The South Florida Environment—

By Benjamin F. McPherson and Robert Halley

U.S. Geological Survey Circular 1134

National Water-Quality Assessment Program

A

Region

Under

Stress
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1996

For sale by the

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

Library of Congress Cataloging in Publication Data

McPherson, Benjamin F.

 The south Florida environment : a region under stress / by Benjamin F. McPherson 

and Robert Halley.

p. cm. -- (United States Geological Survey circular ; 1134)

 “National Water-Quality-Assessment Program.”

 Includes bibliographical references.

 1. Florida--Environmental conditions. 2. Environmental management--Florida. 

I. Halley, Robert B. II. National Water-Quality Assessment Program (U.S.) III. Title. 

IV. Series: U.S. Geological Survey circular ; 1134

GE155.F6M37 1997 97-20480

363.7′009759--dc21 CIP

ISBN 0-607-87154-7

Any use of trade, product, or firm names in this publications is 

for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

The Florida Everglades
III

Contents

Abstract .................................................................................. 1

Introduction ............................................................................ 2

Acknowledgments .................................................................. 5

Environmental Setting—The Natural System ........................ 6

Physiography .................................................................. 6

Climate ............................................................................ 8

Geology ........................................................................... 12

Hydrology ....................................................................... 14

Watersheds and Coastal Waters ...................................... 16

Kissimmee–Okeechobee–Everglades Watershed ... 16

Big Cypress Watershed ........................................... 19

Charlotte Harbor Watershed .................................... 19

Estuaries and Bays .................................................. 20

Florida Reef Tract ................................................... 22

Coral Reefs and Sea Level ...................................... 23

Environmental Setting—The Altered System ........................ 24

Drainage and Development ............................................ 24

Public Lands ................................................................... 27

Agriculture ...................................................................... 28

Urbanization ................................................................... 32

Water Use ........................................................................ 34

Water Budget ................................................................... 34

Water and Environmental Stress ............................................ 40

Loss of Wetlands and Wetland Functions ....................... 40

Soil Subsidence ............................................................... 42

Degradation of Water Quality.......................................... 42

Urban Lands ............................................................ 42

Agricultural Lands and Everglades Region ............ 45

Lake Okeechobee .................................................... 47

Big Cypress Swamp ................................................ 47

Charlotte Harbor Watershed .................................... 48

Mercury Contamination .................................................. 50

Effects on Estuaries, Bays, and Coral Reefs ................... 52

Summary and Research Needs ................................................ 54

References .............................................................................. 56
IV

Figures

1-6. Maps showing:

1. Regional ecosystem and watersheds in the study unit boundary, 

south Florida................................................................................. 2

2. Urban and agricultural lands in south Florida ............................. 3

3. Lands in south Florida under public ownership or control........... 4

4. Physiographic provinces of south Florida ................................... 7

5. Generalized topography and bathymetry in south Florida .......... 8

6. Average annual rainfall in south Florida, 1951–80 ...................... 9

 7-8. Graphs showing:

7. Average monthly rainfall for the lower east coast (1915–85) 

and average pan evaporation at selected locations in south

Florida in 1988 ............................................................................. 10

8. Rainfall above and below the average annual rainfall for 20 

stations in south Florida, 1895–1990............................................ 10

19-15. Maps showing:

9. Average annual temperature in south Florida............................... 11

10. Total numbers of hours where temperatures fell to 32 degrees 

Fahrenheit or lower between November 1937 and March 1967 .... 11

11. Generalized surficial geology of south Florida............................. 12

12. Generalized geohydrologic section A-A′ of 

south Florida ................................................................................ 13

13. Generalized subsurface section B-B′ showing aquifers of 

south Florida ................................................................................ 13

14. Wetlands and deepwater habitats of south Florida ...................... 14

15. Soils of organic and recent limestone origin in south Florida ..... 15

16. Long-term hydrograph showing water-level fluctuations at well 

S-169A in southern Dade County, 1932–39 and 1982–89 .................. 15

17. Map showing hydrologic features and the natural direction of 

surface-water and coastal-water flows under predevelopment 

conditions in south Florida ................................................................. 16

18. Aerial photograph of Everglades wetlands in the Shark River 

Slough, and a generalized section of the slough ................................. 17

 19-21. Photographs of:

19. Wading birds in south Florida ...................................................... 18

20. Cypress pine forests in the Big Cypress Swamp and a 

generalized section of a cypress dome ........................................ 20

21. Western Florida Bay at Cape Sable, a coral reef in south 

Florida, and a generalized section from the Florida Keys 

to the reef ..................................................................................... 21

22. Graph showing sea-level fluctuations on three time scales ................ 23
V

23. Map showing major canals, control structures, direction of water 

flow, and other hydrologic features in south Florida ......................... 25

 24-25. Photographs of:

24. Miami Canal in Water Conservation Area 3 ................................ 26

25. Sugarcane fields south of Lake Okeechobee ............................... 28

26. Map showing estimated fertilizer sales, in tons of phosphate and 

nitrogen, in counties within the study unit, south Florida................... 30

27. Photograph of cattle in a drainage ditch near Lake Okeechobee ....... 30

 28-30. Maps showing:

28. Estimated number of cattle per square mile, per county, in south 

Florida, 1992 ................................................................................ 31

29. Population density in south Florida, 1990 ................................... 33

30. Location of major wastewater facilities and landfills in 

south Florida ................................................................................ 33

31. Schematic diagram of water storage and movement in 

south Florida ....................................................................................... 35

32. Map showing average annual discharge from major canals in 

south Florida ....................................................................................... 36

33. Map showing major water budget subbasins of southeastern 

Florida ................................................................................................ 37

34. Pie chart showing comparison of estimated average annual 

inflows and outflows (as percentages) for the modeled region in 

south Florida........................................................................................ 38

35. Graph showing total discharge to Shark River Slough, Taylor 

Slough, C–111, and the Atlantic Ocean, 1980–89 .............................. 39

36. Map and graph showing total phosphorus concentrations in water at 

selected sites in south Florida, 1984-93 .............................................. 43

37. Map showing areas in south Florida where mercury concentrations 

in largemouth bass tissue equaled or exceeded one-half parts per 

million ................................................................................................ 51

38. Long-term changes on a coral reef community are illustrated by 

photographs taken in 1976 and 1992 at the same location on a reef 

near Key Largo ................................................................................... 53

Tables

1. Major pesticides used on agricultural crops in the south Florida study 

unit, listed in order of estimated total pounds of active ingredient 

applied annually (1989–91) ...................................................................... 29

2. Population characteristics, by hydrologic cataloging unit, in the south

Florida study unit, 1990............................................................................. 34

3. Preliminary estimates of natural water budget in 1,000 acre-feet, 

for the lower east coast area, south Florida, 1980-89 ............................... 38
VI

Multiply By To obtain

inch (in.) 25.4 millimeter

inch per year (in/yr) 25.4 millimeter per year

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

acre 4,047 square meter

0.4047 hectare

square mile (mi2

) 2.590 square kilometer

acre-foot (acre-ft) 1,233 cubic meter

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year

gallon per day (gal/d) 0.003785 cubic meter per day

cubic foot per second (ft3

/s) 0.02832 cubic meter per second

million gallon per day (Mgal/d) 0.04381 cubic meter per second

pound 0.4536 kilogram

ton, short 0.9072 megagram

Fahrenheit (°F) can be converted to degrees Celsius (°C) as follows:

°C = 5/9 × (

o

F – 32)

Sea Level: In this report, “sea level” refers to the National Geodetic

Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived

from a general adjustment of the first-order level nets of both the

United States and Canada, formerly called Sea Level Datum of 1929.

μg/kg..... microgram per kilogram

mg/L..... milligram per liter

μ..... micron

ppt..... parts per thousand

C&SF..... Central and Southern Floridan Project for Flood Control 

and other Purposes

EAA..... Everglades Agricultural Area

NAWQA..... National Water-Quality Assessment

PCB..... Polychlorinated biphenyls

SFWMD..... South Florida Water Management District

SFWMM..... South Florida Water Management Model

USGS..... U.S. Geological Survey

WCA..... Water-conservation area

Vertical Datum

Acronyms

Conversion

Factors

Temperature

Conversion

Abbreviations
 1

When Europeans first arrived in North America, south Florida was a lush,

subtropical wilderness of pine forest, hardwood hammocks, swamps,

marshes, estuaries, and bays. Wetlands dominated the landscape. The region

contained one of the largest wetlands in the continental United States, the

Everglades, which itself was part of a larger watershed that extended for more

than half the length of the Florida Peninsula. 

During the last 100 years, the Everglades and its

watershed have been greatly altered by man, primarily

through drainage and development; however, parts of

the natural system remain in public lands and waters

that are protected at the southern end of the peninsula.

In the remaining natural system, drainage and development have had severe environmental effects, such as

large losses of soil through agriculturally induced

subsidence, degradation of water quality, nutrient

enrichment, contamination by pesticides and mercury,

fragmentation of the landscape, loss of wetlands and

wetland functions, widespread invasion by exotic

species, impairment of estuarine and coastal resources,

and significant declines in populations of native plant

and animal species. Additionally, the large and increasing human population and the active agricultural

development in the region are in intensive competition

with the natural system for freshwater resources.

Recently, a consensus has developed among Federal

and State agencies and environmental groups that

the south Florida ecosystem, and the Everglades in

particular, should be protected and

restored, to the extent possible, to its predevelopment condition. A first and

primary step in this undertaking would be

the restoration of the predevelopment

hydrologic conditions to the remaining

natural system. As part of an interagency

effort, the U.S. Geological Survey (USGS)

is providing scientific information that

will contribute to the protection and restoration effort in south Florida. This information will be generated through such

programs as the National Water-Quality

Assessment and the South Florida Initiative. This report serves as an environmental review and framework for developing

USGS programs in the region and stresses

the critical role of water in natural and

human systems and its importance as a

link between those systems within south

Florida. 

The South Florida Environment—

By Benjamin F. McPherson and Robert Halley

A

Region

Under

Stress

Abstract
2

At the time of settlement by Europeans (mid-1800’s), the south Florida region was a lush, subtropical

wilderness of pine forest, hardwood

hammocks, swamps, marshes, estuaries, and bays (Davis, 1943). Wetlands dominated the landscape. The

region contained one of the largest

wetlands in the continental United

States, the Everglades. The Everglades was part of a larger watershed: the Kissimmee–Okeechobee–

Everglades that extended for more

than half the length of the Florida

Peninsula. The Everglades and the

Big Cypress Swamp stretched as a

continuous wetland across the

southern part of the peninsula south

of Lake Okeechobee. These wetlands and the entire watershed

(fig. 1) provided the freshwater that

sustained the high productivity and

abundant fisheries of the coastal

waters (McIvor and others, 1994).

The wetlands of south Florida

were regarded as being inhospitable

and without intrinsic value. In the

early 1900’s, draining the wetlands

was considered to be essential for

commerce and safety. Loss of lives

as a result of hurricane flooding in

the 1920’s accelerated drainage

projects, primarily in the Everglades. Today, much of south Florida’s wetlands are intensively

managed, with more than 1,400 mi

of primary canals and more than 100

water control structures. Because of

drainage and development, the

south Florida ecosystem has experienced a variety of environmental

problems such as loss of soil, nutrient enrichment, contamination by

pesticides, mercury buildup in the

biota, fragmentation of landscape,

loss of wetlands and wetland functions, widespread invasion by exotic

species, increased algal blooming in

coastal waters, seagrass die off, and

declines in fishing resources.

Water is life for the human and

natural systems in south Florida.

Clean, abundant water was a fundamental characteristic of the original

south Florida system. Increased

human population and activity in

south Florida have brought, not only

an increased need for water, but

also a decrease in water supply

and a deterioration in water quality.

Figure 1. Regional ecosystem and watersheds in the study unit boundary, 

south Florida.

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

EXPLANATION

SOUTHERN FLORIDA NATIONAL WATERQUALITY ASSESSMENT

STUDY UNIT BOUNDARY

MAJOR WATERSHED

NAME AND BOUNDARY

NATURAL KISSIMMEE−

OKEECHOBEE−EVERGLADES

WATERSHED BOUNDARY

BIG

CYPRESS

0 25 50 MILES

0 50 KILOMETERS 25

25°

83° 82° 81° 80°

28°

27°

26°

Florida Bay

Whitewater

Bay

Ten Thousand

Islands

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

Myakka

River Peace

River

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

Caloosahatchee

River

Approximate Location of Florida Reef Tract

EVERGLADES

BIG

CYPRESS

CHARLOTTE

HARBOR

KISSIMMEE

Lake

Okeechobee

LOCATION

OF

STUDY

UNIT

Introduction
3

Changes in the hydrologic system

are thought by many to be the root

cause of the dramatic declines in

fish and wildlife populations and

habitat across the south Florida ecosystem.

Today in south Florida, competition for water is intense and

divided between a large, rapidly

growing population along the coast

and agriculture north and south of

Lake Okeechobee (fig. 2), on the

one hand, and the remaining natural

ecosystem mostly within State and

Federal parks, reserves, sanctuaries,

and preserves (fig. 3), on the other.

Satisfying the water-resource

demands of these competing interests is a complicated and difficult

task. The quantity of water required

for urban and agricultural uses may,

at times, exceed supply. Plants and

animals also have critical requirements with respect to the quantity of

water, because they are dependent

on the timing and duration of wet

and dry periods. Water-quality

requirements also vary markedly.

The Everglades natural biota require

water that is extremely low in phosphorus concentration, yet agricultural activities produce waters that

contain high levels of phosphorus.

Such conditions result in direct

competition because the natural

biota are “downstream” from the

agricultural areas.

Recently, a consensus has

begun to emerge among Federal and

State agencies and environmental

groups that south Florida and the

Everglades should be restored, to

the extent possible, to patterns similar to those of the original system.

For concerned parties to discuss

productively, let alone implement,

such recommendations requires a

substantial increase in available scientific data and understanding of the

hydrology, geology, and ecology of

south Florida and the Everglades

specifically (Holloway, 1994). 

The investigations needed to support restoration have been outlined

recently by the Science Sub–Group

of the South Florida Ecosystem Task

Force. These investigations include

characterizing the predrainage system and comparing it with the

present system, particularly hydrologically; determining the key characteristics of the former natural

hydrologic system that supported

the rich diversity and abundance of

wildlife that has been lost; designing

structural and operational modifications of the Central and Southern

Figure 2. Urban and agricultural lands in south Florida. (Modified from 

South Florida Water Management District and Southwest Florida Water 

Management District digital data, 1988-90.) Land use and land cover 

categories based on South Florida Water Management District classification codes. Equivalent categories are shown for the area within the Southwest Florida Water Management District.

EXPLANATION

STUDY−UNIT BOUNDARY

URBAN OR BUILT−UP LAND

(DOES NOT INCLUDE

EXTRACTIVE LAND USES)

AGRICULTURAL LAND

LAND−USE PARCELS

GREATER THAN OR

EQUAL TO 1 SQUARE MILE

25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES

0 50 KILOMETERS 25

Florida Bay

Whitewater

Bay

Ten Thousand

Islands

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

Myakka

River Peace

River

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

Caloosahatchee

River

Lake

Okeechobee
4

Florida Project for Flood Control

and other Purposes (C&SF) that

would recreate the key characteristics of the natural hydrologic system; assessing the hydrologic and

ecological results of these modifications through pre- and postmodification monitoring; and modifying the design to make improvements (Science Sub–Group, written

commun., 1994).

The U.S. Geological Survey is

providing some of the scientific

information necessary for protection

and restoration in south Florida

through several Survey programs

that include the South Florida Initiative and the National Water–Quality

Assessment (NAWQA) Program.

The USGS is coordinating these

efforts with other Federal agencies

through the South Florida Ecosystem Interagency Working Group

and the Science Sub–Group and

through regularly scheduled liaison

meetings of the Southern Florida

NAWQA study unit. The South

Florida Initiative is a collaborative

effort by the U.S. Geological Survey

and other Federal and State agencies

to provide scientific insight into

conflicting land-use demands and

water-supply issues in the south

Florida regional ecosystem. The

NAWQA Program is described in

the adjacent box.

This report provides an overview of the environmental setting in

south Florida and serves as a review

and framework for developing

USGS programs in the region. In the

report, we describe the predevelopment and the current (present-day)

environmental conditions in south

Florida, with emphasis on the quantity and quality of water. The geographical area covers the southern

half of the State, and includes the

Southern Florida NAWQA study

unit and adjacent coastal waters.

The Southern Florida NAWQA

study unit covers about 19,500 mi2

and is the watershed of the larger

regional ecosystem (fig. 1). We

define the regional ecosystem to

include coastal waters between

Charlotte Harbor on the Gulf of

Mexico and the St. Lucie River on

the Atlantic Ocean and the lands that

drain into these waters (fig. 1). 

Figure 3. Lands in south Florida under public ownership or control. 

(Modified from South Florida Water Management District and Southwest 

Florida Water Management District digital data, 1994.)

EXPLANATION

STUDY−UNIT BOUNDARY

PUBLIC LAND PARCELS

GREATER THAN OR EQUAL

TO 1 SQUARE MILE

0 25 50 MILES

0 50 KILOMETERS 25

25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

St Lucie

Canal

Caloosahatchee

Canal

Cape

Romano

St Lucie

River

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

Myakka

River Peace

River

Biscayne

Bay

Florida Bay

Charlotte

Harbor

Caloosahatchee River

Lake

Okeechobee
5

NAWQA

Acknowledgments

National Water-Quality

Assessment Program

In 1991, the USGS began to implement a full-scale NAWQA Program. 

