In cooperation with the

ST. JOHNS RIVER WATER MANAGEMENT DISTRICT

SOUTH FLORIDA WATER MANAGEMENT DISTRICT

U.S. Geological Survey

Circular 1137

Hydrology of

Central Florida Lakes

—A Primer
Library of Congress Cataloging-in-Publication Data

The use of firm, trade, and brand names in this report is for identification purposes only and

does not constitute endorsement by the U.S. Government

U.S. GOVERNMENT PRINTING OFFICE : 

Free on application to the

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Mark Schaefer, Acting Director

1998

Schiffer, Donna M.

Hydrology of central Florida lakes : a primer / 

by Donna M. Schiffer : illustrations by Rafael Medina.

p. cm. — (U.S. Geological Survey circular : 1137)

“In cooperation with the St. Johns River Water Management District

and the South Florida Water Management District.”

Includes bibliographical references.

Supt. of Docs. no.: I 19.42: 1137

1. Lakes—Florida. I. St. Johns River Water Management District.

II. South Florida Water Management District. III. Title.

IV. Series.

GB1625.F6S35 1997

551.48'2'09759—dc21 97–822

 CIP

ISBN 0–607–88561–0
Contents

CONTENTS

iii

Introduction ........................................................................................................................................................................... 1

Classification of Lakes .......................................................................................................................................................... 4

The Hydrologic Cycle ........................................................................................................................................................... 5

Rainfall ........................................................................................................................................................................ 7

Evaporation and Transpiration .................................................................................................................................... 8

Surface Runoff............................................................................................................................................................. 10

Infiltration and Ground-Water Recharge and Discharge ............................................................................................. 10

Geology of Central Florida.................................................................................................................................................... 10

Ground Water......................................................................................................................................................................... 12

Geomorphology and the Origin of Central Florida Lakes..................................................................................................... 13

Physical Characteristics of Lakes .......................................................................................................................................... 19

What Causes Lake Water-Level Fluctuations........................................................................................................................ 20

How Rainfall and Evapotranspiration Affect Lake Water Levels ............................................................................... 20

How Surface Water Affects Lake Water Levels.......................................................................................................... 21

How Ground Water Affects Lake Water Levels.......................................................................................................... 21

How Human Activities Affect Lake Water Levels...................................................................................................... 24

Examples of Lake Water-Level Fluctuations............................................................................................................... 25

Artificial Control of Lake Water Levels...................................................................................................................... 27

Quality of Water in Central Florida Lakes ............................................................................................................................ 28

Lake Water-Quality Classification Systems ................................................................................................................ 29

Water Quality: A Function of the Source Water.......................................................................................................... 30

Water Quality: A Function of Residence Time ........................................................................................................... 31

Water-Quality Problems .............................................................................................................................................. 31

Water-Quality Solutions .............................................................................................................................................. 32

Summary................................................................................................................................................................................ 35

Selected References............................................................................................................................................................... 36

FIGURES

1. Approximate geographic area covered by this report and distribution of lakes in Florida ........................................ 2

2. Lakes and weather stations in this report.................................................................................................................... 3

3. Illustration summarizing hydrologic characteristics of seepage and drainage lakes .................................................. 4

4. Hydrologic cycle......................................................................................................................................................... 6

5. Major components of the hydrologic budget for a central Florida lake in a ground-water recharge area.................. 7

6. Mean monthly rainfall at DeLand and Orlando for the period 1961–90 and lake evaporation based on 

pan evaporation (1960–88) ......................................................................................................................................... 8

7. Annual rainfall at Jacksonville, the station with the longest record in Florida .......................................................... 9

8. Ground-water movement in recharge and discharge areas ......................................................................................... 10

9. Generalized summary of geologic and geohydrologic units in central Florida.......................................................... 11

10. Geology and major aquifers in central Florida ........................................................................................................... 12

11. Major physiographic regions of central Florida ......................................................................................................... 14

12. Formation of a collapse sinkhole................................................................................................................................ 15

13. Formation of a subsidence sinkhole............................................................................................................................ 16

14. Present-day lakes formed when sea level fell and freshwater filled depressions in the sea floor 

(Upper St. Johns River chain of lakes) ....................................................................................................................... 18

15. Lakes formed by solution processes (Ocklawaha River chain of lakes) .................................................................... 19
Contents iv

16. Water level in Lake Umatilla, Lake County, and the cumulative difference between rainfall and 

lake evaporation.......................................................................................................................................................... 21

17. Interactions between ground-water and lake water levels.......................................................................................... 22

18. Cross section through a sinkhole lake showing ground-water seepage...................................................................... 23

19. Surficial-aquifer spring flow and Floridan-aquifer spring flow to a lake................................................................... 24

20. Water-level fluctuations in a drainage lake (Lake Apopka) and a seepage lake (Lake Francis)................................ 25

21. Water level in Sand Hill Lake, Brooklyn Lake, and in a well near Brooklyn Lake ................................................... 26

22. Water levels in Lake Apopka and Lake Apshawa illustrating the effect of seiche on water levels ........................... 27

23. Examples of lake water-level control devices ............................................................................................................ 28

24. Examples of an oligotrophic, mesotrophic, and eutrophic lake.................................................................................. 29
v Glossary

GLOSSARY

Alkalinity: A measure of the capacity of the water to 

neutralize acids.

Aquifer: A geologic formation, group of formations, 

or part of a formation that contains sufficient saturated, 

permeable material to be able to yield significant 

quantities of water to wells and springs.

Artesian: A condition in which ground water in a well rises 

above the top of the water-bearing formation that is 

tapped by the well.

Buffered: The resistance of water to a change in pH.

Carbonates: Rock composed chiefly of carbonate minerals 

(calcium, magnesium); examples are limestone and 

dolomite.

Condensation: The process by which water changes from 

the vapor state into the liquid or solid state.

Conductivity: See specific conductance.

Dissolved oxygen: Atmospheric oxygen that is dissolved 

and held in solution in water. Only a fixed amount of 

oxygen can be dissolved in water at a given temperature and atmospheric pressure.

Dissolved solids: The sum of all the dissolved constituents 

in a water sample. Major components of dissolved 

solids are the ions of the following: calcium, 

magnesium, sodium, postassium, bicarbonate, sulfate, 

and chloride.

Drainage basin: A part of the surface of the Earth that 

drains to a body of water by way of overland flow or 

stream flow.

Drainage lake: A lake that has a surface-water outlet.

Evaporation: The process by which water is changed from 

the liquid state into the gaseous state through the 

transfer of heat energy.

Evapotranspiration: The sum of water lost from a given 

land area during any specified time by transpiration 

from vegetation and building of plant tissue; by 

evaporation from water surfaces, moist soil, and snow; 

and by interception (rainfall that never reaches the 

ground but evaporates from surfaces of plants and 

trees).

Flushing rate: The rate (volume per unit time) at which 

water leaves a lake, either through a surface-water 

outlet or through ground-water seepage.

Geomorphology: The study of the configuration and 

evolution of land forms.

Ground water: Water below the land surface in the zone of 

saturation.

Hydraulic gradient: The difference in water levels at two 

points divided by the distance between the two points. 

Either horizontal or vertical hydraulic gradients can be 

measured. 

Hydrologic budget: An accounting of the inflow to, outflow 

from, and storage in a drainage basin.

Hydrologic cycle: A term denoting the circulation of water 

from the ocean, through the atmosphere, to the land; 

and then, with many delays, back to the ocean by 

overland and subterranean routes, and in part by way 

of the atmosphere; also includes the many paths by 

which water is returned to the atmosphere without 

reaching the ocean.

Hydrology: The science of the water of the Earth.

Hydrostatic pressure: The pressure exerted by the water at 

any given point in a body of water at rest. 

Infiltration: The flow of water into the surface of the Earth 

through the pores of the soil at land surface. Distinct 

from percolation (see definition).

Interception: The process and the amount of rain stored 

on leaves and branches of vegetation that eventually 

evaporates back to the atmosphere.

Limnology: The branch of hydrology pertaining to the study 

of lakes.

Overburden: The loose soils, sand, gravel, or other 

unconsolidated materials overlying a rock stratum. 

Nitrogen: An essential plant nutrient. High concentrations 

can lead to excessive plant growth and water-quality 

problems.

Pan coefficients: Mathematical ratios that relate lake 

evaporation to measured pan evaporation.

Percolation: Flow of water through a porous substance, 

usually in a vertical direction (downward). Rainfall, as 

it reaches the land surface, first infiltrates the surface, 

then percolates downward.

pH: A measure of how acidic or alkaline water is, based on 

the concentration of hydrogen ions in the water.

Phosphorus: An essential plant nutrient. High concentrations can lead to excessive plant growth and waterquality problems.

Potential evapotranspiration: The maximum amount of 

water that would be evaporated and transpired if there 

was no deficiency of water in the soil at any time for 

the use of vegetation.

Potentiometric surface: An imaginary surface that 

represents the height to which water will rise in a 

tightly cased well.

Precipitation: The discharge of water, in liquid or solid 

state, out of the atmosphere, generally upon a land or 

water surface. It is the common process by which 

atmospheric water becomes surface or subsurface 

water. Precipitation includes rain, hail, sleet, and snow.

Residence time: The time necessary for the total volume of 

water in a lake to be completely replaced by incoming 

water.
Glossary vi

Retention pond: A pond constructed for the purpose of 

retaining stormwater runoff. Water in retention ponds 

evaporates or infiltrates the bottom of the pond, 

eventually recharging the underlying ground water. If 

there is a surface outlet to another body of water, the 

pond is called a detention pond.

Seepage: The process of water moving slowly through the 

subsurface environment, or the actual water involved 

in the process of seepage.

Seepage lake: A lake that has no surface-water outflow; a 

landlocked lake.

Seiche: The free oscillation of the bulk of water in a lake 

and the motion caused by it on the surface of the lake. 

Sinkhole: A funnel-shaped depression in the land surface 

that connects with underground passages or caverns.

Solution processes: The chemical processes by which rock 

is dissolved by interactions with water.

Specific conductance: A measure of the property of water to 

conduct a current of electricity. Specific conductance is 

commonly used as an indicator of the dissolved solids 

content of water.

Spring: Site at which ground water flows through a natural 

opening in the ground onto the land surface or into a 

body of surface water. 

Surface runoff: That part of precipitation that does not 

infiltrate the land surface, but travels along the land 

surface. 

Surface water: Water that is present on the land surface, 

generally referring to lakes and streams.

Transpiration: The process by which plants take water from 

the soil, use it in plant growth, and then transpire it to 

the atmosphere in the form of water vapor. Evaporation 

and transpiration are often combined in one term, 

Evapotranspiration.

Unsaturated zone: The zone between land surface and the 

water table where the pores in the soil matrix are filled 

with air and water.

