U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3118

October 2006

Prepared in cooperation with

Pinellas County, Southwest Florida Water Management 

District, and Tampa Bay Water

Strength in Numbers: Describing the Flooded Area of 

Isolated Wetlands

Printed on recycled paper

Introduction

Thousands of isolated, freshwater wetlands are scattered 

across the karst1 landscape of central Florida. Most are small 

(less than 15 acres), shallow, marsh and cypress wetlands that 

flood and dry seasonally. Wetland health is threatened when 

wetland flooding patterns are altered either by human activities, such as land-use change and ground-water pumping, or 

by changes in climate. Yet the small sizes and vast numbers of 

isolated wetlands in Florida challenge our efforts to characterize them collectively as a statewide water resource. In the 

northern Tampa Bay area of west-central Florida alone, water 

levels are measured monthly in more than 400 wetlands by 

Tampa Bay Water and the Southwest Florida Water Management Distirct (SWFWMD). Many wetlands have over a decade 

of measurements.

 The usefulness of long-term monitoring of wetland water 

levels would greatly increase if it described not just the depth of 

water at a point in the wetland, but also the amount of the total 

wetland area that was flooded. Water levels can be used to estimate the flooded area of a wetland if the elevation contours of 

the wetland bottom are determined by bathymetric mapping.

Despite the recognized importance of the flooded area 

to wetland vegetation, bathymetric maps are not available to 

describe the flooded areas of even a representative number of 

Florida’s isolated wetlands. Information on the bathymetry of 

isolated wetlands is rare because it is labor intensive to collect 

the land-surface elevation data needed to create the maps. 

Five marshes and five cypress wetlands were studied by 

the U.S. Geological Survey (USGS) during 2000 to 2004 as 

part of a large interdisciplinary study of isolated wetlands in 

central Florida. The wetlands are located either in municipal 

well fields or on publicly owned lands (fig. 1). The 10 wetlands 

share similar geology and climate, but differ in their groundwater settings. All have historical water-level data and multiple 

vegetation surveys. 

A comprehensive report by Haag and others (2005) 

documents bathymetric mapping approaches, the frequency 

of flooding in different areas of the wetlands, and the relation 

between flooding and vegetation in these wetlands. This fact 

sheet describes bathymetric mapping approaches and partial 

results from two natural marshes (Hillsborough River State 

Park Marsh, and Green Swamp Marsh) and one impaired marsh 

(W-29 Marsh) that is located on a municipal well field and is 

affected by ground-water withdrawals. (fig. 1). 

Wetland Perimeters

Defining wetland perimeters is fundamental to creating the 

bathymetric maps. At 9 of the 10 wetland sites in this study, a 

saw palmetto fringe delineated the perimeter of the wetland 

and defined the elevation corresponding to 100 percent flooded 1

Terms defined in the glossary are in bold print where first used in 

this fact sheet.

    Why describe the flooded area of

    isolated wetlands?

To identify changes in flooded area that signal changes in the

acreage of wetland vegetation

To summarize wetland water levels into a regional view of

wetland status

To help identify wetland/ground-water interactions

To provide information that can be used to increase the 

similarity of mitigation wetlands and natural wetlands

•

•

•

•
area (fig. 2). The palmetto fringe 

was shown in separate studies by 

the SWFWMD to be a dependable 

indicator of a wetland boundary 

in the study area. At one wetland 

where the palmetto fringe and other 

vegetative indicators were absent, 

the presence of hydric soils was used 

to delineate the perimeter. 

Areas determined for four of 

the cypress wetlands and five of the 

marshes in this study were compared 

to wetland areas in the National 

Wetlands Inventory (NWI) that were 

mapped by the U.S. Fish and Wildlife Service using aerial photointerpretation. The comparison indicates 

a greater discrepancy in the NWI 

area estimates for marsh wetlands 

than for cypress wetlands of this 

size. For four of the five marshes, 

the NWI area differed by 25 to 35 

percent from USGS study estimates. 

Areas for the four cypress wetlands 

and the remaining marsh wetland 

differed by 8 percent or less.