The three major objectives of the NAWQA Program are to provide a consistent 

description of current water-quality conditions for a large part of the Nation’s 

water resources, to define long-term trends (or lack of trends) in water quality, and 

to identify, describe, and explain the major factors that affect observed water-quality 

conditions and trends. These objectives are being met (1) by conducting retrospective 

analyses of existing data, (2) by establishing a long-term nationwide monitoring network 

designed to assess existing water-quality conditions and provide a data base for trend analyses, and (3) by conducting process-oriented studies designed to provide a better understanding 

of the relation between land- and water-use activities and water-quality conditions. The NAWQA 

Program is providing an improved scientific basis for evaluating the effectiveness of water-quality 

management programs and practices.

The NAWQA Program is being implemented through investigations of hydrologic systems in 60 

study units that include parts of most major river basins and aquifer systems in the United States. 

Study units range in size from 1,200 to about 65,000 mi2 and incorporate 60 to 70 percent of the 

Nation’s water use and population served by public water supply. The south Florida study unit 

includes most of the southern half of the Florida Peninsula and contains a major urban complex 

of more than 5 million people. The study unit in this report was included in the NAWQA Program 

in 1993.

The authors of this report and the U.S. Geological Survey wish to

acknowledge the following Survey individuals for their contributions:

David McCulloch, Geographer, who digitized and compiled 

the Geographic Information System (GIS) that enabled him to 

generate the maps in this report.

Ronald S. Spencer, Scientific Illustrator, who received the 

electronically prepared illustrations and prepared them for 

camera-ready copy.

Twila D. Wilson, Writer-Editor, who created the report’s 

camera-ready design and layout, and performed the editorial 

and verification review.
6

Environmental Setting—

the Natural System

The south Florida ecosystem

formed during the last several thousand years after the world-wide climatic changes and sea-level rise at

the end of the Pleistocene age. The

ecosystem consists primarily of

wetlands and shallow-water habitats

set in a subtropical environment.

Physiography

The south Florida ecosystem

includes coastal waters between

Charlotte Harbor on the Gulf of

Mexico and the St. Lucie River on

the Atlantic Ocean and all or part of

the following physiographic provinces: the Lake Wales Ridge, the

Flatwoods, the Atlantic Coastal

Ridge, the Big Cypress Swamp, the

Everglades, the Mangrove and

Coastal Glades, and the Florida

Keys (fig. 4). The coastal waters

consist of a system of interconnected estuaries, bays, lagoons, and

coral reefs that include Charlotte

Harbor, Ten Thousand Islands,

Whitewater Bay, Florida Bay, Biscayne Bay, and the Florida Reef

Tract (fig. 1). 

The highest altitude in the

region is on Lake Wales Ridge,

where sand hills range from 70 to

300 ft above sea level (fig. 5). Adjacent to the ridge, the relatively lower

Flatwoods range in altitude from

about sea level to 100 ft in the north. 
7

The Atlantic Coastal Ridge

extends along the eastern coast as a

low ridge of sand over limestone

that ranges in altitude from about 10

to 50 ft above sea level. The ridge

averages about 5 mi wide and is

breached in places by shallow

sloughs or transverse glades.

The Everglades, which is

located west of the Atlantic Coastal

Ridge, is slightly lower in altitude

than the ridge or the Flatwoods and

extends southward from Lake

Okeechobee to the Mangrove and

Coastal Glades near Florida Bay.

The Everglades has an almost

imperceptible slope to the south,

which averages less than 2 in. per

mile. Altitudes range from 14 ft near

Lake Okeechobee to sea level at

Florida Bay. Under predeveloped

conditions, the Everglades was seasonally inundated, and water

drained slowly to the south through

what was referred to as the “River of

Grass” by Florida author and conservationist Marjory Stoneman

Douglas.

The Big Cypress Swamp to the

west of the Everglades is on slightly

higher land. Water inundation and

peat deposition in the swamp are

less extensive than in the Everglades. The land surface of the

swamp is flat except for numerous

low-mounded limestone outcrops

and small, circular, elongated

depressions in the limestone. Water

in the swamp drains slowly to the

south and southwest through a number of cypress strands into the

coastal mangrove forest.

The Mangrove and Coastal

Glades consists of a broad band of

swamps and marshes south of the

Everglades and the Big Cypress

Swamp. The land is at or near sea

level and is often flooded by tides or

by freshwater runoff. Salinities

range from freshwater to hypersaline, depending on the amount rainfall and runoff, and on tide levels.

The gradual slope of the land continues offshore across the broad west

Florida platform into the Gulf of

Mexico. Much of the southern Florida Gulf Coast receives low wave

energy which is favorable to the

development of tidal marshes, seagrass beds, and mangrove forests.

The Florida Keys are a series of

low limestone islands that extend

140 mi southwest of the mainland.

Altitudes in the islands rarely

exceed 5 ft above sea level. A narrow shelf is present along the Atlantic Coast, where the seafloor drops

sharply into the Straits of Florida.

Figure 4. Physiographic provinces of south Florida. (Modified from Davis, 

1943; and Parker and others, 1955.)

STUDY−UNIT BOUNDARY

EXPLANATION

NATURAL KISSIMMEE− OKEECHOBEE−EVERGLADES

WATERSHED BOUNDARY

0 25 50 MILES

0 50 KILOMETERS 25

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

MANGROVE AND

COASTAL GLADES

EVERGLADES

BIG CYPRESS

SWAMP ATLANTIC COASTAL RIDGE

25°

83° 82° 81° 80°

28°

27°

26°

F L A T W O O D S

Caloosahatchee

Peace

River

Myakka

River

Kissimmee

River

River

ATLANTIC OCEAN

Florida Bay

STRAITS OF FLORIDA

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Lake

Okeechobee

Lake Wales Ridge
8

Mean, maximum, and minimum inches of rainfall for the lower east coast of 

Florida, 1915-85 (South Florida Water Management District, 1993)

Period Mean Maximum Minimum

Annual 51.9 77.5 36.7

Wet season 34.5 53.5 23.4

Dry season 17.4 30.9 7.3

The Atlantic Coast is bathed in the

clear, tropical waters of the Florida

Current which is favorable to the

development of coral reefs several

miles offshore of the keys.

Climate

Annual average rainfall in south

Florida ranges from about 40 to 65

in. (fig. 6). The east coast usually

receives the greatest amount of rainfall, whereas the Florida Keys and

the areas near Lake Okeechobee and

Charlotte Harbor usually receive the

least. More than half the rain falls in

the wet season from June through

September (fig. 7) and is associated

with thundershowers, squalls, and

tropical cyclones. Afternoon thundershowers are frequent over land,

where moisture-laden air from sea

breezes and wetlands warms, and

rises, and the moisture condenses to

form clouds (Pardue and others,

1992). The wet season often has a

bimodal rainfall pattern with two

maxima, one in early and one in late

summer (Thomas, 1974; Duever and

others, 1994). Rainfall during the

remainder of the year is usually the

result of large frontal systems and is

broadly distributed rather than localized. April and May typically have

the lowest rainfall. Annual and seasonal rainfalls, however, vary from

year to year, as shown in figure 8 and

the table below.

Duever and others, (1994) analyzed severe droughts at several stations from 1910 through 1980 and

reported that, whereas some

droughts were fairly widespread,

others were more localized, even

over distances of only 30 mi. The

variability in rainfall is often characterized by multiyear wet and dry

cycles (fig. 8). These cycles are

apparent in the average annual rainfall in the South Florida Water

Figure 5. Generalized topography and bathymetry in south Florida. (Topography generalized from U.S. Geological Survey 1:24,000 quadrangles 

provided in digital form by South Florida Water Management District, 1992. 

Bathymetry modified from Fernald, 1981.)

0 25 50 MILES

0 50 KILOMETERS 25

EXPLANATION

BATHYMETRY,

IN FATHOMS

GREATER

THAN 100

GREATER

THAN 100

0 TO 10

10 TO 20 10 TO 20

20 TO 50 20 TO 50

50 TO 100 50 TO 100

5 TO 10

0 TO 5

STUDY−UNIT BOUNDARY

OUTSIDE

STUDY UNIT

LAND SURFACE

ALTITUDE,

IN FEET

25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

Florida Bay

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Lake

Okeechobee
9

Management District (SFWMD)

network where average annual rainfall decreased by 1.3 in/yr from

1900 through 1920, increased by 1.3

in/yr from 1921 through 1970, and

decreased by 3 in/yr from 1971

through 1991 (Robert Hammeric,

South Florida Water Management

District, oral commun., 1994).

Analysis of the longest period of

rainfall record in south Florida (Fort

Myers) indicates, however, that the

trend for total annual rainfall and

variability in mean annual rainfall

for 10-year intervals, has not

changed at that location since the

late 1800’s (Duever and others,

1994). 

Tropical cyclones (hurricanes

and tropical storms) produce the

most severe weather conditions in

south Florida. The high tides and

heavy rains associated with these

storms can produce coastal and

inland flooding, and strong winds

can cause extensive damage. Rainfall often exceeds 5 in. Tropical

cyclones have repeatedly passed

through the region, most frequently

in late summer or early fall.

Between 1871–81, 138 tropical

cyclones, passed near or over the

region (Neumann and others, 1981;

Duever and others, 1994); some evidence indicates that hurricane

strikes have declined during this

span (Robert Hammeric, South

Florida Water Management District,

oral communication, 1994). These

storms varied greatly in size,

amount of rainfall, and windspeed

and, thus, in their effects on the

region. Generally, cyclones have

had their greatest effects near the

coast and on coral reefs where storm

surges have eroded and buried natural communities (Tabb and Jones,

1962; Ball and others, 1967). Noncoastal areas are primarily affected

by heavy rains that cause flooding

and by strong winds that damage

plant communities (Craighead and

Gilbert, 1962; Alexander, 1967;

Loope and others, 1994). Despite

the immediate damage, natural communities in south Florida have

evolved with these storms and have

adapted to them (Pimm and others,

1994).

Evapotranspiration in south

Florida has been estimated to be

from 70 to 90 percent of the rainfall

in undisturbed wetlands (Kenner,

1966; Dohrenword, 1977). Evaporation from open water is greatest in

late spring when temperatures and

Figure 6. Average annual rainfall in south Florida, 1951–80.

(Water Resources Atlas of Florida, 1984.)

STUDY-UNIT BOUNDARY

EXPLANATION

AVERAGE ANNUAL

RAINFALL, IN INCHES

LESS THAN 48

48 TO 52

52 TO 56

56 TO 60

GREATER THAN 60

LINES OF EQUAL AVERAGE

ANNUAL RAINFALL, IN INCHES 52

56

56

52

48

60

48

48

40

40

44

44

48

52

52

56

56

52

48

52

52

52

52

25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES

0 50 KILOMETERS 25

Florida Bay

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Lake

Okeechobee
10

Figure 8. Rainfall 

above and below 

the average annual 

rainfall for 20

stations in south 

Florida, 1895-1990. 

(Data from the 

National Climatic 

Center.)

80

70

60

50

40

30

-30

-20

-10

0

10

20

30

1900 1910 1920 1930 1940 1950 1960 1970 1980

DIFFERENCE FROM AVERAGE, IN INCHES

ANNUAL RAINFALL, IN INCHES

Above average annual rainfall is shown above the line

Below average rainfall is shown below the line

1890 1990

AVERAGE ANNUAL

RAINFALL IS

50.7 INCHES

windspeeds are high and relative

humidity is low, and is least in winter when temperatures, windspeeds,

and humidity are low (fig. 7).

Evapotranspiration is greatest during the summer wet season when

water is available for surface evaporation and vegetative transpiration

(Duever and others, 1994).

The climate in south Florida is

subtropical and humid. Average

temperatures are in the mid-70’s °F

annually, ranging from about 60 °F

in midwinter to about 80 °F in summer (Florida Department of

National Resources, 1974). Temperatures in coastal areas are moderated

by the Gulf of Mexico and the

Atlantic Ocean (fig. 9). The summer

heat is tempered by sea breezes near

the coast and by frequent afternoon

and evening thundershowers. The

southern one-third of central Florida

and the lower two-thirds of the

coastline have nearly freeze-free climates (fig. 10). Although freezes do

occur, their pattern and severity is

erratic from year to year (Duever

and others, 1994). The low frequency of freezes has allowed a

number of tropical species to colonize and survive in the area.

Figure 7. Average monthly rainfall (South Florida Water Management 

District, 1993) for the lower east coast (1915-1985) and average pan 

evaporation (Duever and others, 1994) at selected locations in south 

Florida in 1988.

1

2

3

4

5

6

7

8

9

0

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

PAN EVAPORATION

AVERAGE MONTHLY RAINFALL AND

PAN EVAPORATION, IN INCHES
Figure 10. Total numbers of hours where temperatures 

Fahrenheit or lower between November 1937 and Ma

(Winsberg, 1990.)

25°

83° 82° 81° 

28°27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972Albers Equal-Area Conic projectionStandard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES0 50 KILOMETERS 25

Florida Bay

GULF OF MEXICO 

Charlotte

Harbor

Lake

Okeecho

100

200

300

2,000

1,500

1,000

500

500

EXPLANATION

LINE OF EQUAL TIME THAT

TEMPERATURE WAS 32DEGREES FAHRENHEITOR LOWER. INTERVAL,IN HOURS, IS VARIABLE.

200

STUDY−UNIT BOUNDARY

500

Figure 9. Average annual temperature in south Florida. (Thomas, 1974.)

25°

83° 82° 81° 80°

28°27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972Albers Equal-Area Conic projectionStandard Parallels 29°30′ and 45°30′, central meridian -83°00′

STUDY−UNIT BOUNDARY

EXPLANATION

LINE OF EQUAL AVERAGE

ANNUAL TEMPERATURE.INTERVAL IS 1 DEGREEFAHRENHEIT.

74

0 25 50 MILES0 50 KILOMETERS 25

77

77

76

76

75

72

74

74

73

73

72

72

71

71

75

Florida Bay

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO St Lucie River

Biscayne Bay

Charlotte

Harbor

Lake

Okeechobee
12

Geology

The surficial geology of south Florida (fig. 11) is a result of marine and

freshwater processes that have alternated with the rise and fall of sea level. At

high sea level, limestone was deposited and beaches and dunes were created.

The Atlantic Coastal Ridge (fig. 4), for example, resulted from marine deposition that occurred during an interglacial age (about 125,000 years before

present) when sea level was as much

as 25 ft above the present level

(Scott and Allmon, 1992; Gleason

and Stone, 1994). At low sea level,

the limestone was dissolved and

eroded by drainage of acidic freshwater to create the riddled solution

features that are characteristic of the

region.

With the recession of glaciers in

northern North America at the end

of the Pleistocene Epoch, sea level

rise began and has continued to the

present day. The rising sea level

retarded runoff and downward leakage in south Florida and, with abundant rainfall, helped establish the

broad expansion of wetlands in the

region (Gleason and Stone, 1994).

The rise in sea level slowed about

3,200 years ago, and this slow rise

favored the expansion of coastal and

freshwater wetlands (Wanless and

others, 1994). The Atlantic Coastal

Ridge helped retain freshwater in

the Everglades Basin and this, in

turn, allowed thick layers of peat to

(up to 18 ft) to accumulate within

the northern parts of the Basin

(Gleason and Stone, 1994). Parts of

the present-day Everglades area had

become short-term flooded calcitic

mud marshes by about 6,500 years

ago. Peat deposition began in the

Everglades area about 5,000 years

ago, which indicates that conditions

favorable to long-term flooding had

begun (Gleason and Stone, 1994).

By the time Europeans came to

south Florida, Everglades peat lands

covered nearly 2 million acres. 

South Florida is underlain by a

huge volume of shallow marine carbonate sediments (fig. 12). The

deeper sediments, which exceed

20,000 ft, are almost pure limestone,

dolomite and anhydrite, and were

deposited from Cretaceous through

early Tertiary time as a carbonate

platform (Klitgord and others,

1988). During much of this time,

south Florida was isolated from the

mainland by the deep water of the

Suwannee Strait (Chen, 1965).

Figure 11. Generalized surficial geology of south Florida. 

(Puri and Vernon, 1964.)

Lake

Okeechobee

B B′

A′

A

25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

STRAITS OF FLORIDA

Florida Bay

ATLANTIC OCEAN

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

0 25 50 MILES

0 50 KILOMETERS 25

LOCATION OF

GEOLOGIC SECTION

FORT PRESTON

FORMATION

EXPLANATION

ANASTASIA

FORMATION

MIAMI OOLITE KEY LARGO

LIMESTONE

FORT THOMPSON

FORMATION

CALOOSAHATCHEE

FORMATION

LOWER MARINE AND

ESTUARINE TERRACE DEPOSITS

LAKE FLIRT MARL

MIOCENE SERIES

PLEISTOCENE SERIES

RECENT AND

PLEISTOCENE SERIES

HAWTHORN

FORMATION

BONE VALLEY

FORMATION

TAMIAMI

FORMATION

STUDY−UNIT BOUNDARY

A A′

OUTSIDE

STUDY UNIT
13

Later, as the strait filled, south Florida was connected to the

mainland and clastic sediments were transported to the south

to form the younger Tertiary deposits (fig. 12) that consist of

shallow marine sandy limestone, marls, and sands (Pinet and

Popenoe, 1985). 

The marine carbonate sediments in south Florida contain three major aquifer systems—the Floridan, the intermediate, and the surficial (fig. 13). The confined Floridan

aquifer system is at or near the land surface in central Florida

but dips deeply beneath the surface to the south. The semiconfined intermediate aquifer system overlies the Floridan

and serves as a confined unit for the Floridan. The surficial

aquifer system includes the highly permeable Biscayne

aquifer. The Biscayne aquifer is more than 200 ft thick under

parts of the Atlantic Coastal Ridge and wedges out about

40 mi to the west in the Everglades. The shallow aquifer

of southwest Florida is about 130 ft thick along the Gulf

Coast and wedges out in the eastern Big Cypress Swamp

(fig. 13; Klein, 1972). The surficial aquifers are recharged by

abundant rainfall.