Water table: The upper surface of the zone of saturation in 

the ground. The water table commonly is at 

atmospheric pressure.

Zone of saturation: The zone in which the soil or rock is 

saturated with water under hydrostatic pressure.

Sources: Langbein, W.B., and Iseri, K.T., 1966; Fernald, E.A., and Patton, D.J., 1984; and Lane, Ed, ed., 1994
1 Introduction

almost seems there is more water

than land. Florida has more naturally formed lakes than other southeastern States, where many lakes

are created by building dams across

streams. The abundance of lakes on

the Florida peninsula is a result of

the geology and geologic history of

the State. An estimated 7,800 lakes

in Florida are greater than 1 acre in

surface area. Of these, 35 percent

are located in just four counties

(fig. 1): Lake, Orange, Osceola, and

Polk (Hughes, 1974b). Lakes add to

the aesthetic and commercial value

of the area and are used by many

residents and visitors for fishing,

boating, swimming, and other types

of outdoor recreation. Lakes also

are used for other purposes such as

irrigation, flood control, water

supply, and navigation. Residents

and visitors commonly ask questions such as “Why are there so

many lakes here?”, “Why is my lake

drying up (or flooding)?”, or “Is my

lake spring-fed?” These questions

indicate that the basic hydrology of

lakes and the interaction of lakes

with ground water and surface

water are not well understood by the

general population.

Because of the importance of

lakes to residents of central Florida

and the many questions and

misconceptions about lakes, this

primer was prepared by the

U.S. Geological Survey (USGS) in

cooperation with the St. Johns River

Water Management District and the

South Florida Water Management

District. The USGS has been

collecting hydrologic data in central

Florida since the 1920’s, obtaining

valuable information that has been

used to better understand the

hydrology of the water resources of

central Florida, including lakes. In

addition to data collection, as of

1994, the USGS had published

66 reports and maps on central

Florida lakes (Garcia and Hoy,

1995).

The main purpose of this primer

is to describe the hydrology of lakes

in central Florida, the interactions

between lakes and ground- and

surface-waters, and to describe how

these interactions affect lake water

levels. Included are descriptions of

the basic geology and geomorphology of central Florida, origins

of central Florida lakes, factors that

affect lake water levels, lake water

quality, and common methods of

improving water quality. The

geographic area discussed in this

primer is approximate (fig. 1) and

includes west and east-central

Florida, extending from the Gulf of

Mexico to the Atlantic Ocean coastlines, northward into Marion,

Putnam, and Flagler Counties, and

southward to Lake Okeechobee.

The information presented here was

obtained from the many publications available on lakes in central

Florida, as well as from publications on Florida geology, hydrology, and primers on ground water,

Hydrology of Central Florida Lakes—A Primer

By Donna M. Schiffer

akes are among the

most valued natural

resources of central

Florida. The landscape of central

Florida is riddled

with lakes—when L

viewed from the air, it

INTRODUCTION

Many lakes in central Florida are used for recreation. (Photograph provided by

St. Johns River Water Management District.)
Hydrology of Central Florida Lakes—A Primer 2

surface water, and water quality.

Many publications are available

that provide more detailed information on lake water quality, and this

primer is not intended as an extensive treatise on that subject. The

reader is referred to the reference

section of this primer for sources of

more detailed information on lake

water quality. Lakes discussed in

this report are identified in figure 2.

Technical terms used in the report

are shown in bold italics and are

defined in the glossary.

The classification of some water

bodies as lakes is highly subjective.

What one individual considers a

“lake” another might consider a

“pond.” Generally, any water-filled

depression or group of depressions

in the land surface could be

considered a lake. Lakes differ from

swamps or wetlands in the type

and amount of vegetation, water

depth, and some water-quality

characteristics. Lakes typically

have emergent vegetation along the

shoreline with a large expanse of

open water in the center. Swamps or

wetlands, on the other hand, are

characterized by a water surface

interrupted by the emergence of

many varieties of plant life, from

saw grasses to cypress trees.

Lakes may be naturally formed

or manmade; however, the distinction between naturally formed and

manmade lakes is not always clear.

For example, retention ponds,

which are required for the treatment

of stormwater, can be constructed

so that they serve multiple purposes

of stormwater treatment and

aesthetic enhancement of property.

Larger retention ponds sometimes

are used by residents for boating

and fishing and are considered by

some to be lakes.

In addition to aesthetic value

and recreational uses, lakes in

central Florida are extremely

important as habitats for fish, alligators, turtles, and birds such as

hawks, eagles, ducks, and herons.

Because Florida lakes are used and

enjoyed by many, they need to be

appreciated, understood, and

managed for the benefit of all.

Figure 1. Approximate geographic area covered by this report and distribution of

lakes (dark blue) in Florida. Outlined counties contain 35 percent of the lakes in

Florida.

Alligators are 

common in 

Florida lakes, 

although their 

population 

varies from 

one lake to 

the next. 

(Photograph 

provided by 

South Florida 

Water 

Management 

District.)
3 Introduction

Figure 2. Lakes and weather stations in this report.
Hydrology of Central Florida Lakes—A Primer 4

Hydrologic characteristics of

central Florida lakes vary widely.

The surface areas of lakes can range

from several hundred square miles,

such as Lake Okeechobee, to less

than an acre. Water levels in some

lakes may vary by 10 feet or more,

whereas in other lakes, the water

level may vary by only 1 or 2 feet.

The quality of water among lakes in

central Florida also is variable, from

pristine lakes such as Lake Butler in

west Orange County to the peagreen-colored waters of Lake

Apopka, a short distance to the

north in Orange and Lake Counties

(although the clarity of water is not

necessarily an indication of the

quality of the water). Some lakes

have natural surface-water inlets

and outlets. Other lakes are landlocked, receiving water only from

rainfall and losing water only from

evaporation and seepage into the

surrounding soils. This great variety

in hydrologic characteristics is one

of the reasons why water levels vary

among lakes and why lakes respond

differently to rainfall.

CLASSIFICATION 

OF LAKES

Lakes are classified according

to different criteria including location, origin, drainage characteristics, trophic state (a measure of the

amount of nutrient enrichment of

the water), and water chemistry.

Lakes commonly are classified by

geologists according to the physiographic region in which the lakes

are located. Many lakes in Florida

were formed by sinkhole activity

and thus are called sinkhole lakes.

Environmental scientists may

classify lakes according to the state

of water quality (this classification

system is described in the section on

water quality). Residents of and

visitors to central Florida have

added their own classification

system by describing a particular

lake as a good fishing or waterskiing lake.

Lakes in Florida and elsewhere

commonly are classified by

drainage characteristics; it is these

characteristics that differentiate

Florida lakes from lakes in other

parts of the country. This general

classification divides lakes into one

of two major types—seepage and

drainage lakes. Although there are

many other hydrologic characteristics associated with each type

(fig. 3), perhaps the most easily

Figure 3. Hydrologic characteristics of seepage and drainage lakes.
5 The Hydrologic Cycle

recognized property used to

distinguish between seepage and

drainage lakes is the absence or

presence (respectively) of surfacewater outflow. Lakes that have no

surface-water outflow lose water

primarily through the ground-water

system or through evapotranspiration and are called seepage lakes.

Lakes that lose water primarily

through a surface-water outlet are

called drainage lakes (Wetzel,

1975). Most Florida lakes are

seepage lakes—nearly 70 percent of

the lakes in Florida have no surfacewater streams flowing into or out of

them (Palmer, 1984). The drainage

basin of a seepage lake commonly

is referred to as a closed basin

because of the lack of surface-water

outflow from the basin; however,

there is outflow from the basin

through ground-water seepage. The

drainage basins of drainage lakes

are commonly referred to as opendrainage basins.

The dominance of ground-water

flow over surface-water flow in

much of Florida makes the

hydrology of lakes here different

from that of lakes in other parts of

the country. The well-drained

porous soils and rocks that are

characteristic of the karst landscape

of Florida are what make this

difference. One difference between

Florida lake hydrology and that of

other parts of the country is

expressed in the classical definition

of a drainage basin. A drainage

basin or watershed (the area

contributing water) of a lake

commonly refers to the land surface

area draining to a water body as

defined by topography. However,

because most of the inflow to

seepage lakes in Florida is from

ground water, the ground-water

basin must be considered part of the

contributing drainage basin. Ultimately, however, the most significant difference between seepage

and drainage lakes is not the source

of the water, but the controlling

factors affecting lake water volume

(ground-water or surface-water

outflows).

THE HYDROLOGIC CYCLE

Water in the environment

moves from the atmosphere, to the

land surface, to the ground-water

system, and back to the atmosphere

in a cycle called the hydrologic

cycle (fig. 4). In this section, the

components of the hydrologic cycle

are described to provide the necessary background for an understanding of lake hydrology.

The primary components of the

hydrologic cycle in central Florida

are rainfall, runoff, infiltration

(including recharge), evaporation,

transpiration, and condensation.

When rain falls, some of the water

infiltrates the ground and recharges

Most Florida lakes are 

seepage lakes—nearly 

70 percent of the lakes 

in Florida have no 

surface-water streams 

flowing into or 

out of them.

Florida has numerous wading birds that make use of the many

lakes here. (Photographs provided by South Florida Water Management District.)
Hydrology of Central Florida Lakes—A Primer 6

aquifers, some of it is intercepted by

plants, and some of it flows over the

land surface (surface runoff or

overland flow). Beneath the land

surface, water moves through the

aquifers toward areas of discharge

such as the ocean or streams. Water

returns to the atmosphere through

the processes of evaporation and

plant transpiration (collectively

labeled “evapotranspiration” in

fig. 4). Once in the atmosphere as

water vapor, the hydrologic cycle is

completed when this vapor

condenses and forms rain droplets

that subsequently fall on the land

surface again.

Some of the components of the

hydrologic cycle can be quantified

by using measuring devices to

collect data, and a “water budget” of

a lake (an accounting of the total

volume of water entering and

exiting the lake) can be determined

from these data. Much of the

research on Florida lakes has

focused on water budgets in an

effort to better understand the

complex hydrologic system of a

lake and to determine how each

component of the hydrologic cycle

affects lake water levels and water

quality. A schematic of the components of a lake water budget is

shown in figure 5. Lakes receive

water from rain falling on the

surface of the lake, from surface

runoff within the drainage basin,

from streamflow, and from ground

water entering the lake (labeled as

lateral seepage in fig. 5). Lakes lose

water to evaporation, seepage

through the bottom, and streamflow

(in lakes with surface-water

connections). Rainfall and streamflow are relatively easy to measure,

but recharge, evaporation, and

seepage are much more difficult to

determine accurately. The components of the hydrologic cycle are

discussed in more detail in the

following sections.

