Hydrology of Central Florida Lakes —A Primer...the Florida peninsula is a result of the geology and geologic history of the State. An estimated 7,800 lakes in Florida are greater

In cooperation with theST. JOHNS RIVER WATER MANAGEMENT DISTRICTSOUTH FLORIDA WATER MANAGEMENT DISTRICT

U.S. Geological SurveyCircular 1137

Hydrology ofCentral 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 anddoes not constitute endorsement by the U.S. Government

U.S. GOVERNMENT PRINTING OFFICE :

Free on application to theU.S. Geological Survey

Branch of Information ServicesBox 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 Districtand the South Florida Water Management District.”

Includes bibliographical references.Supt. of Docs. no.: I 19.42: 11371. Lakes—Florida. I. St. Johns River Water Management District.

II. South Florida Water Management District. III. Title.IV. Series.GB1625.F6S35 1997551.48'2'09759—dc21 97–822

CIP

ISBN 0–607–88561–0

Contents

CONTENTS

iii

Introduction ........................................................................................................................................................................... 1Classification of Lakes .......................................................................................................................................................... 4The Hydrologic Cycle ........................................................................................................................................................... 5

Rainfall ........................................................................................................................................................................ 7Evaporation and Transpiration .................................................................................................................................... 8Surface Runoff............................................................................................................................................................. 10Infiltration and Ground-Water Recharge and Discharge ............................................................................................. 10

Geology of Central Florida.................................................................................................................................................... 10Ground Water......................................................................................................................................................................... 12Geomorphology and the Origin of Central Florida Lakes..................................................................................................... 13Physical Characteristics of Lakes .......................................................................................................................................... 19What Causes Lake Water-Level Fluctuations........................................................................................................................ 20

How Rainfall and Evapotranspiration Affect Lake Water Levels ............................................................................... 20How Surface Water Affects Lake Water Levels .......................................................................................................... 21How Ground Water Affects Lake Water Levels .......................................................................................................... 21How Human Activities Affect Lake Water Levels...................................................................................................... 24Examples of Lake Water-Level Fluctuations............................................................................................................... 25Artificial Control of Lake Water Levels ...................................................................................................................... 27

Quality of Water in Central Florida Lakes ............................................................................................................................ 28Lake Water-Quality Classification Systems ................................................................................................................ 29Water Quality: A Function of the Source Water.......................................................................................................... 30Water Quality: A Function of Residence Time ........................................................................................................... 31Water-Quality Problems .............................................................................................................................................. 31Water-Quality Solutions .............................................................................................................................................. 32

Summary................................................................................................................................................................................ 35Selected References ............................................................................................................................................................... 36

FIGURES

1. Approximate geographic area covered by this report and distribution of lakes in Florida ........................................ 22. Lakes and weather stations in this report.................................................................................................................... 33. Illustration summarizing hydrologic characteristics of seepage and drainage lakes .................................................. 44. Hydrologic cycle......................................................................................................................................................... 65. Major components of the hydrologic budget for a central Florida lake in a ground-water recharge area.................. 76. Mean monthly rainfall at DeLand and Orlando for the period 1961–90 and lake evaporation based on

pan evaporation (1960–88) ......................................................................................................................................... 87. Annual rainfall at Jacksonville, the station with the longest record in Florida .......................................................... 98. Ground-water movement in recharge and discharge areas ......................................................................................... 109. Generalized summary of geologic and geohydrologic units in central Florida.......................................................... 11

10. Geology and major aquifers in central Florida ........................................................................................................... 1211. Major physiographic regions of central Florida ......................................................................................................... 1412. Formation of a collapse sinkhole ................................................................................................................................ 1513. Formation of a subsidence sinkhole............................................................................................................................ 1614. Present-day lakes formed when sea level fell and freshwater filled depressions in the sea floor

(Upper St. Johns River chain of lakes) ....................................................................................................................... 1815. 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 tempera-ture 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 concentra-tions can lead to excessive plant growth and water-quality 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 waterthan land. Florida has more natu-rally formed lakes than other south-eastern States, where many lakesare created by building dams acrossstreams. The abundance of lakes onthe Florida peninsula is a result ofthe geology and geologic history ofthe State. An estimated 7,800 lakesin Florida are greater than 1 acre insurface area. Of these, 35 percentare located in just four counties(fig. 1): Lake, Orange, Osceola, and

Polk (Hughes, 1974b). Lakes add tothe aesthetic and commercial valueof the area and are used by manyresidents and visitors for fishing,boating, swimming, and other typesof outdoor recreation. Lakes alsoare used for other purposes such asirrigation, flood control, watersupply, and navigation. Residentsand visitors commonly ask ques-tions such as “Why are there somany lakes here?”, “Why is my lakedrying up (or flooding)?”, or “Is mylake spring-fed?” These questionsindicate that the basic hydrology oflakes and the interaction of lakeswith ground water and surfacewater are not well understood by thegeneral population.

Because of the importance oflakes to residents of central Floridaand the many quest ions andmisconceptions about lakes, this

p r imer was p repared by theU.S. Geological Survey (USGS) incooperation with the St. Johns RiverWater Management District and theSouth Florida Water ManagementDistr ict . The USGS has beencollecting hydrologic data in centralFlorida since the 1920’s, obtainingvaluable information that has beenused to better understand thehydrology of the water resources ofcentral Florida, including lakes. Inaddition to data collection, as of1994, the USGS had published66 reports and maps on centralFlorida lakes (Garcia and Hoy,1995).

The main purpose of this primeris to describe the hydrology of lakesin central Florida, the interactionsbetween lakes and ground- andsurface-waters, and to describe howthese interactions affect lake waterlevels. Included are descriptions ofthe basic geology and geomor-phology of central Florida, originsof central Florida lakes, factors thataffect lake water levels, lake waterquality, and common methods ofimproving water quality. Thegeographic area discussed in thisprimer is approximate (fig. 1) andincludes west and east-centralFlorida, extending from the Gulf ofMexico to the Atlantic Ocean coast-l ines, northward into Marion,Putnam, and Flagler Counties, andsouthward to Lake Okeechobee.The information presented here wasobtained from the many publica-tions available on lakes in centralFlorida, as well as from publica-tions on Florida geology, hydro-logy, and primers on ground water,

Hydrology of Central Florida Lakes—A PrimerBy Donna M. Schiffer

akes are among themost valued naturalresources of centralFlorida. The land-s c a p e o f c e n t r a lFlorida is r iddledwith lakes—whenviewed from the air, it

LINTRODUCTION

Many lakes in central Florida are used for recreation. (Photograph provided bySt. Johns River Water Management District.)

2Hydrology of Central Florida Lakes—A Primer

surface water, and water quality.Many publications are availablethat provide more detailed informa-tion on lake water quality, and thisprimer is not intended as an exten-sive treatise on that subject. Thereader is referred to the referencesection of this primer for sources ofmore detailed information on lakewater quality. Lakes discussed inthis report are identified in figure 2.Technical terms used in the reportare shown in bold italics and aredefined in the glossary.

The classification of some waterbodies as lakes is highly subjective.What one individual considers a“lake” another might consider a“pond.” Generally, any water-filleddepression or group of depressions

in the land sur face could beconsidered a lake. Lakes differ fromswamps or wetlands in the typeand amount of vegetation, waterdepth, and some water-quality

characteristics. Lakes typicallyhave emergent vegetation along theshoreline with a large expanse ofopen water in the center. Swamps orwetlands, on the other hand, arecharacterized by a water surfaceinterrupted by the emergence ofmany varieties of plant life, fromsaw grasses to cypress trees.

Lakes may be naturally formedor manmade; however, the distinc-tion between naturally formed andmanmade lakes is not always clear.For example, retention ponds,which are required for the treatmentof stormwater, can be constructedso that they serve multiple purposesof s tormwater t reatment andaesthetic enhancement of property.Larger retention ponds sometimesare used by residents for boatingand fishing and are considered bysome to be lakes.

In addition to aesthetic valueand recreational uses, lakes incentral Florida are extremelyimportant as habitats for fish, alliga-tors, turtles, and birds such ashawks, eagles, ducks, and herons.Because Florida lakes are used andenjoyed by many, they need to beappreciated, understood, andmanaged for the benefit of all.

Figure 1. Approximate geographic area covered by this report and distribution oflakes (dark blue) in Florida. Outlined counties contain 35 percent of the lakes inFlorida.

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.


Hydrologic characteristics ofcentral Florida lakes vary widely.The surface areas of lakes can rangefrom several hundred square miles,such as Lake Okeechobee, to lessthan an acre. Water levels in somelakes may vary by 10 feet or more,whereas in other lakes, the waterlevel may vary by only 1 or 2 feet.The quality of water among lakes incentral Florida also is variable, frompristine lakes such as Lake Butler inwest Orange County to the pea-green-colored waters of LakeApopka, a short distance to thenorth in Orange and Lake Counties(although the clarity of water is notnecessarily an indication of thequality of the water). Some lakeshave natural surface-water inletsand outlets. Other lakes are land-locked, receiving water only fromrainfall and losing water only fromevaporation and seepage into thesurrounding soils. This great varietyin hydrologic characteristics is oneof the reasons why water levels varyamong lakes and why lakes responddifferently to rainfall.

CLASSIFICATION OF LAKES

Lakes are classified accordingto different criteria including loca-tion, origin, drainage characteris-tics, trophic state (a measure of theamount of nutrient enrichment ofthe water), and water chemistry.Lakes commonly are classified bygeologists according to the physio-graphic region in which the lakesare located. Many lakes in Floridawere formed by sinkhole activityand thus are called sinkhole lakes.Environmental scientists mayclassify lakes according to the stateof water quality (this classificationsystem is described in the section on

water quality). Residents of andvisitors to central Florida haveadded their own classificationsystem by describing a particularlake as a good fishing or water-skiing lake.