Figure 12. Generalized geohydrologic section A-A’ of south Florida. (Miller, 1986.) (Trace of section shown in 

figure 11.)

0 25

0 25 50 KILOMETERS

50 MILES

EXPLANATION Vertical Scale Greatly Exaggerated

Rocks of Paleocene age

Post-Miocene rocks

Rocks of Miocene age

Late Eocene

Rocks of Oligocene age

Rocks of early

Eocene age

Rocks of late Eocene age

UPPER CONFINING UNIT

Tertiary Period includes Paleocene, Eocene, Oligocene, Miocene and Pliocene

Post-Miocene rocks include Pliocene, Pleistocene, and recent rocks

HIGHLY PERMEABLE ROCKS

MIDDLE CONFINING UNIT

LOCAL CONFINING BED

BOULDER ZONE

LOWER CONFINING UNIT

NO DATA

FEET

500

SEA

LEVEL

1,000

2,000

3,000

4,000

5,000

6,000

Upper

Floridan

aquifer

Surficial

aquifer

(includes

Biscayne)

Upper

confining unit

Upper

Floridan

aquifer

Middle

confining unit

Lower

Floridan

aquifer

Lower

confining unit

Middle

confining unit

Lower

Floridan

aquifer

Lower

confining unit

FLORIDAN AQUIFER SYSTEM

FLORIDAN AQUIFER SYSTEM

Rocks of Cretaceous age

A A′

Rocks of middle Eocene age

Figure 13. Generalized subsurface section B–B’ 

showing aquifers of south Florida. (Klein and 

others, 1975.) (Trace of section shown in 

figure 11.)

SHALLOW AQUIFER

BISCAYNE AQUIFER

FLORIDAN AQUIFER SYSTEM

INTERMEDIATE AQUIFER SYSTEM

(zone of low yield)

250

250

500

750

1000

SEA

LEVEL

FEET

GULF OF

MEXICO ATLANTIC

OCEAN

NAPLES BIG CYPRESS SWAMP THE

EVERGLADES

SURFICIAL AQUIFER SYSTEM

VERTICAL SCALE GREATLY EXAGGERATED

B B′

0 10 20 30 40 50 KILOMETERS

0 10 20 30 40 50 MILES
14

Hydrology

Wetlands are the predominant landscape feature of south Florida (fig. 14).

The prevalence of wetlands is a result of abundant rainfall and a low, flat

terrain. Rainfall becomes ponded in wetlands where it is evapotranspired, infiltrates shallow aquifers, or moves slowly by sheetflow toward tidal waters.

Peat develops in wetlands that are

flooded for extensive periods during

the year, and calcitic muds develop

in wetlands where hydroperiods

(time land is flooded) are shorter and

limestone is near the surface (fig.

15). During the wet season, and for

several weeks afterwards, much of

the land surface in south Florida is

inundated. 

Before development, wetlands

were more extensive, and water

levels fluctuated over a wider range;

water management has tended to

reduce peaks and minimums in

water levels and to lessen flooding

and drought (fig. 16). Hydrologic

models developed by the South

Florida Water Management District

for south Florida indicate that, in

predevelopment times, surface

water covered larger areas for

longer periods of time than it does

today. The models also indicate that

the quantity and timing of surface

and ground-water flow were significantly different than they are today

(Fennema and others, 1994).

The type of wetland drainage

varies from north to south in the

region. In the northern part of the

region, wetlands are drained by several large rivers, which include the

Kissimmee, the Caloosahatchee, the

Myakka, and the Peace Rivers. The

Kissimmee River meanders through

a broad floodplain and discharges

into Lake Okeechobee. The Caloosahatchee, the Myakka, and the

Figure 14. Wetlands and deepwater habitats of south Florida. (U.S. 

Department of the Interior, Fish and Wildlife Service, National Wetlands 

Inventory, 1979-81.)

25°

83° 82° 81° 80°

28°

27°

26°

0 25 50 MILES

0 50 KILOMETERS 25

Florida Bay

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

EXPLANATION

STUDY-UNIT BOUNDARY

ESTUARINE UNVEGETATED

Estuarine open water and unconsolidated

shore (Aquatic beds not identified)

UPLANDS

Neither wetlands or

deepwater habitat

FRESHWATER FORESTED

Palustrine forested and/or

scrub-shrub

DEEPWATER HABITATS

Marine, riverine, or lucustrine

(Does not include estuarine)

ESTUARINE VEGETATED

Estuarine emergent, scrub-shrub,

and emergent

FRESHWATER HERBACEOUS

Palustrine Emergent

WETLANDS AND

DEEPWATER HABITATS

Lake

Okeechobee
15

Figure 15. Soils of organic and recent limestone origin in 

south Florida (above). (Fernald, 1981.)

25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES

0 50 KILOMETERS 25

Florida Bay

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

Myakka

River Peace

River

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

Caloosahatchee

River

Lake

Okeechobee

EXPLANATION

STUDY−UNIT BOUNDARY

PEATS (FRESHWATER)

CALCITIC MUDS

SOILS OF ORGANIC AND

RECENT LIMESTONE ORIGIN

Peace Rivers discharge into Charlotte Harbor and into the Gulf of

Mexico. In the southern part of the

region, streams are smaller, and

freshwater discharge to coastal

waters is more dispersed.

With the exception of the Peace

River, which drains a phosphaterich area in central Florida and

discharges large amounts of phosphorus to coastal waters, nutrient

concentrations of water that drains

south Florida’s wetlands are

typically low, and loading of nutrients to coastal waters is dispersed

over broad areas by sheetflow. The

freshwater typically flows through

extensive mangrove forests into

numerous tidal creeks, estuaries,

and bays where it mixes with saltwater and becomes brackish. Mangrove trees contribute detrital

materials that enrich the brackish

water with nutrients that support a

highly productive estuarine system.

Along the southwestern Gulf

Coast, the gentle slope of the West

Florida Shelf provides a broad, shallow zone where brackish water

mixes with marine water of the open

Gulf of Mexico. This shallow zone

extends south to the Florida Keys.

Several miles south and east of the

Keys, the Florida Current flows

north in the deep Straits of Florida.

Water of the Florida Current is

warm, clear, and salinity is constant.

MONTHLY MEAN GROUND−WATER ALTITUDE AT WELL S−169A, IN FEET ABOVE OR BELOW SEA LEVEL

-2

0

2

4

6

8

10

1982 1983 1984 1985 1986 1987 1988 1989

1932 1933 1934 1935 1936 1937 1938 1939

1982 to 1989

1932 to 1939

Figure 16. Longterm hydrograph 

showing water-level 

fluctuations at well 

S–169A in southern 

Dade County, 1932-

39 and 1982-89 

(right). (Data from 

U.S. Geological 

Survey.)
16

Watersheds and Coastal Waters

Kissimmee–Okeechobee–Everglades Watershed

The Kissimmee-Okeechobee-Everglades watershed, an area of about 9,000

mi2

, once extended as a single hydrologic unit from present-day Orlando to

Florida Bay, about 250 mi to the south (fig. 17). In the northern half of the

watershed, the Kissimmee River and other tributaries drained slowly through

large areas of wetlands into Lake

Okeechobee, a shallow water body

of about 730 mi2

. The lake periodically spilled water south into the

Everglades (Davis, 1943; Parker,

1974), a vast wetland of about

4,500 mi2

. Under high water-level

conditions, water in the Everglades

moved slowly to the south by sheetflow, thus forming the area known

as the River of Grass. Water discharged from the Everglades into

Florida Bay and the Gulf of Mexico,

and under high-flow conditions,

also into the Atlantic Ocean through

small rivers or transverse glades in

the Atlantic Coastal Ridge or as

seepage and spring flow into Biscayne Bay.

The Everglades was a complex

mosaic of wetland plant communities and landscapes with a central

core of peatland that extended from

Lake Okeechobee to mangrove forest that border Florida Bay (Davis

and others, 1994). The peatland was

covered by a swamp forest of custard

apple (Annona glabra) and willow

(Salix caroliniana) along the southern shore of Lake Okeechobee and

by a vast plain of monotypic

sawgrass (Cladium jamaicense) to

the south and east of the swamp forest. Farther southeast, the sawgrass

was broken by sloughs and small

islands of brush. Tree islands and

sloughs became increasingly numerous to the south where the vegetation

formed a mosaic of sawgrass strands

interwoven with lily-pad-covered

sloughs, wet prairies, and tree islands

in Shark River Slough (fig. 18).

A similar mosaic of sawgrass,

slough, wet prairie, and tree islands

was on peatland in the northeastern

Everglades in the Hillsborough

Lake Slough. The peatland was

bounded by peripheral wet prairies,

southern marl marsh, and, cypress

(Taxodium distichum) forest. The

peripheral wet prairies were sandbottomed wetlands of mixed grasses,

sedges, and other macrophytes that

Figure 17. Hydrologic features and the natural direction of surface-water 

and coastal-water flows under predevelopment conditions in south Florida.

STUDY-UNIT BOUNDARY

DIRECTION OF SURFACE

WATER FLOW

NATURAL KISSIMMEE−

OKEECHOBEE−EVERGLADES

WATERSHED BOUNDARY

APPROXIMATE BOUNDARY

OF SLOUGH OR MARSH

EXPLANATION

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

25°

83° 82° 81° 80°

28°

27°

26°

0 25 50 MILES

0 50 KILOMETERS 25

Fakahatchee

Strand Transverse

Glades

Card Sound

Barnes Sound Tay ol r Slough

Devil’s

Garden

Shark River Slough

Hillsborough

Lake Marsh

River

ATLANTIC OCEAN

Myakka

River

Florida Bay

STRAITS OF FLORIDA

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

Lake

Okeechobee

Whitewater

Bay

EVERGLADES

Peace

River

Caloosahatchee

Corkscrew

Swamp

Ocaloacoochee

Slough

Fisheating Creek
17

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

DENSE

SAWGRASS

DENSE

SAWGRASS SLOUGH, SPARSE VEGETATION TREE ISLAND

ALTITUDE, IN FEET

ABOVE SEA LEVEL 8

6

4

2

0 40 78 118 157 197 236

NORTH SOUTH

WATER LEVEL JAN 26, 1972

(vegetation

not to scale)

FEET

Figure 18. Aerial photograph of Everglades wetlands in the Shark River 

Slough (above), and a generalized section of the slough (below). 

(McPherson, 1973a.)

intermingled with higher altitude

pine flatwoods and extended as

transverse glades through parts of the

Atlantic Coastal Ridge. To the south,

the peatland and prairie vegetation

blended into marl and rock-bottomed marshes of herbaceous

plants, short sawgrass, and bay head

tree islands and tropical hammocks.

Water levels and flows in the

Everglades fluctuated seasonally in

response to rainfall and runoff.

Much of the land was inundated during the year, and during heavy rains,

all but the highest tree islands were

flooded. During flood periods,

water moved with enough force to

cause tree islands to develop an

alignment pattern that was parallel

to the lines of surface-water flow

(Parker, 1974). During the dry season, water levels generally were

close to the land surface, but

extreme droughts during some years

lowered water levels substantially

below the land surface, and severe

fires swept over the land, burning

vegetation and peat (Craighead,

1971).

Animal populations in the

Everglades have adapted to and are

dependent upon the seasonal hydrologic fluctuations (McPherson and

others, 1976). Fishes and macroinvertebrates, which form the central

link in the food chain, require

flooded conditions for their growth

and survival. As water levels decline

during the dry season, fishes and

other aquatic animals concentrate in

deeper parts of the marsh and

sloughs where they become prey for

several groups of predators, especially wading birds (fig. 19) whose

nesting season is especially timed to

coincide with the high availability

of food in the remaining pond water

(Kushland, 1991). The animal populations in the adjacent tidal waters

also are dependent upon the seasonal flows of freshwater that create

the salinity conditions that support

productive estuarine and marine fish

populations (Lindall, 1973).

The Everglades has been

dynamic during the approximately

5,500 years of its existence (Davis

and others, 1994). Numerous shifts

have occurred between marl- and

peat-forming marshes and between

sawgrass marshes and water-lily

sloughs (Gleason and Stone, 1994).

Fire, climate, sea level, topography,

hydroperiods, alligator activity, and

recently, man, have had long-term

effects on the vegetation (Craighead, 1971; Parker, 1974; Gunderson and Snyder, 1994).
18

“When I first came to Florida, about 50 yrs ago, I found the country an

almost untouched wilderness filled with beautiful wild life. Wild life fairly

swarmed, especially the birds. Vast numbers of roseate spoonbills,

snowy herons, American egrets, and the great white heron....winged

their way far out over the pineland as they visited swamps in search of

food. And food was abundant. In the Gulf of Mexico....there were schools

of mullet, packed like sardines in a box and reaching away up and down

the coast as far as the eye could carry.” 

Charles Torrey Simpson, 1920

Figure 19. Wading birds in south Florida.
19

Big Cypress Watershed

The Big Cypress Watershed covers

about 2,470 mi2

 of southern Florida

west of the Everglades and south of

the Caloosahatchee River (fig. 1). The

northern part of the watershed is

poorly drained sandy flatlands. Much

of this area is dotted with small, shallow circular ponds, which generally

are less than a foot deep and several

hundred feet in diameter. Altitudes in

the watershed are highest on the

Immokolee Rise (25–42 ft), a sandy

ridge that was formed at higher sea

levels and which now contains the

divide that separates the Caloosahatchee River drainage from that of

the Big Cypress. In the northwestern

part of the watershed, water drains

westward into Estero Bay. On the

Immokolee Rise and to the south, the

sandy flatlands are dissected by several drainage ways that include the

Okaloacoochee Slough, the Devil’s

Garden, and the Corkscrew Swamp

(fig. 17). The Okaloacoochee Slough

extends southward about 50 mi from

the vicinity of the Caloosahatchee

River into the Big Cypress Swamp.

Its average width is a little more than

2 mi. The Okaloacoochee Slough

drains northward and southward from

about the latitude of the Devil’s Garden, a prong of the Slough that

extends to the northeast. The Devil’s

Garden normally drains westward to

the Okaloacoochee Slough, but in

times of high water, it may overflow

in all directions. The southern end of

the Okaloacoochee Slough drains into

the Fakahatchee Strand in the Big

Cypress Swamp. 

Corkscrew Swamp, which begins

near Lake Trafford and extends southwesterly, contains one of the last virgin

cypress forests in north America.

Some trees tower 130 ft and have a

girth of 25 ft. Part of Corkscrew

Swamp is a National Audubon Society

Sanctuary.

The Big Cypress Swamp lies

south of the sandy flatlands and west

of the Everglades. The swamp is characterized by an abundance of small,

stunted cypress trees and by cypress

trees of moderate size associated

with depressions in the bedrock.

Pine and hammock forests occur on

land slightly higher than cypress

forest land. Numerous ponds and

cypress domes are in deeper water

areas (fig. 20). Water levels fluctuate seasonally, and during prolonged

droughts, water levels fall below

even the deep ponds (fig. 20). Natural drainage is by slow, overland

flow to the south. Well defined

streams do not exist except along the

southwestern coast where the

swamp merges with the estuarine

mangrove forest. The soil in the

swamp is usually a thin (less than

2 ft) layer of marl, sand, or a mixture

of the two or is absent where limestone is at the surface. Muck and

peat, however, accumulate to depths

of 3 ft or more in depressions in the

bedrock (Davis, 1943).

Charlotte Harbor Watershed

The Charlotte Harbor watershed is

an area of about 4,685 mi2

 that

drains into the 270 mi2

 Charlotte

Harbor Estuary (fig. 1). Three major

rivers flow into the estuary—the

Peace, the Myakka, and the

Caloosahatchee. The Peace River,

draining an area of 2,350 mi2

, flows

southward for about 75 mi from a

group of lakes at its headwaters to

Charlotte Harbor. Land-surface altitudes range from about 200 ft above

sea level at the headwaters of the
20

Peace River to sea level at the mouth (Hammett, 1990).

The Myakka River, draining an area of 602 mi2

, flows

about 50 mi in a southerly direction to Charlotte Harbor.

The Caloosahatchee River drains an area of 1,378 mi2

.

The river was originally a shallow, meandering stream

with headwaters near Lake Hicpochee. In its natural

state, upstream parts of the river could go dry during the

dry season, and saltwater could move upstream to

within about 10 mi of the lake (Fan and Burgess, 1983). 

Estuaries and Bays

A series of interconnected bays—Biscayne and Florida Bays, Card and Barnes Sounds–lie between the

mainland and the Florida Keys (fig. 17). The bays are

semitropical environments that support a variety of biological communities that are dependent on the distribution of sediment, salinity, temperature, and tidal flow.

The bays and estuaries are protected in a shallow

depression formed between the Keys and the mainland.

On the Keys side, bedrock consists of coralline Key

Largo Limestone (Hoffmeister and Multer, 1968). This

generally low ridge of limestone that forms the upper

Florida Keys is as much as 18 ft above sea level at

Windley Key and extends from Soldier Key in the north

to Big Pine Key in the southwest. On the mainland side

of the bays, the bedrock is the Miami Limestone which

is dominated by two lithologies–an oolitic facies forming the Atlantic Coastal Ridge and a nonoolitic bryozoan facies beneath the Everglades to the west

(Hoffmeister and Multer, 1968).