Figure 4. Hydrologic cycle. (Modified from Fernald and Patton, 1984.)
7 The Hydrologic Cycle

Rainfall

The climate of central Florida is

subtropical, with warm, wet

summers and mild, fairly dry

winters. The wet season, which

begins in June and ends in

September, accounts for more than

half (56 percent) of the rainfall

during the year (Schiner, 1993).

Rainfall during the wet season

generally is associated with local

showers and thunderstorms.

Rainfall during the dry season

(October through May) generally is

associated with frontal systems

moving from northern latitudes

southward. Rainfall during the wet

season can be highly localized with

heavy thunderstorms producing

significantly more rain in some

areas than in others, whereas

rainfall during the dry season

affects larger geographic areas.

Some of the differences in lake

water levels, particularly during

summer months, can be the result of

local differences in rainfall amount

or intensity. 

Total monthly or annual rainfall also is variable from location to

location. For example, the average

annual rainfall, based on 30 years

of record (1961–90), ranges from

43.92 inches in Tampa to

54.09 inches in St. Leo (see fig. 2

for station locations). The

variability from one location to

another is indicated by the average

monthly rainfall during a 30-year

period (1961–90) at two rainfall

stations in DeLand and Orlando

(fig. 6, locations shown in fig. 2).

Both stations are inland and are

only about 30 miles apart, so one

might expect similar rainfall

amounts. Total rainfall at these two

stations is similar for some months,

but the rainfall at DeLand tends to

be greater than the rainfall at

Orlando, particularly during the

months of August, September, and

October, illustrating regional variability. Mean annual rainfall at

DeLand (56.05 inches) is nearly

8 inches greater than mean annual

rainfall at Orlando (48.11 inches)

for the period 1961–90, indicating

that long-term rainfall patterns can

differ significantly even at locations

that are relatively close and in

similar settings.

Some of the differences 

in lake water levels, 

particularly during 

summer months, can 

be the result of local 

differences in rainfall 

amount or intensity.

The annual rainfall for

Jacksonville, the site with the

longest rainfall record in Florida, is

shown in figure 7. Rainfall recordkeeping started in 1851, but there is

a break in the otherwise continuous

record for 1861–66. Total annual

rainfall is shown in the upper graph

of figure 7. In the middle graph, the

difference between the rainfall for

each year and the long-term average

is shown (based on the period

1866–1993). Just as daily rainfall

varies from one day to the next, total

annual rainfall varies from one year

to the next. The difference between Figure 5. Major components of the hydrologic budget for a central Florida lake

in a ground-water recharge area.
Hydrology of Central Florida Lakes—A Primer 8

annual rainfall and the long-term

average annual rainfall is called the

“departure” from average for that

year. These annual departures from

the average can have a cumulative

effect on lake water levels. For

example, a series of years with less

than average rainfall may result in

lake water levels that are lower than

the long-term average level for that

lake. The bottom graph of figure 7

illustrates how the difference

between annual rainfall and the

average rainfall, when accumulated

from one year to the next, can

produce trends of excess or deficit

rainfall. Excess rainfall early in the

period, from 1866 through about

1889, produced enough of a cumulative surplus of rainfall (bars

shown above the zero line) that it

was 25 years before lower-thanaverage annual rainfall produced a

deficit (bars shown below the zero

line, beginning in 1917).

Evaporation and 

Transpiration

Most of the rain that falls is

returned to the atmosphere by evaporation and by transpiration from

plants. Commonly, evaporation and

transpiration are considered

together and called evapotranspiration. Water that is in the soil near the

land surface can return to the atmosphere through evaporation. The

highest evaporation rate is from

open water surfaces such as lakes.

Evaporation from a lake surface is

referred to specifically as lake evaporation. The evaporation rate is

affected by several variables such as

the amount of moisture in the air

(humidity), the amount of sunlight,

wind, and temperature. Thus, evaporation rates are variable; in central

Florida, evaporation rates have

been estimated to be as low as

25 inches per year and as high as

50 inches per year (Tibbals, 1990).

One way that lake evaporation

is estimated is from evaporation

measured using a standard 4-foot

diameter, shallow metal pan. Panevaporation rates are greater than

lake-evaporation rates and must be

corrected using pan coefficients,

which are mathematical ratios

that relate lake evaporation to pan

evaporation.

Annual lake evaporation for

the central Florida area was estimated to be about 51 inches, based

on 28 years (1960–88) of panevaporation data recorded at the

Lisbon and Gainesville weather

stations (locations shown in fig. 2)

and pan coefficients determined in

studies at Lake Hefner, near Lake

Okeechobee, by Kohler (1954).

Other estimates of annual lake

evaporation reported by researchers

in central Florida have ranged from

47 inches (Phelps and German,

1996) to 58 inches (Lee and

Swancar, 1994). The annual lakeevaporation rate of 51 inches is

greater than the average yearly

rainfall at Orlando (48.11 inches,

based on rainfall from 1961–90);

however, lake evaporation represents the maximum possible rate of

evaporation. The actual evaporation rate over a large region is

considerably lower than the lakeevaporation rate because the water

is evaporating from land surfaces as

well as water surfaces. Although the

actual evaporation in the drainage

basin of a lake can be estimated for

a water budget, the computation can

be complex because it requires

information about land use, depth to

the water table, and other hydrologic variables.

Figure 6. Mean monthly rainfall at DeLand and Orlando for the period 1961–90

and lake evaporation based on pan evaporation (1960–88).
9 The Hydrologic Cycle

Figure 7. Annual rainfall at Jacksonville, the station with the longest record in Florida.
Hydrology of Central Florida Lakes—A Primer 10

Surface Runoff

Rainfall that has not infiltrated

the soil and has not been returned

to the atmosphere by evapotranspiration flows over the land surface

and eventually reaches a lake or

stream. Surface runoff refers to

water flowing over the land surface

and to water in streams and rivers.

Surface runoff is a significant

component of the hydrologic

budget of drainage lakes. In seepage

lakes, however, ground-water

inflows and outflows dominate the

hydrologic budget, and the component represented by surface runoff

is relatively small.

Infiltration and GroundWater Recharge and 

Discharge

Part of the rainfall that reaches

the land surface infiltrates the soil,

where it gradually percolates

downward until it reaches the surficial aquifer system (fig. 8). This

water replenishes the surficial

aquifer system, which in turn

replenishes or recharges the

Floridan aquifer system. This

downward movement can occur

only where the water table of the

surficial aquifer system is higher (at

a greater altitude) than the potentiometric surface of the Floridan

aquifer system. The areas in the

State where ground water is replenished are referred to as recharge

areas. In areas where the potentiometric surface of the Floridan

aquifer system is at a higher altitude

than the water table of the surficial

aquifer that overlies it, water moves

upward toward the land surface.

These areas are referred to as

discharge areas or areas of artesian

flow (fig. 8). The many springs in

Florida are located in discharge

areas. Lakes in central Florida are

present in both recharge and

discharge areas.

GEOLOGY OF 

CENTRAL FLORIDA

The Florida peninsula is

composed of carbonate rock (limestone and dolomite) that was

deposited over a period of about

100 million years. For about

75 million years, during the

Cretaceous Period (138 to

63 million years ago), Florida was

covered by relatively deep oceans.

Later, from about 63 to 38 million

years ago, the Florida peninsula was

a shallow carbonate reef. Sea level

then receded and rose several times,

eroding and depositing carbonates,

until about 5 million years ago.

Since that time, land-derived sediments such as sand, silt, and clay

have been deposited, rather than

marine carbonates. Subsequent

fluctuations of sea level during

episodes of glaciation have eroded,

reworked, and transported the

unconsolidated sand, silt, and clay

sediments.

The Florida peninsula is a

product of sedimentary processes,

and Florida’s geology reflects these

processes. The geology of central

Florida is shown in figure 9, which

also shows the hydrogeologic units,

or aquifers and confining units,

corresponding to the geology.

Underlying the deepest rocks

shown in the diagram in figure 9 are

several thousand feet of igneous,

metamorphic, and sedimentary

rocks that form the foundation of

the peninsula. Above these rocks

are the Cedar Key, Oldsmar, and

Avon Park Formations, and the

Ocala and Suwanee Limestones

(fig. 9). These formations consist of

Figure 8. Ground-water movement in recharge and discharge areas.
11 Geology of Central Florida

limestone, dolomite, anhydrite, and

gypsum and were deposited when

most of the Florida peninsula was

below sea level. The total thickness

of these formations ranges from

5,500 to 12,000 feet (Tibbals,

1990). The overlying Hawthorn

Group, deposited about 25 million

years ago, represents a transition

between marine-derived and landderived sediments. The lower layers

of the Hawthorn Group generally

are marine-derived and contain

limestone, whereas the upper layers

of clay, fine sand, and silt are landderived. These upper layers of the

Hawthorn Group generally restrict

ground-water movement. Overlying the Hawthorn Group and

continuing upward to the present

land surface are unconsolidated

sediments generally consisting of

quartz sand, clay, and some organic

material.

The thickness of the Hawthorn

Group, which varies greatly in

central Florida, is a key element in

the lake formation process

(described in a later section). When

rocks of the marine-formed Ocala

Limestone and Suwanee Limestone

Formations were exposed at land

surface, the rock was eroded by

wind, rain, flowing water, and

thermal changes (expansion and

contraction due to seasonal temperature change). Because the

erosional process is not necessarily

uniform from one location to

another, the eroded surface of the

limestone can be very irregular.

Later in geologic time, the sediments of the Hawthorn Group were

deposited on top of this irregular,

eroded limestone surface. Therefore, the thickness of the Hawthorn

Group varies depending on the

original altitude of the surface

where it was deposited. In those

areas where the sediments of the

Hawthorn Group are thin, more

surface water can penetrate to the

underlying rock of the Ocala and

Suwanee Limestone Formations

and continue the erosional process.

These are areas prone to sinkhole

development. In those areas where

the Hawthorn Group is thicker, the

underlying rock is more shielded

from the effects of continued

erosion, and sinkholes are less

common.

Figure 9. Generalized summary of geologic and geohydrologic units in central Florida. (Modified from Miller, 1986; Tibbals,

1990; Bradner, 1994; Phelps, 1987; Lopez and Fretwell, 1992.)
Hydrology of Central Florida Lakes—A Primer 12

GROUND WATER

In this section of the primer,

the ground-water system in central

Florida is described to provide the

background for understanding lake

formation, water levels in lakes, and

the quality of water in lakes

discussed in subsequent sections.

Florida’s ground water is contained

in aquifers. An aquifer is a layer or

a combination of several layers of

permeable soils or rocks that yield

usable quantities of water. The

major aquifers in central Florida are

identified in figure 9 under the

column labeled “Geohydrologic

Unit” and in the generalized cross

section shown in figure 10. The

term “system” is used to indicate

the presence of more than one

water-bearing layer or aquifer; for

regional analysis these multiple

layers of aquifers are collectively

considered as a single geohydrologic unit (fig. 9).