Wetland Bathymetry

Bathymetric maps of the 10 wetlands were created using 

two principal approaches, because the wetlands differ in their 

depth, size, extent of flooding, and density of vegetation. 

Wetland water levels (in elevations relative to the National 

Geodectic Vertical Datum of 1929) were read from staff gages 

and monitored using electronic water-level recorders.

For the five wetlands that were dry at the time of the 

measurements, total station surveying equipment was used to 

survey land-surface elevations. This method was precise and 

readily generated a high density of measurements ranging from 

62 to 18 points per acre.

At three wetlands, where water was present over 

approximately 100 percent of the wetland area, land-surface 

elevations were derived at fewer points by subtracting measured 

water depths from the elevation of the water surface. Measurements were made along lines or transects through the wetland. 

A digital global positioning sytem (GPS) instrument or distance 

measurements were used to determine the location of measuring 

points. At wetlands that were partially flooded, a combination 

of these two approaches was used.

Land-surface elevations were interpolated between 

measured points using spatial-interpolation software. Elevation 

contour lines typically were drawn at 0.5-foot contour intervals. 

Flooded area and volume were calculated and graphed for 

various stages between dry and full. The results for a 6.5-acre 

natural marsh, W-29 Marsh, are shown in figure 3.

The wetland contours revealed by bathymetric mapping 

showed that over certain ranges of stage, the flooded area and 

volume changed substantially with small changes of stage. 

For 7 of the 10 wetlands, including W-29 Marsh, the area 

versus stage curves were S-shaped instead of parabolic (fig. 3). 

Generally, the wetland area changed most over the lowest 

0 5 10 MILES

0 5 10 KILOMETERS

FLORIDA

LOCATION

OF STUDY

AREA

EXPLANATION

1

2

3

4

5

6

7

8

9

10

Duck Pond Marsh

Green Swamp Cypress

Green Swamp Marsh

Hillsborough River State Park Marsh

S-63 Cypress

S-68 Cypress

W-03 Marsh

W-05 Cypress

W-19 Cypress

W-29 Marsh

GREEN SWAMP 6 SELECTED WETLAND AND NUMBER

PARKS AND WELL FIELDS

PHYSIOGRAPHIC REGIONS

Gulf Coast Lowlands

Western Valley

Base from U.S. Geological Survey digital data, 1:24,000, datum nad83

Albers Equal-Area Conic Projection,

Standard parallels 29° ° 30' ' and 45 30 , central meridian -83°00'

River

Pithlachascotee

River Anclote

River

Creek

Cypress

BlackwaterCreek

Alafia

River

8 7

9

10

2

3

4

1

5 6

28°30'

15'

28°00'

82°45' 30' 15' 82°00'

Hillsborough

River State Park

Cypress

Creek

Well field

Cross Bar

Ranch

Well field

Jay B.

Starkey

Wilderness

Park

HERNANDO COUNTY

PASCO

COUNTY

HILLSBOROUGH

COUNTY POLK

COUNTY PINELLAS

COUNTY

SUMTER

COUNTY

Brooksville

Land O Lakes

New Port

Richey

Temple

Terrace

Tarpon

Springs

Clearwater Tampa

Brandon

Plant

City

Zephyrhills

Lakeland

Dade

City

Old

Tampa

Bay

Gulf of Mexico

LAKE

COUNTY

Northern Tampa Bay Area

Figure 1. Location of study wetlands in the northern Tampa Bay area, 

west-central Florida.

Figure 2. Saw palmetto fringe at edge of 

wetland. (Image courtesy of Southwest 

Florida Water Management District.)

ranges of stage. The flat bottoms 

of most of the wetlands resulted 

in a large increase in wetted area 

with a small increase in wetland 

stage.

In contrast, as the flooded 

area approached the perimeter 

(near 100 percent inundation), 

a large increase in wetland 

stage and volume was required 

to produce a relatively small 

increase in wetland surface area. 

For example, adding 3.0 acre-feet 

of water to W-29 Marsh when 

it is dry would flood 63 percent 

of the total surface area, but 

doubling the volume of water in 

the wetland would inundate only 

18 percent more area (fig. 3). Elevation of wetland

perimeter

Saw Palmetto Fringe
Potential Effects of Mapping Approach

The approach used to collect elevation data in wetlands can 

effect the resulting bathymetric map, and therefore our understanding of flooded area and volume at different water levels. 