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

14

13

12

11

10

9

8

7

6

5

4

3

2

1

ALTITUDE, IN FEET

ABOVE MEAN SEA LEVEL

FEET

0 100 200 300 400 500 600 700

WATER LEVEL

SEPT 12, 1960

WATER LEVEL

DEC 4, 1969

CYPRESS DOME

COMMUNITIES

WET

PINE - PALMETTO FOREST PRARIES

PALMETTOS

PINES

S E D I M E N T

WATER LEVEL MAY 30, 1962

LIMESTONE

Figure 20. 

Cypress pine 

forests in the Big 

Cypress Swamp 

(above), and a 

generalized section 

of a cypress 

dome (at left). 

(McPherson, 

1974.)
21

Florida Bay is a triangular area of about 850 mi

2

(fig. 1) whose western side opens directly to the Gulf

of Mexico (fig. 21). Except for tidal channels between

the Keys, it is almost completely enclosed to the south

and east. The rock under Florida Bay is porous limestone, which is similar to the bryozoan facies of the

Miami Limestone in the eastern Everglades to the

north. As much as 16 ft of marine sediments, mud,

peat, and sand has accumulated on the limestone and

formed a latticework of mud banks in Florida Bay. The

banks enclose more than 40 shallow depressions,

locally termed “lakes” that are between 4 and 6 ft deep

(Enos and Perkins, 1978). Mangroves grow where the

banks come near the surface and create small islands

(Enos, 1989). The irregular lattice pattern was created

by a complex process of erosion and deposition during

the slow inundation of an Everglades-like marsh by a

rising sea (Davies and Cohen, 1989). Mangroves that

grow inland along storm berms, rills, and sloughs

joined with mangroves on elevated shorelines to form

a perpendicular meshwork that trapped marine sediments, thus forming the lattice pattern of banks and

depressions (Wanless and Tagett, 1989). Thin layers of

freshwater mud and peat that fill depressions in the

limestone indicate that Florida Bay was similar to the

Everglades when sea level was lower (Davies and

Cohen, 1989).

Numerous interconnected bays and estuaries also

lie along the western coast between Florida Bay and

Charlotte Harbor, including the Ten Thousand Islands.

In the Ten Thousand Islands area, oyster bars and mangrove islands create an intricate pattern of protected

backwaters. Longshore currents from the north have

deposited silica sand to form offshore bars that are

parallel to the coastline. On top of these sandbars,

dense mats of oysters grow perpendicular to the tidal

flow and thus gain a feeding advantage. Mangroves

grow on these intertidal bars and with time deposit

tough, fibrous layers of peat. Eventually, further

growth of the oyster bars may so restrict tidal flow that

oyster growth declines. The mangroves, however, will

continue to cover the bars and to connect adjacent

islands. Later, sediments will fill the lagoons between

the bars. Even so, mangrove land building appears to

be balanced by the gradual drowning of offshore

islands by a rising sea (Scholl, 1964; Scholl and others,

1969; Parkinson, 1989).

Figure 21. Western Florida Bay at Cape Sable.
22

Florida Reef Tract

Corals and coral reefs (fig. 21) are

most abundant and best developed

offshore of the Florida Keys seaward

of Hawk Channel. These reefs which

started growing about 6,000 years

ago (Shinn and others, 1988), form a

tract that is almost 150 mi long and

about 4 mi wide extending to the

edge of the Florida Straits (fig. 1).

The reef tract is not uniform, but

consists of a series of ridges and

channels parallel to the Keys. Two

zones of discontinuous, but parallel,

ridges are evident—the inner ridge,

White Bank, composed of skeletal

sand and scattered patch reefs; and

the outer ridge, a discontinuous shelf

margin of reefs and hard banks, composed of coral rubble and skeletal

sand that form the seaward edge of

the reef tract (Ginsburg and James,

1974). The corals that formed the

Key Largo Limestone and built the

upper Florida Keys are living today

on the reefs. The best examples of

living reefs exist off Key Largo

where the islands retard exchange

with Florida Bay and the offshore

water is dominated by the waters of

the Florida Current (Ginsburg and

Shinn, 1964; Hoffmeister, 1974;

Lidz and Shinn, 1991).

Small patch reefs and large

grass beds, as well as large areas of

bare sand, are scattered on White

Bank. Most of the common life

forms of the outer reef occur on the

patch reefs with the interesting

exception of elkhorn coral, which is

absent in patch reefs and in the Key

Largo Limestone, but is a prominent

reef-building coral of the outer reefs

(Shinn, 1963). Also, the dominance

of marine species differs between the

outer reefs and the patch reefs, as

does the growing shape of some corals. Sea fans and whips seem more

common on the patch reefs than on

the outer reef. The percentage of

grass-feeding fishes is higher on

patch reefs than on the outer reefs.

These fishes utilize the patch reefs as

a daytime resting place and then

move onto nearby grass beds to feed

at night. Patch reefs often have a

halo of white sand around their

perimeter; this is usually caused by

the browsing of the black-spined sea

urchins on the adjacent seagrasses

(Ogden and others, 1973).

The greatest variety of corals

and coral-reef animals live on the

outer reefs (Jaap, 1984). Dustin

(1985) listed more than 40 species of

stony corals from the Florida Reef

Tract. The outer reefs have the most

stable temperature and salinity and

the clearest water because of the

proximity of the Florida Current.

Most reef-building corals require

clear water for photosynthetic algae

living in their soft tissues. The corals,

Figure 21—Continued. 

Coral reef in south Florida (at 

left) and generalized section 

from the Florida Keys to the 

reef (below). (Shinn, 1993.)

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

HAWK CHANNEL

SEA

LEVEL

OUTER

REEF PATCH

REEF

WHITE

BANK

MILES

FEET

KEY

EXPLANATION

PLEISTOCENE LIMESTONE

MUD

LIME SAND

REEF

aaaa

50

40

30

20

10

0

10

20

30

40

50

60

0 1 2345 6 7 8
23

Figure 22. Sea-level fluctuations on 

three time scales. (Sea level is relative 

to 1992 in graph C.) 

[Fairbanks, 1989 (graph A); Scholl and 

others, 1969 (graph B); Maul and Martin, 

1993 (graph C).]

in turn, benefit from the oxygen and

nutrients produced by the algae

(Muscatine, 1990). The Florida Current, which moves northward parallel to the reef tract, provides a rich

source of plankton, an important

food source for many fishes and

invertebrates of the outer reefs. The

fish population on the outer reefs,

which is one of the most varied in the

world, contains more than 500

recorded species (Starck, 1968). The

varied morphologies of reef corals

provide a haven for fish, crustaceans, mollusks, worms, and sea

urchins. Also, the dead coral limestone offer attachment surfaces to a

multitude of marine algae and invertebrates. Nearly 1,400 species of

marine plants and animals were

recorded for a small area of the Florida Reef Tract (Voss and others,

1969).

These coral reefs are, perhaps,

the most diverse and colorful marine

habitats within the continental

United States. Although they have

developed near the northern limit for

coral reefs (Mayor, 1914) and are

subjected to winter water temperatures that can be fatal to corals

(Vaughan, 1918), the reefs of Florida

rival those of many other areas of the

Caribbean in diversity and beauty.

Like coral reefs elsewhere, they are

among the most highly productive

marine ecosystems (Erez, 1990).

They thrive in regions typically low

in nutrients because they have

evolved mechanisms for conserving

and efficiently recycling food. This

ability of reefs to thrive in a nutrientpoor setting led Odum (1971) to

describe a coral reef as “an oasis in a

desert ocean.”

Coral Reefs and Sea Level

In south Florida, most luxuriant reef

growth has long been observed to be

located near the shelf edge seaward

of the largest Florida Keys. Ginsburg

and Shinn (1964, 1994) noted that

this distribution, as well as similar

relations between reefs and islands

in the Bahamas, indicate that reefs

grow best where islands protect

them from extremely shallow bay

waters. Reefs are absent in Florida

Bay because the water is too cold in

the winter, too hot in the summer,

and too variable in salinity to support

the growth of reef-building coral

(Tabb and others, 1962; Holmquist

and others, 1989; Robblee and others, 1989). Similarly, reef growth is

limited where bay water passes

between the Keys to the shelf (Shinn

and others, 1988; Ginsburg and

Shinn, 1994).

During a rising sea level, circulation increases between shelves and

coastal bays on low-relief carbonate

platforms because water depths

increase and larger areas of the platforms are flooded, thus increasing

the volume of the tidal wedge. On

platform tops, water may become

increasingly warm, saline, or, in the

case of the Bahama Platform, cold in

the winter (Roberts and others,

1982). Reefs that become established along the margins of platforms may thrive for thousands of

years until the platform is flooded

and bays develop water conditions

that limit the growth of coral. During

thousands of years of sea-level rise,

shelf margin reefs may be “shot in

the back” by their own bays and

lagoons as circulation increases

between the platform interior and

shelf edge (Neumann and McIntyre,

1985).

Sea level is continuing to rise in

south Florida (fig. 22), and the measured rise at Key West has been

almost 6 in. since 1913 (Emery and

Aubrey, 1991) and about 1-ft since

1850 (Maul and Martin, 1993).

Although the result of sea-level rise

is not fully understood, a 1-ft

increase is thought to have significantly changed the circulation

dynamics of Florida Bay. Florida

Bay, on average, is about 5 ft deep,

so the natural increase represents a

25-percent increase in depth and,

presumably, a significant increase in

water exchange between the bay and

reef tract. Water depth also is determined, in part, by sedimentation

rates in the bay that are, to date,

undetermined. The relation between

current sea-level rise and the health

of reefs is a topic that requires more

study before specific forecasts of

these processes can be applied to the

Florida Keys. If sea level continues

to rise for geologically significant

periods, however, then the long-term

fate of the coral reefs of Florida is

toward continued decline. ELEVATION BELOW PRESENT SEA LEVEL, IN FEET

YEARS BEFORE PRESENT

SOUTH FLORIDA

AREA SHOWN IN C

B 0

5

10

150 1,000 2,000 3,000 4,000 5,000 6,000

0

0.2

0.4

0.6

0.8

1.0

1992

1913

YEARS BEFORE PRESENT

0 10 20 30 40 50 60 70 80

KEY WEST C

AREA SHOWN IN B A 0

50

100

150

200

250

300

350

ATLANTIC

0 5,000 10,000 15,000

YEARS BEFORE PRESENT

20,000
24

Environmental Setting—

the Altered System

The south Florida ecosystem has

been greatly altered by man (fig.

23). Although the hydrology, water

quality, and ecology of much of the

region has been changed by drainage and development, parts of the

old ecosystem remain, primarily on

protected lands and in protected

waters at the southern end of the

peninsula.

Drainage and 

Development

Drainage of the Everglades watershed began in the early 1880’s and

continued into the 1960’s. The first

drainage canals were dug in the

upper Kissimmee River and

between Lake Okeechobee and the

Caloosahatchee River. Beginning

with the Miami River in 1903,

canals were cut through the Atlantic

Coastal Ridge and into the northern

Everglades. By the late 1920’s five

canals had been dug between Lake

Okeechobee and the Atlantic

Ocean–one that passed north of the

Everglades and connected the lake

with the St. Lucie River, and four

that passed through the Everglades

(figs. 23, 24). Drainage has enabled

agriculture to develop south of Lake

Okeechobee. In the late 1920’s, a

low, muck levee was constructed

along the southern and southwestern
25

shore of the lake to prevent flooding; but during the hurricanes of

1926 and 1928, the levee was

breached, which resulted in the

destruction of property and lives. In

response to these catastrophes, the

Federal Government initiated floodcontrol measures, which included

the construction of a levee around

the southern shore of the lake and

the enlargement of the Caloosahatchee and the St. Lucie Canals.

Drainage and development irreversibly altered the natural Everglades watershed and had severe

environmental consequences. Early

drainage lowered water tables up to

6 ft below predevelopment levels

and resulted in conditions favorable

for severe fires that damaged and

eliminated vegetation (Alexander

and Crook, 1973; 1975). Peat burned

and oxidized, and the land subsided

as much as 6 ft below predevelopment levels. Saltwater from the

ocean intruded inland via canals and

filtered into the previously freshwaters of the aquifer (Parker and others,

1955; Klein and others, 1975).

In 1948, Congress authorized

the Central and Southern Florida

Project for Flood Control and other

Purposes to provide flood protection

for urban and agricultural development and an adequate water supply

for development. A water-management plan was adopted that included

Lake Okeechobee and three water

conservation areas (WCA’s) and

that provided flood protection and

water supply through a complex

series of canals, levees, pumps, and

control structures. The northern

Everglades was identified as an area

that was suitable for agricultural

development on the basis of soil

thickness and geologic formations.

As a result, 800,000 acres was designated agricultural land and termed

the “Everglades Agricultural Area”

(EAA). Subsequently, most of this

land was drained and farmed. The

WCA’s were constructed in the central Everglades and consisted of

levees and canals that enclosed

areas that total about 900,000 acres

(fig. 23). These areas, which were

completed by 1962, provided flood

protection during the wet season by

storing water and discharging

excess water to the ocean and supplied water during the dry season for

irrigation and municipal uses (Klein

and others, 1975).

Congress authorized the channelization of the Kissimmee River

for flood control in 1954. Canal construction began in 1961 and was

Figure 23. Major canals, control structures, direction of water flow, and 

other hydrologic features in south Florida.

NATIVE AMERICAN

LANDS

BISCAYNE BAY

NATIONAL PARK

LOXAHATCHEE NATIONAL

WILDLIFE REFUGE

EVERGLADES NATIONAL

PARK

BIG CYPRESS

NATIONAL PRESERVE

FAKAHATCHEE STRAND

STATE PRESERVE

CONTROL STRUCTURE

WELL

WATER CONSERVATION AREA

MODIFIED KISSIMMEE−

OKEECHOBEE−EVERGLADES

BOUNDARY

DIRECTION OF SURFACE

WATER FLOW

EXPLANATION

SELECTED PUBLIC LANDS

S18C

S−196A

WCA

3 25°

83° 82° 81° 80°

28°

27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES

0 50 KILOMETERS 25

S−197

Turner

River

Canal

Tamiami Trail

Florida Bay

WCA

2

S5A

WCA

1

GULF OF MEXICO

L−67A

S−79

STUDY−UNIT

BOUNDARY

Caloosahatchee River

Caloosahatchee Canal

Golden

Gate

Canal

System

Barron

River

Canal

Faka

Union

Canal

Miami Canal

North New

River Canal

West Palm Beach Canal

St Lucie

Canal

Corkscrew Swamp

Sanctuary

WCA

3

Herbert

Hoover

Dike

ATLANTIC OCEAN

STRAITS OF FLORIDA

Myakka

River Peace

River

St Lucie

River

Charlotte

Harbor

Kissimmee

River

Lake

Okeechobee

Hillsborough

Canal

S−12’s

C111

S−333

S−196A

S18C
26

completed 10 years

later. The 90-mi-long

meandering river was

replaced by a 52-milong canal, and seasonal

flooding of much of the

river’s flood plain,

which averaged about

2 mi in width, was eliminated. Flow into Lake

Okeechobee was controlled by six locks and

dams along the rivers

(Carter, 1974).

Construction of the

Kissimmee River Canal

(C-38) considerably

altered the hydrology,

water quality, and wetlands in the Kissimmee

Basin. The dams in the

canal created pond

areas that were permanently flooded, whereas

farther upstream from

each dam, wetlands

were drained and replaced by terrestrial vegetation. As a result, about

40,000 to 50,000 acres of flood plain

marsh disappeared, resulting in a

significant loss of habitat for wading

birds and other aquatic animals

(South Florida Water Management

District, 1989) and in a loss of the

natural nutrient-filtering effects of

these wetlands. Drainage also eliminated the river’s natural oxbows and

stimulated agricultural development

in flood plain and adjacent wetlands,

all of which have contributed to the

increased nutrient loading to Lake

Okeechobee (Lamonds, 1975; Federico, 1982). The environmental

impacts of channelization were

quickly recognized, and calls for

restoration of the river began even

during canal construction. Plans are

underway to restore the river and its

flood plain by increasing water storage in the upper Kissimmee Basin

and by physical modifications to the

lower basin. This will include backfilling 22 mi of C-38, recarving 9 mi

of river channel, removing two

water-control structures, and removing flood-plain levees. Construction

will require approximately 15 years

(South Florida Water Management

District, written commun., 1993).

A large percentage of the water

that originally flowed from the

Kissimmee River and Lake Okeechobee into the Everglades is now

diverted directly to the Gulf of

Mexico by the Caloosahatchee Canal

and to the Atlantic

Ocean by the St. Lucie

Canal. The remaining

outflow from the lake

is released in canals

that pass through the

EAA (South Florida

Water Management

District, 1989). Water

is pumped from the

EAA into the WCA’s,

but the timing and spatial distribution of this

water delivery is

altered from natural

flows, and the amount

Figure 24. Miami Canal in Water Conservation Area 3.
27

of water discharged is greatly

reduced. As a result, water levels in

the Everglades generally are shallower and have shorter hydroperiods than those of predevelopment

time, and the timing and distribution

of flows have changed (Parker,

1974; Fennema and others, 1994).

In the northern parts of the WCA’s,

water levels are drawn down rapidly

by canals, and in the southern parts,

water ponds as flow is impeded by

levees (Dineen, 1972). The ponding

effect began in the mid-1960’s in

WCA–3 and resulted in extensive

flooding of tree islands. During

droughts, water is released from

Lake Okeechobee to the EAA and

the WCA’s. Most of this water, however, never reaches the interior

marshes, because it is confined to

canals and their nearby marshes.

Drainage of the Big Cypress

Swamp began in the 1920’s with the

construction of the Barron River

Canal (fig. 23). Subsequently, the

Turner River Canal was dug 5 mi to

the east. Even though these canals

have not been effective in lowering

water levels, both intercept substantial quantities of water from the

Okaloacoochee Slough and divert

them directly to the estuaries. A

major drainage system, the Golden

Gate Canals, was started in the Big

Cypress in the early 1960’s. As a

result of this system, water levels in

the western part of the swamp have

been lowered an average of 2 ft, and

seasonal flooding has been reduced.

