A Beginner's Guide to
Water Management -
Information Circular 104
Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
-. UNIVERSITY OF 4
Intitute of Foodd ad Agri itural Sciences LAK A CH
Many thanks to Dr. Roger Bachmann for his contributions
concerning the dynamics of wind mixing in lakes.
This publication was produced by:
Florida LAKE ATCH 2001
University of Florida / Institute of Food and Agricultural Sciences
Department of Fisheries and Aquatic Sciences
7922 NW 71st Street
Gainesville, Florida 32653-3071
Phone: 352/392-4817 or 1-800-LAKEWATCH (1-800-525-3928)
Web Address: http:/i,'.:: T.-atch.ifas.ufl.edu
Copies are available for download from the Florida LAKEWATCH web site:
A Beginner's Guide to
Water Management -
Information Circular 104
InAtitute of Food ndi Agrul tural Scieni es
.- lake is the landscape's most
beautiful and expressive feature.
It is the earth eye looking into
which the beholder measures the
depth of his own nature.
-Henry Da\ id Thoreau
U f asked to describe a lake, most of us would probably begin by discussing the waterbody's
more obvious characteristics such as its size, water clarity, aquatic plant growth, the color of
the water, or fishing potential whichever characteristics are most important to us as lake
users. However, there are several other less visible lake characteristics that are just as significant,
yet rarely discussed: namely the shape and structure of a lake basin.
The study of these lake basin features is known as lake morphology and familiarity with the subject
is as important to lake management professionals as human anatomy is to a physician. Just as physicians
rely on their knowledge of human anatomy to understand a patient better, lake management professionals
(and anyone else) can learn a great deal about how a lake functions by studying its morphometric character-
istics. For example, when we know the shape and structure of a lake basin, we can sometimes predict
how weather conditions or human-induced events may affect water levels in that system. This is important
because, as many lake residents already know, changes in water levels can influence the water quality of
their lake including the amount of algae and/or aquatic plants growing in the water, fish species and abun-
dance, and water clarity. It can even play a role in determining the types of birds and wildlife that are
attracted to a waterbody.
From a scientist's or lake manager's standpoint, this type of information can be helpful in anticipating
changes in a lake system and predicting how it might affect the lake's inhabitants increasing the
chances of mitigating any undesirable impacts with carefully planned management techniques.
Lake residents can use their knowledge of lake morphometry when deciding where to build a
lakefront home, boathouse, or dock. Some people have even learned to foresee changes in their lake and
use them to enhance their own personal management goals. For instance, by anticipating drought-
induced low water levels in his lake, one homeowner took advantage of the circumstances and applied
for muck removal permits for his beachfront. Being able to plan ahead and obtain permits before condi-
tions changed was a real advantage as the permitting process had traditionally proven to be a lengthy and
Lastly, studying lake morphometry can also help us appreciate lakes for what they are and manage
them with more realistic expectations. For example, residents living on a large shallow lake in central
Florida might find it useful to know that the size and shape of their lake makes it more susceptible to
wave action, which stirs up bottom sediments and often results in low water clarity. With this insight,
they may decide to create man-made islands to help buffer the effects of the wind or they may choose to do
nothing, content with the idea that the lake has naturally limited water clarity. Whatever they decide,
knowledge is an effective tool for the decision making process.
Because lake morphometry can play such a critical role in the dynamics of a lake system, we
encourage anyone who is interested in learning more about their lake to become familiar with the follow-
ing terminology and techniques currently used to study lakes. Such information will provide a solid basis
for comparison with other waterbodies, as well as invaluable tools for developing a lake management
plan for your lake, or others in your area.
Part 1 How Lakes Are Formed
Part 2 Bathymetric Maps and What They Tell Us About Lakes
Part 3 Commonly Measured Morphometric Features and What They Tell Us About Lakes
Part 4 Wind, Waves and Water Mixing in Lakes
Appendix A Measuring Lake Surface Area
Appendix B Measuring Lake Volume
.- --. i- :I
":... .. ..
Sakes are formed in a variety of ways, depending on their geographic location and the geological and
biological forces at work within that region. In some instances, they are the result of catastrophic
events such as earthquakes, volcanoes, landslides, or even meteorites.' Others were formed by more
gradual processes such as glacial activity, changes in a river's course, wind action, solution processes, or the
scouring effects of underwater currents in an ancient sea.2 Even the accumulation of decaying organic materials
has been attributed to the formation of some lakes. For example, it's thought that Lake Okeechobee, an enormous
shallow bowl-shaped lake in Florida, was largely the result of erosion caused by ocean currents from long ago
when the sea covered the area. But some scientists speculate that an accumulation of decaying organic materials
around the rim of the lake (i.e., aquatic organisms, plants, and sediments) complemented its development by
helping to contain water within its shores early in its formation.3
The majority of lakes in the northern regions of the United States and Canada are glacier-related: the
result of large ice flows that scoured the land's surface, creating and removing great quantities of loose
material and leaving behind thousands of depressions or basins that eventually became lakes. In some
instances, lakes are formed simply from the accumulation of ice in pre-existing depressions within the
landscape. Further south and throughout much of the continental United States, a majority of lakes are man-
made, created for flood control, the generation of hydroelectric power, recreation, agriculture, or drinking
water storage. In many cases, small reservoirs were created to reduce soil erosion.
Florida, however, is a major exception with more than 7,800 naturally formed bodies of water that are
as diverse as the creatures inhabiting them. Some lakes are nearly circular in shape, appearing to have been
cut out of the landscape with an immense cookie cutter. Many resemble shallow bowls, and others are
contained within meandering shorelines that change seasonally with water levels and shoreline vegetation.
They vary in size as well. While the majority are less than 30 acres in size, one waterbody, Lake
Okeechobee, spans more than 450,000 acres. Its surface area is larger than some counties!
Of course, such an abundance and diversity of lakes is closely linked to Florida's watery geologic
history. Fossilized seashells and coral fragments found many miles inland from today's coastline are a sure
clue that, for a time, much of the state was submerged. As the oceans receded and the peninsula gradually
began to rise out of the water, it became a shallow reef for an estimated 25 million years. During that time,
warm ocean currents deposited materials such as limestone and phosphates and then later scoured and re-
worked the porous carbonate rock, transporting and redepositing sand, silt, and clay sediments many times
over. Eons later, after Florida emerged as a land mass, the resulting limestone formation has proven to be
particularly susceptible to solution processes, the most common origin of Florida lakes.4 It's important to note
however, that most Florida lakes are likely the result of numerous geological and biological forces that occurred
over a period of years, often centuries, and continue even today. The following is a discussion of these processes.
1 Listed respectively: Lake Tahoe in California, Crater Lake in Oregon, Lost Lake in California, ( Lake in Quebec.
2 Listed respectively: the Great Lakes, Catahoula Lake in Louisiana, K ~ Lake in New Mexico, Lake Jackson in Florida,
and Lake Okeechobee in Florida.