In central Florida, the aquifer

nearest the land surface is the surficial aquifer. The water table refers

to a surface below which all the

openings or spaces in the soil or

rock are filled with water (saturated). Pores in the soil between the

land surface and the water table are

filled with both water and air; this

uppermost layer above the water

table is referred to as the unsaturated zone. The term ground water

refers to water below the water

table. The water table is in contact

with the atmosphere through the

unsaturated zone; therefore, the

water table is at atmospheric pressure. The depth to the water table

can range from 50 feet or more

below land surface to above land

surface in wetlands and lakes.

(Because the water surface of a lake

is the water table, and the land

surface is actually the submerged

lake bottom, the water table is

above land surface in lakes.)

Beneath the surficial aquifer

system lies the intermediate

confining unit, which is composed

of layers of clays, fine silts and, in

some areas, limestone beds. The

Figure 10. Geology and major aquifers in central Florida.
13 Geomorphology and the Origin of Central Florida Lakes

materials that make up the intermediate confining unit in most of

central Florida generally are not

very permeable, thus, this unit

restricts the movement of water

from above and below. In some

areas, the lower part of the intermediate confining unit may consist of

highly fractured limestone and

dolomite, which can produce usable

quantities of water; however, in

most areas, the intermediate

confining unit is not used for water

supply.

The Floridan aquifer system

underlies the intermediate confining unit and is the primary source

of nearly all the drinking water in

central Florida. The rock that

contains the Floridan aquifer

system generally consists of highly

permeable limestone and dolomite.

The top of the aquifer is at or near

land surface in west-central Florida

(including Marion and Sumter

Counties) and can be as deep as

several hundred feet below land

surface in other areas of central

Florida. The Floridan aquifer

system is further divided into

the Upper Floridan and Lower

Floridan aquifers (fig. 9). The

Upper Floridan aquifer is the

primary source of drinking water,

although the Lower Floridan

aquifer is used in some areas.

The Floridan aquifer 

system is the primary 

source of nearly all 

the drinking water in 

central Florida.

Water in the Floridan aquifer

system is under pressure in much of

central Florida because the overlying clays and silts of the

Hawthorn Group act to confine the

water (thus the name “intermediate

confining unit”). This condition is

commonly called artesian because

water levels in wells drilled into the

aquifer rise above the top of the

aquifer and, in some places, above

land surface (fig. 10). The level to

which the water rises is referred to

as a potentiometric surface. Wells

that tap the Floridan aquifer system

in ground-water discharge areas are

commonly referred to as artesian

wells, particularly if the potentiometric level in the well is greater

than the land surface; where this

occurs, water will flow out of the

well as shown in figure 10. In areas

where the limestone of the Floridan

aquifer system is at or near land

surface and there are no confining

sediments, water in the aquifer is

not under pressure; in these areas,

the aquifer is not artesian.

GEOMORPHOLOGY AND 

THE ORIGIN OF CENTRAL 

FLORIDA LAKES

The geomorphology of the

Florida peninsula provides clues to

geologic history and the origin of

lakes. Physical features of the land

surface of central Florida have been

used to define physiographic

regions, some of which are associated with the abundance of lakes.

The physical features of Florida are

quite varied, although compared

to other parts of the country the

State is rather flat and featureless.

Physical features of central Florida

range from highlands, ridges, and

upland plains to lowlands and

coastal marshes. Cooke (1945)

described two major physiographic

regions in central Florida: the

Central Highlands and the Coastal

Lowlands. Land elevations in the

Central Highlands region range

from about 40 to 325 feet above sea

level. The Coastal Lowlands border

the entire coast of Florida, and land

elevations generally are less than

100 feet (Cooke, 1945). 

The central Florida area is characterized by discontinuous highlands separated by broad valleys.

One of the effects of the rise and fall

of sea level was the formation of

relict shoreline features such as

beach ridges, which parallel the

present-day Atlantic coastline

(fig. 11). These beach ridges have

had an effect on the shape, location,

and orientation of lakes in central

Florida. Swales between the beach

ridges (shown as plains in fig. 11)

are areas where surface water

collected when sea level dropped,

causing a decline in the water table

and increasing the downward

movement of water. This increase in

vertical water movement in turn

accelerated the lake formation

processes, which partly explains the

abundance of lakes in central

Florida. Within the Central

Highlands of the Florida peninsula

is the most extensive area of closely

spaced lakes in North America. The

nearest counterparts to the lake

district of Florida are in Canada and

in the northern United States from

Minnesota to Maine.

Lakes in Florida were formed

by several processes. The limestones and dolomites that underlie

central Florida are “soft” rocks that

are easily dissolved by rainwater

entering the subsurface environment from the land surface and by

the ground water moving through

the rock matrix. The chemical

processes by which rock is

dissolved by interactions with water

are commonly referred to as
Hydrology of Central Florida Lakes—A Primer 14

solution processes. The past and

continuing solution of the limestone

beneath the land surface by ground

water results in a landform called

karst. Common characteristics of a

karst landform include a lack of

surface drainage (nearly all the

rainfall infiltrates into the ground

and few drainage channels or

streams develop) and the presence

of sinkholes (depressions in the

land surface), springs, circularshaped lakes, and large cavities in

the limestone and dolomite rocks.

By far, the most common origin

of Florida’s lakes is by solution

processes. Solution processes are

the underlying cause of sinkholes,

which over time become lakes.

Sinkholes are most common in

areas where the intermediate

confining unit between the unconsolidated materials of the surficial

aquifer system and the underlying

limestone of the Floridan aquifer

system is thin, breached, or absent.

With few exceptions, sinkholes

form in areas of recharge to the

Floridan aquifer system. In these

areas, ground water easily moves

downward from the land surface to

the limestone. Carbon dioxide from

the atmosphere and soil zone is

carried with the ground water and

forms carbonic acid, a weak acid

that dissolves the limestone,

eventually forming cavities and

caverns. In contrast, in areas where

the intermediate confining unit is

intact and water in the aquifer is

confined, less water from the

surface reaches the limestone and

sinkholes are not as common.

By far, the most 

common origin of Florida’s 

lakes is by 

solution processes.

The three general types of

sinkholes—subsidence, solution,

and collapse—in central Florida

generally correspond to the thickness of the sediments overlying the

limestone of the Floridan aquifer

system. The sediments and water

contained in the unsaturated zone,

surficial aquifer system, and intermediate confining unit collectively

are termed overburden in the

following description of sinkhole

formation. Collapse sinkholes are

most common in areas where the

overburden is thick, but the intermediate confining unit is breached

or absent. Subsidence sinkholes

form where the overburden is thin

and only a veneer of sediments is

present overlying the limestone.

Figure 11. Major physiographic regions of central Florida. (Modified from Cooke,

1945.)
15 Geomorphology and the Origin of Central Florida Lakes

Solution sinkholes form where the

overburden is absent and the limestone is exposed at land surface.

Collapse sinkholes are the most

dramatic of the three sinkhole

types; they form with little warning

and leave behind a deep, steeply

sided hole. Collapse occurs because

of the weakening of the rock of the

aquifer by erosional processes and

is often triggered by changes in

water levels in the surficial and

Floridan aquifer systems. The

progression of a collapse sinkhole is

shown in figure 12. A small cavity

in the limestone of the Floridan

aquifer system gradually grows

larger as the rock is dissolved by the

ground water flowing through it.

The weight of the overburden above

the cavity is supported by the roof

of the cavity and is partly supported

by pressure from the water in the

aquifer. As the cavity grows larger,

the rock that forms the roof of the

cavity becomes progressively

thinner. As water levels decline

during dry periods or because of

pumping, water pressure in the

limestone cavity decreases and the

weight of the overburden overcomes the structural integrity of the

cavity roof. At this point, the roof

and overburden collapse into the

cavity, forming the surficial

“pothole” that is associated with

sinkholes and karst terrain. The

Winter Park sinkhole that formed in

May 1981 (see photo, p. 17)

probably is the best known collapse

sinkhole in central Florida.

Although collapse sinkholes

generally form during dry periods,

they also can form as a result of

heavy rainfall during an extended

period of time. The rise in the water

table of the surficial aquifer system

as a result of the increased rainfall

causes an increase in the weight of

the overburden. A collapse occurs

when the weight of the overburden

on the roof of a limestone cavity

increases more rapidly than the

water pressure in the cavity that is

partially supporting the cavity roof

and exceeds the structural strength

of the roof.

Figure 12. Formation of a collapse sinkhole.
Hydrology of Central Florida Lakes—A Primer 16

The progression of a subsidence

sinkhole is shown in figure 13.

Rainwater percolates through

overlying sediments and reaches

the limestone, dissolving the rock

and gradually weakening its

structural integrity. As the limestone continues to dissolve, the

unconsolidated sediments above the

limestone are carried downward

with the percolating water and

occupy the spaces formed by the

dissolving limestone. With time, the

movement of sediments into the

openings in the limestone causes

the formation of a bowl-shaped

depression at the land surface. As

the depression enlarges with time,

water begins to collect in the

depression. Sand and clay are

carried to the newly formed

sinkhole in runoff from the

surrounding land, eventually

settling, lining the bottom of the

depression, and restricting the flow

of water through the bottom. As

water continues to accumulate in

the depression, a lake is formed. An

example of a lake formed in this

way is Lake Tsala Apopka, in Citrus

County (see photo, p. 17). In this

area of central Florida, the limestone is covered with a veneer of

unconsolidated sediments or is

exposed at the land surface. Lake

Tsala Apopka is actually a series of

lakes and marshes; the aerial view

shows the many depressions that

together form the lake.

With the passing of time, the

difference in appearance of lakes

formed by these different types of

sinkholes is less distinct. For

example, the photograph of the

Winter Park sinkhole was taken in

1994, 13 years after the sinkhole

formed. In the photo, the steep sides

of the sinkhole are no longer visible

because the water level has risen in

the sinkhole and the “sink” looks

very much like nearby lakes that

may have been formed by other

processes. Thus the specific type of

sinkhole that caused the lake to

form is not readily apparent.

Not all lakes in central Florida

were formed by solution processes.

Some central Florida lakes originally were natural depressions in

the ancient sea floor. These depressions formed as a result of scouring

and redepositing of material by

wave action and water currents.

Freshwater eventually filled these

Figure 13. Formation of a subsidence sinkhole. depressions when sea level fell.
17 Geomorphology and the Origin of Central Florida

Lake Tsala-Apopka in west-central

Florida is an example of a lake

formed by solution processes. The

limestone in the area near this lake

is commonly seen at land surface.

(Photograph by L.A. Bradner,

U.S. Geological Survey.)

The top photograph shows the

Winter Park sinkhole shortly

after it formed in May 1981.

(Photograph by A.S. Navoy.)