Public Lands

Observation tower (looking south)

at Everglades National Park;

Shark River Slough

on the horizon

Today, a fragmented part of the

old, predevelopment landscape

exists in south Florida at the southern end of the peninsula. Most of

this area is wetlands (fig. 14) and is

in public ownership or under public

control (fig. 3). These lands include

Everglades National Park, Big

Cypress National Preserve, Loxahatchee National Wildlife Refuge,

the water-conservation areas, the

Fakahatchee Strand State Preserve,

and other State lands.

Parts of the Everglades were set

aside for preservation and protection

of wildlife by the Federal Government in the mid-1900’s. The Everglades National Park was established

in 1947 on marshland south of the

WCA’s and now covers about 1.4

million acres. The Loxahatchee

National Wildlife Refuge was established in 1951 in WCA–1 and covers

145,000 acres. The refuge, park, and

the other WCA’s contain most of the

remaining natural Everglades.

Everglades National Park

depends on seasonal flows of goodquality freshwater from outside its

boundaries from such sources as the

headwaters of the Shark River and

the Taylor Sloughs. However, flows

into the park from these sloughs

have been greatly altered. Federal

legislation was passed in 1968 to

assure that a minimum monthly

water delivery be made to the park.

With the minimum flows, however,

the wetlands and the aquatic animals

in the park still suffered from lack of

water in dry periods. Also, the legislation did not protect the park from

receiving too much water in

extremely wet years. Water delivery

to the park is now based on rainfall

measurements to the north of the

park and mimics natural seasonal

flow, but does not provide the prolonged flows and hydroperiods of

the predevelopment Everglades

when flows were attenuated from

one wet season into the next dry

season (Davis and others, 1994).

Also, water releases to the park are,

for the most part, outside the natural

flow path in the Shark River Slough.

In addition to alterations in flow,

deterioration of water quality in the

northern Everglades through nutrient enrichment also threatens the

integrity of the park (Amador and

others, 1992).
28

Agriculture

Despite the obvious benefit of a year-round growing season, most of south

Florida was originally not suited to farming because of annual flooding. Agricultural activity increased in the 1920’s as more and more peat soil in the

northern Everglades was drained. Drainage also opened land for farming

between the Everglades and the Atlantic Coastal Ridge (fig. 4) and in parts of

the Western Flatlands. Increasing availability of farm machinery, fertilizers,

and pesticides allowed for intensive farming and farming of marginal lands.

On rock land, for example, machinery was used to break up the original rock

surface and produce a coarse soil suitable for farming (Nicholas, 1973).

Today, farming is concentrated primarily in the northern Everglades, the

Western Flatlands, and the rocky glades west of the Atlantic Coastal Ridge

(figs. 2 and 4). Sugarcane (fig. 25), vegetables, and citrus are the most important crops. Vegetables from the region provide a large part of the Nation’s

winter supply. More than $750 million is earned annually from production of

sugarcane, vegetables, sod, and rice, and has provided more than 20,000 fulltime equivalent jobs (Snyder and Davidson, 1994).

Much of the farming in south Florida depends on the

rich muck land for the production of sugarcane, snap beans,

celery, cabbage, sweet corn, as well as other crops. However,

oxidation is progressively removing this important rich

organic soil. As water levels are regulated for agricultural development,

muck is alternately covered by

water and exposed to the air. During

low-water periods of drying, the

muck oxidizes and the probability of

fire increases. Subsidence rates in

the EAA have averaged about

1 in/yr. An elevated water table

reduces oxidation, and in recent

years, management to maintain elevated water levels in the soil has

probably reduced the average rate of

soil subsidence (Barry Glaz, Department of Agriculture, written commun., 1994). Forced abandonment

of farms is predicted in the intensively farmed muck land because of

soil subsidence. Other farms will be

abandoned or sold to urban and residential development as land taxes

and land values increase (Alexander

and Crook, 1973). 

Figure 25. Sugarcane fields south of Lake Okeechobee.
29

Pesticides are widely used in south Florida to control insects, fungi,

weeds, and other undesirable organisms. These compounds vary in their toxicity, persistency, and transport. Some of the more persistent pesticides, such

as DDT, chlordane, dieldrin, and aldrin, have been banned for use in the State,

but their residues still occur in the environment. Although pesticides are usually applied to specific areas and directed at specific organisms, these compounds often become widely distributed and pose potential hazards to

nontarget biota.

The major pesticides used for agricultural crops in south Florida and their

estimated application rate are listed in table 1. Herbicides having the highest

application rates, including atrazine, bromocil, simazine, 2-4-D, and diuron,

are among the most frequently detected pesticides in Florida’s surface waters

(Shahane, 1994). Insecticides currently applied in south Florida, such as

ethion and endosulfan, are sometimes detected in Florida’s surface waters. By

far the greatest frequency of insecticide detection is from chlorinated hydrocarbon insecticides that are no longer used in the State, such as DDD, DDE,

DDT, dieldrin, and heptachlor. These insecticides also are the most frequently

detected pesticides in bottom sediments (Shahane, 1994).

Fertilizers are widely used in south Florida to maintain high levels of

agricultural productivity. Fertilizers sold in the study unit from July 1, 1990,

through June 30, 1991, contained about 140,000 tons

of inorganic nitrogen and

56,000 tons of phosphate

(U.S. Environmental Protection Agency, written

commun., 1991). The rates

of fertilizer application for

this period, which were

based on county sales, are

shown in figure 26. The

average rates of fertilizer

application for the study

unit were 7 tons/mi2

 for nitrogen

and 3 tons/mi2

 for phosphate. The

highest rates of application

(19 tons/mi 2 for nitrogen and

8 tons/mi2

 for phosphate) were in

Palm Beach County.

Cattle ranching and dairy

farming are important agricultural

activities in south Florida (fig. 27).

The total number of cattle in the

study unit in 1994 was almost 1

million (U.S. Department of Agriculture, written commun., 1994).

Cattle ranching and farming is most

intensive near and to the north of

Lake Okeechobee, where densities

can exceed 200 head of cattle per

square mile (fig. 28). High densities

of cattle are a potential source of

nutrients that can degrade the surface and ground waters of the

region.

Table 1. Major pesticides used on agricultural crops in the south Florida study unit, listed in order of estimated 

total pounds of active ingredient applied annually (1989–91)

[Based on data compiled by Resources of the Future, 1990; 1992]

Insecticides 

Oil ........................................... 13,300,000

Ethion ......................................... 810,000

Phorate ........................................ 320,000

Methomyl ................................... 270,000

Aldicarb ...................................... 240,000

Endosulfan .................................. 230,000

Chlorpyrifos ............................... 200,000

Fenbutatin oxide.......................... 191,000

Ethoprop ..................................... 170,000

Methamidophos .......................... 170,000

Dicofol ........................................ 150,000

Fungicides

Sulfur ...........................................2,300,000

Copper ........................................ 1,900,000

Maneb ........................................... 600,000

Chlorothalonil ............................... 600,000

Fosetyl-Al ..................................... 170,000

Mancozeb ...................................... 120,000

Metalaxyl ...................................... 120,000

Benomyl .......................................... 65,000

Ziram ............................................... 45,000

Captan ............................................... 7,000

____________________________________

Herbicides 

Atrazine .......................................1,250,000

Asulam .......................................... 880,000

Bromacil ........................................ 730,000

Simazine ........................................ 490,000

2-4-D ............................................. 260,000

Diuron ........................................... 260,000

Dicamba ........................................ 240,000

Dalapon ......................................... 210,000

Paraquat ........................................ 170,000

Metribuzin ..................................... 140,000

Norflurazon ................................... 130,000
30

Figure 26. Estimated 

fertilizer sales, in tons of 

phosphate and nitrogen, 

in counties within the 

study unit, south Florida. 

(USEPA, 1990.) (1991 

data calculated by Jerald 

Fletcher, West Virginia 

University, using a method 

described in the USEPA 

publication.)

EXPLANATION

38,000

14,000

10,000

2,000

4,000

7,000

500

< 250

15,200

5,600

4,000

2,800

1,600

800

200

< 100

NITROGEN TONS

PER COUNTY

IN STUDY UNIT

ESTIMATED FERTILIZER

SALES, 1991

PHOSPHATE TONS

PER COUNTY

IN STUDY UNIT

STUDY−UNIT

BOUNDARY

83° 82° 81° 80°

28°

27°

26°

25°

0 25 50 MILES

0 25 50 KILOMETERS

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

GULF OF MEXICO

ATLANTIC OCEAN

BROWARD

DADE

PALM BEACH

GLADES

HENDRY LEE

CHARLOTTE

DE SOTO

HIGHLANDS

POLK

OSCEOLA

HARDEE

MARTIN

ST LUCIE

COLLIER

M O N R O E

MANATEE

HILLSBOROUGH

PASCO

HERNANDO

P I N E L L A S

CITRUS

SUMTER LAKE

OKEECHOBEE

INDIAN RIVER

ORANGE BREVARD

SEMINOLE

SARASOTA

Figure 27. Cattle in a 

drainage ditch near 

Lake Okeechobee.
31

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

25°

83° 82° 81° 80°

28°

27°

26°

0 25 50 MILES

0 50 KILOMETERS 25

MIAMI

Charlotte

Harbor

STRAITS OF FLORIDA

TAMPA

ORLANDO

Florida Bay

ATLANTIC OCEAN

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Lake

Okeechobee

M O N R O E

MARTIN

ST LUCIE

MANATEE

HILLSBOROUGH

BROWARD

DADE

PALM BEACH

GLADES

HENDRY LEE

CHARLOTTE

DE SOTO

HIGHLANDS

POLK

OSCEOLA

HARDEE

COLLIER

PASCO

HERNANDO

P I N E L L A S

CITRUS

SUMTER LAKE

OKEECHOBEE

INDIAN RIVER

ORANGE BREVARD

SEMINOLE

SARASOTA

EXPLANATION

NUMBER OF CATTLE

PER SQUARE MILE

0

1 to 15

40 to 80

90 to 105

130 to 170

GREATER THAN 200

AREA OUTSIDE

STUDY UNIT

STUDY−UNIT BOUNDARY

SELECTED CITY, TOWN,

OR VILLAGE

MIAMI

Figure 28. Estimated 

number of cattle per 

square mile, per county, in 

south Florida, 1992. (U.S. 

Department of Agriculture, 

National Agricultural 

Statistics Service, June 

1994.)
32

Urbanization

Urbanization in south Florida began along the Atlantic Coastal Ridge where

the land is high and close to the marine transport that was essential to the original coastal settlements. Extension of a railroad to Miami at the turn of the

century sparked phenomenal growth. Through the years, flood-control and

water-management practices have made some land west of the ridge available

for development. Today, more than 4 million people live in an urban complex

along or near the eastern coast (fig. 29). A second urban complex of more

than 0.5 million people has developed along the Gulf Coast between Charlotte Harbor and Cape Romano. The Everglades and the Big Cypress Swamp

presented a formidable barrier to development between the two coasts.

The major effects of urbanization on water resources are reduction of

infiltration, increase of flood potential, and degradation of the quality of water

bodies receiving water. Trash and litter deposited on streets and parking lots,

erosion of exposed ground as a result of construction, lawn and landscape fertilization, pet wastes, automobile emissions, atmospheric deposition from

industrial and thermoelectric powerplants, and seepage from landfills, septic

tanks, and disposal wells have been identified as sources of urban stormwater

loads. These sources of generally distributed substances are grouped together

under the classification of “nonpoint” to distinguish them from the more

readily identifiable industrialand domestic-sewage plant effluents, called “point sources”

(fig. 30).

Consequences of drainage for

urban and agricultural development have included seawater

intrusion into coastal parts of the

shallow aquifers. During the prolonged droughts of the 1930’s and

1940’s, seawater moved inland along

canal channels and infiltrated aquifers. At the end of the 1945 dry season, seawater intrusion had affected

large segments of the Biscayne aquifer, and several of Miami’s municipal

supply wells yielded salty water.

Water levels in southern Dade County

and in what is now the eastern part of

Everglades National Park were as low

as 2 ft below sea level (Parker and

others, 1955).

Uncontrolled drainage in southeastern Florida was halted in 1946 by

the installation of control structures

(barriers) near the outlets of most

drainage canals. These structures mitigated the recurring problems of seawater intrusion and excessively low

water levels. During the rainy season,

the controls are opened to release

water for flood prevention in the

urban and agricultural areas, and during

the dry season, they

are closed to prevent

overdrainage and to

retard seawater intrusion. Canal flows

have been controlled

since 1946, and regional water management in southeastern Florida was

begun in 1962 with

storage of water in

Conservation Area 3.

Water control in the

1950’s and management in the 1960’s

reduced canal outflows from the Everglades (Leach and

others, 1972) but resulted in increased

water losses from the

coastal ridge area

where flood prevention was necessary in

the rapidly expanding urban areas.
Figure 30. Location of major wastewater facilities south Florida. (Florida Department of Environmen

1990-94.)

25°

83° 82° 81° 

28°27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972Albers Equal-Area Conic projectionStandard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES0 50 KILOMETERS 25

Florida Bay

GULF OF MEXICO 

Charlotte

Harbor

Lake

Okeecho

STUDY−UNIT BOUNDARY

EXPLANATION

WASTEWATER FACILITY WITHDISCHARGE GREATER THANOR EQUAL TO 1 MILLIONGALLONS PER DAY IN 1990

ACTIVE LANDFILLS IN 1990

Key West SELECTED CITY, TOWN, OR VILLAGE

TAMPA

Arcadia

Clewiston

La Belle

Naples

Ho

Key West

Marathon

Okee

FortMyers

Port Charlotte

Immokalee

Sebring

Lakeland

Lake Wales

ORLANDO

Figure 29. Population density in south Florida, 1990. (Derived from 

U.S. Department of Commerce, Bureau of Census, 1990 Census of Population and Housing Public Law 94-171.)

STUDY-UNIT BOUNDARY

EXPLANATION

MEAN AREAL CENTER OF CENSUS

TRACT OR BLOCK NUMBERING AREA

PERSONS PER SQUARE MILE

1,000 TO 9,99910,000 TO 19,999

25° GREATER THAN OR EQUAL TO 20,000

83° 82° 81° 80°

28°27°

26°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972Albers Equal-Area Conic projectionStandard Parallels 29°30′ and 45°30′, central meridian -83°00′

0 25 50 MILES0 50 KILOMETERS 25

Florida Bay

STRAITS OF FLORIDA

ATLANTIC OCEAN

GULF OF MEXICO St Lucie River

Biscayne Bay

Charlotte

Harbor

Lake

Okeechobee
34

Water Use

Most public water supplies in south Florida are withdrawn from shallow

aquifers, generally from wells less than 250 ft deep. The most productive and

widespread of these aquifers are the Biscayne aquifer in the southeast and the

shallow aquifer in the southwest. The Biscayne aquifer has been designated

as a “sole-source water supply” by the U.S. Environmental Protection

Agency. Important also, but of lower yield, are the coastal aquifer, which

extends northward from West Palm Beach, and the local aquifers that are scattered through the remaining area, particularly those that supply water to the

western coast urban region.

The Floridan aquifer system is a source of water primarily north of Lake

Okeechobee. South of the lake, this aquifer is deeper and more brackish.

Although it is capable of large yields of saline water by artesian flow, it is not

generally used as a source of water supply, but rather used for wastewater

injection.

Freshwater withdrawals within the Southern Florida NAWQA study unit

were about 4,110 Mgal/d in 1990. Most of this water was used for public supply (22 percent) and agriculture (67 percent). Public water supply for most of

the 5.8 million people that live in the study unit is from ground-water sources

(table 2). Ground water supplied 94 percent (872 Mgal/d) of the water used

for public supply in 1990. Water withdrawn for agricultural purposes is nearly

divided between ground-water and surface-water sources. In 1990, ground

water accounted for 45 percent (1,230 Mgal/d) and surface water accounted

for 55 percent (1,505 Mgal/d) (Richard L. Marella, U.S. Geological Survey,

written commun., 1994).

Water Budget

The movement and storage of water

in south Florida is represented in a

schematic diagram in figure 31, and

the average annual flows from 1980

through 1989 in the area are summarized in figure 32. The Kissimmee

River annually discharged about 0.8

million acre-ft to Lake Okeechobee.

In turn, the lake annually discharged

about 0.4, 0.2, and 0.5 million acre-ft

south into the EAA, east into the St.

Lucie Canal, and west into the

Caloosahatchee Canal, respectively.

Table 2. Population characteristics, by hydrologic cataloging unit, in the south Florida study unit, 1990 

[From Richard L. Marella, U.S. Geological Survey, written commun., 1994] 

Name of unit Catalog unit 

number

Total 

population

Served by public supply Selfsupplied 

population Total water Ground 

water

Surface 

water

Kissimmee River 03090101 482,871 272,810 272,810 0 210,061

Northern Okeechobee inflow 03090102 24,466 17,035 0 17,035 7,431

Western Okeechobee inflow 03090103 5,036 0 0 0 5,036

Lake Okeechobee 03090201 4,152 0 0 0 4,152

Everglades 03090202 4,268,450 3,941,135 3,845,228 95,907 327,315

Florida Bay–Florida Keys 03090203 77,212 77,212 77,212 0 0

Big Cypress Swamp 03090204 233,601 211,917 193,172 18,745 21,684

Caloosahatchee River 03090205 250,712 190,103 163,043 27,060 60,609

Peace River 03100101 361,709 311,330 261,330 50,000 50,379

Myakka River 03100102 29,552 27,565 7,600 19,965 1,987

Charlotte Harbor 03100103 31,144 23,188 23,188 0 7,956

Total 5,768,905 5,072,295 4,843,583 228,712 696,610
35

Figure 31. Water storage and movement in south Florida (arrows indicate dominate flow direction). 