3 Hutchinson, G.E., 1957.
4 The solution process is a chemical process by which rock is dissolved by interactions with water.
Solution Depression lakes are often different in
(- shape and size from solution lakes. Due to the
(Sinkhole) scouring action of underwater currents and
Lakes waves, some depression lakes are elongated or
Florida's karst5 appear threaded together like a string of pearls,
geology is unique in often along coastal areas or near rivers.
that a large portion of Examples of depression lakes can be found
the state is comprised le S. along Florida's Upper St. Johns River, including
Sinkhole lake in Sebring, Florida.
of major deposits of Lakes Helen Blazes, Washington, Winder, and
limestone (i.e., calcium and magnesium carbon- Poinsett in Brevard County all remnants of an
ates) that are particularly susceptible to solution ancient estuary.7
processes. Lake Okeechobee is another example of a
Rainwater tends to become slightly acidic depression lake and is thought to have originated as
as it mixes with carbon dioxide from the atmo- a rather large dip in the Pliocene seabed.8 Scientists
sphere and soils. As it percolates down from the believe that additional biological and geological
surface or flows horizontally as groundwater,6 forces (i.e., solution processes, wind and wave
small pockets of Florida's limestone rock matrix erosion, and sedimentation) helped to modify the
are continually being dissolved. As a result, lake over time.
underground cavities and/or caverns are some- River lakes
times formed. Under certain conditions, the
roofs of these caverns eventually collapse and A few lakes in
the resulting sinkhole can, over time, fill with a are e resu
water, creating a solution lake (a.k.a. sinkhole of rivers carrying
lake). See Figure 1-1 on page 3. and delivering large
loads of sediment
Florida has the largest concentration of solution
lakes in North America. Many are nearly circular, along their borders and creating natural levies that
but others are irregular in shape because adjacent eventually separate newly formed lakes from the river.
sinkholes or additional ground subsidences allow Similar to depression lakes, river lakes are
typically elongated in shape and appear to be
cavities to fuse together to form compound depres- typically elongated in shape and appear to be
threaded together. Some are even connected as a
sions. Solution lakes are still being formed today!. Te a
series or "chain" of lakes. The Tsala-Apopka Chain-
of-Lakes in Citrus county is an example of this
c North Floridao' Faimotu. Disappearing type of lake system. Created years ago when the
Like\, i,' -i Withlacoochee River was much wider than it is
today, numerous lakes (e.g., Floral City, Freds,
Depression lakes Hemando, and Henderson) were formed, in part, as
Although the majority of lakes in Florida the result of changes in the river's course. River
are the result of solution processes, there are lakes are similar to oxbow lakes, but are generally
a few other varieties. For example, there are wider and larger than ox-bow lakes, which are
large shallow bowl-shaped waterbodies, only a typically U-shaped.
few meters deep, known as depression lakes.
While a number of these lakes were carved out of 5 Karst refers to limestone and dolomite
beneath the land that have been altered by solution
the seafloor by wave action or underwater currents, processes rhat continue eve today. Karst
processes that continue even today. Karst
others are the result of land subsidence or even lack major ". drainage such as rivers and streams. Instead,
earthquake activity. For instance, Long Pond in nearly ; down through the porous rock.
Levy County is thought to have been formed 6 Groundwater is the water below the land ".. in the
along a fault line, caused by earthquake activity zone ofsaturation.
7 White, William A. 1970, pages 102-104.
several thousand years ago. 8 Hutchinson, G.E., 1957.
No evidence of land subsidence, small- to medium-sized cavities in the rock
marix. Water from surface percolates Ihrough TO rock and the erosion process
Caviiles in limestone continue to grow larger Note missing confninng layer that
allows more water to flow through o0 the rock matrix. Roof of the cavern is
As ground-water levels drop dunng Ihe dr season the weight of the overburden
exceeds the strength of the cavern roof. and the overburden collapses into the
cavern forming a sinkhole
Figure 1-1 Formation of a collapse sinkhole.
T he north central region of Florida is
1 dotted with numerous examples of
solution lakes. However, there are several
that are particularly interesting as they have
a rather disconcerting habit of disappearing.
Alligator Lake in Lake City, Lake
Alachua (i.e., Payne's Prairie) in Gainesville,
and Lake Jackson in Tallahassee, are all
known to have done this at various times in
the past and they have at least one morpho-
logical characteristic in common. Their lake
beds are situated above local groundwater
Paynes's Prairie in Gainesville was once known as levels and as a result, they are even more
LakeAlachua, locals plugged up a nearby sinkhole, susceptible to solution processes and the
S -, formation of sinkholes. See Figure 1-1 on
S. page 3.
Lake Jackson performed its most recent
disappearing act in September of 1999 and
May 2000. Due to drought conditions and
steadily dropping groundwater levels, a
Portion of the lakebed collapsed into an
.'. .eight-foot wide sinkhole that drained the
Porter Sink that drained Lake Jackson in September 1999. entire central portion of the lake. Six months
later, a second sinkhole opened up in a
different section of the lake. And "though
shallow pools still occurred in portions of
I- the lake, more than 90% of the 4,000-acre
lake had vanished!" (Jess Van Dyke, Aquatics,
.This type of activity has been docu-
S' mented several times since the late 1800s
and always coincides with long-term
..u. drought conditions.
Not to worry, once the sinkhole becomes
S.. blocked with sediment and debris and
A brave Jess Van Dyke, with Florida DEP, descends into rainfall resumes, the lake will probably fill
Porter Sink to explore underneath Lake Jackson its up again.
infamous disappearing act in September 1999.
Reservoirs and other Lake Ocklawaha (a.k.a. Rodman Reservoir) in
man-made lakes Putnam County is a remnant of the now defunct
Cross-Florida Barge Canal project from the 1960s.9
Reservoirs are generally formed by building Since then, it has become a popular fishing lake.
a dam or weir across a body of flowing water and
allowing water to fill up a valley or low lying area. Aside from reservoirs, hundreds of thousands
People have been building these structures for of stormwater retention lakes exist throughout the
more than 3,000 years to supply drinking water, state. Some were built for stormwater treatment, while
hydroelectric power, fish and other aquatic products, others were made for the diversion of floodwaters,
aids to navigation, even defense. While many and/or landscape enhancement. Many are managed
lakes in Florida were formed naturally, thousands for a variety of recreational uses including fishing,
of small reservoirs can be found throughout the water gardening, bird watching, and boating.
state, as well as several larger ones, including: Florida is also possibly the nation's leader for
real estate lakes, the majority of which are found
* Manatee Lake in Manatee County was built to in south Florida in areas that were mined for fill
serve as a drinking water supply; dirt. The remaining pits were turned into lakes
* Lake Talquin in Gadsden County is used prima- and surrounded by homes.
rily for recreational activities; Coastal _,
* Lake Seminole, which spans across the Florida/ C s
Georgia border into both Jackson and Seminole Dune
Counties, is used for barge traffic/access, via the Lakes
Apalachicola River, to Georgia and Alabama; Coastal
* Lakes Karick and Hurricane in Okaloosa dune lakes are .
County, and Bear lake in Santa Rosa County are most likely the .
managed as fishing lakes by the Florida Fish and result of several
Wildlife Conservation Commission; forces. Some
SL R in r n rn are the result of longshore currents flowing along
* Lake Rousseau in Citrus County was originally
a stretch of coastline, depositing sediments
created to help generate hydroelectric power for,
across some irregularity or indentation of the
the phosphate industry there. It is now surrounded
ih m y l h is a is a a coast, such as an inlet or bay. Eventually, a
with many lakefront homesites and is also a
popular fishing lake. newly formed sandbar becomes large enough to
popular fishing lake.
cut off the inlet or bay from the ocean, and a
lagoon or saltwater lake is created. Years later,
after the development of additional sand hills or
dunes, the lake becomes increasingly landlocked,
F .with less influence or intrusion of saltwater.
Occasionally, these lakes can become saline due
to surrounding dunes opening up again.
In the Florida panhandle there is a prominent
area of coastal dune lakes. Examples include
lakes Big Red Fish, Camp Creek, Draper, Eastern
and Western, to name a few. Depending on
hurricane activity and rainfall, they oscillate
between being saline and freshwater lakes.
9 The canal project was an attempt to connect the Atlantic
Ocean with the G ~ > in Norrth Florida.
Shen planning a trip to unfamiliar map depending on the number of depth measure-
territory, the first thing many of us will ments taken. To put it simply, the more depth
do is reach for a map of the area. If measurements one is able to record, the more
traveling by car, one might use a road map. accurate the map will be.
