Prepared in cooperation with

Pinellas County, Southwest Florida Water Management 

District, and Tampa Bay Water

Flooding Frequency Alters Vegetation in Isolated Wetlands

Printed on recycled paper

different elevations in a wetland and 

describe the effects of flooding frequency 

on wetland vegetation at those elevations. 

Five cypress swamps and five 

marshes were studied by the U.S. 

Geological Survey (USGS) during 

2000-2004, as part of an interdisciplinary study of isolated wetlands 

in central Florida (Haag and others, 

2005). Partial results from two of these 

marshes are described in this fact sheet.

Methods

Hillsborough River State Park 

(HRSP) Marsh has a maximum size of 

about 2 acres and a maximum depth of 

about 2.6 feet (fig. 1). This natural marsh 

is largely unaffected by human activities. W-29 Marsh has a maximum size 

of about 6.5 acres and a maximum depth 

of about 2.8 feet (fig. 1). This impaired 

marsh is located on a municipal well 

field. The effects of ground-water withdrawal in the well field and prolonged 

periods of less than average rainfall have 

lowered wetland water levels and reduced 

the frequency of flooding in this wetland.

The wetland perimeter was delineated at both marshes, and the perimeter 

elevation was determined (see Haag 

and others, 2005). Wetland stage (water 

level) was measured at staff gages, 

and bathymetric data were collected 

at many points throughout the interior 

of each wetland. Bathymetric contour 

maps were drawn, and wetland area and 

stored water volume were calculated.

Vegetation was assessed semiannually. At each wetland, a transect line 

was established from the perimeter to 

the deepest point in the wetland. Three 

vegetation zones were established along 

the transect line and were defined as 

Transition, Intermediate, or Deep based 

on their elevation relative to the elevation at the wetland perimeter. Vegetation 

was identified to species in fixed plots 

in each of the vegetation zones.

Introduction

Many isolated wetlands1 in central 

Florida occur as small, shallow depressions scattered throughout the karst 

topography of the region. In these 

wetlands, the water table approaches land 

surface seasonally, and water levels and 

flooding frequency are largely determined by differences between precipitation and evapotranspiration. Because 

much of the region is flat with little topographic relief, small changes in wetland 

water levels can cause large changes in 

wetland surface area. Persistent changes 

in wetland flooding frequencies, as a 

result of changes in rainfall or human 

activity, can cause a substantial change 

in the vegetation of thousands of acres of 

land. Understanding the effect that flooding frequency has on wetland vegetation is important to assessing the overall 

ecological status of wetlands. Wetland 

bathymetric mapping, when combined 

with water-level data and vegetation 

assessments, can enable scientists to 

determine the frequency of flooding at 

1Terms defined in the glossary are in bold print

where first used in this fact sheet.

Flooded-area frequency 

data combined with 

vegetation assessments 

improve our ability to 

assess the ecological 

status of wetlands Saw palmetto

Pine lily
Results

Establishing vegetation plots at 

distinctly different elevations allowed 

vegetation assessments to be directly 

related to wetland stage. Bathymetric data 

were used to further describe the relation between vegetation, stage data, and 

changes in flooded area in each wetland. 

The percent of wetland area 

flooded and the percent of time flooded 

at HRSP Marsh (natural) and W-29 

Marsh (impaired) were compared during 

2 consecutive years. The first year 

(July 17, 2001 - July 16, 2002) was a 

period of average rainfall and average 

ground-water withdrawal in the well 

field, and is referred to in this fact sheet 

as an “Average Year”. The second year 

(July 17, 2002 - July 16, 2003) was a 

period of greater than average rainfall 

and less than average ground-water withdrawal, and is referred to as a “Wet Year”

During the Average Year, 20 

percent or less of the surface area at 

W-29 Marsh was flooded about 85 

percent of the time (fig. 2), and for 

most of that time, the marsh was almost 

completely dry (less than 5 percent 

flooded). For the remaining 15 percent 

of the year only 21-60 percent of the 

total surface area of the wetland was 

flooded. In contrast, more than 40 percent 

of the surface area at HRSP Marsh 

was flooded about 75 percent of the 

time during the Average Year (fig. 3). 