Lakes in Florida and elsewhereco m m o nly a re c l as s i f i ed bydrainage characteristics; it is thesecharacteristics that differentiateFlorida lakes from lakes in otherparts of the country. This generalclassification divides lakes into oneof two major types—seepage anddrainage lakes. Although there aremany other hydrologic characteris-tics associated with each type(fig. 3), perhaps the most easily

Figure 3. Hydrologic characteristics of seepage and drainage lakes.

5 The Hydrologic Cycle

recognized p roper ty used todistinguish between seepage anddrainage lakes is the absence orpresence (respectively) of surface-water outflow. Lakes that have nosurface-water outflow lose waterprimarily through the ground-watersystem or through evapotranspira-tion and are called seepage lakes.Lakes that lose water primarilythrough a surface-water outlet arecalled drainage lakes (Wetzel,1975). Most Florida lakes areseepage lakes—nearly 70 percent ofthe lakes in Florida have no surface-water streams flowing into or out ofthem (Palmer, 1984). The drainagebasin of a seepage lake commonlyis referred to as a closed basinbecause of the lack of surface-wateroutflow from the basin; however,there is outflow from the basinthrough ground-water seepage. Thedrainage basins of drainage lakesare commonly referred to as open-drainage basins.

The dominance of ground-waterflow over surface-water flow inmuch o f F lo r ida makes thehydrology of lakes here differentfrom that of lakes in other parts ofthe country. The well-drainedporous soils and rocks that arecharacteristic of the karst landscapeof Florida are what make thisdifference. One difference betweenFlorida lake hydrology and that ofother par ts of the country isexpressed in the classical definitionof a drainage basin. A drainagebasin or watershed (the areacontr ibuting water) of a lakecommonly refers to the land surfacearea draining to a water body asdefined by topography. However,because most of the inflow toseepage lakes in Florida is fromground water, the ground-waterbasin must be considered part of the

contributing drainage basin. Ulti-mately, however, the most signifi-cant difference between seepageand drainage lakes is not the sourceof the water, but the controllingfactors affecting lake water volume(ground-water or surface-wateroutflows).

THE HYDROLOGIC CYCLE

Water in the envi ronmentmoves from the atmosphere, to theland surface, to the ground-watersystem, and back to the atmospherein a cycle called the hydrologiccycle (fig. 4). In this section, thecomponents of the hydrologic cycleare described to provide the neces-sary background for an under-standing of lake hydrology.

The primary components of thehydrologic cycle in central Floridaare rainfall, runoff, infiltration(including recharge), evaporation,transpiration, and condensation.When rain falls, some of the waterinfiltrates 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 manylakes here. (Photographs provided by South Florida Water Management District.)


aquifers, some of it is intercepted byplants, and some of it flows over theland surface (surface runoff oroverland flow). Beneath the landsurface, water moves through theaquifers toward areas of dischargesuch as the ocean or streams. Waterreturns to the atmosphere throughthe processes of evaporation andplant transpiration (collectivelylabeled “evapotranspiration” infig. 4). Once in the atmosphere aswater vapor, the hydrologic cycle iscomp le ted when th i s vaporcondenses and forms rain dropletsthat subsequently fall on the landsurface again.

Some of the components of thehydrologic cycle can be quantifiedby using measuring devices tocollect data, and a “water budget” ofa lake (an accounting of the totalvolume of water entering andexiting the lake) can be determinedfrom these data. Much of theresearch on Florida lakes hasfocused on water budgets in aneffort to better understand thecomplex hydrologic system of alake and to determine how eachcomponent of the hydrologic cycleaffects lake water levels and waterquality. A schematic of the compo-nents of a lake water budget isshown in figure 5. Lakes receive

water from rain falling on thesurface of the lake, from surfacerunoff within the drainage basin,from streamflow, and from groundwater entering the lake (labeled aslateral seepage in fig. 5). Lakes losewater to evaporation, seepagethrough the bottom, and streamflow( in lakes wi th sur face-waterconnections). Rainfall and stream-flow are relatively easy to measure,but recharge, evaporation, andseepage are much more difficult todetermine accurately. The compo-nents of the hydrologic cycle arediscussed in more detail in thefollowing sections.

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


Rainfall

The climate of central Florida issubt rop ical , wi th warm, wetsummers and mild, fairly drywinters. The wet season, whichb e g i n s i n J u n e a n d e n d s i nSeptember, accounts for more thanhalf (56 percent) of the rainfallduring the year (Schiner, 1993).Rainfall during the wet seasongenerally is associated with localshowers an d thun ders to rms .Rainfall during the dry season(October through May) generally isassociated with frontal systemsmoving from northern latitudessouthward. Rainfall during the wetseason can be highly localized withheavy thunderstorms producing

significantly more rain in someareas than in others, whereasrainfall during the dry seasonaffects larger geographic areas.Some of the differences in lakewater levels, particularly duringsummer months, can be the result oflocal differences in rainfall amountor intensity.

Total monthly or annual rain-fall also is variable from location tolocation. For example, the averageannual rainfall, based on 30 yearsof record (1961–90), ranges from43.92 inches in Tampa to54.09 inches in St. Leo (see fig. 2fo r s ta t i on loca t ions ) . Thevariability from one location toanother is indicated by the average

monthly rainfall during a 30-yearperiod (1961–90) at two rainfallstations in DeLand and Orlando(fig. 6, locations shown in fig. 2).Both stations are inland and areonly about 30 miles apart, so onemight expect simi lar rainfal lamounts. Total rainfall at these twostations is similar for some months,but the rainfall at DeLand tends tobe greater than the rainfall atOrlando, particularly during themonths of August, September, andOctober, illustrating regional vari-ability. Mean annual rainfall atDeLand (56.05 inches) is nearly8 inches greater than mean annualrainfall at Orlando (48.11 inches)for the period 1961–90, indicatingthat long-term rainfall patterns candiffer significantly even at locationsthat are relatively close and insimilar 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 annua l ra in fa l l f o rJacksonville, the site with thelongest rainfall record in Florida, isshown in figure 7. Rainfall record-keeping started in 1851, but there isa break in the otherwise continuousrecord for 1861–66. Total annualrainfall is shown in the upper graphof figure 7. In the middle graph, thedifference between the rainfall foreach year and the long-term averageis shown (based on the period1866–1993). Just as daily rainfallvaries from one day to the next, totalannual rainfall varies from one yearto the next. The difference between

Figure 5. Major components of the hydrologic budget for a central Florida lakein a ground-water recharge area.


annual rainfall and the long-termaverage annual rainfall is called the“departure” from average for thatyear. These annual departures fromthe average can have a cumulativeeffect on lake water levels. Forexample, a series of years with lessthan average rainfall may result inlake water levels that are lower thanthe long-term average level for thatlake. The bottom graph of figure 7il lustrates how the differencebetween annual rainfall and theaverage rainfall, when accumulatedfrom one year to the next, canproduce trends of excess or deficitrainfall. Excess rainfall early in theperiod, from 1866 through about1889, produced enough of a cumu-lative surplus of rainfall (barsshown above the zero line) that itwas 25 years before lower-than-average annual rainfall produced adeficit (bars shown below the zeroline, beginning in 1917).

Evaporation and Transpiration

Most of the rain that falls isreturned to the atmosphere by evap-oration and by transpiration fromplants. Commonly, evaporation andtranspi ra t ion are consideredtogether and called evapotranspira-tion. Water that is in the soil near theland surface can return to the atmo-sphere through evaporation. Thehighest evaporation rate is fromopen water surfaces such as lakes.Evaporation from a lake surface isreferred to specifically as lake evap-oration. The evaporation rate isaffected by several variables such asthe amount of moisture in the air(humidity), the amount of sunlight,wind, and temperature. Thus, evap-oration rates are variable; in centralFlorida, evaporation rates havebeen estimated to be as low as25 inches per year and as high as50 inches per year (Tibbals, 1990).

One way that lake evaporationis estimated is from evaporationmeasured using a standard 4-footdiameter, shallow metal pan. Pan-evaporation rates are greater thanlake-evaporation rates and must becorrected using pan coefficients,which are mathematical ratiosthat relate lake evaporation to panevaporation.

Annual lake evaporation forthe central Florida area was esti-mated to be about 51 inches, basedon 28 years (1960–88) of pan-evaporation data recorded at theLisbon and Gainesville weatherstations (locations shown in fig. 2)and pan coefficients determined instudies at Lake Hefner, near LakeOkeechobee, by Kohler (1954).Other estimates of annual lakeevaporation reported by researchersin central Florida have ranged from47 inches (Phelps and German,1996) to 58 inches (Lee andSwancar, 1994). The annual lake-evaporation rate of 51 inches isgreater than the average yearlyrainfall at Orlando (48.11 inches,based on rainfall from 1961–90);however, lake evaporation repre-sents the maximum possible rate ofevaporation. The actual evapora-tion rate over a large region isconsiderably lower than the lake-evaporation rate because the wateris evaporating from land surfaces aswell as water surfaces. Although theactual evaporation in the drainagebasin of a lake can be estimated fora water budget, the computation canbe complex because it requiresinformation about land use, depth tothe water table, and other hydro-logic variables.

Figure 6. Mean monthly rainfall at DeLand and Orlando for the period 1961–90and lake evaporation based on pan evaporation (1960–88).


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


Surface Runoff

Rainfall that has not infiltratedthe soil and has not been returnedto the atmosphere by evapotranspi-ration flows over the land surfaceand eventually reaches a lake orstream. Surface runoff refers towater flowing over the land surfaceand to water in streams and rivers.Surface runoff is a significantcomponent of the hydrologicbudget of drainage lakes. In seepagelakes, however, ground-waterinflows and outflows dominate thehydrologic budget, and the compo-nent represented by surface runoffis relatively small.