However, if traveling "off road," a topographic It's also important to note that the outermost
map would be especially helpful as it would contour line, as well as the rest of a lake's
provide details about the actual terrain such as bathymetry, is subject to change depending on
distances and elevations of mountain ranges, as rainfall patterns and resulting lake levels.
well as the location of rivers and streams. Bathymetric maps are the primary
Bathymetric maps are similar to topographic method used to describe a lake's
maps, in that they provide details about the physical characteristics.
terrain of a landscape. In the case of a bathymetric e e ae a a m
Once we have a bathymetric map, we can
map, the terrain that is described is underwater. i .
map, tt is dcrid is ur calculate several measurements that are crucial
As you can see in Figure 2-1 on page 7,
to understanding how a lake system functions,
a bathymetric map is generally depicted as a including surface area, maximum length, mean
f n r ," a including surface area, maximum length, mean
grouping of concentric contour lines, with the w ma m th, ea et, a
prd d l th width, maximum width, mean depth, maximum
outermost contour line representing the shoreline dept, sreline lent, srline level
depth, shoreline length, shoreline development,
of the lae at a given point in time. Lines within and volume. These measurements are discussed in
the map are obtained by recording water depths et .
greater detail in Part 3.
throughout the lake and connecting the recorded
points of equal water depth. Contour lines drawn See Part 3 Commonly MeasuredMorphometric
close together indicate rapid changes in water Features and What They Tell UsAbout Lakes on
depth and lines that are far apart indicate water page 10 for detailed information about these features.
depthsthhat change gradually.
The contour lines are only estimates of olin e
The following are a few examples of how
water depth between two points of a known
water depth b n to points of a known bathymetric maps may be useful to scientists or
depth. There may be discrepancies in any given anyone interested in learning more about a lake:
anyone interested in learning more about a lake:
Figure 2-1 below is an example of a bathymetric map made by
Notticc o\\ tho I' lthc o 'lt'll. t Inll' idc'lll'n l t lk l .c'l s~ lhorc ll.k' LllI' % \\ ithlln th11 t outL llhk' nic.
called contour Iinis Tlhc' arc obtained b\ recoiding a\t'r depths thiouvihouit di lakc anid
con0ik'ctill thc' i'cordc'd jpolll( Of (.'qi ill \\%atc dcpth
Conbrtitr In ics di" l closc to'cthcr i ndlt ica( rapid clhancJ Is \\n t' a (l dc).')til andl Iictsl t t a 'c
fai apl)at Ilinical \\a'r dcptl. h tl t chan1'1 _' g ia la;ill All 1oftl rcOS conII l In' i a1 si.' at1ii 'cs of \\ 'l
dcilLrh tl l s ll'l' tl r 111 bd dilSlcr`.'lcilCL'S I amll gi- cl\ i ll p1 dcill' nd'il l' on11 th u Illllli'c f :' drl hil nica-
sML''l 'ltcl l.t takcll to niakc' tll' n11ip A '_'cnc I IIl of r llll t llb r i'c1 d il1 llc'pr .icl lll'lit. i o IS
Nblc to iclcoid. thc' oll 1 `rL;t' tllc 11n1; ic \\ i ll 'I '
lakc is isuscc.ptiblc' to chang_'c For c'\ani plL'. di in, a pl)nori oflidro, uht. it s Ii'Ilk.'c that thc' lakc '
tirtli' anvl. coIlid shll illnk III l ich cI 1thc o11 tc'lilOst contIIll lll.' ,r i d 'cach of thc dcpI. h
contIIolli n II .' \\ould clhanc' 'l-hit alon1-'
I\ th thc 'atcLr Ic'\ cl Jackson/ Highlands County
A well-made bathymetric 8.. .2"5
map will usually include:
A The name. coin l t\ and eouialhc a s c.ae = 1. .--I
scale = 10.000 II
location of rthe aterlbod\.
B An outline of the lake shoreline.
dra\\ n to a kno" n scale.
C Depth contour I nes dra\\n at .- 2 -, -. -
kno" n inter\ als. '
D S\ mbol indicating, ueou.ralphic / -1'i/i .
orientation I e 110noth1).
E Name of the mallpmnakers andte dlare # \
While the map shown here \ \ '
is not designed for navigation A-
purposes, it can be used to _;
calculate important morphometric
features of a lake such as:
sirtifacc aca. Iiia\iliIIl lcngtl. Ri FlorIc LAKEWJTCH personal crealed hms map tung dlteienlalfy arrested
lc'i-tIl. 111m\1111i1i \\ Itl" Illlr \\ idtrl. gloapS lmumel pmnt( S DIal r eiCttdl uguS16 ItS IMap
cOnourb ar h tlel a d mw gelnmlen asg ng9 hedknque In Sumgaer
11\ 1IIIIII dcIp rtl1. IIic'a1 dcpr I hl, l lll' otiwar( pacrtage (GCoian SfwaiOo Daide CO) On lrmi ale thir laws surlfe
n m i\lll lldcptl. ll ll 'ptl. l01Illl e* arwac lr.latse 19.212 are..3.728nelr) Tnltr&s an onml .appro.'mate
lcgi. Siolcrllc (1 c1 clopicl ad b nlrc v maUana houl eall be used fai nivlqan.
SLllicth. h l l \ ll'llt. 111
Local residents ofLake Alice in Gainesville, Florida.
+ Lake surface area can be calculated from a
See Shoreline Development on pages 18.
bathymetric map. This measurement determines
the size of the lake and is usually expressed in Mention bathymetc maps to an angler and he
acres or hectares.
acres or hectaresor she is likely to get starry-eyed at the prospect
For more on this, see Part 3 Commonly Measured of finding a fishing "hotspot." Anglers use these
Morphometric Features on page 10. maps to spot areas where lake depth changes
rapidly; they know that larger predatory fish can
* Bathymetric maps can be used to help calculate often be found there.
lake volume, which is usually expressed in acre-
feet or cubic meters. Making a bathymetric map
Making a bathymetric map can be a simple
See Hypsographic curves on page 12, Volume on M
page 15, andAppendixB onpages 31- 32. process or a complex one. LAKEWATCH uses a
page 15, andAppendix B on pages 31- 32. .
technique that is somewhere in between. Regard-
Sm c a b less of their complexity, a well made bathymetric
* Bathymetric maps can also be used to calculate c
average depth, which can help predict biological map generally consists of a line drawing of the
average depth, which can help predict biological
rdt h ae edt shoreline, to scale, along with depth measurements
productivity (i.e., shallow lakes tend to be moredifferent areas of the lake.
taken at different areas of the lake.
productive than deep lakes).
+ Using the scale provided in a bathymetric map, See Anatomy of a Bathymetric Map on page 7.
one can calculate fetch distances from all directions.
Beyond that, the amount of detail in a
See Fetch onpage 19. bathymetric map depends on the amount of
time and effort expended in making it, as well
* The irregularity of a lake's shoreline, as
as consideration of its intended use. For example,
depicted by bathymetric maps, can tell us much
some bathymetric maps are designed for navi-
about a lake's potential for biological habitat nation, requiring many, many data points or
gation, requiring many, many data points or
(i.e., its ability to support animals such as fish,
. .depth measurements.
birds, alligators, etc.).
Complex bathymetric maps are Simple bathymetric maps can be
constructed by completing a survey of the made by sketching a general outline of a lake
shoreline using standard surveying methods and basin and then measuring and recording water
then combining the survey with electronically depths at a number of locations within the lake.
measured water depths at known locations The more depth measurements one is able
throughout the lake. Lake water levels, in relation to record, the more accurate the map will be.
to mean sea levels, are often represented. Water depths can be measured with an electronic
Water depth readings are collected with an depth recorder or something as basic as a
electronic depth recorder (a.k.a. fathometer) and weighted line, marked in increments of feet or
simultaneously linked with global positioning meters.
system (GPS) coordinates.10 The procedure is This approach can be used by anyone with
repeated numerous times on the lake, generally a boat and can be a valuable exercise, especially
following a grid pattern. This type of mapping for those who live on or frequently use a lake.
procedure allows for more data points to be While these maps may not be appropriate
recorded and is considered to be very accurate. for navigation purposes, they are perfectly
It's important to point out however that not adequate for developing aquatic plant management
all bathymetric maps provide lake level data in strategies or planning a fishing trip.
relation to mean sea level (MSL). LAKEWATCH
athymetric maps, for example, provide data only 10 This type of system utilizes satellite technology to
bathymetric maps, determine one geographic location.
for one point in time and not in relation to MSL.