To gain insights into the effect of different mapping approaches 

on flooded area, three mapping approaches were applied to an 

oblong wetland, Marsh W-29, and the resulting bathymetric 

maps were compared. The results of the detailed total-station 

surveying approach are shown in figure 4a, and the results of 

the two simple approaches are shown in figures 4b and 4c. 

The first of the two simpler mapping approaches eliminated 

most of the elevation data points collected by total-station 

surveying, leaving three lines of points (fig. 4b). This approach 

reduced the average density of elevation points from 18.2 to 4.6 

points per acre.

The second of the simpler approaches assumes that the 

outline of the flooded area at some stage between dry and the 

perimeter is known (fig. 4c). This elevation contour is added 

to the three lines of data points. For W-29 Marsh, an elevation 

contour was selected from the suite of contours developed for 

this wetland. In the field, however, the data points defining a 

contour line could be collected by using a digitial GPS unit to 

map the water’s edge.

The results of the comparisons show that the simplest 

approach (fig. 4b) missed a substantial amount of information, 

particularly about the nonlinear changes in the flooded area with 

stage. Contouring sparse data points along transects resulted in 

a more linear interpolation of the elevation between data points 

than actually existed. As a result, wetland area and volume over 

certain ranges in water level were 50 to 100 percent lower than 

the more detailed total-station approach (fig. 5). Adding a single 

elevation contour to the transect data greatly improved the 

agreement with the detailed mapping approach.

Choosing the appropriate density of data points needed to 

determine wetland bathymetry depends on the intended use of 

the data. For example, a simple approach that uses fewer data 

points could be sufficient when comparing the percentage of 

flooded area in multiple wetlands over the same time period, as 

illustrated in figure 6. In contrast, a detailed approach that uses 

as many data points as possible would benefit a wetland waterbudget study that must closely account for changes in wetland 

volume over time. All three methods applied here were subject 

to similar bathymetry measurement errors attributable to the 

vegetative cover and the rough surface of the wetland landscape.

Uses for Wetland Flooded-Area Data

Once stage-area curves are generated from bathymetric 

data, they can be used to expand routine observations of wetland 

stage into routine observations of wetland flooded area. Flooded 

area, expressed as a percentage of the total wetland area, is a 

versatile and descriptive measurement that can be tracked over 

time for an individual wetland or compared across a population 

of wetlands.

Recording flooded area over time at a given wetland has 

another advantage. Observations can be analyzed statistically 

to describe the length of time that different areas of the wetland 

were flooded. Summarizing the flooded-area data in this manner 

results in what Haag and others (2005) refer to as flooded-area 

frequency distributions. These distributions can characterize 

the amount of time different areas of wetlands were flooded in a 

given year or historical time period (fig. 7).

Ideally, wetland mitigation using augmentation or other 

means could attempt to re-establish natural wetland flooding 

cycles. This would entail not only timing the presence or 

1

2

VOLUME,

IN ACRE-FEET

AREA, IN ACRES

10

8

6

4

2

0

67 68 69 70

8

6

4

2

0

Volume

Area

WETLAND STAGE, IN FEET

ABOVE NGVD OF 1929

W-29 Marsh

Bathymetric

Data Points

(A)

(B)

(C)

BATHYMETRIC CONTOURS–Depth in feet

below wetland perimeter elevation of

69.92 feet above NGVD of 1929. Interval

is 0.5 foot

VEGETATION PLOTS

DEEPEST POINT–Showing elevation in

feet below elevation of wetland perimeter

STAFF GAGE LOCATION

BATHYMETRIC POINT–Location where

bathymetric measurement made

DIRT ROADWAY–Wetland outflow at an

elevation higher than elevation of

wetland perimeter

EXPLANATION

0

2.76

50 METERS

150 FEET

0

0

50 METERS

150 FEET

0

0

W-29 Marsh

EXPLANATION

DEPTH, IN FEET

0-0.5

0.5-1.0

1.0-1.5

1.5-2.0

2.0-2.5

>2.5

WETLAND PERIMETER AT 100 PERCENT

INUNDATION

WETLAND PERIMETER AT A SELECTED

WETLAND STAGE BELOW 100

PERCENT INUNDATION

CONTOUR LINE GENERATED BY SURFER

BATHYMETRIC DATA POINT

0 150 300 FEET

0 50 100 METERS

(A) (B) (C)

Figure 3. (A) Wetland bathymetric contours, (B) density of bathymetric data 

points, and (C) stage-volume and stage-area curves for W-29 Marsh.