Infiltration and Ground-Water Recharge and Discharge

Part of the rainfall that reachesthe land surface infiltrates the soil,where it gradually percolatesdownward until it reaches the surfi-cial aquifer system (fig. 8). Thiswater replenishes the surficialaquifer system, which in turnrep len ishes or recharges theFloridan aquifer system. Thisdownward movement can occuronly where the water table of thesurficial aquifer system is higher (ata greater altitude) than the potentio-metric surface of the Floridanaquifer system. The areas in theState where ground water is replen-ished are referred to as rechargeareas. In areas where the potentio-metric surface of the Floridanaquifer system is at a higher altitudethan the water table of the surficialaquifer that overlies it, water movesupward toward the land surface.These areas are referred to asdischarge areas or areas of artesianflow (fig. 8). The many springs in

Florida are located in dischargeareas. Lakes in central Florida arepresent in both recharge anddischarge areas.

GEOLOGY OF CENTRAL FLORIDA

T h e F lo r i d a p e n i n su l a i scomposed of carbonate rock (lime-stone and dolomite) that wasdeposited over a period of about100 mill ion years . For about75 mi l l ion yea rs , dur ing theC r e t a c e o u s P e r i o d ( 1 3 8 t o63 million years ago), Florida wascovered by relatively deep oceans.Later, from about 63 to 38 millionyears ago, the Florida peninsula wasa shallow carbonate reef. Sea levelthen receded and rose several times,eroding and depositing carbonates,until about 5 million years ago.Since that time, land-derived sedi-ments such as sand, silt, and clay

have been deposited, rather thanmarine carbonates. Subsequentfluctuations of sea level duringepisodes of glaciation have eroded,reworked, and transported theunconsolidated sand, silt, and claysediments.

The Florida peninsula is aproduct of sedimentary processes,and Florida’s geology reflects theseprocesses. The geology of centralFlorida is shown in figure 9, whichalso shows the hydrogeologic units,or aquifers and confining units,corresponding to the geology.Underlying the deepest rocksshown in the diagram in figure 9 areseveral thousand feet of igneous,metamorphic, and sedimentaryrocks that form the foundation ofthe peninsula. Above these rocksare the Cedar Key, Oldsmar, andAvon Park Formations, and theOcala 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, andgypsum and were deposited whenmost of the Florida peninsula wasbelow sea level. The total thicknessof these formations ranges from5,500 to 12,000 feet (Tibbals,1990). The overlying HawthornGroup, deposited about 25 millionyears ago, represents a transitionbetween marine-derived and land-derived sediments. The lower layersof the Hawthorn Group generallyare marine-derived and containlimestone, whereas the upper layersof clay, fine sand, and silt are land-derived. These upper layers of theHawthorn Group generally restrictground-water movement. Over-lying the Hawthorn Group andcontinuing upward to the present

land surface are unconsolidatedsediments generally consisting ofquartz sand, clay, and some organicmaterial.

The thickness of the HawthornGroup, which varies greatly incentral Florida, is a key element int h e l a k e f o r m a t i o n p r o c e s s(described in a later section). Whenrocks of the marine-formed OcalaLimestone and Suwanee LimestoneFormations were exposed at landsurface, the rock was eroded bywind, rain, flowing water, andthermal changes (expansion andcontraction due to seasonal temper-a t u r e c h a n g e ) . B e c a u s e t h eerosional process is not necessarilyuniform from one location toanother, the eroded surface of thelimestone can be very irregular.

Later in geologic time, the sedi-ments of the Hawthorn Group weredeposited on top of this irregular,eroded limestone surface. There-fore, the thickness of the HawthornGroup varies depending on theoriginal altitude of the surfacewhere it was deposited. In thoseareas where the sediments of theHawthorn Group are thin, moresurface water can penetrate to theunderlying rock of the Ocala andSuwanee Limestone Formationsand continue the erosional process.These are areas prone to sinkholedevelopment. In those areas wherethe Hawthorn Group is thicker, theunderlying rock is more shieldedfrom the effects of continuederosion, and sinkholes are lesscommon.

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.)


GROUND WATER

In this section of the primer,the ground-water system in centralFlorida is described to provide thebackground for understanding lakeformation, water levels in lakes, andthe quali ty of water in lakesdiscussed in subsequent sections.Florida’s ground water is containedin aquifers. An aquifer is a layer ora combination of several layers ofpermeable soils or rocks that yieldusable quantities of water. Themajor aquifers in central Florida areidentified in figure 9 under thecolumn labeled “GeohydrologicUnit” and in the generalized crosssection shown in figure 10. The

term “system” is used to indicatethe presence of more than onewater-bearing layer or aquifer; forregional analysis these multiplelayers of aquifers are collectivelyconsidered as a single geohydro-logic unit (fig. 9).

In central Florida, the aquifernearest the land surface is the surfi-cial aquifer. The water table refersto a surface below which all theopenings or spaces in the soil orrock are filled with water (satu-rated). Pores in the soil between theland surface and the water table arefilled with both water and air; thisuppermost layer above the watertable is referred to as the unsatur-ated zone. The term ground water

refers to water below the watertable. The water table is in contactwith the atmosphere through theunsaturated zone; therefore, thewater table is at atmospheric pres-sure. The depth to the water tablecan range from 50 feet or morebelow land surface to above landsurface in wetlands and lakes.(Because the water surface of a lakeis the water table, and the landsurface is actually the submergedlake bottom, the water table isabove land surface in lakes.)

Beneath the surficial aquifersystem l ies the intermediateconfining unit, which is composedof layers of clays, fine silts and, insome 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 interme-diate confining unit in most ofcentral Florida generally are notvery permeable, thus, this unitrestricts the movement of waterfrom above and below. In someareas, the lower part of the interme-diate confining unit may consist ofhighly fractured limestone anddolomite, which can produce usablequantities of water; however, inmost areas , the in termedia teconfining unit is not used for watersupply.

The Floridan aquifer systemunderlies the intermediate con-fining unit and is the primary sourceof nearly all the drinking water incentral Florida. The rock thatcontains the Floridan aquifersystem generally consists of highlypermeable limestone and dolomite.The top of the aquifer is at or nearland surface in west-central Florida(including Marion and SumterCounties) and can be as deep asseveral hundred feet below landsurface in other areas of centralFlorida. The Floridan aquifersystem is further divided intothe Upper Floridan and LowerFloridan aquifers (fig. 9). TheUpper Floridan aquifer is theprimary source of drinking water,al though the Lower Flor idanaquifer 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 aquifersystem is under pressure in much ofcentral Florida because the over-l y i n g c l a y s a n d s i l t s o f t h eHawthorn Group act to confine the

water (thus the name “intermediateconfining unit”). This condition iscommonly called artesian becausewater levels in wells drilled into theaquifer rise above the top of theaquifer and, in some places, aboveland surface (fig. 10). The level towhich the water rises is referred toas a potentiometric surface. Wellsthat tap the Floridan aquifer systemin ground-water discharge areas arecommonly referred to as artesianwells, particularly if the potentio-metric level in the well is greaterthan the land surface; where thisoccurs, water will flow out of thewell as shown in figure 10. In areaswhere the limestone of the Floridanaquifer system is at or near landsurface and there are no confiningsediments, water in the aquifer isnot under pressure; in these areas,the aquifer is not artesian.

GEOMORPHOLOGY AND THE ORIGIN OF CENTRAL FLORIDA LAKES

The geomorphology of theFlorida peninsula provides clues togeologic history and the origin oflakes. Physical features of the landsurface of central Florida have beenused to def ine physiographicregions, some of which are associ-ated with the abundance of lakes.The physical features of Florida arequite varied, although comparedto other parts of the country theState is rather flat and featureless.Physical features of central Floridarange from highlands, ridges, andupland plains to lowlands andcoastal marshes. Cooke (1945)described two major physiographicregions in central Florida: theCentral Highlands and the CoastalLowlands. Land elevations in the

Central Highlands region rangefrom about 40 to 325 feet above sealevel. The Coastal Lowlands borderthe entire coast of Florida, and landelevations generally are less than100 feet (Cooke, 1945).

The central Florida area is char-acterized by discontinuous high-lands separated by broad valleys.One of the effects of the rise and fallof sea level was the formation ofrelict shoreline features such asbeach ridges, which parallel thepresent-day Atlantic coastline(fig. 11). These beach ridges havehad an effect on the shape, location,and orientation of lakes in centralFlorida. Swales between the beachridges (shown as plains in fig. 11)are areas where surface watercollected when sea level dropped,causing a decline in the water tableand increasing the downwardmovement of water. This increase invertical water movement in turnaccelerated the lake formationprocesses, which partly explains theabundance of lakes in centralF lo r ida . Wi th in the Cen t ra lHighlands of the Florida peninsulais the most extensive area of closelyspaced lakes in North America. Thenearest counterparts to the lakedistrict of Florida are in Canada andin the northern United States fromMinnesota to Maine.

Lakes in Florida were formedby several processes. The lime-stones and dolomites that underliecentral Florida are “soft” rocks thatare easily dissolved by rainwaterentering the subsurface environ-ment from the land surface and bythe ground water moving throughthe rock matrix. The chemicalp rocesses by wh ich rock i sdissolved by interactions with waterare commonly referred to as


solution processes. The past andcontinuing solution of the limestonebeneath the land surface by groundwater results in a landform calledkarst. Common characteristics of akarst landform include a lack ofsurface drainage (nearly all therainfall infiltrates into the groundand few drainage channels orstreams develop) and the presence

of sinkholes (depressions in theland surface), springs, circular-shaped lakes, and large cavities inthe limestone and dolomite rocks.