E very summer since 1996, LAKEWATCH -
staff work with students and volunteers
to create bathymetric maps for a limited .
number of LAKEWATCH lakes. The maps .
are designed to compliment LAKEWATCH
data on individual lakes, providing a snapshot
of the lake's bathymetry at a given time, and at
a minimum of cost and effort.
LAKEWATCH uses a technique that
involves the use of Global Positioning (GPS)
equipment in coordination with a depth recorder
(i.e., echosounding equipment). The depthfinder
is used for recording actual lake depth measure-
ments, while the GPS equipment simultaneously
determines and records the location of each depth
measurement. Bathymetric maps are completed
with a computer software program that merges
the information together and "draws" the lake's
A good number of these bathymetric See Figure 2-1 on page 7for an example
maps (200+) are available on the Florida ofa LAKEWATCHBathymetric map.
LAKEWATCH web site:
http://lakewatch. ifas. ufl. edu
As we learned in Part 2, bathymetric maps are essential tools for anyone interested in
learning about a lake system. The following is a continuation of this discussion, as we will
now introduce the various lake features that are commonly associated with bathymetric
maps. Note: These terms and concepts are not listed alphabetically but are instead
presented in the order of their significance to the lake management process.
Surface area -- Surface area is often represented in scientific
refers to the size of a lake and is generally expressed literature as the symbol A.
in units of acres or hectares (abbreviated ha).
Note: One hectare equals 2.47 acres. A square that is Lake Okeechobee, with a surface area of
100 meters on a side would have an area of hectare. 450,000 acres (183,000 ha).
In Florida, the majority of lakes have small It's important to remember that the water level
surface areas. About 80% of the named lakes and surface area of many Florida lakes can change
have surface areas less than 100 acres (40 ha) dramatically with drought and/or flood conditions.
and only 31 lakes have surface areas greater than This can be a shocking realization for homeowners
5,000 acres (2,024 ha). Florida's largest lake is who bought lakefront property during a lake's
/i high water stage, and later find themselves living
hundreds of feet from the water's edge due to
drought-induced shrinkage of the lake.
Surface area is one of the most
important morphometric parameters of
a lake because it not only describes the
size of a lake, but also plays a major
role in a lake system in the following
Lake surface area can be used to help predict
the potential effects of wind on a lake.
In general, lakes with more surface area are
subject to larger waves during windy conditions.
This photo ofLake Hampton in Citrus County is a good example This is significant because larger waves have the
of how drastically a lake shoreline can change. Obviously, when ability to mix water at greater depths, in some
a lake shoreline "shrinks this much, its surface area has been
reduced as well. instances reaching all the way to the bottom of the
Lake Eloise at Cypress Gardens, Florida. Lakes with more surface area are generally subject to larger waves during windy conditions.
lake. Waves can also result in extensive shore erosion. with a smaller surface area. If a lake has a greater
The ability to create mixing at the bottom of a dilution capacity, it is less likely to be affected by
lake is extremely important because it can result nutrients or other substances that may be introduced
in the resuspension of sediments, and/or the from human activity. In this instance, the adage "the
disturbance of submersed aquatic plants. As a result, solution to pollution is dilution" rings true.
other lake characteristics, such as water clarity Consideration of a lake's surface area is also
and the availability of nutrients, can be affected. useful when trying to choose the appropriate
A lake's surface area also influences the type of boat to use on a lake.
dilution capacity a lake may have. If you've ever been on a large lake during
Consider two lakes, one small and one windy conditions, it doesn't take long to figure
large, but with the same average water depth (i.e., out that lakes with more surface area are generally
mean depth). Visualize an equal amount of capable of generating larger waves. And when
nutrients, let's say 100 kg, are introduced into winds do increase, you don't want to be caught
each lake over the course of a year. Although the out in the middle of the lake in a small boat.
amount entering both lakes is the same, the larger
lake, with a greater surface area and volume of See Fetch onage 19
water, will have a reduced concentration of the Some boats are even designed to fit the
material than the smaller lake. In other words, the dynamics of the waterbody that they are used in.
larger lake simply has more water available to For example, commercial fishermen who spend a
dilute materials coming into the lake. lot of time fishing on Lake Okeechobee have
The ability of a lake to dilute materials, designed, and continue to build, custom fishing
whether they be naturally occurring from the boats with a high prow and tapering gunnels in an
watershed or from a human-induced spill, is known effort to lessen the effects of choppy waves that
as dilution capacity. are common on Lake Okeechobee. These boats
In general, lakes with a greater surface area provide a smoother ride and a safer boat during
will have a greater dilution capacity than lakes windy conditions.
outermost contour line. It's also relatively easy to
s a a a f i picture what the lake would look like if water levels
were to shrink down to the 6-, 11- or 16-foot depth
contours. Notice that if the lake were to shrink down
th ls et e u e to the 21-foot depth contour line, its surface area
would be reduced to about half its original size.
laebsidnats w- a n ie But this approach gives us a visual estimation
g m b - s only. If we were to calculate the surface area
within each one of the contour lines in Figure 3-1
and then plot them on a graph, we'd have a
LHypso graphic curves hypsographic curve a visual image that can
are graphs used to provide a visual representation give us accurate information at a glance.
of the relationship between the surface area of a
lake basin and its depth. With these graphs, we See Drawing Hypsographic Curves on page 13
can be more accurate in predicting how a lake's e a t h e gra
surface area could change based on changes in Interpreting hypsographic curves
water depth. See Figure 3-2on page 13. Using the "gauge" provided in the vertical
To help explain this concept, let's refer a y-axis of Figure 3-2, we can choose a hypothetical
LAKEWATCH bathymetric map: When looking change in lake level and then compare it with the
at the bathymetric map in Figure 3-1, we have a horizontal x-axis for an estimate of what the lake's
"bird's eye" view of Lake Denton. From this surface area would be under those circumstances.
perspective, it's easy to picture how the lake For example, let's say that we want to know
would look if it were full of water, right up to its what the surface area of Lake Denton would be if
the water level were to
Denton (Highlands County) drop 10 feet.
fcorid. LAKEWA*CH Bahym.erol.Map To do this, we would
draw a horizontal imaginary
t 41 -foot depth contour line line across the graph in
Figure 3-2 from the 10 unit
S-mark. As you can see, it
_- 21would intersect the
"3 hypsographic curve at
,_ e Iabout the 45 unit mark
along the x-axis. This
Stmeans that if the lake level
were to drop 10 feet, the
SJlake's surface area would
/ 4/ be reduced to approximately
6efoot 7\ 45 acres.
contour ; Notice that a larger
line .':,. drop in the water level
would have an even more
foodpt profound effect. For instance,
deh/ ;- if the lake level were to
1 6-foot I o t
depth oo drop 30 feet, the surface
area of the lake would be
Figure 3-1 A bathymetric map created by Florida LAKEWATCH. reduced to 25 acres.
0 10 20 30 40 50 60 lake surface (full)
,- Q 20
\ lake bottom
Figure 3-2 i .i < ,.i i, i /,i, ..i .. *,q i -
i,, l k. /.I III /,o 0 1 i. II /. o .1 ,, 11 r.i ," h I. Io l ,, I.I i. i0. ,i ,,, o
One way to explain hypsographic curves is to describe the steps that were taken
to draw one. To do this, we'll need to revisit what we know about bathymetric
maps. Why? Because hypsographic curves are based on lake bathymetry and it
works like this:
1 while e looking at the bath\ metric map in Fl uure 3-1. \\e can see that the outermost
contoIu I'ne of the map is used to represent the lakes shoreline at its hiuh \\ ate le\ el
2 Lsinu ain\ one of the tecIhniques from A.llppendii\ A. on ipave 2' (i .. lc.'asurIn Siufac.'
Ai'a). thle area i \\ th that ouLtelmost shoreline contoLur can be measured aind calculated
In this Instalnce. (the surface area of Lake Denton in Fluiure 3-1 \\as found to be oi aces
I 'I I- .f f \ I, U, *ia i I'. a 1 I ;, "0 -
3 Lake Dentons suritace area imeasuirement of oO units \\Ias then i)lotted on the \-a\is
of the urapih in FI lre 3-2 Notice that It \\"as plotted to corresplond \\lith a I0i" \ lueon
the \ -ais e depth) In other \\ words. the surface area of the lake. at 0 units belo\\ the
suilace. is o" acres
,. h, ... I ,ll, h. IllI ,. l l %,. 0ll lll ll /,.' lll 1 ,. '1 / l .l /.. l I,. -1 l 1l l I,./ I .I I, / ,. 0 ,. 1 ,.