In the bottom photograph, the

sinkhole is shown 13 years later,

in October 1994. (Photograph by

L.A Bradner, U.S. Geological

Survey.)
Hydrology of Central Florida Lakes—A Primer 18

Examples of lakes that were formed

in this way are the lakes that form a

chain along the Upper St. Johns

River, including Lakes Helen

Blazes, Washington, Winder, and

Poinsett in Brevard County

(fig. 14). These lakes, through

which the Upper St. Johns River

flows, are remnants of an ancient

estuary (White, 1970, p. 103).

Probably the least common

origin of lakes in Florida is by

fluvial, or riverine, processes. More

commonly, fluvial processes

combine with other lake-formation

processes to enlarge or change the

shape of lakes. For example,

although Lakes Harney, Monroe,

and George (fig. 2) all originated

from depressions in the ancient sea

floor, riverine processes have

continued to enlarge these lakes. 

Lakes formed in ancient sea

floor depressions or by riverine

processes differ in shape, grouping,

and orientation of inlets and outlets

from lakes formed by solution

processes. For example, the lakes

that form the Upper St. Johns River

chain of lakes are elongated in the

direction of the river flow, so that

the lakes appear to be threaded

together by the river (fig. 14). The

inlet and outlet of each lake line up

with the river and are oriented along

the long axis of the lake. Solutionformed or sinkhole lakes tend to

appear in groups or clusters, rather

than in linear chains as in the Upper

St. Johns River lake chain. An

example of solution-formed lakes

are the lakes of the Ocklawaha

chain: Apopka, Harris, Griffin,

Dora, Eustis, and Yale (fig. 15). The

inlets and outlets of these lakes tend

to be randomly oriented with

respect to their long axes. Regardless of the orientation of inlets and

outlets of central Florida lakes, a

group of connected lakes are

commonly referred to as a chain of

lakes.

Manmade lakes in Florida often

are created for practical as well as

aesthetic reasons. Sometimes these

lakes are referred to as “real-estate

lakes,” and are used in housing

developments for stormwater treatment and for enhancement of the

natural landscape. Lakes of this

type usually are not named, even

though some are quite large. One

Lakes formed in ancient 

sea floor depressions or by 

riverine processes differ in 

shape, grouping, and 

orientation of inlets and 

outlets from lakes formed 

by solution processes.

Figure 14. Present-day lakes formed

when sea level fell and freshwater filled

depressions in the sea floor (Upper

St. Johns River chain of lakes).
19 Physical Characteristics of Lakes

type of manmade lake is formed by

building a dam across a river.

Examples of this type of lake are

Lake Rousseau in west-central

Florida (located along the county

lines of Levy, Marion, and Citrus

Counties), which was created when

the Inglis Dam was built in 1969,

and Lake Ocklawaha in Putnam

County, which was created when

the Rodman Dam was built in 1968

(fig. 2). Both of these impoundments were created as part of the

Cross-Florida Barge Canal, which

was partially constructed in the

1960’s to connect the Atlantic

Ocean and the Gulf of Mexico for

shipping purposes but was halted

after concerns about environmental

impacts arose. 

PHYSICAL 

CHARACTERISTICS 

OF LAKES

Some of the physical characteristics of lakes include shape (length,

width), depth, surface area, and

total volume. As described in the

last section, the shape of a lake,

which may be circular, elliptical, or

irregular, is the result of the process

by which it was formed and the

environment in which it has existed

since it was formed. Because the

environment of a lake is dynamic,

physical characteristics may change

with time, either as a result of

natural environmental processes or

because of human activity.

Although the unnamed lake in this aerial photograph taken in an area near

Sebring, Florida, has a nearly perfect circular shape indicating its sinkhole origin,

most solution-formed lakes do not have this perfect a shape! (Photograph by

E.P. Simonds, U.S. Geological Survey.)

Figure 15. Lakes formed by solution

processes (Ocklawaha River chain of

lakes).
Hydrology of Central Florida Lakes—A Primer 20

The physical characteristics of

lakes in Florida differ from those of

lakes in other parts of the country,

primarily because of differences in

how they were formed and in local

geology. Many lakes in the northern

United States, such as the Great

Lakes, the Finger Lakes of New

York, and the numerous lakes in

Minnesota and Wisconsin, were

formed by glacial processes. Lakes

of glacial origin can be very deep,

with steeply sloped sides and an

irregular shape. Florida’s lakes tend

to be very shallow (7–20 feet deep),

with gently sloping sides. Sinkhole

lakes generally are circular but can

be irregularly shaped if a number of

circular depressions coalesce. 

The physical characteristics of a

lake also are related to the age of the

lake. A recently formed sinkhole

lake may be quite deep and have

steeply sloped sides, but gradually

the lake fills in with sediments

carried into the lake by runoff, and

the side slopes become more gradual

and stabilize as a result of erosional

processes. As a lake continues to

age, it may resemble a marsh or

wetland rather than a lake. Eventually, the lake may become little more

than a broad open field (or prairie),

sometimes with a stream running

through what once was the deepest

part of the lake. An example of this

can be seen in Payne’s Prairie near

Gainesville, Florida.

WHAT CAUSES

LAKE WATER-LEVEL

FLUCTUATIONS

Water levels in lakes can vary

over a wide range of time scales,

from short-term, storm-related rises

over a period of days, to long-term

fluctuations caused by seasonal

variations during the year. Water

levels also can vary as a result of

long-term trends in rainfall—

patterns in rainfall may vary over

periods of 20 to 30 years. For many

people, the “normal” level of a lake

is the level at which they first

observed the lake. However, the

normal or average water level

of a lake must be defined for a

particular period of time. The

average water level in a lake during

the 20-year period 1950–70 may be

quite different from the average for

1970–90, for example, and the

difference in the averages may be

primarily due to the difference in

rainfall during those periods. Thus,

the average or normal water level for

any lake can change with time. 

Water levels in Florida lakes,

though driven by inflows (rainfall,

runoff, and ground-water seepage),

ultimately are determined by

outflows. In drainage lakes,

outflows generally increase and

decrease rapidly with small changes

in water levels. Although the water

levels fluctuate frequently, the fluctuations fall within a narrow range.

Seepage lakes are characterized by

less frequent water-level fluctuations within a greater range. The

long-term fluctuations in seepage

lakes reflect the larger seasonal and

annual variations in rainfall and the

altitude of the potentiometric

surface. Examples of these waterlevel fluctuations are presented in

this section of the report.

The variability in lake water

levels is a function of the hydrologic budget of the lake. The

balance of inflows and outflows in

the hydrologic budget of a lake is

reflected in the water levels. This

section presents a description of

how each component of the hydrologic cycle affects lake water levels.

How Rainfall and 

Evapotranspiration 

Affect Lake Water 

Levels

Rainfall, either directly or indirectly, probably is the single

greatest driving factor affecting

water levels in lakes. When rain

falling on the surface of a lake or on

the land surface within the drainage

basin exceeds outflows from the

lake, an increase in the volume of

water in the lake causes a rise in

water levels. A lack of rainfall

causes lake levels to fall because

water losses (from evaporation,

seepage out of the lake, and

pumping from the lake) are not

balanced by inflows from rainfall.

Evaporation from the lake

surface causes lake levels to

decline. Lake evaporation generally

is greatest during May through July

(fig. 6). Lower rainfall during April

and May, combined with relatively

high lake evaporation, result in

greater water losses from lakes

during these months. The losses

resulting from lake evaporation

during low-rainfall periods is most

pronounced in seepage lakes, where

evaporation is a primary output; in

drainage lakes or in lakes where

water levels are controlled, the

effects of high evaporation losses

are less noticeable because they are

masked by other, larger water

losses.

For many people, the 

“normal” level of a 

lake is the level at 

which they first 

observed the lake.
21 What Causes Lake Water-Level Fluctuations

The cumulative difference

between rainfall and evaporation

over time affects long-term

seasonal and annual lake water

levels, particularly in seepage lakes.

Lake Umatilla in Lake County

(fig. 2), for example, has characteristics of a seepage lake because it

does not overflow except at

extremely high water levels. Lake

evaporation, in inches, was

subtracted from monthly rainfall for

a 4-year period; the result for each

month then was accumulated. This

cumulative difference is shown in

figure 16 along with the weekly

water levels in Lake Umatilla. The

4-year period shown represents a

time when many lakes in central

Florida reached record-low water

levels (1981) because of a number

of years of below-normal rainfall.

The lag in the response of the lake

water level to the cumulative difference between rainfall and lake

evaporation (fig. 16) is the result of

the storage of water in the hydrologic system. Inflows of this stored

water sometimes mask the effects of

high or low rainfall. 

The cumulative difference 

between rainfall and 

evaporation over time 

affects long-term seasonal 

and annual lake water 

levels, particularly in 

seepage lakes.

How Surface Water 

Affects Lake Water 

Levels

Surface-water inflows to lakes

include overland flow within the

drainage basin; inflow from

contributing streams; inflow (from

other lakes or ponds) through

surface-water connections such as

canals, streams, or pipes; and

stormwater transported through

storm sewers. Surface-water

connections generally allow more

rapid inflow and outflow of water

than ground-water connections; for

this reason, these inflows and

outflows can affect lake levels and

water quality more rapidly than can

similar volumes of inflow and

outflow of ground water. Most lakes

in Florida are seepage lakes and

thus are not subject to the effects

caused by directly connected

surface-water inflows and outflows.

In drainage lakes in central Florida,

the inlets and outlets may not

function for periods ranging from a

few months to many years because

water levels are below the level of

the inlet or outlet (Deevey, 1988);

thus, even in many drainage lakes,

water levels generally are affected

more by rainfall and ground water

than by surface water.

How Ground Water 

Affects Lake Water 

Levels

The interaction between lakes

and ground water can be very

complex. Flow direction and

volume are functions of the

position of the water surface of the

lake relative to the water table of

the surficial aquifer system and the

potentiometric surface of the

Floridan aquifer system. The

hydraulic gradient is a term used

to refer to the difference or change

in water level over a given

distance; for example, the difference in altitude between the water

table of the surficial aquifer system

and the potentiometric surface of

the Floridan aquifer system,

divided by the vertical distance

between the two water surfaces,

would be the vertical hydraulic

gradient.

Figure 16. Water level in Lake Umatilla, Lake County, and the cumulative

difference between rainfall and lake evaporation.
Hydrology of Central Florida Lakes—A Primer 22

Some “typical” configurations

of lake- and ground-water levels in

recharge and discharge areas that

also illustrate the effects of the

hydraulic gradient are presented in

figure 17. A lake in a ground-water

recharge area is shown in figure 17

(A and B). In figure 17A, water

moves out of the lake and into the

adjacent and underlying aquifer

because the lake water level is

higher than the water table of the

surficial aquifer system, perhaps

because of recent rainfall. During

periods of intense rainfall, the lake

water level may rise above the local

water table and the general flow

direction will be away from the

lake. This condition usually is

temporary. As the rain enters the

soil and recharges the surficial

aquifer system, the water table near

a lake rises above the lake water

level (fig. 17B), the direction of

flow reverses, and the lake receives

seepage from the surficial aquifer

system.