By far, the most common originof Florida’s lakes is by solutionprocesses. Solution processes arethe underlying cause of sinkholes,which over time become lakes.Sinkholes are most common in

areas where the intermediateconfining unit between the uncon-solidated materials of the surficialaquifer system and the underlyinglimestone of the Floridan aquifersystem is thin, breached, or absent.With few exceptions, sinkholesform in areas of recharge to theFloridan aquifer system. In theseareas, ground water easily movesdownward from the land surface tothe limestone. Carbon dioxide fromthe atmosphere and soil zone iscarried with the ground water andforms carbonic acid, a weak acidthat d issolves the l imestone,eventually forming cavities andcaverns. In contrast, in areas wherethe intermediate confining unit isintact and water in the aquifer isconfined, less water from thesurface reaches the limestone andsinkholes are not as common.

By far, the most common origin of Florida’s

lakes is by solution processes.

The three general types ofsinkholes—subsidence, solution,and collapse—in central Floridagenerally correspond to the thick-ness of the sediments overlying thelimestone of the Floridan aquifersystem. The sediments and watercontained in the unsaturated zone,surficial aquifer system, and inter-mediate confining unit collectivelyare termed overburden in thefollowing description of sinkholeformation. Collapse sinkholes aremost common in areas where theoverburden is thick, but the inter-mediate confining unit is breachedor absent. Subsidence sinkholesform where the overburden is thinand only a veneer of sediments ispresent 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 theoverburden is absent and the lime-stone is exposed at land surface.

Collapse sinkholes are the mostdramatic of the three sinkholetypes; they form with little warningand leave behind a deep, steeplysided hole. Collapse occurs becauseof the weakening of the rock of theaquifer by erosional processes andis often triggered by changes inwater levels in the surficial andFloridan aquifer systems. Theprogression of a collapse sinkhole isshown in figure 12. A small cavityin the limestone of the Floridanaquifer system gradually growslarger as the rock is dissolved by theground water flowing through it.The weight of the overburden abovethe cavity is supported by the roofof the cavity and is partly supportedby pressure from the water in theaquifer. As the cavity grows larger,the rock that forms the roof of thecavity becomes progressivelythinner. As water levels declineduring dry periods or because ofpumping, water pressure in thelimestone cavity decreases and theweight of the overburden over-comes the structural integrity of thecavity roof. At this point, the roofand overburden collapse into thecavi ty, forming the surf ic ia l“pothole” that is associated withsinkholes and karst terrain. TheWinter Park sinkhole that formed inMay 1981 (see photo, p. 17)probably is the best known collapsesinkhole in central Florida.

Although collapse sinkholesgenerally form during dry periods,they also can form as a result ofheavy rainfall during an extendedperiod of time. The rise in the watertable of the surficial aquifer systemas a result of the increased rainfallcauses an increase in the weight ofthe overburden. A collapse occurswhen the weight of the overburden

on the roof of a limestone cavityincreases more rapidly than thewater pressure in the cavity that is

partially supporting the cavity roofand exceeds the structural strengthof the roof.

Figure 12. Formation of a collapse sinkhole.


The progression of a subsidencesinkhole is shown in figure 13.Rainwater percolates throughoverlying sediments and reachesthe limestone, dissolving the rockand gradual ly weakening i t sstructural integrity. As the lime-stone continues to dissolve, the

unconsolidated sediments above thelimestone are carried downwardwith the percolating water andoccupy the spaces formed by thedissolving limestone. With time, themovement of sediments into theopenings in the limestone causesthe formation of a bowl-shaped

depression at the land surface. Asthe depression enlarges with time,water begins to collect in thedepression. Sand and clay areca rr i ed to the newly formeds i nk h ol e in r un o ff f ro m t hesurrounding land, eventuallysettling, lining the bottom of thedepression, and restricting the flowof water through the bottom. Aswater continues to accumulate inthe depression, a lake is formed. Anexample of a lake formed in thisway is Lake Tsala Apopka, in CitrusCounty (see photo, p. 17). In thisarea of central Florida, the lime-stone is covered with a veneer ofunconsolidated sediments or isexposed at the land surface. LakeTsala Apopka is actually a series oflakes and marshes; the aerial viewshows the many depressions thattogether form the lake.

With the passing of time, thedifference in appearance of lakesformed by these different types ofsinkholes is less distinct. Forexample, the photograph of theWinter Park sinkhole was taken in1994, 13 years after the sinkholeformed. In the photo, the steep sidesof the sinkhole are no longer visiblebecause the water level has risen inthe sinkhole and the “sink” looksvery much like nearby lakes thatmay have been formed by otherprocesses. Thus the specific type ofsinkhole that caused the lake toform is not readily apparent.

Not all lakes in central Floridawere formed by solution processes.Some central Florida lakes origi-nally were natural depressions inthe ancient sea floor. These depres-sions formed as a result of scouringand redepositing of material bywave action and water currents.Freshwater eventually filled thesedepressions when sea level fell.Figure 13. Formation of a subsidence sinkhole.

17 Geomorphology and the Origin of Central Florida

Lake Tsala-Apopka in west-centralFlorida is an example of a lakeformed by solution processes. Thelimestone in the area near this lakeis commonly seen at land surface.(Photograph by L.A. Bradner,U.S. Geological Survey.)

The top photograph shows theWinter Park sinkhole shortlyafter it formed in May 1981.(Photograph by A.S. Navoy.)

In the bottom photograph, thesinkhole is shown 13 years later,in October 1994. (Photograph byL.A Bradner, U.S. GeologicalSurvey.)


Examples of lakes that were formedin this way are the lakes that form achain along the Upper St. JohnsRiver, including Lakes HelenBlazes, Washington, Winder, andP o in s e t t i n B r e v a rd C o u n t y(fig. 14). These lakes, throughwhich the Upper St. Johns Riverflows, are remnants of an ancientestuary (White, 1970, p. 103).

Probably the least commonorigin of lakes in Florida is byfluvial, or riverine, processes. Morecommonly, f luvia l processescombine with other lake-formation

processes to enlarge or change theshape of lakes. For example,although Lakes Harney, Monroe,and George (fig. 2) all originatedfrom depressions in the ancient seafloor, riverine processes havecontinued to enlarge these lakes.

Lakes formed in ancient seafloor depressions or by riverineprocesses differ in shape, grouping,and orientation of inlets and outletsfrom lakes formed by solutionprocesses. For example, the lakesthat form the Upper St. Johns Riverchain of lakes are elongated in the

direction of the river flow, so thatthe lakes appear to be threadedtogether by the river (fig. 14). Theinlet and outlet of each lake line upwith the river and are oriented alongthe long axis of the lake. Solution-formed or sinkhole lakes tend toappear in groups or clusters, ratherthan in linear chains as in the UpperSt. Johns River lake chain. Anexample of solution-formed lakesare the lakes of the Ocklawahachain: Apopka, Harris, Griffin,Dora, Eustis, and Yale (fig. 15). Theinlets and outlets of these lakes tendto be randomly oriented withrespect to their long axes. Regard-less of the orientation of inlets andoutlets of central Florida lakes, agroup of connected lakes arecommonly referred to as a chain oflakes.

Manmade lakes in Florida oftenare created for practical as well asaesthetic reasons. Sometimes theselakes are referred to as “real-estatelakes,” and are used in housingdevelopments for stormwater treat-ment and for enhancement of thenatural landscape. Lakes of thistype usually are not named, eventhough 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 formedwhen sea level fell and freshwater filleddepressions in the sea floor (UpperSt. Johns River chain of lakes).

19 Physical Characteristics of Lakes

type of manmade lake is formed bybuilding a dam across a river.Examples of this type of lake areLake Rousseau in west-centralFlorida (located along the countylines of Levy, Marion, and CitrusCounties), which was created whenthe Inglis Dam was built in 1969,and Lake Ocklawaha in PutnamCounty, which was created whenthe Rodman Dam was built in 1968(fig. 2). Both of these impound-ments were created as part of theCross-Florida Barge Canal, whichwas partially constructed in the1960’s to connect the AtlanticOcean and the Gulf of Mexico forshipping purposes but was haltedafter concerns about environmentalimpacts arose.

PHYSICAL CHARACTERISTICS OF LAKES

Some of the physical character-istics of lakes include shape (length,width), depth, surface area, andtotal volume. As described in thelast section, the shape of a lake,which may be circular, elliptical, orirregular, is the result of the processby which it was formed and theenvironment in which it has existedsince it was formed. Because theenvironment of a lake is dynamic,physical characteristics may changewith time, either as a result ofnatural environmental processes orbecause of human activity.

Although the unnamed lake in this aerial photograph taken in an area nearSebring, Florida, has a nearly perfect circular shape indicating its sinkhole origin,most solution-formed lakes do not have this perfect a shape! (Photograph byE.P. Simonds, U.S. Geological Survey.)

Figure 15. Lakes formed by solutionprocesses (Ocklawaha River chain oflakes).


The physical characteristics oflakes in Florida differ from those oflakes in other parts of the country,primarily because of differences inhow they were formed and in localgeology. Many lakes in the northernUnited States, such as the GreatLakes, the Finger Lakes of NewYork, and the numerous lakes inMinnesota and Wisconsin, wereformed by glacial processes. Lakesof glacial origin can be very deep,with steeply sloped sides and anirregular shape. Florida’s lakes tendto be very shallow (7–20 feet deep),with gently sloping sides. Sinkholelakes generally are circular but canbe irregularly shaped if a number ofcircular depressions coalesce.