(Modified from South Florida Water Management District, 1993.) Gulf of Mexico

Atlantic Ocean

Biscayne

Bay

Barnes Sound

Florida Bay

Boundary of

modeled area

Charlotte

Harbor

Myakka River

Peace River

Lower East Coast

Kissimmee River

Taylor Creek

St Lucie

Canal

Lake Okeechobee

Everglades Agricultural Area

Fisheating

Creek

Caloosahatchee

River

Big Cypress

Swamp

Everglades

National

Park

Shark River Slough

Everglades National Park

Taylor Slough

Everglades National

Park Panhandle

Canal 111 Basin

Water

Conservation

Area 1

Water

Conservation

Area 2A

Water

Conservation

Area 3A

Water

Conservation

Area 2B

Water

Conservation

Area 3B
36

The average annual discharge to

coastal waters was about 1.3, 0.3, and

1.4 million acre-ft, respectively, from

the Caloosahatchee and St. Lucie

Canals, and into the Atlantic Ocean

from 11 canals south of the St. Lucie

Canal. The average discharge under

the Tamiami Trail to the southern

Everglades and the Big Cypress

Swamp was about 1.2 million acre-ft.

The movement and storage of

water in southeastern Florida from

1980 through 1989 was evaluated

by the South Florida Water Management District (1993) by using a

water-budget approach. Budgets

were developed for the region and

for subbasins in the region, which

includes Lake Okeechobee, the

three water conservation areas, the

Everglades Agricultural Area, the

eastern part of Everglades National

Park, and the developed Atlantic

Coastal Ridge (fig. 33). Hydrologic

components used in developing the

water budgets were those estimated

directly from measurement data,

which included rainfall, canal flows,

and consumptive pumpage, and

those estimated by using a computer

SURFACE−WATER−

FLOW DIRECTION

21,758

70,000

120,000

220,000

420,000

820,000

1,327,090

AVERAGE ANNUAL DISCHARGE,

IN ACRE−FEET (1980−1989)

STUDY−UNIT BOUNDARY

EXPLANATION

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

25°

83° 82° 81° 80°

28°

27°

26°

0 25 50 MILES

0 50 KILOMETERS 25

Lake

Okeechobee

Myakka

River

Caloosahatchee

Canal

Cape Romano

STRAITS OF FLORIDA

River

Florida Bay

ATLANTIC OCEAN

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

St Lucie

Canal

Caloosahatchee

Peace

River

Figure 32. Average annual discharge from major canals in south Florida.

The word, swamp, as we 

understand it, has no application 

whatever to the Everglades ... it is 

a country of pure water ... [that] is 

moving in one direction or another 

depending on the natural topography of the county; ... the air is 

wholesome, pure, free from 

disease ... Near the coast, the 

mangroves and the mosquitoes 

thrive; but deep in the Everglades, in the winter time at least, 

you can sleep comfortably without a net. No stagnant pools exist 

for the larvae to thrive in. 

Hugh L. Willoughby, 1898
37

model, the South Florida Water

Management Model (SFWMM),

which includes evapotranspiration,

overland flow, ground-water flow,

levee seepage, and both surfaceand ground-water storage changes.

There are varying degrees of

uncertainty in the measured and

the model estimates. Model results

probably overestimate the coastal

outflows and underestimate the

evapotranspiration in developed

areas (South Florida Water Management District, 1993). Although

the model is capable of simulating a

25-year period, lack of contiguousstructure-flow and canal-flow data

in the developed area of the lower

eastern coast prevented the development of water budgets for longer

than the 10-year period (South Florida Water Management District,

1993, p. C–8). The 1980 through

1989 period includes typical hydroperiods although the entire period is

considered to be somewhat dry compared with the long-term average

(South Florida Water Management

District, 1993).

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

25°

83° 82° 81° 80°

28°

27°

26°

0 25 50 MILES

0 50 KILOMETERS 25

STUDY−UNIT BOUNDARY

WATER BUDGET SUBBASIN

BOUNDARY

EXPLANATION

Myakka

River

Caloosahatchee

Peace

River

Cape Romano

STRAITS OF FLORIDA

River

Caloosahatchee

Canal

Florida Bay

ATLANTIC OCEAN

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

St Lucie

Lake Canal

Okeechobee

Everglades

Agricultural Area

Water−

Conservation

Area

System

Everglades

National

Park

East

Lower East Coast

Developed Area

Figure 33. Major water budget subbasins of southeastern Florida. (South 

Florida Water Management District, 1993.)
38

The average annual budget

summary for the modeled area

shows that rainfall dominates inflow

and that evapotranspiration dominates outflow (fig. 34). Rainfall

directly accounts for 89 percent of

the total inflow, and river and stream

inflows indirectly account for

another 11 percent. Ground water

contributes less than 1 percent of the

total inflow, and evapotranspiration

accounts for 66 percent of the total

outflow. Canal discharge to tidewater is the next largest outflow (22

percent). Overland flow, which is

primarily to the Shark River and

Taylor Sloughs, and pumpage for

consumptive use contribute about 6

and 4 percent, respectively, to the

average total outflow. 

A summary of the water budget

(table 3) shows a negative change in

storage, which indicates more

GROUND-WATER

INFLOWS

0.1

RAINFALL

88.7

ESTIMATED AVERAGE ANNUAL INFLOWS, 1980-89

TOTAL EQUALS 17.35 MILLION ACRE−FEET PER

YEAR OR 5,650 BILLION GALLONS PER YEAR

OTHER INFLOWS

5.7

KISSIMMEE RIVER

INFLOWS

4.6

OVERLAND INFLOWS

0.9

ESTIMATED AVERAGE ANNUAL OUTFLOWS, 1980-89

TOTAL EQUALS 17.77 MILLION ACRE−FEET PER

YEAR OR 5,790 BILLION GALLONS PER YEAR

GROUND-WATER

OUTFLOWS

1.7%

EVAPOTRANSPIRATION

66.0%

(EVAPOTRANSPIRATION IS

LIKELY TO BE UNDERESTIMATED

AND COASTAL OUTFLOWS ARE

LIKELY TO BE OVERESTIMATED.)

STRUCTURE/CANAL FLOWS

22.3

OVERLAND FLOWS

5.9%

PUMPAGE FOR

CONSUMPTIVE USE

4.1%

Table 3. Preliminary estimates of natural water budget, in 1,000 acre-feet, for the 

lower east coast area, south Florida, 1980-89

[area, 5,814 square miles. From South Florida Water Management District, 1993]

Component

Average

annual

(Jan.–Dec.)

Wet season

(June–Oct.)

Dry season

(Nov.–May)

1980–81

(June–May)

1988–89

(June–May)

Rainfall 15,398 9,336 6,050 12,237 13,694

Evapotranspiration 11,729 5,668 6,024 11,359 11,285

Net groundwater outflow 298 150 151 274 289

Structure/tributary outflow 1,794 904 934 473 1,113

Structure/canal outflow 3,956 1,953 2,032 2,857 3,015

Wellfield pumpage 723 301 424 663 847

Net overland outflow 905 583 326 753 1,212

Changes in storage –338 +1,632 –1,937 –3,526 –1,971

Figure 34. Comparison of estimated average annual inflows 

and outflows (as percentages) for 

the modeled region in south 

Florida. (South Florida Water 

Management District, 1993.)
39

outflow than inflow. Most of this

change (–338,000 acre-ft/yr) occurs

in the Lake Okeechobee part of the

model and would amount to a lowering of the lake level by about 5 ft

during the 1980s. According to the

South Florida Water Management

District (1993), the negative change

in storage is merely the result of

including a low-storage drought

year (1989) as the last year in the

model simulation. The report indicates that, if a longer time period

had been used for the simulation, the

change in annual storage would

have more nearly approached zero

(South Florida Water Management

District, 1993, p. 11-10).

Most of the surface-water outflow from the modeled area goes

into the Atlantic Ocean (fig. 35). An

annual average of 3.3 million acre-ft

is estimated to have been discharged

to the Atlantic from 1980 through

1989, whereas the combined discharges to the Shark River and Taylor Sloughs and C–111 averaged

813,100 acre-ft. (South Florida

Water Management District, 1993;

Light and Dineen, 1994). Annual

measured discharges to the Atlantic

Ocean (1980-89) at the 12 canals

shown in figure 32, which represents a substantial but incomplete

estimate of outflow to the ocean,

was about 1.8 million acre-ft or

about 1.5 million acre-ft less than

the model estimate. Although

coastal outflows to the Atlantic

Ocean may be overestimated by the

model because of uncertainties in

the budget (South Florida Water

Management District, 1993), the

magnitude of the outflows suggest

that capture or redirection of these

flows represents a potential for additional water supply for Everglades

restoration (Light and Dineen,

1994). 

Models, like the SFWMM,

have proven to be extremely useful

management tools in the short term

(periods generally less than a few

years). Great care must be taken,

however, when interpreting the

results of simulated decadal-scale

processes from models that are

designed for short-term management objectives. Oreske and others

(1994) observed that verification

and validation of numerical models

of natural systems is impossible.

They concluded that models are

most useful when used to challenge

existing assumptions, rather than to

validate them. 

Figure 35. Total discharge to Shark River Slough, Taylor Slough, C–111, 

and the Atlantic Ocean, 1980-89. (Light and Dineen, 1994.)

1,200

1,000

800

600

400

200

0

SHARK

RIVER

SLOUGH

TOTAL DISCHARGE,

IN BILLION GALLONS

559,000

acre-feet

86,300

acre-feet

167,700

acre-feet

3,300,000 acre-feet

TAYLOR

SLOUGH C−111 ATLANTIC

OCEAN
40

As stated throughout this report,

drainage and development have had

severe environmental effects on the

natural system of south Florida. The

landscape has been greatly altered,

and its natural functions eliminated

or changed over large areas. Water

quality has deteriorated. The native

plant and animal communities have

been reduced and stressed by the

altered hydrologic system and by

competing exotic species. Changes

in the natural system also are

adversely affecting the growing

human population of the region.

Loss of Wetlands and 

Wetland Functions

Drainage and development have

eliminated or severely stressed wetlands in south Florida. About half

the original Everglades has been

eliminated (Davis and others, 1994).

All the custard apple and willow

swamps south of Lake Okeechobee

(148,260 acres), and most of the

peripheral wet prairie (289,110

acres) and the cypress forest (30,000

acres) in the eastern Everglades

have disappeared. About 50 percent,

or 897, 000 acres, of the sawgrassslough communities and 24 percent,

or 146,000 acres, of the southern

Water and Environmental

Stress
41

marl marshes have been eliminated. Many of the remaining

wetlands have been adversely affected by a reduced hydroperiod that has been caused by accelerated runoff in drainage

canals. The increased frequency and spatial extent of wetland

drying has reduced aquatic production at all levels of the food

chain. Compared with the predrainage system, surface-water refugia that support populations of aquatic fauna and their predators

during droughts are smaller and fewer and are relocated and subdivided in the currently managed system. In addition, these

hydrologic changes have disrupted wading bird nesting, which

depends on concentrated food supplies that occur under normal

dry-season conditions (Kushland, 1991).

Loss of wetlands in south Florida has significantly reduced

landscape heterogeneity, habitat options, and long-term population

survival for animals with large spatial requirements. Wading birds,

snail kites, and panthers, for instance, have become increasingly

stressed by the fragmentation and loss of habitat (Robertson and

Frederick, 1994). Wildlife populations generally have declined. At

present, 18 species have been designated as threatened or endangered by the U.S. Fish and Wildlife Service and 12 more are under

review to determine their status (South Florida Water Management

District, 1992). The number of wading birds has decreased from

about 2.5 million in 1870 to about 70,000 in 1973 (Crower, 1974).

The large decreases in wading birds is a direct result of hydrologic

alterations (Kushland and others, 1975).

Drainage and development in south Florida have increased the

opportunities for exotic species to become established. Exotic species

often compete with and replace native species. Perhaps the most dramatic invasion into the region is the rapid spread of melaleuca tree into

the Everglades (Kushland, 1991). Melaleuca was introduced to Florida in

the early 1900’s and now grows as dense strands that have

replaced native vegetation in parts

of the eastern Everglades.

Photo courtesy of the Florida Panther National Wildlife Refuge
42

Degradation of Water Quality

Water quality has been degraded by human activities in large areas of south

Florida. Water in urban and agricultural canals commonly has high concentrations of nutrients and toxic compounds compared to water in marshes that are

remote from canals. Figure 36 illustrates the wide variability in the concentration of the nutrient, phosphorus, in south Florida waters. Drainage of nutrient and toxic-ladened water from urban and agricultural lands has degraded

lakes, streams, canals, estuaries, and bays of the region. 

Urban Lands

A degradation of water quality has been noticeable in recent years in the

urban-industrial areas along the eastern coast. Most degradation has been in

urban canals, especially during periods of low flow when many of these

canals are covered with algae and scum and are choked with aquatic weeds.

These conditions have been brought about through the discharge of nutrientladen sewage and stormwater runoff into canals. Runoff also carries bacteria,

viruses, oil and grease, toxic metals, and pesticides into urban canals from

which they can be discharged into coastal waters or can enter the groundwater system and the public water supply (Klein and others, 1975).

An ample supply of dissolved oxygen is most important for water of good

quality, especially in urban areas where much of the oxygen is used in the

decomposition of sewage. In contaminated canals with a luxuriant growth of

plants, the dissolved-oxygen content is high during daylight. During the night,

however, the dissolved-oxygen content may approach zero as the oxygen is

depleted to oxidize sewage. Thus, many urban canals lack popular sport fish,

such as bass, and are inhabited

instead by gar and mullet, which are

able to tolerate low levels of dissolved oxygen (Klein and others,

1975).

Chlorine chemicals, such as

polychlorinated biphenyls (PCB’s),

dioxin, and furans, which are generated and used primarily in urban and

industrial areas, pose serious health

hazards to fish, wildlife, and human

populations (Colborn and others,

1993). PCB’s are a diverse family of

compounds with a wide variety of

industrial applications, which

include use in transformers, plasticizers, rubber, adhesives, inks, sealants,

caulking compounds, and other products. Although most uses of PCB’s

have been banned since the late

1970’s, these persistent chemicals are

still found in the environment. Dioxins and furans are byproducts of

chlorinated industrial processes,

which include incineration of hazardous, medical, and solid wastes;

chemical manufacturing; plastic production; and chlorine bleaching of

pulp and paper. Chlorine chemicals

Soil Subsidence

The intensive drainage and associated agriculture south of Lake Okeechobee

in the EAA has caused a tremendous loss of organic soils. The compaction

and oxidation of organic soils in the agricultural lands south of the lake was

one of the first observed environmentally destructive effects of large-scale

drainage. In most areas, 5 ft or more of organic soil had been lost by 1984

(Stephens and others, 1984). A recent calculated rate of loss in the EAA is

about 1 in/yr (Barry Glaz, Department of Agriculture, written commun.,

1994). The maximum thickness of this soil, which is underlain by limestone,

was initially only 12-14 ft. The process of oxidative loss of soil continues,

although the process has been slowed in some locations by reflooding fallow

fields and maintaining a high water table.

Such a large loss of soil has affected hydrology and ecology of the Everglades in many ways. The altitude gradient from the upper to the central Everglades has been greatly affected by the soil loss. The loss of altitude has meant

a loss of the hydraulic head that once caused water to flow south. The movement of water from north to south

now requires pumpage, and the

pumpage effort necessary to move

water continues to increase with

time as the soil continues to subside.

The soil loss also has reduced waterstorage capacity, which has caused a

reduction in the ability of the area to

absorb water and mediate seasonal

and long-term variations in rainfall.

The problems caused by soil loss

are magnified by the enormous spatial extent of the loss. In fact, the

loss is not confined to the EAA, but

actually extends into the northern

parts of WCA–1 and WCA–3A,

where additional soil has been lost

as a result of the diversion of water.
43

Figure 36. Total phosphorous concentrations in water at selected sites in south Florida, 1984-93.

(All data from South Florida Water Management District, except USGS data at Peace River.)

83° 82° 81° 80°

25°

28°

27°

26°

EXPLANATION

90th percentile

75th percentile

Median

25th percentile

10th percentile

TOTAL PHOSPHORUS, IN MILLIGRAMS PER LITER

ARC

S79

S78

S77

S65

S65E

S308C

S80

S4

S3

S2

S8

S7

S6

S5A

S155

MR08

BL01

AR03

BR105

S12

P34

P33

P36

P35

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Lake

Okeechobee

Kissimmee

River

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

Myakka

River

St Lucie

River

Biscayne

Bay

Florida Bay

Charlotte

Harbor

River

St Lucie

Canal

Peace

River

Caloosahatchee

Canal

Caloosahatchee

0 25 50 MILES

0 25 50 KILOMETERS

STUDY−UNIT BOUNDARY

SURFACE-WATER SITE

EXPLANATION

P34

S78

S5A

S-155

P33

P36 P35

S12

P34 BL01

S80

S2 S3

S4

S77

S79

AR03

BR105

S65

S6

S7 S8

ARC

S65E

S308C

MR08
44

have long been known to have toxic

effects on fish and wildlife, but it has

been recently reported that these

chemicals can disrupt the endocrine

systems of fish and wildlife, which

cause such problems as reproductive

failure, birth deformities, demasculinization, defeminization and

immune system disorders (Colborn

and others, 1993).