Rainfall amounts were almost identical 

at the two wetlands during the Average 

Year. Ground-water withdrawals in the 

well field were probably responsible 

for the low flooding frequency and 

small flooded area observed at W-29 

Marsh compared to HRSP Marsh.

During the Wet Year, more than 60 

percent of the surface areas at both W-29 

Marsh and HRSP Marsh were flooded 

about 60-65 percent of the year (figs. 2, 3). 

The similarity in flooded areas between 

the natural wetland and the impaired 

wetland during the Wet Year reflected 

the substantially reduced quantities of 

ground-water withdrawals in the vicinity 

of W-29 Marsh, and the increased rainfall 

throughout the region during the year. 

W-29 Marsh received about 50 percent 

more rainfall and HRSP Marsh received 

about 25 percent more rainfall during 

the Wet Year than during the Average 

Year. Upper Floridan aquifer water levels 

beneath W-29 Marsh averaged about 14 ft 

higher during the Wet Year than during 

the Average Year. This change is likely 

the combined result of increased rainfall 

and reduced ground-water withdrawal.

Figure 1. Bathymetric maps of Hillsborough River 

State Park (HRSP) Marsh (at left) and W-29 Marsh 

(below).

2.65

0

1

1

0.5

1

1.5

1.5

1.5

Hillsborough River State Park Marsh

0.5

Intermediate

BATHYMETRIC CONTOURS – Depth in feet

below wetland perimeter elevation.

Contour interval 0.5 foot

VEGETATION PLOTS – Transition,

Intermediate, and Deep

DEEPEST POINT – Showing elevation in feet

below elevation of wetland perimeter

EXPLANATION

1

Transition

Deep

25 METERS

0 100 FEET

0

Abundant Plants

in a Wet Year

Leafy Bladderwort

Maidencane

Pickerelweed

BlueMaidencane

Combleaf mermaidweed

Grassy Arrowhead

Leafy Bladderwort

Narrowfruit Horne Beaksedge

Southern Cutgrass

Swamp Smartweed

Blue Maidencane

Broomsedge Bluestem

Maidencane

Southern Cutgrass

Switchgrass

Deep Plots

ntermediate Plots

Transition Plots

I

Abundant Plants

in an Average Year

Maidencane

Pickerelweed

Blue Maidencane

Narrowfruit Horne Beaksedge

Blue Maidencane

Maidencane

Deep Plots

Intermediate Plots

Transition Plots

W-29 Marsh

2.76 0

50 METERS

150 FEET

0

0

Intermediate Transition

1

2

Deep

BATHYMETRIC CONTOURS – Depth in feet below wetland perimeter elevation. Contour interval 0.5 foot

VEGETATION PLOTS – Transition, Intermediate, and Deep

DEEPEST POINT – Showing elevation in feet below elevation of wetland perimeter

EXPLANATION

1

Abundant Plants

in a Wet Year

Maidencane

American White Waterlily

Dog Fennel

Eastern Purple Bladderwort

Grassy Arrowhead

Peelbark St. John’s-wort

Slender Flattop Goldenrod

Maidencane

Maidencane

Fringed Yelloweyed Grass

Deep Plots

ntermediate Plots

Transition Plots

I

Abundant Plants in

an Average Year

Dog Fennel

Maidencane

Blue Maidencane

Broomsedge Bluestem

Dog Fennel

(No living plants)

Deep Plots

Intermediate Plots

Transition Plots

River

Creek Cypress

W-29 Marsh 28°15’

28°0 0'

82°45' 30' 15' 82°00'