The physical characteristics of alake also are related to the age of thelake. A recently formed sinkholelake may be quite deep and havesteeply sloped sides, but graduallythe lake fills in with sedimentscarried into the lake by runoff, andthe side slopes become more gradualand stabilize as a result of erosionalprocesses. As a lake continues toage, it may resemble a marsh orwetland rather than a lake. Eventu-ally, the lake may become little morethan a broad open field (or prairie),sometimes with a stream runningthrough what once was the deepestpart of the lake. An example of thiscan be seen in Payne’s Prairie nearGainesville, Florida.

WHAT CAUSESLAKE WATER-LEVELFLUCTUATIONS

Water levels in lakes can varyover a wide range of time scales,from short-term, storm-related risesover a period of days, to long-termfluctuations caused by seasonalvariations during the year. Water

levels also can vary as a result oflong-term trends in rainfall—patterns in rainfall may vary overperiods of 20 to 30 years. For manypeople, the “normal” level of a lakeis the level at which they firstobserved the lake. However, thenormal or average water levelof a lake must be defined for aparticular period of time. Theaverage water level in a lake duringthe 20-year period 1950–70 may bequite different from the average for1970–90, for example, and thedifference in the averages may beprimarily due to the difference inrainfall during those periods. Thus,the average or normal water level forany lake can change with time.

Water levels in Florida lakes,though driven by inflows (rainfall,runoff, and ground-water seepage),ul t imately are determined byout f lows. In dra inage lakes,outflows generally increase anddecrease rapidly with small changesin water levels. Although the waterlevels fluctuate frequently, the fluc-tuations fall within a narrow range.Seepage lakes are characterized byless frequent water-level fluctua-tions within a greater range. Thelong-term fluctuations in seepagelakes reflect the larger seasonal andannual variations in rainfall and thealt itude of the potentiometricsurface. Examples of these water-level fluctuations are presented inthis section of the report.

The variability in lake waterlevels is a function of the hydro-logic budget of the lake. Thebalance of inflows and outflows inthe hydrologic budget of a lake isreflected in the water levels. Thissection presents a description ofhow each component of the hydro-logic cycle affects lake water levels.

How Rainfall and Evapotranspiration Affect Lake Water Levels

Rainfall, either directly or indi-rect ly, probably is the singlegreatest driving factor affectingwater levels in lakes. When rainfalling on the surface of a lake or onthe land surface within the drainagebasin exceeds outflows from thelake, an increase in the volume ofwater in the lake causes a rise inwater levels. A lack of rainfallcauses lake levels to fall becausewater losses (from evaporation,seepage out of the lake, andpumping from the lake) are notbalanced by inflows from rainfall.

Evaporation from the lakesur face causes lake levels todecline. Lake evaporation generallyis greatest during May through July(fig. 6). Lower rainfall during Apriland May, combined with relativelyhigh lake evaporation, result ingreater water losses from lakesduring these months. The lossesresulting from lake evaporationduring low-rainfall periods is mostpronounced in seepage lakes, whereevaporation is a primary output; indrainage lakes or in lakes wherewater levels are controlled, theeffects of high evaporation lossesare less noticeable because they aremasked by other, larger waterlosses.

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 cumulat ive differencebetween rainfall and evaporationov e r t im e a ff ec t s lo n g- t e r mseasonal and annual lake waterlevels, particularly in seepage lakes.Lake Umatilla in Lake County(fig. 2), for example, has character-istics of a seepage lake because itdoes no t overf low except a textremely high water levels. Lakeev a p or a t io n , i n i nc h es , wassubtracted from monthly rainfall fora 4-year period; the result for eachmonth then was accumulated. Thiscumulative difference is shown infigure 16 along with the weeklywater levels in Lake Umatilla. The4-year period shown represents atime when many lakes in centralFlorida reached record-low waterlevels (1981) because of a numberof years of below-normal rainfall.The lag in the response of the lakewater level to the cumulative differ-ence between rainfall and lakeevaporation (fig. 16) is the result ofthe storage of water in the hydro-logic system. Inflows of this storedwater sometimes mask the effects ofhigh 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 lakesinclude overland flow within thedra inage bas in ; inf low f romcontributing streams; inflow (fromother lakes or ponds) through

surface-water connections such ascanals, streams, or pipes; andstormwater transported throughstorm sewers . Surface-waterconnections generally allow morerapid inflow and outflow of waterthan ground-water connections; forthis reason, these inflows andoutflows can affect lake levels andwater quality more rapidly than cansimilar volumes of inflow andoutflow of ground water. Most lakesin Florida are seepage lakes andthus are not subject to the effectscaused by directly connectedsurface-water inflows and outflows.In drainage lakes in central Florida,the inlets and outlets may notfunction for periods ranging from afew months to many years becausewater levels are below the level ofthe inlet or outlet (Deevey, 1988);thus, even in many drainage lakes,water levels generally are affectedmore by rainfall and ground waterthan by surface water.

How Ground Water Affects Lake Water Levels

The interaction between lakesand ground water can be verycomplex. Flow direc t ion andvo lume are func t ions o f theposition of the water surface of thelake relative to the water table ofthe surficial aquifer system and thepotentiometric surface of theFloridan aquifer system. Thehydraulic gradient is a term usedto refer to the difference or changein wa te r l e v e l o ve r a g i ve ndistance; for example, the differ-ence in altitude between the watertable of the surficial aquifer systemand the potentiometric surface ofthe Flor idan aquifer sys tem,divided by the vertical distancebetween the two water surfaces,would be the vertical hydraulicgradient.

Figure 16. Water level in Lake Umatilla, Lake County, and the cumulativedifference between rainfall and lake evaporation.


Some “typical” configurationsof lake- and ground-water levels inrecharge and discharge areas thatalso illustrate the effects of thehydraulic gradient are presented infigure 17. A lake in a ground-waterrecharge area is shown in figure 17(A and B). In figure 17A, watermoves out of the lake and into theadjacent and underlying aquiferbecause the lake water level ishigher than the water table of thesurficial aquifer system, perhapsbecause of recent rainfall. Duringperiods of intense rainfall, the lakewater level may rise above the localwater table and the general flowdirection will be away from thelake. This condition usually istemporary. As the rain enters thesoil and recharges the surficialaquifer system, the water table neara lake rises above the lake waterlevel (fig. 17B), the direction offlow reverses, and the lake receivesseepage from the surficial aquifersystem.

A lake in a ground-waterd ischarge a rea is shown infigure 17C. In this illustration, thepotent iometr ic surface of theFloridan aquifer system is abovethe lake water surface (and thewater table of the surficial aquifersystem); thus, the hydraul icgradient and the direction of flow isupward. Water from the Floridanaquifer system moves upward to thesurficial aquifer system or directlyto the lake bottom, providing asource of water to the lake. Thewater moves under pressure bydiffuse upward leakage (migratingthrough the pores of the rock andsoil), through fractures, or through alarge opening, such as a spring.

Water levels in lakes located inrecharge areas generally fluctuatemore than levels in lakes indischarge areas (Hughes, 1974b).This may be because a lake in a

discharge area receives a fairlyconstant supply of water from theunderlying aquifer, whereas theleakage of water out of a lake in arecharge area is more variable.

Figure 17. Interactions between ground-water and lake water levels. (Note: Onlysurficial materials are illustrated.)


Because so many of the lakes incentral Florida were formed bysolution processes, there is a stronglink between lakes and groundwater. The illustration in figure 18of a sinkhole lake in a ground-waterrecharge area shows some of thepathways of ground-water move-ment. The lake shown in figure 18 isunderlain by sand and organicmaterial from the collapse thatformed the lake; these materialsprovide an avenue for water tomove easily from the lake to theunderlying Floridan aquifer system.Also shown in figure 18 is the“f low-through” condition thatexists in many central Florida lakes.Ground-water flows into the lakealong one shoreline and out of thelake along the opposite shoreline, inthe d i rect ion o f the posi t ivehydraulic gradient of the water tableof 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 ground-water flow also depends on thepermeability of the materials inwhich the lake exists and the thick-ness and characteristics of the inter-mediate confining unit beneath thelake. For example, ground watermoves very slowly through a thickintermediate confining unit, thusslowing the rate and reducing thevolume of water entering or leavingthe lake. The rate of leakage from alake increases with increasingpermeability of the soil. Lakes with

high rates of leakage generally willhave greater ranges of water levelsthan lakes with low rates of leakage(Hughes, 1974b).

One common misconceptionabout lakes in central Florida is thatmany of them are spring-fed. Manylakes are spring-fed, but the originof the water of the spring is notnecessarily the Floridan aquifersystem, the same aquifer thatsupplies the major springs forwhich Florida is known. Observedsprings in lakes commonly arespecific locations in the lake bottomwhere water is entering the lakefrom the adjacent surficial aquifer.Some lakes are directly connectedto the Floridan aquifer systemthrough fractures in the underlyingrock or through the remains of theoriginal sinkhole that formed thelake. Such connections provide a

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


path for water from the Floridanaquifer system to enter the lake, andsprings of sufficient magnitude tobe detected are manifestations ofthese connections. The crosssections in figure 19 show theconditions of spring flow from thesurficial aquifer system (fig. 19A)

and the Floridan aquifer system(fig. 19B) to lakes. Examples oflakes that receive spring flow fromthe Floridan aquifer system areLake Apopka in Orange County(Apopka Spring) and Lake Georgein northwestern Volusia County(Croaker Hole).

How Human Activities Affect Lake Water Levels

Many Florida lakes have beenmodified to decrease the range offluctuation in lake levels and reducethe risk and extent of floodingaround lakes. Natural drainagesystems have been altered, forexample, through the addition ofcanals or by the dredging orstraightening of existing outflowchannels . Many of the canalsconnecting lakes in Florida wereconstructed during the 1800’s,before the arrival of the railroad,when Florida lakes and riversformed a major transportationnetwork. Some of these canals weredug to lower the water levels andprovide more usable land adjacentto the lakes because the land wasvaluable for farming and citruscultivation. The effect of this typeof modification of natural drainageis the reduction in the overallsurface area of the lake and in therange of lake level fluctuations,particularly for natural seepagelakes.