S / 11.f lh llh. i'1'll .,.. ll l l 0 11ll-1 /lPl '. hl. 0 1 ll -. .*I 1/1 %. lll \- i \ i
4 B\ caIlculatinl anid pilottinu sulAi-tace area measurements tfo the remaninu conItoul Iines III
Flure 3-1. \\ e \\ ere able to complete tihe Ii pisorap|ilhc curII e for Lake Denton as shlo\\ n
in Fluure 3-2 abo\ e
Why are hypsographic curves
* Hypsographic curves are used for predicting the
best time to implement various lake management
strategies such as aquatic plant management,
habitat restoration, muck removal activities, etc.
From a lake resident's standpoint, being able to
visualize and/or predict a lake's surface area
during high, medium, or low water levels can A-
certainly be helpful in planning the location for a .. ,-
new lakefront home or dock. ,.
c c s ae a u o Now you see it, now you don t. This boathouse and dock
SHypsographic curves are also useful for re high and dry after a prolongedperiod of drought. The
comparing lakes and explaining why some lakes structure of the lake basin, with a shallow sloping shoreline
are more susceptible to changes in lake surface and an average lake depth of only 2 meters has contributed
area while others of similar size (i.e., surface to the drastic loss of lake surface area.
area) may show very little change. For example, .
during dry weather conditions, property owners
on a shallow lake basin often see a dramatic
recession of water from what they considered to
be the original shoreline. In contrast, those living
on a deeper lake basin would probably notice very
little change in lake surface area when lake levels
If both the shallow and deep lakes happen
to be located near one another, it can be quite
confusing to the shallow lake property owners
who are trying to figure out why their lake has so
little water, while a neighboring lake seems unaf-
fected. This scenario occurs on a regular basis in
Florida and is cause for much concern to some
lake residents. c--
Most of the time, such discrepancies can
be explained simply by differences in lake Deeper lakes, with steep bottom slopes, are often less
on.,. ,.../by periods of low rainfall.
morphometry (i.e., size, depth and shape).
Both of these dynamics are particularly
Scientists use hypsographic curves i important because they have much to do with the
for predicting two lake dynamics in concentration of nutrients in a lake and a lake's
particular: ability to support life its biological productivity.
(1) a lake's ability to dilute incoming materials, For instance, it's been found that, in Florida
See SurfaceArea on pages 10-11 and Volume shallow lakes tend to be more productive than
on page 15for more about dilution capacity. deep lakes, meaning that shallow lakes often
have greater concentrations of nutrients and also
(2) the potential for lake water mixing. produce more fish and wildlife.
See Part 4 Wind, Waves and Water Mixing See Figure 3-3 on page 15 for examples of
on pages 20-27for more about this dynamic. hypsographic curves drawn from both a
shallow lake and a deep lake.
Volume knowledge, we can use several techniques to
is the total amount of water in a lake basin, and it
is usually expressed in units of acre-feet or cubic See Appendix B Measuring Lake Volume
meters depending on which measurement system on pages 31-32 for details.
being used (i.e., English or Metric).
+ If you happen to know the mean depth of a
Volume is often represented in scientific lake and also its surface area, volume can be
literature with the symbol V. found by multiplying the two:
Sv d a ai f o a Volume (V) = mean depth (Z) X surface area (A)
Lake volume data are available for only a
limited number of lakes in Florida. As a whole,
Florida lakes tend to have less volume than See surface area on pages 10-11 and mean depth
deeper lakes in the northern United States. It is on page 16.
also important to remember that lake volume can
fluctuate dramatically depending on rainfall. ori L.W.C s h r
Lake volume is an important consideration i m rth 0la i
to lake management, as it can influence a
lake's dilution capacity. c e t
As mentioned earlier in the Surface Area
segment on pages 10-11, the ability of a lake to
dilute materials, whether they be naturally occur-
ring from the watershed or from human activity,
is known as a lake's dilution capacity. Lakes x A
with larger volumes of water have a greater ability
to dilute materials coming into the lake basin. O Full lake surface
* Dilution capacity is an important consideration 0
when applying some herbicides to algae or aquatic 2 lake bottom
plants in a lake. Extra care must be taken to apply 21-
the correct concentration of chemical based on the 2- 4 lake volume
lake's volume as herbicides are absorbed by plants depth
from the water column. If concentrations are too
weak, they would be less effective and if the deep lake
dosage is too strong, the herbicide application
would cost more than it should. (Herbicide Lakerea
treatment for submersed aquatic weeds can cost as Full akesurface 50 100
much as $300 $400 per acre.) 3
* Scientists also consider lake volume when 2 lakeovolume
estimating nutrient loads or flushing rates, as max...
0 depth lake bottom
both can impact algal populations in a lake.
There are several ways to measure
the volume of a lake. shall
* Hypsographic curves are used to determine
lake volume. In the graphs in Figure 3-3, the area Figure 3-3
between the x-axis, the y-axis and the curve itself Hypsographic curves can be used to make comparisons
between deep lakes and shallow lakes.
is proportional to the lake's volume. Based on this
Maximum length Maximum width is also important to consider
when determining the potential for waves to mix
is the distance, in a straight line, between the two
Sp o a e water and/or sediments at the bottom of a lake.
farthest points on a lake. The distance must be
measured without intersecting a land mass. Mean depth -
See Figure 3-5 on page 19.
See Figure 3-5 onpage 19. is the average water depth of a lake.
Maximum length is often represented in scientific Mean depth is often representedin scientific
literature by the symbol L
litrmax literature by the symbol Z or Z.
Maximum length is important because it can Pro io p
influence the depth at which waves can mix
water andlor bottom sediments in a lake. dividing the volume of a lake by its surface area.
For example, if a lake should happen to However, if lake volume and surface area measure-
have a sizable maximum length, with no landform ments are not available, one can simply collect
to disrupt the wind, waves have the potential to grow numerous lake depth measurements and then
quite large under windy conditions, influencing average them. This technique can be useful for
boating safety and shoreline erosion. smaller waterbodies though it will be less accurate
In contrast, when a lake has a small maximum than the first method described.
length, waves are prevented from becoming very
large and lake water mixing is reduced. SeeAppendixA Measuring Lake Surface
As a general rule, the larger the maximum Area on pages 29-30 andAppendix B
length, the larger the waves, and the greater potential Measuring Lake Volume on pages 31-32.
there is for mixing or disruption of bottom sedi-
ments. Of course, there are exceptions. For instance, Mean depth is important because early
Mean depth is important because early
deep lakes are less likely to experience disruption of studies of algae, aquatic invertebrates,
bottom sediments and oxbow lakes are usually and fish populations have shown that
less affected by wind-induced wave action due to shallow lakes are generally more productive
their narrow shape. A lake's orientation to prevailing than deep lakes.
winds is another consideration. Mean depth also has much to do with the
potential for waves to disrupt bottom sediments.
For more on waves and their potential to mix or For example, lakes with greater mean depths
disrupt bottom sediments, see Part 4 Wind, Waves mi
usually don't experience as much mixing of
and Water Mixing in Lakes on page 20.
bottom sediments, as wave action is less likely
Mean width to reach the bottom.
is calculated by dividing a lake's surface area by Maximum depth -
its maximum length. This measurement is also
used to predict the amount of water mixing that is the latest depth of a lake. The location of a
can occur in a lake during strong winds. lake's maximum depth is sometimes (but not
See definition of maximum length. always) indicated in bathymetric maps with an
"X." See Figure 3-5 on page 19.
is the maximum distance between the two widest Maximum depth is often represented in scientific
points of a lake, that can be measured without literature by the symbol zma
crossing land, and at a right angle to the maximum
length. In other words, the lake's maximum Maximum depth cannot be estimated, and can
width must be measured at a 900 angle to the only be obtained by locating and actually measuring
lake's "axis." See Figure 3-5 onpage 19. the deepest point in a lake.