Figure 4. Contour lines developed for W-29 Marsh from (A) all bathymetric 

data points, (B) a subset of bathymetric data points along three transect 

lines, and (C) a subset of bathymetric data points along three transect lines 

and a contour line.
absence of water at the deepest point in the wetland, but also 

restoring the natural frequency of flooding across a range of 

wetland elevations. Improved information on wetland stage, 

area, and volume, would enable water managers to recommend 

target stages to flood predetermined wetland surface areas with 

a certain frequency. Bathymetric data for wetlands of different 

sizes also could be used to help construct artificial wetlands 

with a range of bottom depths comparable to natural wetlands.

Bathymetry and long-term stage data can be used to 

characterize the flooded-area frequencies in natural wetlands for 

a given year, for different climatic cycles, and for longer historical 

periods. The contrast in flooded-area frequencies of natural 

wetlands and impaired wetlands could indicate which impaired 

wetlands would make the best candidates for mitigation. By 

comparing the flooded-area frequencies of natural wetlands to 

historically augmented wetlands, a quantitative guidepost could 

be established for evaluating the success of mitigation efforts.

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

Augmentation—adding water from an external source to increase the water level in a 

surface-water body

Bathymetric mapping—mapping the water depth of the wetland

Flooded-area frequency—the percent of the time when water covers some part of the 

wetland bottom

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

rivers, estuaries, or the ocean 

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

physical features of the land surface include large and small depressions

Saw-palmetto fringe—a long-lived terrestrial plant that occurs in the transition zone 

between wetlands and the surrounding uplands 

Total station—an optical instrument used in modern surveying that can determine 

angles and distances from the instrument to the points to be surveyed

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

water is withdrawn from the aquifer and sent to a treatment and distribution system

Authors— Terrie M. Lee and Kim H. Haag, 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

Figure 7. The frequency of flooding in different areas of a natural marsh 

and an impaired marsh for the 16-year period from 1988 to 2003 (marsh 

locations are shown on fig. 1).

Figure 5. Effects of a reduced density of bathymetric data points at W-29 

Marsh on (a) stage-area and (b) stage-volume relations.

7

6

5

4

3

2

1

0

AREA, IN ACRES

67 67.5 68 68.5 69 69.5 70

WETLAND STAGE, IN FEET ABOVE NGVD OF 1929

VOLUME, IN ACRE-FEET10

9

8

7

6

5

4

3

2

1

0

(A) W-29 Marsh

(B) W-29 Marsh

Three transects

Three transects plus 68.1-foot contour

Complete data set

Three transects

Three transects plus 68.1-foot contour

Complete data set

Figure 6. Weekly average flooded area at a natural marsh and an impaired 

marsh for the same time period, using three bathymetric mapping 

approaches.

(B) Impaired Marsh (W-29 Marsh)

(A) Natural Marsh (Green Swamp Marsh)

WEEKLY AVERAGE FLOODED AREA, IN PERCENT

100

80

60

40

20

0

100

80

60

40

20

0

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

2001 2002

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

2001 2002

ORIGINAL DATA

TRANSECTS ONLY

TRANSECTS PLUS CONTOUR

0

20

40

60

80

0

20

40

60

80

0-40 41-60 61-80 81-100

and above

PERCENT OF TOTAL TIME

Impaired Marsh (W-29 Marsh)

Natural Marsh (Hillsborough River State Park)

FLOODED AREA, IN PERCENT OF THE TOTAL AREA

This report can be accessed at http://pubs.water.usgs.gov/fs2006-3118