Other human activities thataffect the water budget of lakes andthus, water levels, include urbaniza-tion, the use of lake water byresidents for lawn ir r igat ion(lowering lake levels), and thedisposal of water from varioussources to lakes (increasing lakelevels). Urbanization increases theimpervious surface area contrib-uting runoff to a lake, causing agreater volume of water to reach thelake unless other provisions aremade ( re ten t ion fac i l i t i es ) .Although some of the lake waterused for irrigation returns to thelake through ground-water seepage,most of it is lost to evaporation andFigure 19. Surficial-aquifer spring flow (A) and Floridan-aquifer spring flow (B) to a

lake.


uptake by vegetation. Examples ofwater contributed to lakes throughhuman activities include storm-water runoff, water from water-to-air heat-pump systems (whichobtain water from ground-watersources), treated sewage effluent,and discharge water from agricul-tural activities. Recent regulationspertaining to discharging of treatedwastewater to lakes and streamshave led to development and imple-mentation of alternative disposalmethods and, in some cases, havereduced or eliminated these inflowsto lakes.

Ground-water interactions withlakes also are affected by humanactivities. For example, one sourceof ground water to lakes is water(leachate) from drainfields of septictanks (shown in fig. 18). Otheractivities cause declines in lakewater levels. Water pumped forirrigation or other applications from

the surficial aquifer system near alake reduces the natural seepage tothe lake by lowering the water table,which may cause a decrease in lakewater level. Pumping water fromthe Floridan aquifer system inground-water recharge areas canlower the potentiometric surface ofthe aquifer, increasing the hydraulicgradient between the lake waterlevel and the po tent iometr icsurface. This increase would in turninduce more seepage from the lake,thus lowering the lake water level.

Examples of Lake Water-Level Fluctuations

The range of water-level fluctu-ations can vary significantly amonglakes. As described previously,water-level fluctuations in seepagelakes (without artificial water-levelcontrols) generally are less frequentbut vary through a greater range

than water-level fluctuations indrainage lakes. To illustrate this,water levels for two lakes are shownin figure 20—one is a seepage lake(Lake Francis in northwesternOrange County), and the other is adrainage lake with a controlledoutlet (Lake Apopka in Orange andLake Counties). Although the waterlevels are at different elevations, thelake levels are shown to the samerelative scale. The scale for LakeFrancis is shown on the left axis ofthe graph and the scale for LakeApopka is shown on the right. Lakelevel changes are more frequent inLake Apopka than in Lake Francis,but the range in water levels forLake Francis is much greater thanthe range in water levels for LakeApopka. Observed water levels inLake Francis range from about53 feet (May 25, 1990) to about66 feet (October, 1960), whichrepresents a range of 13 feet.

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


Observed water levels in LakeApopka range from about 64 feetabove sea level (August 13, 1956)to about 69 feet above sea level(October 12, 1936), which repre-sents a range of water level of5 feet. Thus, the water level in land-locked Lake Francis has varied byas much as 8 feet more than thewater level in Lake Apopka.

Extreme water-level fluctua-tions in some lakes in Florida havebeen a matter of great concernamong residents. For example,water levels in Brooklyn Lake,located in Clay County in north-central Florida, have ranged from

90 feet above sea level (recorded in1992) to 118 feet above sea level( reported in 1948 by a localresident), a range of 28 feet. Incontrast, Sand Hill Lake, about2.5 miles northeast of BrooklynLake, historically has fluctuatedthrough a range of only 3.3 feet(fig. 21). Brooklyn Lake receivessurface inflow, but surface outflowfrom the lake occurs only when thelake level rises above about 115 feet(Clark and others, 1963). Thus,water-level fluctuations are similarto what might be expected in aseepage lake. Brooklyn Lake islocated in a ground-water recharge

area, receiving inflow from the sur-ficial aquifer system and rechargingthe underlying Floridan aquifersystem. Water levels during an11-year period (1985–95) in a welljust west of Brooklyn Lake andwater levels for Brooklyn and SandHill Lakes are shown in figure 21.Note that there is a greater simi-larity between the fluctuations inthe level of Brooklyn Lake (middleline in graph) and the potentio-metric level of the Floridan aquifersystem (as represented by the waterlevel in the well, bottom line ingraph) than there is between waterlevels for Brooklyn Lake and SandHill Lake (top line in graph).Research has indicated that in someareas near Brooklyn Lake, the inter-mediate confining unit is relativelypermeable and al lows watermovement from the lake and sur-ficial aquifer system to the Floridanaquifer system. The identificationof a filled sinkhole in the lake alsoindicates a path of preferential flowfrom the lake to the underlyingFloridan aquifer system. Thishydraulic connection is reflected inthe similarity of the Brooklyn Lakewater levels and ground-waterlevels shown (fig. 21).

Lake water levels may fluctuatebecause of the effect of wind on thewater surface and sometimesbecause of tidal effects. Seiche is aterm applied to a wave that oscil-lates in a water body, causingperiodic water-level fluctuationsthat can vary in time from a fewminutes to several hours. Seiche iscaused by seismic or atmosphericdisturbances and is aided by windand tidal currents. The effect ofseiche is related to the physicaldimensions of a lake (surface areaand depth); large lakes will exhibitmore of the effects of seiche than

Figure 21. Water level in Sand Hill Lake, Brooklyn Lake, and in a well nearBrooklyn Lake.


will small lakes, and shallow lakeswill be affected more than deeplakes. The effect of seiche isindicated by the oscillations inwater levels in Lake Apopka duringa 48-hour period (fig. 22). Lakelevels for the same time period areshown for Lake Apshawa, which isnear Lake Apopka and thus subjectto similar wind patterns. LakeApopka has a surface area of30,630 acres (47.9 square miles).Water levels in Lake Apshawa, witha surface area of only 110 acres(0.17 square mile), indicate nonoticeable seiche effect during the48-hour period.

Artificial Control of Lake Water Levels

Water levels in lakes may beartificially controlled by increasingthe volume of water entering orleaving the lake through the use ofcontrol structures, which can befixed or adjustable. Some examplesof fixed control structures includecanals, dams, and drainage pipes(culverts). These fixed controls can

be made adjustable by the additionof devices that can be raised orlowered. Examples of devices usedto artificially control lake waterlevels are shown in figure 23. Ac u l v e r t c o n n e c t s t w o l a k e s(fig. 23A) and allows water to flowin either direction until the levels ofthe two lakes are equal. Canals canbe fitted with gates that can beadjusted to allow more or less flowout of (or into) a lake, which in turnaffects the lake water level. Radialgates, shown in figure 23B, are usedin many canals and can be set at afixed opening size to allow only acertain amount of water to flowbeneath the lower edge of the gate.A culvert riser is a way of control-ling inflow or outflow to or from aculvert using boards, or stop-logswhich are removed or added tocontrol flow (fig. 23C). A screw-type gate on a culvert (fig. 23D) canbe adjusted to regulate the volumeof water flowing into or out of a lake(depending on the flow direction).

Lake drainage wells (fig. 23E),which allow lake water to flowdirectly into the aquifer (usually the

Floridan aquifer system), have beenused to control lake water levels incentral Florida, particularly in theOrlando area, since 1904. Thevolume of water that can leave alake through a drainage well isprimarily a function of the diameterof the well; debris is prevented fromentering the well by a protectivecage over the top of the well. If thedrainage-well structure is not regu-larly maintained by removingdebris from the protective cage, thebuildup of debris causes a restric-tion in flow, resulting in higher lakewater levels. Drainage wells, usedprimarily on seepage lakes, signifi-cantly affect lake water levels andreduce the risk of flooding byproviding rapid drainage. However,the direct connection between lakewater and water in the Floridanaquifer system increases the risk ofcontaminating the aquifer. Adrainage well on a seepage lakeperforms the same function that asurface out let performs on adrainage lake. Thus, the range ofwater- level f luc tuat ions in aseepage lake that has a drainagewell may be similar to those of adrainage lake.

The range of water-level fluctu-ations in artificially controlled lakestends to be smaller than the range oflevels in uncontrolled lakes. Manyof these controls were built in thepast when it was considered desir-able to control lake levels foraesthetic reasons and for floodcontrol. In more recent years,however, the negative effects onwater quality caused by minimizingthe natural rise and fall of lake waterlevels has been recognized. Forexample, reducing the natural rangeof fluctuations in a lake decreases

Figure 22. Water levels in Lake Apopka and Lake Apshawa illustrating the effectof seiche on water levels.


the area of vegetated flood plain andwetlands around many lakes, areasthat serve as natural fish nurseriesand nutrient filters (through plantuptake). The loss of fish habitat andincreased nutrient enrichmentresulting from the decreased rangeof water levels in controlled lakeshas been detrimental to the overallrecreational and ecological qualityof these lakes. Greater variation inwater levels is now allowed in mostcontrolled lakes because of therecognized benefits to lake floraand fauna.

QUALITY OF WATER IN CENTRAL FLORIDA LAKES

The quality of water in Floridalakes is affected by many factors,including the sources of water to thelake, the quality and quantity ofwater from these sources, the lengthof time a volume of water remainsin the lake (residence time), rainfalland runoff amount and quality, timeof year, depth of the lake, and thetype and amount of plants andanimals in the lake. Common lakeclassification systems, factors orhydrologic characteristics thataffect lake water quality, and someof the methods used for water-quality improvement are discussedin the following sections.