Maximum depth is important because it
can influence the movement of fine organic
sediments found on the bottom of a lake.
For example, water currents or waves can
move sediments along the bottom of a lake or
resuspend them into the water column. If a lake
has deep areas or holes, the sediments usually find
their way into these areas first. However, if the
holes were to eventually become filled-in with
sediments, there is no place for new and/or
remaining sediments to go except back and forth
across the lake bottom or into the water column. A Lake Mystery
1 t-I L.4 iwl- r'%w_T r 1' ] Iln thie. I Lt, ,s. annn Floruida Ialkcliont
onIIOIIcs \\%:crIc Nibtu ol sand iOlllinds that
\c\: cI icdt: cd flolll tllc nIc'al-slolo \\atc is
Shoreline lof Ihk, This act, ,t, c icntad mann, dccpl
is the area where a body of water meets the land. holes in the lalcsIn In sonic \\natcirbodics.
On a bathymetric map, it is represented by the thlc holes consltlitutcd the laIk' s ni a\;iniLin
outermost contour line of the map. See Figure 2-1 dpcthl ()\ cr the Il c'ars. thl\ ha' c actedI as
on page 7. ilcpositoics fo0 finl s.'Illniiits Forti \L ';a
Iatci. Icsldcllltsl on soni' lalcS bcan nIoticnlll,
A lake's shoreline is important because it that th u \\ hitc sand\ beaches had suicknl\
defines the area where a waterbody i ii of i
interfaces with the land. t Ho ti
In Florida, this area of interface can change In oi i it lla
Illn SOI11.' Ill~lnll.ic'S. It Iian\ SllIIip Lxcan
considerably depending on rainfall and lake levels. casc of thc dcc) holcs ha fin II lcil I). anlI
Lakefront property owners also need to be aware no that th I s no piac for t sldimcns to
of the fact that, in Florida, land below the high water ,,. the\ arc b carried alon the lak
mark typically belongs to the state. This is important bottom. all the \\ a to the shorilin and
for planning various activities such as aquatic beach A solution"
plant removal, muck removal, or dock construction, \\h1l drcd' i in is not all\ s a popular
as some of these activities require permits. acti\ it. it could bc hclptil in this instalnc
Changes in a lake shoreline can also be signifi-
cant to aquatic plant management. For instance, at
high water levels, and depending on the slope of the Shoreline length -
land, a lake may have small amounts of aquatic is the linear measurement of a waterbody's entire
vegetation along its shoreline. However, if water perimeter, at a given water level. In Florida,
levels were to fall, the reduction in water depth along shoreline lengths fluctuate considerably, depending
the lake's shoreline on rainfall and lake levels.
could result in a dramatic Shoreline length is important because it
-... increase in aquatic plant provides a measurement of the actual
growth. Why? amount of interface between a waterbody
Because when the and the surrounding land.
water becomes shallow, There are several approaches one can use to
sunlight may be able to measure shoreline length:
reach larger areas of Using an aerial or topographic map of the
the lake botom, provid- waterbody, trace around the image of the lake
ing the necessary energy
ig te essay with a cartometer (a mapmaker's device that
for plants to grow.
measures distances as drawn on a map). If you
don't have a cartometer, trace around the water-
body using a piece of string. Compare the
cartometer measurement or the length of string
with the map's scale to convert the measurement
to the actual shoreline length.
* If the lake has a perimeter road that is in close
proximity to the lakeshore, it might be possible
to drive the perimeter in a car and measure the
distance with an odometer, and then estimate
shoreline length using that distance.
S Using electronic navigation instrumentation,
such as a GPS," measure the distance while
traveling close to the shoreline in a boat.
* If the waterbody is small enough, the distance
can be manually paced off by walking the perimeter.
(One average stride is generally equal to about
Shoreline development the string used to trace Lake B would be longer
refers to the length of a lake's shoreline relative to than that of Lake A.
a circle of the same area. In other words, lakes Determining a lake's shoreline development
with longer, irregularly shaped shorelines are is important because it helps us assess the
considered to have more shoreline development, amount of potential wildlife habitat
while circular lakes are considered to have less available for a lake.
shoreline development. See the following explanation: For example, if Lake B has a greater amount
of shoreline development, there is more of an
Shoieline dev ehsmwt /s ,,nd 1 B"''"N ,t. ,? interface between the water and surrounding
hl mIri /I n I/t.e ,shmb d SLD land (i.e., coves and peninsulas). This interface
often translates into more habitat for fish, birds,
and other wildlife to raise their young.
How does one determine a lake's shoreline
The mathematical equation provided below
can be used to calculate the shoreline development
of a lake. The higher the number, the greater the
Lake A Lake B shoreline development.
Consider Lakes A and B above. Both lakes Note: a lake in the shape ofa perfect circle will always
have the same surface area. Notice how Lake B have a shoreline development value ofl.
on the right has an irregularly shaped shoreline
and Lake A on the left is more circular in shape.
If you were to trace the entire perimeter of each
lake with a piece of string, you would find that shoreline
11 GPS is an acronym for Global i- ..r. ,," System development A
a, ,ni ,o,. -,, device that utilizes satellite technology for
determining one geographic location. L = Shoreline l, glth A = surface area of the lake
is the distance that wind can travel over water Lake Broo
before intersecting a landmass. We can use fetch N
distances to predict the depth at which wave energy
extends below the water's surface.
These predictions are made based on the
relationship between wind velocity and the
amount of fetch distance that a lake may have.
Fetch is also an important consideration Fetch distance
when boating, as wind exerts the greatest amount for a sou thwest
wind on Lake Brooklyn
of energy when there is no landmass in the way scale 5,000 ft
to "break" its effect. The greater the fetch distance,
the greater potential there is for large waves Figure 3-4 Using the map's scale, fetch distances can
and increasingly dangerous boating conditions. be calculated from any direction. Both the depth contour
lines and the scale in this map are recorded in feet.
11/ I/ %Id I/' I 't / I//// /h l
ilic/,'ic//'J Ii/i scale = 5000 ft
ow that we've been introduced to some downwind shore, so that the water level is actually
of the basic concepts and terminology higher on one side of the lake than another -
related to lake morphometry, we can turn usually by a fraction of an inch, but sometimes
our attention toward a dynamic that is extremely much more in a large lake.
important to lake management and yet often An extreme example of this dynamic was
overlooked: wind mixing (a.k.a. water mixing) in seen in 1928 when hurricane winds piled water
lakes. upon Lake Okeechobee's northern shore, causing
In the following pages, we'll explore the the city of Lake Okeechobee to flood. As water
influence that wind and waves can have on the piled up along the north shore of the lake, waters
movement and/or mixing of water within a lake, receded substantially along the southern shore-
as well as the role that lake morphometry plays. line. Hours later, when the hurricane force winds
changed direction, lake waters then returned to
Wind the southern shore and caused massive flooding
As any boater soon learns while navigating there as well, resulting in hundreds of deaths.
across open water, there is a strong correlation This single event prompted the call for the
between wind and waves (i.e., the stronger the construction of what is now known as the
wind, the larger the waves). However, aside Hoover Dike.
from wave activity on the surface, there are other Under normal conditions however, the
types of water movement occurring below the difference in water elevation across a lake is
waves that we never see. For instance, underwater minimal -just enough to generate water currents.
currents tend to move water particles horizontally These currents move water back to the other side
through the water column. At the same time, of the lake to even out the elevation difference.
water particles are also being distributed in an Sometimes the currents flow along the shore,
irregular swirling motion known as turbulence. but often the water flows as a return current
It works like this: below the surface of the lake. Thus the wind may
Currents be moving a surface layer of water in one direc-
tion and the return current moves a layer of water
Winds blowing across the surface of a lake
in the opposite direction. (Anglers sometimes
interact with lake water and cause the water toe di e
S. notice that the direction of the current changes as
move in a downward direction. The resulting
m ,, they lower their baited hook down through the
water currents can move across the lake with the
wind, or they can move along the shore when the water column.)
winds approach the shoreline at an angle. Turbulence
If the wind blows from one direction for a In most lake currents, the water does not flow
while, it can cause the water to pile up along the smoothly, but rather tends to move in a more
chaotic, irregular manner known as turbulent flow. one wave crest to be replaced by another (i.e.,
It is like the motion of smoke as it comes out of a large, the wave period). The radius of these orbits gets
industrial smokestack. While the dominant movement smaller as they move downward in the water
might be in one direction, the particles within the flow column and become negligible at a depth of
are moving in a series of swirls of different sizes. about one-half the wavelength.