PCB’s are widely distributed in

the south Florida environment

(Klein and others, 1975). The wide

distribution is probably a result of

volatization and transport by aerosols and fallout with dust or rain

(Pfeuffer, 1991). Concentrations,

are probably highest in industrial

urban areas. For example, fish from

a canal at Miami International Airport contained PCB’s in a concentration of 1,000 μg/Kg (Freiberger

and McPherson, 1972). The high

concentrations of these chemicals in

industrial urban areas could pose a

hazard for the drinking water supply

by entering the shallow Biscayne

aquifer (Klein and others, 1975).

Generally, ground water is less

susceptible to pollution than surface

water because it is filtered as it

moves through sediments. However, because contamination in

ground water is less readily

detected than in surface water,

and because of the difficulties

associated with working below

land surface, the contaminants

can become widely dispersed

and quite expensive to remediate by the time the magnitude

of the problem is recognized.

In past years, the prime threat

to ground-water quality in

south Florida has been that of

seawater intrusion in coastal

areas and near heavily pumped

municipal wells. In recent years,

however, seawater intrusion generally has been controlled by providing sufficient freshwater so that

adequate high-water levels are

maintained near the coast.

Rapid urbanization of the lower

eastern coast and growth of agricultural areas pose additional threats of

ground-water contamination by

manmade liquid and solid wastes

and by fertilizers and pesticides.

Because of the benefits of seasonally heavy rainfall and dilution,

however, the level of contamination

probably has been excessive only in

some areas. Ground water in the

section of the Biscayne aquifer

beneath the densely populated urban

parts of southeastern Florida contains nutrients and coliform bacteria

at shallow depths as a result of effluent from thousands of septic tanks

and disposal wells and from seepage

from polluted canals. These contaminants presumably have accumulated over the 60 years of

urbanization. Although septic tanks

operate effectively in the southeast,

they provide only partial water treatment. Septic tanks can become ineffective from the lack of periodic

maintenance. In such cases, raw

wastewater seeps into the aquifer

Floating aquatic plants

in a pond bordered

by cypress

trees.
45

and causes local pockets of pollution. The recent constraints on the

use of septic tanks and the construction of sewer systems to service large urban sectors will further

curtail the use of septic tanks in

many areas and thereby reduce a

potential cause of contamination to

the ground-water system.

Solid-waste disposal in dumps

poses another, but less widespread,

source of pollution of shallow aquifers by contributing leachates from

garbage and trash. Many materials

in dumps are toxic and nonbiodegradable. Preliminary data from

widely scattered study sites in

southeastern Florida indicate that

local pollution plumes exist downgradient from the dump sites but

primarily in the shallow parts of the

aquifer. Contaminated ground water near dumps also contains

coliform bacteria and nutrients because sludge from many septic

tanks and small sewage-treatment

plants is disposed at some dumps.

Traces of toxic metals and organics

also have been found in the shallow

ground water beneath the dumps.

Because of the high permeability of

the aquifer, contaminants can move

readily to wells and estuaries in

downgradient areas.

Agricultural Lands and 

Everglades Region

The marshes in the northern Everglades have been replaced, for the

most part, by the EAA which

includes about 700,000 acres of rich

organic soils, most of which has

been converted to intensively managed agriculture; the crops are primarily sugar cane and vegetables.

Nutrients from the EAA soils are

released as the result of periodic

drying and oxidation of these

organic soils by aerobic bacteria

(soil subsidence). Once the soil is

oxidized, large quantities of nitrogen and phosphorus are released and

transported from these fields during

subsequent rains. Nutrients are carried from the field through drainage

ditches, water-control structures,

and flood-control pumps into EAA

canals. Water that drains the EAA

farmlands contains low dissolvedoxygen concentrations; high concentrations of nitrogen, phosphorus,

chloride, sodium and trace metals;

high color; high specific conductivity; and occasional pesticides. Water

quality generally is worse during

periods of pumping than during

those of no discharge.

Present water-quality conditions in some northern parts of the

Everglades are dramatically different than conditions that existed at

the turn of the century. These

changes are a result of the disruption

of the natural flow patterns by drainage canals and the development

of agriculture south of Lake
46

Okeechobee. Nutrient concentrations in water discharged from

canals that drain the EAA are significantly higher than those in the

marsh at locations remote from the

canals. Average flow-weighted

phosphorus concentrations at canalcontrol structures that discharge to

the northern Everglades from the

EAA range from 0.10 to 0.25 mg/L

and represent a tenfold increase in

nutrient levels compared with water

at the marsh sites remote from the

canals (South Florida Water Management District, 1992).

A number of studies have identified the Everglades marshes of the

WCA’s as a natural filtration system

or nutrient sink that has a purifying,

or “kidney effect,” by reducing inorganic forms of nitrogen and phosphorus to background levels as the

waters flow through the marsh (Jones

and Amador, 1992). Much of the

introduced nutrients are assimilated

or incorporated into sediment and

marsh vegetation. The marsh vegetation of the Everglades, however, has

a limited capacity for nutrient uptake

and is sensitive to small increases in

nutrients which are measured in the

parts per billion range. Historically,

the Everglades was a low-nutrient

system, limited primarily by

phosphorus. The algae and vascular plants that comprise the

marsh system have developed

under conditions of low nutrient

inputs that are characteristic of

pristine rainfall. Sawgrass, a

major plant component of the

Everglades system, has a low nutrient requirement and is competitive

with other vascular plants in a low

nutrient environment (Steward and

Ornes, 1973 a,b). Sawgrass has been

replaced by cattail in parts of the

northern Everglades, particularly in

WCA–2 where nutrient concentrations

are high. Cattails thrive under highnutrient conditions and have an advantage over sawgrass under nutrientenriched conditions (Davis, 1991).

Algal species and bacterial populations also have changed at nutrient

enriched locations in the northern

Everglades. These changes include

loss of the native periphyton mat,

changes in algal species present, phytoplankton blooms, and changes in

microbial populations that result in

prolonged low dissolved-oxygen

concentrations or anoxic conditions

(Steward and Ornes 1973 a,b; Swift

and Nicholas, 1987). These changes

in Everglades vegetation appear to be

due to combined effects of nutrient

enrichment, hydroperiod change, and

fire (South Florida Water Management District, 1992).

There is concern that the nutrient enrichment and ecological disruption in the northern Everglades

will spread south and adversely

affect the southern Everglades,

which includes Everglades National

Park. Long-term water-quality data

have shown an increase in specific

conductance and major ion concentrations within the Shark River

Slough in the southern Everglades.

These increases are due to changes

from the natural sheetflow regime to

one dominated by canal delivery

(McPherson and others,

1976; Flora and Rosendahl, 1982).

No changes in metal concentrations

have been observed. Trace levels of

pesticides occasionally have been

detected in waters that enter the park.

However, trace metals and pesticides

are only slightly soluble in water and

their presence in the park may be

mainly in sediments and biota

(Ogden and others, 1974). Average

flow-weighted phosphorus concentrations discharged into the park from

1979 through 1988 was 0.011 mg/L

for the four control structures (S–

12’s). Although these concentrations

are much less than those in the northern Everglades, there is concern that

nutrient concentrations at the S–12’s

may increase unless delivery of

water by canals, such as C–67A, is

replaced or supplemented by natural

sheetflow (South Florida Water Management District, 1992). 

Water quality in the southeastern Everglades (the northeastern

part of the Shark River Slough, C–

111 basin, and the Taylor Slough) is

typically good. Flora and Rosendahl

(1982) and Waller (1982) reported

low nutrient concentrations within

the northeastern part of the Shark

River Slough and the Taylor

Slough.
47

Current data collected by the

SFWMD from 1979 through 1988

indicate that flow-weighted total

phosphorus concentrations at S–333

in the northeastern part of the Shark

River Slough (fig. 23) averaged

0.026 mg/L over the 10-year period

of record. Concentrations were

highest (0.043 mg/L) during the

drought of 1985 as a result of water

being routed from Lake Okeechobee to western Dade County

well fields through L–29 for watersupply purposes. Total phosphorus

values within C–111 at S–18C averaged 0.007 mg/L from 1985 through

1987 and were comparable to values

recorded at interior marsh sites. C–

111 water, however, contained moderate concentrations of dissolved

minerals compared with the Miami

and Hillsborough Canals. Canals

that drain the urban and agricultural

lands that lead to C-111 had detectable levels of such pesticides as atrazine and chlordane (South Florida

Water Management District, 1992).

The most significant water-quality

effects observed within the C–111

basin has been the periodic removal

of the S–197 structure from the

mouth of the canal to alleviate

upstream flooding. This removal

released large amounts of freshwater into Manatee Bay, Barnes

Sound, and other surrounding estuarine areas and caused severe impacts

to marine biota (South Florida

Water Management District, 1992).

Lake Okeechobee

Water quality in Lake Okeechobee

has been degraded by large-scale

inflow from streams that drain agricultural land on the northern side of

the lake and by backpumping from

canals in the EAA to the south

(Klein and others, 1975). Agricultural wastes are washed from farmlands into canals during heavy

runoff. During high-water periods,

excess water is sometimes backpumped from the EAA into Lake

Okeechobee to prevent crop damage. The average concentration of

dissolved solids of the inflow from

the north and from the EAA is

higher than anywhere else in south

Florida, excluding the saltwater

areas, and at times, it is more than

three times the average in the Big

Cypress Swamp. The high concentration of dissolved solids is partly

the result of irrigating with highly

mineralized water (Klein and others,

1975).

Water-quality and biologic data

collected in 1969 and 1970 indicate

that Lake Okeechobee was in an

early eutrophic condition by the late

1960’s (Joyner, 1971; 1974). The

rate of eutrophication is of major

concern because the lake is the primary surface reservoir in southeast

Florida. Overenrichment could seriously impair its water quality and,

thereby, affect downstream water

users. Joyner (1971) reported that

the growth of algae increased

greatly between January and July

1970. The dominant species also

changed from a green alga, which is

indicative of early eutrophic conditions, to a blue-green alga, which is

indicative of late eutrophic conditions. Inflow to the lake increased at

this time because of channel

improvement and accelerated

inflow from its major tributary, the

Kissimmee River. After channelization of the Kissimmee River, water

flow through the flood plain

marshes was reduced.

During the last 15 years, phosphorus concentrations have increased 21

/2 times in Lake Okeechobee; peak levels were reached in

1987-88. Recent occurrences of

massive, lakewide blooms of bluegreen algae are viewed as another

sign that the lake is receiving excessive amounts of nutrients, primarily

phosphorus, which threaten the

overall health of the lake resources.

Preliminary evidence indicates that

the lake’s sediments may be losing

their ability to assimilate additional

phosphorus loadings. Lakewide

nitrogen-to-phosphorus ratios also

have declined over the same period

and may have favored the shift from

the normal algal flora of the lake to

less desirable blue-green species.

These data suggest the lake may be

in a phase of transition from its

present eutrophic condition to a

higher trophic (hypertrophic) state.

Loss of the lake as a recreational

fishery or potable water supply

would be a major economic loss to

the region. Phosphorus has been

considered to be the key element

that controls the growth of these nuisance algae. Therefore, to prevent

further eutrophication of the lake,

the primary water management

strategy has been to limit and control phosphorus inflows (South

Florida Water Management District,

1992). Recent research, however,

has shown that primary productivity

is most often limited by nitrogen or

light (N.G. Aumen, South Florida

Water Management District, written

commun., 1994).

Big Cypress Swamp

The quality of water in the remote,

undrained parts of the Big Cypress

Swamp is good and probably best

reflects south Florida’s pristine

water-quality conditions. The Everglades, on the other hand, has been

affected more than the Big Cypress

Swamp by land-use and water-management practices, and water quality, particularly in the northern part

of the Everglades, is of poorer quality than that in the swamp. Most of

the water-quality data described

below were collected more than 20

years ago.
48

The concentrations of dissolved

solids indicate general water-quality

conditions of a freshwater environment. In samples collected in the

Big Cypress Swamp in the 1960’s,

concentrations averaged about 250

mg/L (Klein and others, 1970). In

the northern Everglades, for comparison, concentrations at three

long-term stations (1950–70) averaged 471 to 541 mg/L (McPherson,

1973). Sources of dissolved

solids include limestone, agricultural and urban runoff,

salty artesian ground water,

and seawater. In the northern

Everglades, dissolved solids

are attributable primarily to

saline ground water and agricultural runoff, whereas in the

Big Cypress they are attributable to exposed, soft

limestone. Interestingly, dissolved-solids concentrations

from samples collected at a

long-term station (1950-65,

1969) in the southern Everglades averaged 205 mg/L

(McPherson, 1973). The relatively low value probably

reflects an environment with

little exposed limestone. Concentrations of nitrogen and

phosphorus in the surface

waters of the Big Cypress also

are usually low compared

with concentrations in water in the

urban and agricultural canals of

south Florida (Klein and others,

1970).

Although water quality in the

Big Cypress Swamp is good, it has

been degraded to some extent by

human activities. Canals in the

western part of the swamp transport

potentially toxic metals to the estuaries in concentrations and amounts

that are greater than those transported by overland flow (Little and

others, 1970). Magnesium, iron,

cobalt, cadmium, copper, zinc, and

lead have been concentrated above

natural levels in Chokoloskee Bay

presumably because of transport

down the Barron River and the

Turner River Canals from agricultural land to the north (Horvath,

1973; Mattraw, 1973). Concentration of pesticides are low in surface

water of the Big Cypress Swamp,

but are concentrated in soils and

biota (Klein and others, 1970).

Although chloride concentrations

tend to be low (averaging about

20 mg/L) in the interior parts of the

Big Cypress, high concentrations

(as much as 5,350 mg/L) have been

detected around several exploration

oil-well sites (Wimberly, 1974).

Nutrients transported from

drained parts of the Big Cypress

Swamp to the estuaries exceed those

transported by overland sheetflow

in the undrained areas. The Faka

Union Canal (fig. 23) transports to

the estuaries almost 5 times as much

Kjeldahl nitrogen (ammonia and

organic nitrogen), 10 times as much

total phosphorus, and 7 times as

much organic carbon as the Fakahatchee Strand transports by sheetflow (Carter and others, 1973).

Trace elements concentrated in finegrained organic sediments (less than

20 microns), also are transported to

the estuaries by canals that extend to agricultural land in the

north (Mattraw, 1973). Finegrained sediments are readily

transported when canal flow

is high and are deposited with

mangrove detritus in the estuaries. Because mangrove detritus is a major food source,

its enrichment may provide a

pathway for the metals to enter into the estuarine and marine food chains (Mathis,

1973).

Metals transported from

the drained parts of the Big Cypress Swamp to the estuaries

also exceeded those transported by overland sheetflow in the

undrained areas. Chokoloskee

Bay, which receives heavy

metals from the Barron River

Canal, has greater loads of

these metals in its waters and

sediments than more remote

bays and estuaries to the south

that do not receive canal flows (Horvath, 1973; Mattraw, 1973). 

Charlotte Harbor Watershed

Much of the Charlotte Harbor

watershed has been altered by

human activities. Streamflow has

declined significantly during 1934–

84 in sections of the Peace River,

probably as a result of ground-water

pumping in the upper basin. Nutrient concentrations generally have

increased in the rivers over the last
49

15 years because of an increase in

wastewater effluent and agricultural

runoff. In 1984, 114 facilities were

permitted to discharge domestic or

industrial effluents to waters tributary to Charlotte Harbor. One of

these facilities is in the Myakka

River Basin, 11 are in the coastal

basin, 14 are in the Caloosahatchee

River Basin, and 88 are in the Peace

River Basin. Citrus and phosphateore processing account for most of

the industrial effluent. Several locations in the headwaters of the Peace

River show significant effects of

receiving wastewater effluent. At

some locations, dissolved-oxygen

concentrations were lower than 2.0

mg/L, which is the minimum State

standard for any class of surface

water (Hammett, 1990). 

The water quality of several

lakes in the headwaters of the Peace

River has been affected by citrusprocessing effluent (Hammett,

1990). Citrus processing produces a

strongly buffered, high carbon

waste that can contain inorganic

debris from washing, and can have a

residue of pesticides and toxic peel

oils (Lackey, 1970). The degradation of the waste produces objectionable odors and a high

biochemical oxygen demand. Citrus

production involves the use of

numerous chemicals that include

fertilizers, insecticides, herbicides,

and fungicides. Benomyl, bromocil,

diuron, dicofol, chlorobenzilate,

ethylene dibromide, and aldicarb

have been used or are currently in

use. The trace elements copper,

manganese, and zinc also are

applied to citrus (Rutledge, 1987).

Runoff or ground-water seepage

from citrus groves has the potential

of transporting any of these substances to the stream system.

The concentrations of phosphorus are naturally high in the Peace

River because of extensive phosphate deposits in its basin. The

phosphate deposits also are rich in

radionuclides of the uranium–238

series, including radium–226. In the

upper basin, these deposits are

exposed in the riverbed. Extensive

phosphate mining and processing

have exposed additional deposits to

surface runoff. Periodic spills of

phosphate industry sediments are

caused by the structural failure of

retaining dikes and have resulted in

the discharge of clayey wastes,

known as slime, to the Peace River

and has contributed additional phosphorus and radium–226 to the river

and estuary. A single spill can contribute a phosphorus load equal to

the annual loading in the Peace

River at Arcadia (Miller and

McPherson, 1987). The effects of

these slime spills have been seen as

long as 2 years after the event (Martin and Kim, 1977).

In phosphate ore processing

plants, a mixture of organic chemicals, which include kerosene and

fuel oil, is used to facilitate separation of phosphate ore from

unwanted sands and clays. Runoff

from sand tailings may represent

diffuse sources of organic-chemical

contamination (Rutledge, 1987).

The chemical processing of phosphate ore into phosphoric acid produces a highly acidic process water.