The basic characteristics of lakewater quality include the physical,chemical, and biological character-istics of a lake. Lake water quality iscomplex, and the diversity of plantsand animals that can be supported ina lake is closely related to therelative amounts of individualchemical constituents in the water.Relative concentrations of selectedchemical constituents can be indica-tors of the source of the water to thelake because of differences in thechemistry of water from thesesources. As an example, water fromthe Floridan aquifer system ishigher in concentrations of calciumand bicarbonate ions than waterfrom the surficial aquifer system orsurface waters.

How “good” the quality ofwater in a lake is depends on who isasking the question! What is theintended use of the water? Is it“good enough” for one use, but notfor another? A lake that contains alarge number of aquatic plants andsupports a large fish population may“look” unappealing to some forswimming 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 andphosphorus) concentrations, clearwater, and a sandy bottom free ofaquatic plants, will be of little use tosomeone who wants to catch fish.The quality of water sometimes isjudged on the basis of its clarity—astrictly subjective opinion, based onan individual’s perception of theclarity of the lake water.

Lake Water-Quality Classification Systems

Sc ien t i s t s have dev iseddifferent classification schemes todefine the quality of lakes relativeto one another. A common methodof classifying lakes is by the trophicstate of the lake. The trophic staterefers to the degree or amount ofenrichment (eutrophication) of thelake with nutrients in the water.Lakes can be c lass i f ied asol igotrophic, mesotrophic, oreutrophic (fig. 24). Oligotrophiclakes have very low levels of nutri-ents, very little organic materialalong the lake bottom, and highlevels of dissolved oxygen near thebottom. Mesotrophic lakes aremoderately enriched, and thenatural processes of accumulationof sediments and growth of aquaticvegetation are occurring. Eutrophiclakes are highly enriched with nutri-ents, have an accumulation oforganic sediments, and low levelsof dissolved oxygen in water nearthe lake bottom. Eutrophic lakestypically have high concentrationsof algae or aquatic vegetation andalso differ from oligotrophic andmesotrophic lakes in the type ofvegetation and animal life that canexist in the lake. The trophic statealso is a measure of the productivityof a lake; lakes that are enriched are

more “productive” in that they havelarge populations of aquatic plantsand animals, though the diversity ofthe organisms may be low. Low-productivity (oligotrophic) lakeshave much smaller, but morediverse, populations of flora andfauna than mesotrophic or eutrophiclakes. However, many species offish and other aquatic animalscannot tolerate the conditions thatexist in eutrophic lakes, where largefluctuations in concentrations ofdissolved oxygen are common.

Thus, there may be more fish in aeutrophic lake than in an olig-otrophic lake, but far fewer speciescan survive in the eutrophic lake.

All lakes eventually becomeeutrophic as a part of the agingprocess. Although the process isnatural, many factors can acceleratethe process, including human influ-ences. A newly formed, “young”lake will be oligotrophic. With time,sediments continue to enter thelake, plant and animal populationsbecome estab l ished, and the

Figure 24. Examples of an oligotrophic, mesotrophic, and eutrophic lake.


chemistry of the water graduallychanges to that of a mesotrophiclake. As the lake continues to age,or eutrophy, it gradually becomes amarsh and, eventually, an open field(as described in the section onphysical characteristics of lakes).

Lakes also can be classifiedaccording to the chemical charac-ter is t ics of the water. In thelate 1960’s and early 1970’s,researchers from the University ofFlorida studied 55 lakes and pondsin north-central Florida and deter-mined that the lakes could bedivided into four distinct groupsbased on the following physicalcharacteristics: alkalinity, specificconductance, color, and calciumconcent ra t ion (Shannon andBrezonik, 1972). The four groupsinto which these 55 lakes innorth-central Florida were placedare 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 Floridalakes generally is controlled by twoimportant properties—the qualityof the water entering the lake, andthe rate at which lake water isreplenished. These two propertiesare related—the effects of poorerquality inflow water can be miti-gated by the rapid replacement ofwater in a lake. Conversely, thequality of water in a lake in whichthe water is replenished slowly (asin seepage lakes) can be maintainedif the inflow water quality is good.In this section, the effects frominflow water from various sourcesare discussed.

The source of water to a lake cansometimes be determined from thechemical characteristics of thewater. For example, lakes thatreceive water from surface-watersources or from the surficial aquifersystem generally have low concen-trations of dissolved solids when

compared with lakes that receivewater from the Floridan aquifersystem. Water in lakes that receiverunoff from wetlands may have arelatively low pH and be dark andreddish brown in color. This colorcomes from the natural tannins inthe plants and soils of the wetlandareas. Lakes that receive most oftheir water from ground-watersources and rainfall (seepage lakes)generally have little color. Seepagelakes are affected by ground-waterseepage to and from the lake and bythe land use immediately around thelake. Leachate from septic tanksystems, fertilized lawns, and agri-cultural lands can potential lycontribute additional nutrients suchas nitrogen and phosphorus to thelake.

Surface-water inflows can carrywith it nutrients and pesticides fromupstream sources. For example,outflow from nutrient-rich LakeApopka enters Lake Beauclair andthe other downstream lakes in theOcklawaha chain of lakes. Surfacerunoff from streets can carry greaseand oils from automobiles. Surfacerunoff from residential areas cancarry fertilizers and pesticides fromlawns, grass clippings, leaves, andanimal wastes to a lake. Runofffrom industrial areas may containresidual amounts of chemicals usedfor cleaning or degreasers that arewashed 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.)


Water Quality: A Function of Residence Time

How much the water from thevarious sources described in the lastsection affects water quality in alake is a function of several factors.One important factor that has adirect effect on the response of thelake to the addition of nutrients isthe residence time of water in thelake. Another term sometimes usedin place of res idence t ime isflushing rate, which is the rate(volume per unit time) at whichwater leaves a lake, either through asurface-water outlet or throughground-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 tothe water quality of seepage anddrainage lakes. Residence times inseepage lakes tend to be naturallylong due to slow rates of ground-water outflow. The long residencetime provides time for aquaticplants to use the nutrients in thewater, encouraging plant growthand settling of nutrient-rich sedi-ments on the lake bottom. Resi-dence times in drainage lakes areshorter than in seepage lakes. Inlakes with short residence times, thenutrients are not available in thewater for plant uptake for as long asin lakes with long residence times.Thus drainage lakes generally areable to receive a greater nutrientload than seepage lakes without

excessive aquatic plant growth.Because seepage lakes tend to havelong residence times, they are morelikely to be affected by the additionof nutrients or changes in chemistrythan are drainage lakes. One studyof 20 Florida lakes indicated thatlakes in northern States (of compa-rable volume to the lakes in thestudy) were flushed five to ten timesmore rapidly than Florida lakes(Brenner and others, 1990, p. 372).The long residence time of mostFlorida lakes makes them morevulnerable to the effects of rela-tively small amounts of pollutants.

Water-Quality Problems

Central Florida lakes are subjectto many influences, both naturaland manmade. Although eutrophi-cation is a natural process, it can beaccelerated by human activities.

One well-documented example of alake in which eutrophication hasbeen accelerated because of humaninfluence is Lake Apopka, a classicexample of a eutrophic lake that ischaracterized by a thick layer oforganic matter on the lake bottomand high concentrations of algae. Inthe late 1800’s, Lake Apopka wasthe second largest lake in Floridaand supported a large population ofbass. Beginning at the turn of the20th century, dikes were built alongthe north shore of the lake and thepart of the lake behind the dike wasdrained so that the rich organic soilsof the lake bottom could be used forfarming. This reduced the size ofthe lake, so that it is now only thefourth largest lake in the State.Nutrients from farms and citrusgroves washed into the lake, andsewage effluent was discharged intothe lake for many years. The lake

When aquatic vegetation grows to nuisance levels, chemical methods may beemployed for control. This type of control can lead to problems with water qualitywhen the plants begin to decompose after they are killed, releasing nutrients intothe water, and encouraging more vegetative growth. (Photograph provided bySt. Johns River Water Management District.)


was used for recreation includingbass fishing competitions up to the1940’s, when a hurricane uprootedmuch of the vegetation in the lake,which further upset the naturalbalance of the lake and degraded thequality of the water. The uprootingof the vegetation, in combinationwith the added nutrients, contrib-uted to the rapid decline of lakewater quality.

Aquatic vegetation is necessaryfor the health of a lake, but sometypes of vegetation are desirablewhereas others are considerednuisance plants. Shoreline plantsprovide habitat for water fowl andother animals and are consideredbeneficial because they removenutrients from the water, therebydecreasing the nutrients availablefor algae growth. Aquatic vegeta-tion, whether along the shore orfarther in the lake, contributes to theclarity of lake water. However,generally more than one-half of the

lake bottom must have aquaticvegetation in order to cause thewater to be clearer or to maintainwater clarity (Canfield, 1992).However, this level of aquatic vege-tation often is considered a nuisancelevel.

One aquatic plant that hascreated problems in Florida lakesand streams is the water hyacinth, aplant brought from Brazil to theUnited States in the late 1800’s(Brenner and others, 1990, p. 382).Hyacinths float on the water surfaceand prevent light from penetratingthrough to the lake bottom. Thislack of light in turn causes thedecline of rooted aquatic vegetationon the lake bottom, which changesthe vegetative characteristics of thelake and alters the natural balanceof plant and animal life. Waterhyacinths grow prolifically andaffect the use of lakes for fishingand recreational purposes. Hydrillais another aquatic plant, introduced

through the aquarium trade in theearly 1960’s (Brenner and others,1990, p. 382), that has spread tomany Florida lakes. Hydrilla alsocan choke out native plant speciesand affect lake water quality.