PASCO COUNTY

HILLSBOROUGH

COUNTY PINELLAS

COUNTY

Tampa

Gulf of Mexico

0 5 10 MILES

0 5 10 KILOMETERS

Marsh Site

Locations

FLORIDA

Hillsborough River

State Park Marsh
The frequency of flooding at the 

elevations of the Deep, Intermediate, 

and Transition vegetation plots also was 

compared at HRSP Marsh and W-29 Marsh 

during the same 2 consecutive years. During 

the Average Year, Deep plots at HRSP Marsh 

were flooded about 80 percent of the time 

(fig. 4). Intermediate plots were flooded 

for more than 70 percent of the time, and 

Transition plots had water at the surface more 

than 40 percent of the time. In contrast, Deep 

plots at W-29 Marsh were flooded only 30 

percent of the time (fig. 5), Intermediate plots 

were flooded less than 10 percent of the time, 

and water was not present in the Transition 

plots at W-29 Marsh during the Average Year. 

During the Wet Year, the flooded-area 

frequencies of these two marshes were more 

similar (figs. 4, 5). Deep and Intermediate 

plots were flooded 90-100 percent of the 

year in both wetlands. The greatest differ-

ence occurred at the elevation of the transi-

tion plots. At W-29 Marsh, transition plots 

were flooded about 50 percent of the year, 

whereas those at HRSP Marsh had water at 

the surface almost 90 percent of the year.

Vegetative surveys alone can be an 

ambiguous indicator of the frequency of flood-

ing in different areas of some wetlands. Many 

marsh plants display broad tolerances for flood-

ing and desiccation. In this study, several species 

were observed across a gradient of elevation 

in both of the wetlands (see plant lists in fig. 

1). For example, at HRSP Marsh, maidencane 

(a wetland grass) was abundant in Deep and 

Transition plots in May 2002 when the ground 

was saturated, with no standing water. The 

species was again found at this site in Deep and 

Figure 4. Duration of inundation at Hillsborough River 

State Park (HRSP) Marsh.

Figure 5. Duration of inundation at W-29 Marsh.

Figure 3. Inundated area at Hillsborough River 

State Park Marsh.

Figure 2. Inundated area at W-29 Marsh. 0

20

40

60

80

100

0-20

PERCENT OF YEAR

21-40 41-60 61-80 >80

INUNDATED AREA, IN

PERCENT OF TOTAL AREA

W-29 Marsh

AVERAGE YEAR

WET YEAR

0

20

40

60

80

100

0-20

PERCENT OF YEAR

21-40 41-60 61-80 >80

INUNDATED AREA, IN

PERCENT OF TOTAL AREA

Hillsborough River

State Park Marsh AVERAGE YEAR

WET YEAR

0

20

40

60

80

100

TRANSITION

PLOTS

Hillsborough River

State Park Marsh

WET YEAR

EXPLANATION

AVERAGE YEAR

PERCENT OF YEAR

INTERMEDIATE

PLOTS

DEEP

PLOTS

0

20

40

60

80

100

W-29 Marsh AVERAGE YEAR

WET YEAR

PERCENT OF YEAR

INTERMEDIATE

PLOTS

DEEP

PLOTS

TRANSITION

PLOTS
Transition plots in October 2003 when 

water depths were 10-15 inches. Dead 

maidencane was present in Deep and 

Intermediate plots at W-29 Marsh in May 

2002 when those plots were dry. At W-29 

Marsh in October 2003, maidencane was 

found in plots in all three zones where 

water depths ranged from 8-36 inches. 