A lake in a ground-water

discharge area is shown in

figure 17C. In this illustration, the

potentiometric surface of the

Floridan aquifer system is above

the lake water surface (and the

water table of the surficial aquifer

system); thus, the hydraulic

gradient and the direction of flow is

upward. Water from the Floridan

aquifer system moves upward to the

surficial aquifer system or directly

to the lake bottom, providing a

source of water to the lake. The

water moves under pressure by

diffuse upward leakage (migrating

through the pores of the rock and

soil), through fractures, or through a

large opening, such as a spring. 

Water levels in lakes located in

recharge areas generally fluctuate

more than levels in lakes in

discharge areas (Hughes, 1974b).

This may be because a lake in a

discharge area receives a fairly

constant supply of water from the

underlying aquifer, whereas the

leakage of water out of a lake in a

recharge area is more variable.

Figure 17. Interactions between ground-water and lake water levels. (Note: Only

surficial materials are illustrated.)
23 What Causes Lake Water-Level Fluctuations

Because so many of the lakes in

central Florida were formed by

solution processes, there is a strong

link between lakes and ground

water. The illustration in figure 18

of a sinkhole lake in a ground-water

recharge area shows some of the

pathways of ground-water movement. The lake shown in figure 18 is

underlain by sand and organic

material from the collapse that

formed the lake; these materials

provide an avenue for water to

move easily from the lake to the

underlying Floridan aquifer system.

Also shown in figure 18 is the

“flow-through” condition that

exists in many central Florida lakes.

Ground-water flows into the lake

along one shoreline and out of the

lake along the opposite shoreline, in

the direction of the positive

hydraulic gradient of the water table

of the surficial aquifer system. 

Because so many 

lakes in central Florida 

were formed by solution 

processes, there is a 

strong link between lakes 

and ground water.

The rate and volume of groundwater flow also depends on the

permeability of the materials in

which the lake exists and the thickness and characteristics of the intermediate confining unit beneath the

lake. For example, ground water

moves very slowly through a thick

intermediate confining unit, thus

slowing the rate and reducing the

volume of water entering or leaving

the lake. The rate of leakage from a

lake increases with increasing

permeability of the soil. Lakes with

high rates of leakage generally will

have greater ranges of water levels

than lakes with low rates of leakage

(Hughes, 1974b). 

One common misconception

about lakes in central Florida is that

many of them are spring-fed. Many

lakes are spring-fed, but the origin

of the water of the spring is not

necessarily the Floridan aquifer

system, the same aquifer that

supplies the major springs for

which Florida is known. Observed

springs in lakes commonly are

specific locations in the lake bottom

where water is entering the lake

from the adjacent surficial aquifer.

Some lakes are directly connected

to the Floridan aquifer system

through fractures in the underlying

rock or through the remains of the

original sinkhole that formed the

lake. Such connections provide a

Figure 18. Cross section through a sinkhole lake showing ground-water seepage.
Hydrology of Central Florida Lakes—A Primer 24

path for water from the Floridan

aquifer system to enter the lake, and

springs of sufficient magnitude to

be detected are manifestations of

these connections. The cross

sections in figure 19 show the

conditions of spring flow from the

surficial aquifer system (fig. 19A)

and the Floridan aquifer system

(fig. 19B) to lakes. Examples of

lakes that receive spring flow from

the Floridan aquifer system are

Lake Apopka in Orange County

(Apopka Spring) and Lake George

in northwestern Volusia County

(Croaker Hole).

How Human Activities 

Affect Lake Water Levels

Many Florida lakes have been

modified to decrease the range of

fluctuation in lake levels and reduce

the risk and extent of flooding

around lakes. Natural drainage

systems have been altered, for

example, through the addition of

canals or by the dredging or

straightening of existing outflow

channels. Many of the canals

connecting lakes in Florida were

constructed during the 1800’s,

before the arrival of the railroad,

when Florida lakes and rivers

formed a major transportation

network. Some of these canals were

dug to lower the water levels and

provide more usable land adjacent

to the lakes because the land was

valuable for farming and citrus

cultivation. The effect of this type

of modification of natural drainage

is the reduction in the overall

surface area of the lake and in the

range of lake level fluctuations,

particularly for natural seepage

lakes.

Other human activities that

affect the water budget of lakes and

thus, water levels, include urbanization, the use of lake water by

residents for lawn irrigation

(lowering lake levels), and the

disposal of water from various

sources to lakes (increasing lake

levels). Urbanization increases the

impervious surface area contributing runoff to a lake, causing a

greater volume of water to reach the

lake unless other provisions are

made (retention facilities).

Although some of the lake water

used for irrigation returns to the

lake through ground-water seepage,

most of it is lost to evaporation and Figure 19. Surficial-aquifer spring flow (A) and Floridan-aquifer spring flow (B) to a

lake.
25 What Causes Lake Water-Level Fluctuations

uptake by vegetation. Examples of

water contributed to lakes through

human activities include stormwater runoff, water from water-toair heat-pump systems (which

obtain water from ground-water

sources), treated sewage effluent,

and discharge water from agricultural activities. Recent regulations

pertaining to discharging of treated

wastewater to lakes and streams

have led to development and implementation of alternative disposal

methods and, in some cases, have

reduced or eliminated these inflows

to lakes.

Ground-water interactions with

lakes also are affected by human

activities. For example, one source

of ground water to lakes is water

(leachate) from drainfields of septic

tanks (shown in fig. 18). Other

activities cause declines in lake

water levels. Water pumped for

irrigation or other applications from

the surficial aquifer system near a

lake reduces the natural seepage to

the lake by lowering the water table,

which may cause a decrease in lake

water level. Pumping water from

the Floridan aquifer system in

ground-water recharge areas can

lower the potentiometric surface of

the aquifer, increasing the hydraulic

gradient between the lake water

level and the potentiometric

surface. This increase would in turn

induce more seepage from the lake,

thus lowering the lake water level.

Examples of Lake 

Water-Level Fluctuations

The range of water-level fluctuations can vary significantly among

lakes. As described previously,

water-level fluctuations in seepage

lakes (without artificial water-level

controls) generally are less frequent

but vary through a greater range

than water-level fluctuations in

drainage lakes. To illustrate this,

water levels for two lakes are shown

in figure 20—one is a seepage lake

(Lake Francis in northwestern

Orange County), and the other is a

drainage lake with a controlled

outlet (Lake Apopka in Orange and

Lake Counties). Although the water

levels are at different elevations, the

lake levels are shown to the same

relative scale. The scale for Lake

Francis is shown on the left axis of

the graph and the scale for Lake

Apopka is shown on the right. Lake

level changes are more frequent in

Lake Apopka than in Lake Francis,

but the range in water levels for

Lake Francis is much greater than

the range in water levels for Lake

Apopka. Observed water levels in

Lake Francis range from about

53 feet (May 25, 1990) to about

66 feet (October, 1960), which

represents a range of 13 feet.

Figure 20. Water-level fluctuations in a drainage lake (Lake Apopka) and a seepage lake (Lake Francis).
Hydrology of Central Florida Lakes—A Primer 26

Observed water levels in Lake

Apopka range from about 64 feet

above sea level (August 13, 1956)

to about 69 feet above sea level

(October 12, 1936), which represents a range of water level of

5 feet. Thus, the water level in landlocked Lake Francis has varied by

as much as 8 feet more than the

water level in Lake Apopka.

Extreme water-level fluctuations in some lakes in Florida have

been a matter of great concern

among residents. For example,

water levels in Brooklyn Lake,

located in Clay County in northcentral Florida, have ranged from

90 feet above sea level (recorded in

1992) to 118 feet above sea level

(reported in 1948 by a local

resident), a range of 28 feet. In

contrast, Sand Hill Lake, about

2.5 miles northeast of Brooklyn

Lake, historically has fluctuated

through a range of only 3.3 feet

(fig. 21). Brooklyn Lake receives

surface inflow, but surface outflow

from the lake occurs only when the

lake level rises above about 115 feet

(Clark and others, 1963). Thus,

water-level fluctuations are similar

to what might be expected in a

seepage lake. Brooklyn Lake is

located in a ground-water recharge

area, receiving inflow from the surficial aquifer system and recharging

the underlying Floridan aquifer

system. Water levels during an

11-year period (1985–95) in a well

just west of Brooklyn Lake and

water levels for Brooklyn and Sand

Hill Lakes are shown in figure 21.

Note that there is a greater similarity between the fluctuations in

the level of Brooklyn Lake (middle

line in graph) and the potentiometric level of the Floridan aquifer

system (as represented by the water

level in the well, bottom line in

graph) than there is between water

levels for Brooklyn Lake and Sand

Hill Lake (top line in graph).

Research has indicated that in some

areas near Brooklyn Lake, the intermediate confining unit is relatively

permeable and allows water

movement from the lake and surficial aquifer system to the Floridan

aquifer system. The identification

of a filled sinkhole in the lake also

indicates a path of preferential flow

from the lake to the underlying

Floridan aquifer system. This

hydraulic connection is reflected in

the similarity of the Brooklyn Lake

water levels and ground-water

levels shown (fig. 21).

Lake water levels may fluctuate

because of the effect of wind on the

water surface and sometimes

because of tidal effects. Seiche is a

term applied to a wave that oscillates in a water body, causing

periodic water-level fluctuations

that can vary in time from a few

minutes to several hours. Seiche is

caused by seismic or atmospheric

disturbances and is aided by wind

and tidal currents. The effect of

seiche is related to the physical

dimensions of a lake (surface area

and depth); large lakes will exhibit

more of the effects of seiche than Figure 21. Water level in Sand Hill Lake, Brooklyn Lake, and in a well near

Brooklyn Lake.
27 What Causes Lake Water-Level Fluctuations

will small lakes, and shallow lakes

will be affected more than deep

lakes. The effect of seiche is

indicated by the oscillations in

water levels in Lake Apopka during

a 48-hour period (fig. 22). Lake

levels for the same time period are

shown for Lake Apshawa, which is

near Lake Apopka and thus subject

to similar wind patterns. Lake

Apopka has a surface area of

30,630 acres (47.9 square miles).

Water levels in Lake Apshawa, with

a surface area of only 110 acres

(0.17 square mile), indicate no

noticeable seiche effect during the

48-hour period. 