The importance of turbulent flow is that it There are other factors that come into play
results in the mixing of the water mass. Among as well. For example, the longer the fetch distance,
other things, turbulent flow keeps plankton (i.e., the greater the wavelengths and wave heights
algae and zooplankton) in suspension, it moves will be. Likewise, the greater the wind speed, the
dissolved oxygen from the surface of the lake to greater the wavelengths and wave heights.
deeper waters, it evens out water temperatures in
the upper part of the lake, and distributes See Fetch on page 19, Figure 4-1 Anatomy
dissolved substances like plant nutrients of a Wave below, and Table 4-1 on page 25.
throughout the lake.
Waves As waves get larger, they are able to exert
While water currents tend to move particles in energy to greater depths, resulting in significant
one direction, simple surface waves are rhythmic water movements. As a rule-of-thumb, if the
movements of water particles that in theory end water depth is less than one-half the wavelength,
up in the same place that they start. These water waves can potentially have a scouring effect on a
particles rotate at the surface in circular orbits, lake's bottom sediments. Lake scientists often
completing one circle in the time that it takes for refer to this scouring effect as resuspension.
L wave crest
L = wavelength
h = iave neighl
Figure 4-1 Aside from the energy that waves exert on the surface of a lake, there is also a
substantial amount of energy exerted below the surface. Notice that the radius of the downward orbits
is reduced as one moves downward in the water column. At a depth of about one-half the wavelength
distance, the orbits become negligible. A rule-of-thumb: if the water depth is less than one-half the
wavelength, the waves have the potential to disturb and resuspend bottom sediments.
Based on the dynamics just described, it is Water temperature changes
possible to use standard engineering equations to In shallow lakes, water movements can keep
calculate the sizes of surface waves for various the temperature uniform from the surface of the lake
combinations of wind speed and fetch. Once the to the bottom. In deep lakes, warm water will float
size of a wave is known, one can then use the on top of the cold water isolating deeper waters
one-half wavelength rule mentioned earlier to from the atmosphere its major source of oxygen
determine the depths at which waves can be expected (i.e., stratification). This can have a detrimental affect
to disturb or resuspend fine bottom sediments in a on fish by reducing the availability of oxygen,
lake. We have provided this information for quick particularly after a sudden thunderstorm.
reference in the Table 4-1 on page 25. Nut t tt w n te
Nutrient transport within the water
Note: The possibility for resuspension also depends column
upon the characteristics of the sediments and the Nutrients are distributed vertically by turbu-
roughness of the bottom. For instance, heavy particles lence in much the same way that oxygen and
like sand are less likely to be resuspended than are
S, water temperatures are mixed. This can facilitate
smaller particles like silts and clays.
the recycling of nutrients from the sediments and
Water Mixing deeper waters and, in some instances, result in an
increase in biological productivity.
Scientists are particularly interested in the
energy that waves set in motion below the surface Disruption of bottom sediments
because they know that this type of water movement Several studies have shown that the
or water mixing has the potential to influence resuspension of sediments by wind-driven waves
one or more of the following processes: can play a significant role in affecting water
Oxygen in the water column quality in large shallow lakes particularly in
Turbulent water movements that are generated by Florida where shallow lakes are abundant.
waves assist the movement of oxygen from the air into Water quality problems caused by
the water. In fact, this is one of the main sources of sediment resuspension
oxygen in lakes. As mentioned earlier, it can help to Resuspended sediments increase the turbidity
move oxygen from surface waters to deeper waters. of the water and reduce light penetration. This
It should be mentioned however, that even reduces the depth at which algae and aquatic
with the help of turbulent water movements distrib- plants can grow in a lake.
uting oxygen from the air, there are times when the
respiration of organisms within a lake (i.e., bacteria, Nutrients stored in bottom sediments are often
macrophytes, and animals) can consume so much introduced back into the water column resulting
oxygen that a fish kill can occur. This is common in an increase in the growth of algae. This may
after several consecutive calm cloudy days when or may not be desirable, depending on the
the loss of sunlight prevents algae and macrophytes intended use of the lake.
from making their usual contribution of oxygen to In some shallow lakes, there is a layer of algae
the water column, via photosynthesis. that grows on the surface of the sediments. These
algae are resuspended along with the sediments
during strong wind events and can result in significant
increases in the amount of algae in the lake water.
+ The resuspension process, along with the
effects of the waves themselves, can form a layer
of fluid-like sediments on the lake bed that is too
unstable to allow for the rooting of aquatic plants.
This can prevent the reestablishment of aquatic
plants in a lake that previously had plants.
lake morphometry and
As \\-e've discussed before. x\ind-
driien w\av es often can cause enough A
turbulence in shallow waters to resuspend
fine sediments Some of these particles
w\ill be suspended in the water colLumnI
and can move about the lake with water
curnents Eventually. they w\ill settle back
to the lakebed when the water becomes A0 ion tI /tiarl et there it lt et.ier wirg tu leairt abut
calm If they settle in a shallow area with /iAk' mu Tp h me- -
exposure to the \\-ind., they \\ill be resuLspended again at some time in the fuIture
On the other hand. particles settling in deep areas of the lake may be protected from
resuspension and \\ill remain undisturbed In such a lake., the fine particles may go through
several cycles of settlement and resuspension but in time they \\ill end tip being trapped in
the deep holes As a result, shallower exposed areas \\ill tend to have sediments dominated by
larger particles such as sands Deep areas will contain fine particles like silts, clays, and
fragments of dead plants and animals
In a shallow lake, there may not be a place that can remain undisturbed by water
motions developed by surface \\-aves As a result, fine sediments can cover the entire lakebed
and sediment resuspension may be a frequent event
r Aquatic plants and bottom sediments
I alge beds ol aquatic plants can alter sedillentation
patterns iin a lake in se\ eral \\ays
The plants themsel\ es greatly reduce the amounLIIlt 01
tllurbulence \\ilhin Ithe plant beds. resulting in an accumL u-
lation of line particles in shallo\\ areas that are dominated
by plaits ThIis caln halpei C\ in lihougli there imay be
_deep areas \\itllill the lake
I*')lant beds ca1 intellere \\Iill the de\eloplment ol
S .,. X \\a\es in a lake Thus. shallow \ lakes filled \\itlh plants
I"'IIa Ino de\ elop large \\a\ es anid the fine sediments \\ill
be protected lronm resuspelnsion Sucli plant-dominated lakes
Ste(nd to appear clear dtie to a lack of turbullenice that would d
.. otlleri\\ise keep line particles and algae in stuspensioln
The effect that plants can have on a lake is demonstrated effectively when, for one
reason or another, a plant-dominated lake loses its aquatic plants This might happen when
plants are removed on purpose with the use of grass carp or herbicides or when they are lost
due to increased water levels or ripped up by hutrricane-force winds If any of these events
should occur, the tiusal effect is for the water to become more turbid as w\ind-driven \waves are
able to resuspend sediments and algae are able to grow due to lack of competition from large
plants and associated algae
How can we estimate if the bottom 2% of the time.
lake bottom sediments are We could then make the same calculations
for winds from a north, south, and west direction.
subject to resuspension? By adding all of these individual percentages
together, we can obtain the per cent of time that
SIf we want to know if a particular spot in we would expect winds to disturb the sediments
a lake is subject to resuspension, we can at that point on the lake.
use the information provided in Table 4-1.