The occurrence of floating and 

submersed aquatic plants is a less 

ambiguous indicator of prior flooding 

in a particular wetland zone, although 

the duration of that flooding cannot 

be inferred reliably without access to 

bathymetric and stage data. For example, 

at HRSP Marsh (fig. 1) during the Wet 

Year, leafy bladderwort (a submersed 

plant) was found in Deep plots where 

water depths were 8-12 inches, but it 

was not present in the wetland during the 

Average Year. The Intermediate plots also 

contained leafy bladderwort and several 

other obligate wetland plants that were 

not present during the Average Year. At 

W-29 Marsh (fig. 1), the occurrence of 

American white water lily (a floating 

plant) in Deep plots coincided with water 

depths of 24-36 inches during the Wet 

Year. Intermediate plots also sustained 

much more flooding during the Wet 

Year, as indicated by the occurrence of 

eastern purple bladderwort (a submersed 

plant) and several other obligate wetland 

plants. During the Average Year, the 

diversity and density of wetland plants 

were much lower in both marshes. 

Spatial and temporal patterns of 

flooding sustained over a period of years 

determine the distribution of long-lived 

wetland plants (trees and shrubs), and 

facilitate the long-term maintenance 

of viable habitat for wetland wildlife. 

Determining flooded-area frequencies 

for natural wetlands based on long-term 

stage records would provide a more 

representative indication of longterm average conditions compared to 

sporadic vegetation assessments with 

little or no associated stage data. The 

utility and robustness of estimates of 

long-term average conditions would be 

enhanced if a large number of natural 

wetlands representing a range of hydrologic conditions could be included.

Conclusions

Wetland water-level changes can 

be related to changes in rainfall, and can 

indicate a connection between groundwater levels and the depth and frequency 

of flooding in isolated wetlands in 

central Florida. Vegetation assessments 

in wetlands were enhanced as a quantitative management tool when they 

were coupled with data on the elevation 

of vegetation plots and the location of 

transects, and when vegetation results 

were combined with information on 

wetland bathymetry. When the elevation 

of vegetation plots is determined, the 

composition of the vegetation community 

in those plots can be extrapolated to 

areas of similar elevation and flooding frequency throughout the entire 

wetland. In this way, bathymetric data 

can be used to expand wetland vegetation community assessment into the 

landscape scale, and the extent and 

duration of wetland flooding over time 

can be interpreted in terms of a loss 

or gain of functional wetland area. 

Reference

Haag, K.H., Lee, T.M., and Herndon, D.C., 2005, 

Bathymetry and Vegetation in Isolated Marsh and 

Cypress Wetlands in the Northern Tampa Bay Area, 

2000-2004: U.S. Geological Survey Scientific 

Investigations Report 2005-5109, 49 p.

Glossary of Terms

Bathymetric mapping—mapping the water depth of the wetland 

Cypress swamp—a wetland with pond cypress (Taxodium ascendens) and other trees, 

shrubs, and vines

Deep plot—vegetation plot located near the deepest part of the wetland

Evapotranspiration—water that vaporizes from water or soil in a wetland, together with 

moisture that passes through plants to the atmosphere

Flooding frequency—the percent of the year when water covers some part of the wetland 

bottom

Intermediate plot—vegetation plot located halfway between the elevation of the deep plot 

and the elevation of the transition plot

Isolated wetland—a wetland with no apparent surface-water connections to streams, 

rivers, estuaries, or the ocean 

Karst topography—a region underlain by limestone that contains solution cavities and 

where the physical features of the land surface (topography) include large and small 

depressions 

Marsh—a wetland with rooted and/or floating aquatic plants but without trees

Obligate wetland plant—plant found in wetlands 99 percent of the time 

Transition plot—vegetation plot located 3-6 inches below the elevation of the wetland 

perimeter, at the transition to upland vegetation

Well field—an area developed by a local or regional water authority where ground water is 

withdrawn and sent to a treatment and distribution system 

Authors— Kim H. Haag and Terrie M. Lee, U.S. Geological Survey, Florida Integrated Science Center, 

Suite 215, 10500 University Center Dr., Tampa, FL 33612

Collaborators—Anthony Ridge, University of Plymouth, Bioscience and the Media Program; and Joel 

Toledo, University of South Florida, Environmental Science and Policy Program

Spikerush

This report can be accessed at http://pubs.water.usgs.gov/fs2006-3117