Artificial Control of 

Lake Water Levels

Water levels in lakes may be

artificially controlled by increasing

the volume of water entering or

leaving the lake through the use of

control structures, which can be

fixed or adjustable. Some examples

of fixed control structures include

canals, dams, and drainage pipes

(culverts). These fixed controls can

be made adjustable by the addition

of devices that can be raised or

lowered. Examples of devices used

to artificially control lake water

levels are shown in figure 23. A

culvert connects two lakes

(fig. 23A) and allows water to flow

in either direction until the levels of

the two lakes are equal. Canals can

be fitted with gates that can be

adjusted to allow more or less flow

out of (or into) a lake, which in turn

affects the lake water level. Radial

gates, shown in figure 23B, are used

in many canals and can be set at a

fixed opening size to allow only a

certain amount of water to flow

beneath the lower edge of the gate.

A culvert riser is a way of controlling inflow or outflow to or from a

culvert using boards, or stop-logs

which are removed or added to

control flow (fig. 23C). A screwtype gate on a culvert (fig. 23D) can

be adjusted to regulate the volume

of water flowing into or out of a lake

(depending on the flow direction).

Lake drainage wells (fig. 23E),

which allow lake water to flow

directly into the aquifer (usually the

Floridan aquifer system), have been

used to control lake water levels in

central Florida, particularly in the

Orlando area, since 1904. The

volume of water that can leave a

lake through a drainage well is

primarily a function of the diameter

of the well; debris is prevented from

entering the well by a protective

cage over the top of the well. If the

drainage-well structure is not regularly maintained by removing

debris from the protective cage, the

buildup of debris causes a restriction in flow, resulting in higher lake

water levels. Drainage wells, used

primarily on seepage lakes, significantly affect lake water levels and

reduce the risk of flooding by

providing rapid drainage. However,

the direct connection between lake

water and water in the Floridan

aquifer system increases the risk of

contaminating the aquifer. A

drainage well on a seepage lake

performs the same function that a

surface outlet performs on a

drainage lake. Thus, the range of

water-level fluctuations in a

seepage lake that has a drainage

well may be similar to those of a

drainage lake.

The range of water-level fluctuations in artificially controlled lakes

tends to be smaller than the range of

levels in uncontrolled lakes. Many

of these controls were built in the

past when it was considered desirable to control lake levels for

aesthetic reasons and for flood

control. In more recent years,

however, the negative effects on

water quality caused by minimizing

the natural rise and fall of lake water

levels has been recognized. For

example, reducing the natural range

of fluctuations in a lake decreases

Figure 22. Water levels in Lake Apopka and Lake Apshawa illustrating the effect

of seiche on water levels.
Hydrology of Central Florida Lakes—A Primer 28

the area of vegetated flood plain and

wetlands around many lakes, areas

that serve as natural fish nurseries

and nutrient filters (through plant

uptake). The loss of fish habitat and

increased nutrient enrichment

resulting from the decreased range

of water levels in controlled lakes

has been detrimental to the overall

recreational and ecological quality

of these lakes. Greater variation in

water levels is now allowed in most

controlled lakes because of the

recognized benefits to lake flora

and fauna. 

QUALITY OF WATER 

IN CENTRAL FLORIDA 

LAKES

The quality of water in Florida

lakes is affected by many factors,

including the sources of water to the

lake, the quality and quantity of

water from these sources, the length

of time a volume of water remains

in the lake (residence time), rainfall

and runoff amount and quality, time

of year, depth of the lake, and the

type and amount of plants and

animals in the lake. Common lake

classification systems, factors or

hydrologic characteristics that

affect lake water quality, and some

of the methods used for waterquality improvement are discussed

in the following sections.

The basic characteristics of lake

water quality include the physical,

chemical, and biological characteristics of a lake. Lake water quality is

complex, and the diversity of plants

and animals that can be supported in

a lake is closely related to the

relative amounts of individual

chemical constituents in the water.

Relative concentrations of selected

chemical constituents can be indicators of the source of the water to the

lake because of differences in the

chemistry of water from these

sources. As an example, water from

the Floridan aquifer system is

higher in concentrations of calcium

and bicarbonate ions than water

from the surficial aquifer system or

surface waters. 

How “good” the quality of

water in a lake is depends on who is

asking the question! What is the

intended use of the water? Is it

“good enough” for one use, but not

for another? A lake that contains a

large number of aquatic plants and

supports a large fish population may

“look” unappealing to some for

swimming or water-skiing, but a

Figure 23. Examples of lake water-level control devices.

Lake drainage wells, 

which allow lake water 

to flow directly into the 

aquifer, have been used 

to control lake water 

levels in central Florida, 

particularly in the 

Orlando area, 

since 1904.
29 Quality of Water in Central Florida Lakes

lake that is attractive for swimming,

with low nutrient (nitrogen and

phosphorus) concentrations, clear

water, and a sandy bottom free of

aquatic plants, will be of little use to

someone who wants to catch fish.

The quality of water sometimes is

judged on the basis of its clarity—a

strictly subjective opinion, based on

an individual’s perception of the

clarity of the lake water. 

Lake Water-Quality 

Classification Systems

Scientists have devised

different classification schemes to

define the quality of lakes relative

to one another. A common method

of classifying lakes is by the trophic

state of the lake. The trophic state

refers to the degree or amount of

enrichment (eutrophication) of the

lake with nutrients in the water.

Lakes can be classified as

oligotrophic, mesotrophic, or

eutrophic (fig. 24). Oligotrophic

lakes have very low levels of nutrients, very little organic material

along the lake bottom, and high

levels of dissolved oxygen near the

bottom. Mesotrophic lakes are

moderately enriched, and the

natural processes of accumulation

of sediments and growth of aquatic

vegetation are occurring. Eutrophic

lakes are highly enriched with nutrients, have an accumulation of

organic sediments, and low levels

of dissolved oxygen in water near

the lake bottom. Eutrophic lakes

typically have high concentrations

of algae or aquatic vegetation and

also differ from oligotrophic and

mesotrophic lakes in the type of

vegetation and animal life that can

exist in the lake. The trophic state

also is a measure of the productivity

of a lake; lakes that are enriched are

more “productive” in that they have

large populations of aquatic plants

and animals, though the diversity of

the organisms may be low. Lowproductivity (oligotrophic) lakes

have much smaller, but more

diverse, populations of flora and

fauna than mesotrophic or eutrophic

lakes. However, many species of

fish and other aquatic animals

cannot tolerate the conditions that

exist in eutrophic lakes, where large

fluctuations in concentrations of

dissolved oxygen are common.

Thus, there may be more fish in a

eutrophic lake than in an oligotrophic lake, but far fewer species

can survive in the eutrophic lake.

All lakes eventually become

eutrophic as a part of the aging

process. Although the process is

natural, many factors can accelerate

the process, including human influences. A newly formed, “young”

lake will be oligotrophic. With time,

sediments continue to enter the

lake, plant and animal populations

become established, and the

Figure 24. Examples of an oligotrophic, mesotrophic, and eutrophic lake.
Hydrology of Central Florida Lakes—A Primer 30

chemistry of the water gradually

changes to that of a mesotrophic

lake. As the lake continues to age,

or eutrophy, it gradually becomes a

marsh and, eventually, an open field

(as described in the section on

physical characteristics of lakes). 

Lakes also can be classified

according to the chemical characteristics of the water. In the

late 1960’s and early 1970’s,

researchers from the University of

Florida studied 55 lakes and ponds

in north-central Florida and determined that the lakes could be

divided into four distinct groups

based on the following physical

characteristics: alkalinity, specific

conductance, color, and calcium

concentration (Shannon and

Brezonik, 1972). The four groups

into which these 55 lakes in

north-central Florida were placed

are 1) acid, colored; (2) alkaline,

colored; (3) alkaline, clear; and

(4) soft water, clear.

Water Quality: A Function 

of the Source Water

The quality of water in Florida

lakes generally is controlled by two

important properties—the quality

of the water entering the lake, and

the rate at which lake water is

replenished. These two properties

are related—the effects of poorer

quality inflow water can be mitigated by the rapid replacement of

water in a lake. Conversely, the

quality of water in a lake in which

the water is replenished slowly (as

in seepage lakes) can be maintained

if the inflow water quality is good.

In this section, the effects from

inflow water from various sources

are discussed. 

The source of water to a lake can

sometimes be determined from the

chemical characteristics of the

water. For example, lakes that

receive water from surface-water

sources or from the surficial aquifer

system generally have low concentrations of dissolved solids when

compared with lakes that receive

water from the Floridan aquifer

system. Water in lakes that receive

runoff from wetlands may have a

relatively low pH and be dark and

reddish brown in color. This color

comes from the natural tannins in

the plants and soils of the wetland

areas. Lakes that receive most of

their water from ground-water

sources and rainfall (seepage lakes)

generally have little color. Seepage

lakes are affected by ground-water

seepage to and from the lake and by

the land use immediately around the

lake. Leachate from septic tank

systems, fertilized lawns, and agricultural lands can potentially

contribute additional nutrients such

as nitrogen and phosphorus to the

lake.

Surface-water inflows can carry

with it nutrients and pesticides from

upstream sources. For example,

outflow from nutrient-rich Lake

Apopka enters Lake Beauclair and

the other downstream lakes in the

Ocklawaha chain of lakes. Surface

runoff from streets can carry grease

and oils from automobiles. Surface

runoff from residential areas can

carry fertilizers and pesticides from

lawns, grass clippings, leaves, and

animal wastes to a lake. Runoff

from industrial areas may contain

residual amounts of chemicals used

for cleaning or degreasers that are

washed into storm sewers.

One important factor that 

has a direct effect 

on the response of the lake 

to the addition of nutrients 

is the residence time of 

water in the lake.

Shoreline vegetation can provide aesthetic value as well as habitat for wildlife.

(Photograph provided by St. Johns River Water Management District.)
31 Quality of Water in Central Florida Lakes

Water Quality: A Function 

of Residence Time

How much the water from the

various sources described in the last

section affects water quality in a

lake is a function of several factors.

One important factor that has a

direct effect on the response of the

lake to the addition of nutrients is

the residence time of water in the

lake. Another term sometimes used

in place of residence time is

flushing rate, which is the rate

(volume per unit time) at which

water leaves a lake, either through a

surface-water outlet or through

ground-water seepage.

Because seepage lakes 

tend to have long 

residence times, they are 

more likely to be affected 

by the addition of nutrients 

or changes 

in chemistry than are 

drainage lakes.

Residence time is significant to

the water quality of seepage and

drainage lakes. Residence times in

seepage lakes tend to be naturally

long due to slow rates of groundwater outflow. The long residence

time provides time for aquatic

plants to use the nutrients in the

water, encouraging plant growth

and settling of nutrient-rich sediments on the lake bottom. Residence times in drainage lakes are

shorter than in seepage lakes. In

lakes with short residence times, the

nutrients are not available in the

water for plant uptake for as long as

in lakes with long residence times.

Thus drainage lakes generally are

able to receive a greater nutrient

load than seepage lakes without

excessive aquatic plant growth.