Organic chemicals, which include

phenols, also are used in processing.

The gypsum stacks, cooling ponds,

and recirculation ditches of the

chemical-processing plants are a

potential source of contamination

of the surficial aquifer (Miller and

Sutcliffe, 1984). Runoff

from phosphate mines

may increase turbidity

and significantly reduce

light in the receiving

bodies of water (Miller

and Morris, 1981). 

There are other

potential sources of

nutrient and pollutant

loads to the Charlotte

Harbor watershed.

Ground-water inflow to

the rivers and Charlotte Harbor is an

apparent source of radium–226

(Miller and Sutcliffe, 1985; Miller

and others, 1990). Background levels of radium–226 in the rivers and

Harbor reported by Miller and others (1990) are an order of magnitude

higher than those found in other

parts of the United States (Elsinger

and Moore, 1980; 1983; 1984).

Runoff from pastures and cropland

carries nutrients and, in some cases,

pesticides to the river system. Septic-tank drainfields are another

source of nutrients and a potential

source of bacterial contamination.

Runoff from urban areas may carry

heavy metals, nutrients, bacteria,

viruses, and pesticides. Marinas

contribute oil and gas, as well as

wastewater and metals, to the rivers

and estuarine system. Rainfall and

dustfall bring contaminants and

nutrients by air to the river system

and estuary.

Numerous deep wells under

high artesian pressure, drilled into

the Floridan aquifer system many

years ago, have been a source of

brackish water contamination of

shallow aquifers in southwestern

Florida and south of Lake

Okeechobee (Parker and others 1955;

McCoy, 1972). These wells were not

cased deeply enough or else the casings have corroded, allowing brackish water that is under pressure from

the Floridan aquifer system to move

upward along open well bores or

through corroded sections and seep

outward to contaminate large parts

of shallow aquifers (Klein and others, 1964; Sproul and others, 1972).
50

Mercury Contamination

Mercury contamination is a significant problem for aquatic organisms and

predators as well as a potential health risk for humans. Many lakes in the

northern United States, southern Canada, and Scandinavia are affected by

mercury contamination (Mierle, 1990; Hurley and others, 1991; Lindquist,

1991). Florida and 27 other states have issued health advisories that restrict

consumption of fish because of high levels of mercury (Tom Atkeson, Florida

Department of Environmental Regulation, written commun., 1992). The

problems with mercury relate to two factors—mercury biomagnifies in the

food chain to toxic concentrations, even though it is found at very low (subnanogram–per–liter) concentrations in water; and mercury is transported and

deposited through atmospheric processes and is, thus, widely distributed.

Methylmercury is the more toxic bioaccumulative species. As a result of environmental changes, increased

rates of methylation may be contributing to toxic mercury problems. For example, a change from an aerobic to an anaerobic environment would favor

increased mercury methylation (Winfrey and Rudd,

1990; Gilmour and Henry, 1991) and mercury

uptake in the biota. Because of the existing burden

of mercury in the environment, recycling of mercury between demethylated and methylated forms

can provide a source of methylmercury even if

atmospheric deposition ceased. However, estimates indicate that atmospheric mercury concentrations have nearly doubled over the last 50

years (Fitzgerald and others, 1991; Slemr and

Langer, 1992). Increasingly evident is the fact

that even modest increases in atmospheric

deposition can translate into mercury levels

in organisms that are of ecological and, perhaps, toxicological concern.

High levels of mercury have

been measured in fish (fig. 37) and

wildlife in south Florida, particularly

in the Everglades. Roughly 1 million

acres of the Everglades are under a

health advisory that recommends that

anglers and others completely avoid

consuming large mouth bass and several other species of fish (Lambou

and others, 1991). Mercury body

burdens of large mouth bass collected in the Everglades were higher

than those taken at Superfund sites

noted for mercury contamination

(Dan Scheidt, U.S. Environmental

Protection Agency, personal commun., 1992). Alligators harvested

from the Everglades cannot be sold

for human consumption because of

elevated mercury levels in their tissue. In 1989, a Florida panther died

from mercury toxicosis (Lambou

and others, 1991), and mercury is

suspected as the causative agent in

the deaths of two other panthers.

Analysis of raccoons, a major prey

of panthers, from certain areas of

south Florida revealed very high

concentrations of mercury in liver

and muscle tissues (Lambou and

others, 1991).
51

The result of a recently completed study in which 50

sediment cores were taken across the Everglades indicate

that mercury accumulation rates increased about sixfold

between 1900 and 1992 (Delfino and others, 1993). The

greatest increases in mercury concentrations were in the

northern Everglades, and the timing of the increases corresponded fairly well with alterations of Everglades hydrology and with agricultural and urban development in south

Florida.

The severity of the mercury

problem in the Everglades may be

the result of a combination of factors. Concerns are focused on two

potential sources of the problem—

local effects of municipal incinerators and other emission sources on

the southeastern coast of Florida,

and possible increased release of

mercury or increased methylation of

mercury from soils in the Everglades

as a result of drainage and soil disturbance (Tom Atkeson, Florida

Department of Environmental Regulation, written commun., 1992).

Additional research on mercury

in south Florida continues. The

Electric Power Research Institute,

Florida Power and Light, and the

Florida Department of Environmental Protection are currently funding

a project to determine atmospheric

deposition patterns and rates of mercury deposition in the region. The

U.S. Environmental Protection

Agency has initiated a study of mercury in the Everglades with the goal

of establishing spatial trends of mercury concentrations in water, sediment, and biota. The USGS is

planning research in the Everglades

on processes that control mercury

transport and cycling. Research

studies are being planned by the

State in the northern Everglades

where a nutrient-removal project is

underway as a result of concern that

the effects of nutrient removal will

promote the release of sedimentbound mercury or an increase in the

methylation of mercury.

Figure 37. Areas in south Florida where mercury concentrations in largemouth bass tissue equaled or exceeded one-half parts per million.

(Lambou and others, 1991.)

EXPLANATION

MERCURY CONCENTRATIONS

IN BASS TISSUE

(in parts per million)

0.5 TO 1.5

Greater than 1.5

STUDY−UNIT BOUNDARY

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30′ and 45°30′, central meridian -83°00′

25°

83° 82° 81° 80°

28°

27°

26°

0 25 50 MILES

0 50 KILOMETERS 25

Canal

Florida Bay

Whitewater

Bay

Ten Thousand

Islands

ATLANTIC OCEAN

STRAITS OF FLORIDA

GULF OF MEXICO

St Lucie

River

Biscayne

Bay

Charlotte

Harbor

Kissimmee

River

River

Caloosahatchee

Peace

River

Myakka

River

St Lucie

Caloosahatchee

Canal

Lake

Okeechobee
52

Effects on Estuaries, Bays, and Coral Reefs

Changes in coastal estuaries are due, in part, to such factors as an overall

reduction of the amounts of freshwater flow through the Everglades, the

effects of constructing levees and canals near the coast to provide drainage

and flood protection, the changes in the quality of runoff water, and the maintenance of lower ground-water levels along the southeastern coast (Smith and

others, 1989; Halley and Hudson, 1993). Although the amount of flow reduction is disputed, the effects of drainage activities are observed in the

encroachment of mangrove forests into northern Everglades National Park in

recent years, the replacement of freshwater marshes by saltwater marshes,

and the decline of coastal mangrove forests in areas that have been deprived

of natural overland flow. Today, many of these coastal estuaries, especially

along the southeastern coast and the central part of Florida Bay, frequently

experience hypersaline conditions during the dry season (Davis and Ogden,

1994; McIvor and others, 1994).

Water management has resulted in more short-duration, high-volume

water flow and less life-sustaining base flows to estuaries. Regulatory

releases to control lake and ground-water levels according to prescribed

flood-prevention formulae result in pulses of freshwater entering estuaries,

causing rapid, drastic decreases in salinity that stress estuarine organisms. In

addition, water flows have been diverted from one receiving basin to another,

which changes the long-term salinity regimes in both systems. The combined

effects of reduced freshwater inflow to Florida Bay as a result of man’s alteration of the natural flow regime and the natural evaporation of water in this

semiconfined embayment, have resulted in salinities that can be more than

double those of the open ocean. Widespread chronic hypersalinity in the bay

is extreme and frequently reaches 50 ppt over large areas, with known maximums of 70 ppt in small areas during severe drought (Tabb and others, 1962;

Tabb, 1963). Biscayne Bay sometimes exhibits negative, or reverse, salinity

gradients, with hypersaline conditions inshore. On the other hand, salinities

in Manatee Bay have declined from 36 to 0 ppt in a matter of hours as a result

of an abrupt release of freshwater.

Florida Bay has undergone changes during the last decade that are unprecedented within the period of recorded observation and that reflect a degradation

of the ecosystem, in terms of its productivity of living resources, biodiversity,

and stability (Bancroft, 1993; Boesch and others, 1993). Seagrasses

have died in large areas (Robblee

and others, 1991) and microscopic

algae have bloomed with increasing

frequency and intensity, thus turning

the once clear waters a turbid green.

Populations of water birds, forage

fish, and juveniles of species of

game fish seem to have been significantly reduced, catches of pink

shrimp have declined, and many

sponges have died, causing a potential threat to the catch of spiny lobsters. These and other ecological

alterations in the Bay were recently

summarized in a research plan by the

Interagency Drafting Committee

(unpublished, 1994). Because the

freshwater flow through the Everglades into Florida Bay has been

greatly reduced by consumptive use

and watershed drainage (McIvor and

others, 1994), much concern has

been directed to flow reduction as

the root cause of the deterioration of

the bay ecosystem. However, other

causes may be implicated in changes

in the bay; for example, nutrient

stimulation from internal sources

(benthic sediment releases) and external sources (runoff and seepage

from the land) (Lapointe and Clark,

1992) and natural influences, such as

the diminished frequency of tropical

storms in the vicinity of the bay

(Boesch and others, 1993).
53

Figure 38. Long-term changes on a coral reef community are illustrated by photographs taken in 1976 and 1992 at 

the same location on a reef near Key Largo. The thick growth of staghorn coral (Acropora cervicornis) evident in 

1976 had largely disappeared in 1992. This change may be natural or related to human activities. (E.A. Shinn, 

written commun. 1993.)

The amount and distribution of

nutrients that flow into coastal waters

of south Florida are extremely

important to the health of the bays,

estuaries, and reefs (Lapointe and

others, 1990; Fourqurean and others,

1992; Lapointe and Clark, 1992).

Denitrification in Everglades sediments is not an effective means of

removing excess nitrogen that may

be introduced as nitrate into coastal

waters (Gordon and others, 1986).

The flow of freshwater and nutrients

from urban and agricultural lands,

extensive mangrove and marsh wetlands, and phosphate-rich river

waters of southwestern Florida may

create a blend of water that contains

adequate concentrations of nutrients

to support algal blooms and enrichment in the western part of Florida

Bay. In addition, if freshwater flow to

the Atlantic Ocean is rediverted to

the Gulf of Mexico and Florida Bay,

as is being discussed as part of the

Everglades restoration plan, then

nutrient loading to the bay and gulf

might substantially increase.

In much of Florida Bay, the Florida Keys, and the reefs, the quality of

marine water is widely recognized as

important, if not more important,

than restoring freshwater flow to Florida Bay (Hallock and others, 1993). Continued residential development in the Florida Keys will intensify problems of

increased stormwater runoff, septic tank and disposal well leachate, and nutrients and heavy metal contamination from marinas and live-aboard vessels

within the Florida Keys area. The EPA and NOAA Florida Keys National

Marine Sanctuary prepared a plan to deal with the continuing development and

associated water-quality changes (U.S. National Oceanic and Atmospheric

Administration, 1995). However, significant changes in water quality may have

already taken place, as indicated by the seepage of sewage components in

marine ground water offshore from the Keys (Shinn, 1993; Shinn and others,

1994).

Changes in Florida coral reefs (fig. 38) have received increasing attention

during the last decade (Ward, 1990). Some damage to the reef corals by ships

or divers, for example, may be easily related to human intervention (Hudson

and Diaz, 1988; Talge, 1992). Other changes, however, are slower and pose difficulties in determining their ultimate cause. For example, coral mortality is

often the result of disease, such as “bleaching” that results when symbiotic

algae are expelled from the coral tissue, thus causing the corals to lose their

color (Brown and Ogden, 1993). 

In the last two decades, coral diversity and the amount of seafloor inhabited by coral has declined in the reef tract off the northern Florida Keys (Dustin and Halas, 1987; Porter and Meier, 1992). The decrease in general coral

health has been variously attributed to global warming, nutrient increases,

disease, and hypersaline water. These possible causes have not been fully

documented. Some changes on the reefs are difficult to identify with local,

regional, or global change. Some of the observed change may be related to

widespread change throughout the Caribbean (Rodgers, 1985) or may be the

result of more local causes. The U.S. Environmental Protection Agency is currently funding studies to monitor reef and hard-bottom community changes in

the Florida Keys National Marine Sanctuary (Continental Shelf Associates,

Inc., 1995).
54

When Europeans first arrived in south Florida, the region was a lush,

subtropical wilderness of pine forest, hardwood hammocks, swamps,

marshes, estuaries, and bays. Wetlands dominated the landscape. The

region contained one of the largest wetlands in the continental United

States, the Everglades, which was part of the larger Kissimmee–

Okeechobee–Everglades watershed that extended more than half the

length of the Florida Peninsula. The Everglades and the Big Cypress

Swamp stretched as a continuous wetland across the southern part of the

peninsula south of Lake Okeechobee. These wetlands and the entire

watershed provided the freshwater that sustained highly productive estuaries and bays of the region.

Drainage of the Everglades watershed began in the early

1880’s and continued into the 1960’s. The first drainage canals

were dug in the upper Kissimmee River and between Lake

Okeechobee and the Caloosahatchee River. Beginning with the

Miami River in 1903, canals were cut through the Atlantic

Coastal Ridge and into the northern Everglades. By the late

1920’s, five canals had been dug between Lake Okeechobee and

the Atlantic Ocean—one that passed north of the Everglades and

connected the lake with the St. Lucie River and four that passed

through the Everglades. Drainage opened land for agriculture to

develop south of Lake Okeechobee. In the late 1920’s, a low

muck levee was constructed along the southern and southwestern shore of the lake to prevent flooding, but during the hurricanes of 1926 and 1928, the levee was breached, which resulted

in destruction of property and lives. In response to these catastrophes, the Federal Government initiated flood-control

measures that included construction of levees around the southern shore of Lake Okeechobee and enlargement of the Caloosahatchee and the St. Lucie Canals.

Drainage and development altered most of south Florida and

caused severe environmental changes. These changes include

large losses of soil through oxidation and subsidence, degradation of water quality, nutrient enrichment, contamination by

pesticides and mercury, fragmentation of the landscape, large

losses of wetlands and wetland functions, and widespread invasion by exotic species. Additionally, the large and growing

human population and the active agricultural development in the

region are in intense competition with the natural system for

freshwater resources.

Recently, a consensus has begun to

emerge among Federal and State agencies,

as well as among environmental groups,

that the Everglades should be restored to

patterns similar to the original system. For

concerned parties to discuss, let alone to

implement, such recommendations

requires a substantial increase in available

scientific data and understanding of Everglades hydrology, geology, and ecology.

The investigations needed to support

restoration, which have been developed by

the Science Sub–Group of the South Florida Ecosystem Task force, include hydrologically characterizing the predrainage

system and comparing it to the present

system; determining the key characteristics of the former natural hydrologic

system that supported the rich diversity

and abundance of wildlife that has been

lost; designing structural and operational

modifications of the Central and Southern

Florida Project for Flood Control and other

Purposes (C&SF) that would recreate the

key characteristics of the natural hydrologic system; assessing the hydrologic and

ecological results of these modifications

through pre- and post-modification monitoring; and modifying the design of the

C&SF project based on the assessment and

monitoring of the hydrologic and ecological results of the changes.

Summary and

Research Needs
55

The USGS is providing some of the scientific information necessary for protection and restoration in south Florida through several

of its programs, which include the South Florida Initiative and the

NAWQA Program. The U.S. Geological Survey is coordinating

these efforts with other Federal agencies through the South Florida Ecosystem Interagency Working Group and the Science Sub–

Group and through regularly scheduled liaison meetings of the

Southern Florida NAWQA study unit. The South Florida Initiative

is a collaborative effort by the U.S. Geological Survey, the fromer

National Biological Service, and other Federal and State agencies

to provide scientific insight into conflicting land-use demands and

water supply issues in the south Florida, Florida Bay, Florida Keys

ecosystem (McPherson and others, 1995). The proposed U.S.

Geological Survey contributions include the following:

• Assessing the availability of water for competing requirements (public water supply, agriculture, fisheries, ecosystem

protection/restoration) in south Florida and the Florida Bay area

by measuring and modeling the movement of water.

• Assessing the water quality within south Florida, Florida Bay,

and the Keys/Reefs by collaborating with other planned efforts

and focusing on the additional needs for information, such as identifying processes that transform and transport nutrients and

mercury.

• Determining the “natural state” of

the south Florida regional ecosystem,

which include the estuaries, bays, and

coral reefs and their watershed.

• Developing a comprehensive and

readily accessible quality-assured relational data base for spatial and point

data, which include water, geology, soils,

topography, vegetation, and remotely

sensed data to support scientific investigation. 

Regardless of the information obtained

and the effort expended, the success of

ecosystem restoration is uncertain

because of the complexity of the interaction between the biological and abiotic

components of the system. In writing

about the Everglades, Davis and Ogden

(1994, p. 789) state “This uncertainty of

information becomes another force in

support of a broad restoration premise;

restoration of hydrology and natural

environmental fluctuations is an appropriate target in the stepwise and still

imprecise process of attempting to

produce ecological restoration.”
56

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