Acid rain may affect lake waterquality. Most of Florida’s lakeshave soft water (Shannon andBrezonik, 1972) and are poorlybuffered (pH of the water is easilyaffected by changes in water chem-istry), thus these lakes are moresusceptible to the effects of acidrainfall. However, many Floridalakes are naturally acidic, andspecies of plants and animals in thelakes are naturally tolerant of moreacidic conditions.

Water-Quality Solutions

The use of the preventive andrestorative techniques describedhere, along with the actions ofresidents today, will ensure thatfuture generations will enjoy thelakes of Florida. Techniques forsolving lake water-quality problemsare numerous, but never easy,because lakes are complex systems.The most common method ofimproving lake water quality isthrough the application of BestManagement Practices, or BMPs,which include the following: aera-tion, pretreatment by detention ofstormwater prior to its entering alake, addition of alum, and moreexotic techniques such as additionof 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 water-quality problems. Some of theBMPs are preventive measureswhereas 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.)


in nature. Other methods of lakerestoration include dredging andlake level d rawdown. Thesepreventive and restorative methodsare discussed briefly below.

Aeration of lake water addsoxygen to the water and aids in themixing and distribution of theoxygen in the water. Aeration helpsprevent fish kills that result fromlow concentrations of oxygen.Other potential benefits of aerationinclude control of algal blooms andgen era l improv emen t o f thecondition of the bottom sediments,which can be highly organic andanaerobic (without oxygen) ineutrophic lakes. Phosphorus is moresoluble in water with low oxygenconcentrations. The addition ofoxygen through aeration causesphosphorus to be less mobile, whichhe lps to keep th e l ake f ro mbecoming weed-choked or algal-rich. The addition of oxygen to thewater provides a better environmentfor aquatic plants and animals, and

the mixing that occurs duringaeration helps distribute the oxygenthrough the water column.

Another common method ofpreventing water-quality degrada-tion in lakes is the detention of

stormwater prior to its entering alake. Stormwater is known to be asource of contaminants and nutri-ents to lakes. By detaining theincoming stormwater, sedimentsand debris carried by the water cansettle, improving the condition ofthe water before it enters a lake.Alum, a chemical compound with ahigh affinity for absorbing otherchemicals, has been added to waterin detention basins to remove nutri-ents from the water prior to releaseof the water to a lake. However, thealum and absorbed material eventu-ally must be removed after it hassettled and accumulated in thedetention basin or lake bottom.

A more exotic method of lakerestorat ion is the addit ion ofspecific species of fish known fortheir capacity to consume algae andaquatic plants. The Asiatic grasscarp has been used successfully insome central Florida lakes tocontrol aquatic vegetation (Canfieldand 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.


Probably the most effective wayto protect the quality of water inlakes is through education andprevention of problems before theyoccur. All of Florida’s residents andvisitors can help protect lakes bybeing aware of human activities thataffect lakes. People living on lake-front property can improve waterquality by leaving aquatic vegeta-tion along the shoreline. Removingthe vegetation to create beachesinterferes with the natural uptake of

nutrients from the water by shore-line plants and eliminates habitatsfor aquatic life. Florida residentscan help prevent water-qualityproblems in neighborhood lakes byreducing the amount of fertilizerapplied to lawns and landscapeplants to that which is required forproper growth, so that excess fertil-izer will not wash into lakes andstreams. Boaters can help improvelake water quality by being aware ofthe possible transfer of undesirable

vegetation (specifically, hyacinthsand hydrilla) from one lake toanother 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 andprovides habitat for wildlife. (Photograph provided by SouthFlorida Water Management District.)

The drawdown or lowering of the water level in an urbanlake sometimes is used to improve water quality byexposing the lake bottom for sediments to becomeoxygenated. Here, Lake Eola in downtown Orlando isshown, August 1994.

Another management method is the use of dry-retentionbasins, which can be incorporated into the landscape oflakeside parks. Shown above is a dry-retention basin nearLake Eola in downtown Orlando, Florida.

It is not uncommon to find the natural shoreline of a lakealtered such as is shown above. (Photograph byE.R. German, U.S. Geological Survey.)

35 Summary

SUMMARY

One of the most common sightsin central Florida are the numerouslakes dotting the landscape. Lakesenhance the beauty of the land-scape, are important for wildlife,and are a treasured natural resource,used by Florida residents andvisitors for recreation and appreci-ated for their beauty.

Lakes in Florida are specialbecause of the hydrologic charac-teristics that set these lakes apartfrom lakes in most other States.Florida’s karst landscape hasprovided a setting for numerouslandlocked, shallow seepage lakesthat are closely tied to the ground-water resources of the State. Inaddition to these seepage lakes,some lakes in central Florida aredrainage lakes, which drain throughsurface-water connections to otherlakes 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 l evel s a re o f g reatc o n c e r n t o m a n y, a n d t h ecomplexity of the hydrologicsystem governing these lake levelsis not commonly understood. Lakelevels are a product of the balanceof the hydrologic budget—inflows(rainfall, runoff, seepage) minusoutflows (evaporation, seepage,surface-outflow). The water level

response to the components of thehydrologic budget differs betweenseepage and dra inage lakes,primarily in the timing of the fluctu-ations of lake levels and in the rangeof lake levels. Water levels indrainage lakes can change rapidlybecause the surface-water outflowsallow for a large volume of water toleave the lake in a relatively shorttime period. However, a smallchange in lake level can produce alarge volume of outflow from thelake, thus the water levels rise andfall within a very narrow range.Seepage lakes, however, lose watermuch more gradually through evap-orat ion and seepage, and thevolume of water exiting is much

aquatic weeds, and many otherproblems, do not have simple orquick solutions. Many problems arethe natural consequence of climaticchanges; others are the result ofmany decades of development. Theimportance of education, improvedunderstanding through research,and cooperation cannot be under-stated. Through the combinedefforts and cooperation of individ-uals, communities, and govern-ment, lakes in Florida can bepreserved and continue to be avaluable natural resource for thebenefit of all.

smaller, so water levels rise togreater altitudes than in drainagelakes. The wide variation in waterlevels in many central Floridaseepage lakes historically haspresented problems for the develop-ment and use of land near theselakes. As a result, many seepagelakes have been artificially drainedthrough the construction of canalsand drainage wells.

The long residence times inFlorida seepage lakes makes theselakes more susceptible to even smallchanges in inflow water quality. Theshorter residence times of drainagelakes helps somewhat in main-taining the lake water quality butdoes not preclude the potential fornegative effects.

Problems of declininglake levels (or flooding caused byrising lake levels), algae blooms,chok ing of lakes wi th


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Anderson, Warren, Lichtler, W.F., andJoyner, B.F., 1965, Control of lakelevels in Orange County, Florida:F lor ida Geo log ica l SurveyInformation Circular 47, 15 p.

Arthur, J.D., Bond, Paulette, Lane, Ed,and Rupert, F.R., 1994, Florida’sglobal wandering through thegeological eras, in Lane, Ed, ed.,Florida’s geological history andgeological resources: Tallahassee,Florida Geological Survey SpecialPublication 35, p. 11–25.

Bates, R.L., and Jackson, J.A., eds.,1987 , G lossary o f geo logy(3d ed.): Virginia, AmericanGeological Institute, 788 p.

Beck, B.F., ed., 1984, Sinkholes—Their geology, engineering andenvironmental impact: Multi-d isc ip l inary conference onsinkholes, 1st, Orlando, Fla.,October 15–17, 1984 [Proceed-ings], 429 p.

Beck, B.F., and Sinclair, W.C., 1986,S inkho les in F lo r ida—Anintroduction: Orlando, Fla., TheFlorida Sinkhole Institute at theUniversity of Central Florida, 18 p.

Bradner, L.A., 1994, Ground-waterresources of Okeechobee County,Florida: U.S. Geological SurveyWater-Resources InvestigationsReport, 41 p.

Brenner, Mark, Binford, M.W., andDeevey, E.S., 1990, Chapter 11—Lakes, in Myers, R.L., and Ewel,J.J., eds., Ecosystems of Florida:Orlando, University of CentralFlorida Press, p. 364–391.

Britton, L.J., Averett, R.C., andFerreira, R.F., 1975, An introduc-tion to the processes, problems,and management o f urbanlakes: U.S. Geological SurveyCircular 601–K, 22 p.

Bush, P.W., 1974, Hydrology of theOklawaha Lakes area of Florida:Florida Bureau of Geology MapSeries 69.

Canfield, D.E., Jr., 1981, Final report,chemical and trophic state charac-teristics of Florida lakes in relationto regional geology: Gainesville,University of Florida, Center forAquatic Weeds, 444 p.

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Canfield, D.E., Jr., Maceina, M.M., andShireman, J.V., 1983, Effects ofhydrilla and grass carp on waterquality in a Florida lake: WaterResources Bulletin, v. 19, no. 5,p. 773–778.

Clark, W.E, Musgrove, R.H., Menke,C.G., and Cagle, Jr., J.W., 1962,Inter im report on the waterresources of Alachua, Bradford,Clay, and Union Counties, Florida:F lor ida Geo log ica l SurveyInformation Circular 36, 92 p.

———1963, Hydrology of BrooklynLake near Keystone Heights,Flor ida: Flor ida GeologicalSurvey Report of Invest iga-tions 33, 43 p.

Cooke, C.W., 1945, Geology of Florida:Tallahassee, Florida GeologicalSurvey, Geological Bulletin 29,339 p.

Chow, V.T., 1964, Handbook of appliedhydrology, Section 23, Hydrologyof lakes and swamps: New York,McGraw-Hill, p. 23–1—23–31.

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37 Selected References

Hendry, C.D., and Brezonik, P.L., 1984,

C h emi ca l compos i t i on o f

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39 Selected References

Documents

Hydrology of Central Florida Lakes —A Primer...the Florida peninsula is a result of the geology and geologic history of the State. An estimated 7,800 lakes in Florida are greater