Because seepage lakes tend to have

long residence times, they are more

likely to be affected by the addition

of nutrients or changes in chemistry

than are drainage lakes. One study

of 20 Florida lakes indicated that

lakes in northern States (of comparable volume to the lakes in the

study) were flushed five to ten times

more rapidly than Florida lakes

(Brenner and others, 1990, p. 372).

The long residence time of most

Florida lakes makes them more

vulnerable to the effects of relatively small amounts of pollutants.

Water-Quality Problems

Central Florida lakes are subject

to many influences, both natural

and manmade. Although eutrophication is a natural process, it can be

accelerated by human activities.

One well-documented example of a

lake in which eutrophication has

been accelerated because of human

influence is Lake Apopka, a classic

example of a eutrophic lake that is

characterized by a thick layer of

organic matter on the lake bottom

and high concentrations of algae. In

the late 1800’s, Lake Apopka was

the second largest lake in Florida

and supported a large population of

bass. Beginning at the turn of the

20th century, dikes were built along

the north shore of the lake and the

part of the lake behind the dike was

drained so that the rich organic soils

of the lake bottom could be used for

farming. This reduced the size of

the lake, so that it is now only the

fourth largest lake in the State.

Nutrients from farms and citrus

groves washed into the lake, and

sewage effluent was discharged into

the lake for many years. The lake

When aquatic vegetation grows to nuisance levels, chemical methods may be

employed for control. This type of control can lead to problems with water quality

when the plants begin to decompose after they are killed, releasing nutrients into

the water, and encouraging more vegetative growth. (Photograph provided by

St. Johns River Water Management District.)
Hydrology of Central Florida Lakes—A Primer 32

was used for recreation including

bass fishing competitions up to the

1940’s, when a hurricane uprooted

much of the vegetation in the lake,

which further upset the natural

balance of the lake and degraded the

quality of the water. The uprooting

of the vegetation, in combination

with the added nutrients, contributed to the rapid decline of lake

water quality.

Aquatic vegetation is necessary

for the health of a lake, but some

types of vegetation are desirable

whereas others are considered

nuisance plants. Shoreline plants

provide habitat for water fowl and

other animals and are considered

beneficial because they remove

nutrients from the water, thereby

decreasing the nutrients available

for algae growth. Aquatic vegetation, whether along the shore or

farther in the lake, contributes to the

clarity of lake water. However,

generally more than one-half of the

lake bottom must have aquatic

vegetation in order to cause the

water to be clearer or to maintain

water clarity (Canfield, 1992).

However, this level of aquatic vegetation often is considered a nuisance

level. 

One aquatic plant that has

created problems in Florida lakes

and streams is the water hyacinth, a

plant brought from Brazil to the

United States in the late 1800’s

(Brenner and others, 1990, p. 382).

Hyacinths float on the water surface

and prevent light from penetrating

through to the lake bottom. This

lack of light in turn causes the

decline of rooted aquatic vegetation

on the lake bottom, which changes

the vegetative characteristics of the

lake and alters the natural balance

of plant and animal life. Water

hyacinths grow prolifically and

affect the use of lakes for fishing

and recreational purposes. Hydrilla

is another aquatic plant, introduced

through the aquarium trade in the

early 1960’s (Brenner and others,

1990, p. 382), that has spread to

many Florida lakes. Hydrilla also

can choke out native plant species

and affect lake water quality. 

Acid rain may affect lake water

quality. Most of Florida’s lakes

have soft water (Shannon and

Brezonik, 1972) and are poorly

buffered (pH of the water is easily

affected by changes in water chemistry), thus these lakes are more

susceptible to the effects of acid

rainfall. However, many Florida

lakes are naturally acidic, and

species of plants and animals in the

lakes are naturally tolerant of more

acidic conditions. 

Water-Quality Solutions

The use of the preventive and

restorative techniques described

here, along with the actions of

residents today, will ensure that

future generations will enjoy the

lakes of Florida. Techniques for

solving lake water-quality problems

are numerous, but never easy,

because lakes are complex systems.

The most common method of

improving lake water quality is

through the application of Best

Management Practices, or BMPs,

which include the following: aeration, pretreatment by detention of

stormwater prior to its entering a

lake, addition of alum, and more

exotic techniques such as addition

of Asiatic grass carp to the lake

(Brenner and others, 1990, p. 383).

Many BMPs are useful for all lakes,

regardless of the lake water quality,

to help prevent or delay waterquality problems. Some of the

BMPs are preventive measures

whereas others are more restorative

The scene above shows the variety of vegetation that is found in and near lakes.

(Photograph provided by South Florida Water Management District.)
33 Quality of Water in Central Florida Lakes

in nature. Other methods of lake

restoration include dredging and

lake level drawdown. These

preventive and restorative methods

are discussed briefly below. 

Aeration of lake water adds

oxygen to the water and aids in the

mixing and distribution of the

oxygen in the water. Aeration helps

prevent fish kills that result from

low concentrations of oxygen.

Other potential benefits of aeration

include control of algal blooms and

general improvement of the

condition of the bottom sediments,

which can be highly organic and

anaerobic (without oxygen) in

eutrophic lakes. Phosphorus is more

soluble in water with low oxygen

concentrations. The addition of

oxygen through aeration causes

phosphorus to be less mobile, which

helps to keep the lake from

becoming weed-choked or algalrich. The addition of oxygen to the

water provides a better environment

for aquatic plants and animals, and

the mixing that occurs during

aeration helps distribute the oxygen

through the water column.

Another common method of

preventing water-quality degradation in lakes is the detention of

stormwater prior to its entering a

lake. Stormwater is known to be a

source of contaminants and nutrients to lakes. By detaining the

incoming stormwater, sediments

and debris carried by the water can

settle, improving the condition of

the water before it enters a lake.

Alum, a chemical compound with a

high affinity for absorbing other

chemicals, has been added to water

in detention basins to remove nutrients from the water prior to release

of the water to a lake. However, the

alum and absorbed material eventually must be removed after it has

settled and accumulated in the

detention basin or lake bottom.

A more exotic method of lake

restoration is the addition of

specific species of fish known for

their capacity to consume algae and

aquatic plants. The Asiatic grass

carp has been used successfully in

some central Florida lakes to

control aquatic vegetation (Canfield

and others, 1983). 

Many central Florida lakes are surrounded by private homes and docks.

(Photograph by E.R. German, U.S. Geological Survey.)

Aeration of lake water increases oxygen in the water, improves water circulation,

and also has aesthetic value.
Hydrology of Central Florida Lakes—A Primer 34

Probably the most effective way

to protect the quality of water in

lakes is through education and

prevention of problems before they

occur. All of Florida’s residents and

visitors can help protect lakes by

being aware of human activities that

affect lakes. People living on lakefront property can improve water

quality by leaving aquatic vegetation along the shoreline. Removing

the vegetation to create beaches

interferes with the natural uptake of

nutrients from the water by shoreline plants and eliminates habitats

for aquatic life. Florida residents

can help prevent water-quality

problems in neighborhood lakes by

reducing the amount of fertilizer

applied to lawns and landscape

plants to that which is required for

proper growth, so that excess fertilizer will not wash into lakes and

streams. Boaters can help improve

lake water quality by being aware of

the possible transfer of undesirable

vegetation (specifically, hyacinths

and hydrilla) from one lake to

another on boats and boat trailers.

Probably the most effective 

way to protect the quality 

of water in lakes is through 

education and prevention 

of problems before 

they occur.

Lakeshore vegetation is important for nutrient uptake and

provides habitat for wildlife. (Photograph provided by South

Florida Water Management District.)

The drawdown or lowering of the water level in an urban

lake sometimes is used to improve water quality by

exposing the lake bottom for sediments to become

oxygenated. Here, Lake Eola in downtown Orlando is

shown, August 1994.

Another management method is the use of dry-retention

basins, which can be incorporated into the landscape of

lakeside parks. Shown above is a dry-retention basin near

Lake Eola in downtown Orlando, Florida.

It is not uncommon to find the natural shoreline of a lake

altered such as is shown above. (Photograph by

E.R. German, U.S. Geological Survey.)
35 Summary

SUMMARY

One of the most common sights

in central Florida are the numerous

lakes dotting the landscape. Lakes

enhance the beauty of the landscape, are important for wildlife,

and are a treasured natural resource,

used by Florida residents and

visitors for recreation and appreciated for their beauty. 

Lakes in Florida are special

because of the hydrologic characteristics that set these lakes apart

from lakes in most other States.

Florida’s karst landscape has

provided a setting for numerous

landlocked, shallow seepage lakes

that are closely tied to the groundwater resources of the State. In

addition to these seepage lakes,

some lakes in central Florida are

drainage lakes, which drain through

surface-water connections to other

lakes or to streams or rivers.

Lakes in Florida are special 

because of the hydrologic 

characteristics that set 

these lakes apart from 

lakes in most other States. 

Florida’s karst landscape 

has provided a setting for 

numerous landlocked, 

shallow seepage lakes that 

are closely tied to the 

ground-water resources 

of the State.

Water levels are of great

concern to many, and the

complexity of the hydrologic

system governing these lake levels

is not commonly understood. Lake

levels are a product of the balance

of the hydrologic budget—inflows

(rainfall, runoff, seepage) minus

outflows (evaporation, seepage,

surface-outflow). The water level

response to the components of the

hydrologic budget differs between

seepage and drainage lakes,

primarily in the timing of the fluctuations of lake levels and in the range

of lake levels. Water levels in

drainage lakes can change rapidly

because the surface-water outflows

allow for a large volume of water to

leave the lake in a relatively short

time period. However, a small

change in lake level can produce a

large volume of outflow from the

lake, thus the water levels rise and

fall within a very narrow range.

Seepage lakes, however, lose water

much more gradually through evaporation and seepage, and the

volume of water exiting is much

aquatic weeds, and many other

problems, do not have simple or

quick solutions. Many problems are

the natural consequence of climatic

changes; others are the result of

many decades of development. The

importance of education, improved

understanding through research,

and cooperation cannot be understated. Through the combined

efforts and cooperation of individuals, communities, and government, lakes in Florida can be

preserved and continue to be a

valuable natural resource for the

benefit of all.

smaller, so water levels rise to

greater altitudes than in drainage

lakes. The wide variation in water

levels in many central Florida

seepage lakes historically has

presented problems for the development and use of land near these

lakes. As a result, many seepage

lakes have been artificially drained

through the construction of canals

and drainage wells.

The long residence times in

Florida seepage lakes makes these

lakes more susceptible to even small

changes in inflow water quality. The

shorter residence times of drainage

lakes helps somewhat in maintaining the lake water quality but

does not preclude the potential for

negative effects.

Problems of declining

lake levels (or flooding caused by

rising lake levels), algae blooms,

choking of lakes with
Hydrology of Central Florida Lakes—A Primer 36

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Hydrology of Central Florida Lakes—A Primer 38

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