First, we would use a map to find the fetches We can compare lakes for their extent of
in all directions and then find the maximum fetch. Wave disturbance by looking at several
Suppose the fetch was 10,000 feet in the north points within a lake.
direction. If we look to see the effect of a 10 mile For example, as shown in Figure 4-2 on
per hour (mph) wind from the north for that page 26, we made the above calculations for
fetch in Table 4-1, we see that we might expect several points within two Florida lakes.
mixing to a depth of 6.0 feet. In other words, if However, instead of using a table like the
the water depth at that point were 6 feet or less one in Table 4-1 we used engineering equations
and the sediments were of a fine consistency, we directly to find the minimum velocities. Also,
might expect some of the sediments to be resus- instead of using the four basic wind directions
pended. If the wind were say 25 mph, the table mentioned earlier, we used 36 different wind
shows a mixing depth of about 16 feet. directions to calculate the per cent of time that
Note: These calculations assume that there we would expect lake sediments to be disturbed
are no beds of aquatic plants along the fetch that by wind-driven waves.
might reduce the buildup of waves. The lakes used for our comparison in Figure
4-2 were chosen because they are so different
See Fetch on page 19, Table 4-1 on page 25 and from one another: one lake being large and
Figure 4-1Anatomy of a Wave on page 21. shallow (Lake Istokpoga) and the other relatively
smaller and deeper (Lake Thonotosassa). Notice
that in Lake Istokpoga every single point on the
2 If we are interested in knowing how lakebed is frequently disturbed by the waves (i.e.,
often sediments might be disturbed at every number is greater than 0). In contrast, Lake
one particular spot in a lake, we could start Thonotosassa has a large area with no sediment
by finding the fetches for the four major
by finding the fetches for the four major disturbance (i.e., numbers are 0 or less than 1).
If we know the depth of the water for that Another way to make comparisons
one location, we can estimate the minimum 4 between lakes is to summarize the
wind velocity from each direction that could calculated percent for all points in a lake.
cause sediments to be disturbed. For example, For instance, we could determine what percent
suppose our location on the lake was six feet of the points or lake locations were disturbed 90%
deep and the east fetch was 8000 feet. From of the time or more, followed by the percent of
Table 4-1 we can see that a wind between 10 points that were disturbed 80% of the time or
and 12 mph (11 mph would be pretty close) more, and so on down to the percent of points
would produce waves sufficient to disturb the disturbed 0% of the time or more.
bottom at 6 feet. These numbers can then be plotted on a graph,
Our next step would be to use wind records with percent of time on the horizontal axis and
from a nearby recording station (i.e., an airport) percent of the lake area on the vertical axis as we
to determine how often we could expect an did in Figure 4-3 on page 27.
easterly wind to exceed 11 mph. For example, if We can use the same approach to make
an east wind blew 11 mph or more for 2% of the comparisons of different water levels within an
time, then we would expect east winds to disturb individual lake. For instance, the graph shown
Depth of wave mixing in feet for various fetch distances and wind velocities
Wind Velocity in Miles Per Hour
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
2,000 0.4 1.1 1.7 2.3 2.9 3.5 4.2 4.8 5.4 6.0 6.6 7.3 7.9 8.5 9.1
4,000 0.5 1.4 2.3 3.1 4.0 4.9 5.7 6.6 7.5 8.4 9.2 10.1 11.0 11.8 12.7
6,000 0.6 1.6 2.7 3.7 4.8 5.9 6.9 8.0 9.0 10.1 11.2 12.2 13.3 14.3 15.4
8,000 0.6 1.8 3.0 4.2 5.5 6.7 7.9 9.1 10.3 11.6 12.8 14.0 15.2 16.4 17.7
0 10,000 0.6 1.9 3.3 4.6 6.0 7.4 8.7 10.1 11.4 12.8 14.2 15.5 16.9 18.3 19.6
.5 12,000 0.5 2.0 3.5 5.0 6.5 8.0 9.5 11.0 12.4 13.9 15.4 16.9 18.4 19.9 21.4
S 14,000 0.5 2.1 3.7 5.3 6.9 8.5 10.1 11.7 13.4 15.0 16.6 18.2 19.8 21.4 23.0
w 16,000 0.5 2.2 3.9 5.6 7.3 9.1 10.8 12.5 14.2 15.9 17.6 19.3 21.0 22.7 24.5
S 18,000 0.5 2.3 4.1 5.9 7.7 9.5 11.3 13.2 15.0 16.8 18.6 20.4 22.2 24.0 25.8
.C 20,000 0.4 2.3 4.2 6.2 8.1 10.0 11.9 13.8 15.7 17.6 19.5 21.4 23.3 25.2 27.1
S 22,000 0.4 2.4 4.4 6.4 8.4 10.4 12.4 14.4 16.4 18.4 20.4 22.4 24.4 26.4 28.4
24,000 0.4 2.4 4.5 6.6 8.7 10.8 12.9 15.0 17.0 19.1 21.2 23.3 25.4 27.5 29.6
26,000 0.3 2.5 4.7 6.8 9.0 11.2 13.3 15.5 17.7 19.8 22.0 24.2 26.4 28.5 30.7
28,000 0.3 2.5 4.8 7.0 9.3 11.5 13.8 16.0 18.3 20.5 22.8 25.0 27.3 29.5 31.8
30,000 0.2 2.6 4.9 7.2 9.6 11.9 14.2 16.5 18.9 21.2 23.5 25.8 28.2 30.5 32.8
Using the table above, we can quickly determine the depth at which a wave's energy is felt below the surface
based on wind speed and fetch. The table was assembled using the one-half wavelength rule in combination with
standard engineering equations to calculate the sizes of surface waves for numerous combinations of wind
speed and fetch.
Note: When evaluating the fetch distances shown here, remember that 1 mile is equivalent to 5,280feet.
percentages for Lake Istokpoga
two Florida lakes 1.2
Water depths shown
3.0 here are in meters.
1km 10 km 6
86 85 84
96 73 73 72
89 61 62 60 59 54
95 87 90 80 66 59 51 48 43 45
3 3 3 3 56 58 59 61 65 67 69 69 61 54 29 46 86
<1 0<1<1 <1 56 46 38 40 42 55 64 72 72 57 39 36 49 89
<1 0 0 <1 <1 285 51 49 41 33 44 58 67 74 73 53 46 44 52 88
<1 0 0 0<1 <1 1 6 52
<1 0 0 0<1 1 652 68 46 51 42 44 46 61 69 62 62 62 47 45 54 60 95
13 0 0 0<1<1 1 269 96 56 65 69 72 73 74 75 50 50 63 48 46 56 63 86
82 <1 0 0 0 0< c <1 1 34
77 47 96 94 73 75 76 31 41 76 75 60 64 66 76 96
6 0 0 0 0 0<1 <1 <1 3 70 73 73 70 66 77 80 82 75 72 62 64 58 84
<1 0 0 0 0<1 <1 <1 3 13 69 71 69 64 72 83 82 83 87 96 7053 55 54 94
<1 0 0 0 0 <<1 <1 28 69 65 62 53 65 83 80 82 88 94 62 56 55 79
<1 0 0 0 0<1 <1 <1 59 50 40 65 77 77 77 75 94 62 56 56 80
31 0 0 0 0 0 <1 <1 <1 69 48 64 76 76 76 775 94 94 68 62 56 95
33 0 0 0 0<1 <1 <1 <1 44 61 74 76 76 76 81 81 73 68 66 54 53
33 0 0 0<1 <1 <1 <1 7 26 72 74 82 88 81 75 74 58 55 51 52 50
32 0 0 0 0<1 <1 <1 <1 3 61 18 85 74 87 87 87 86 72 56 52 55 57
29 <1 0 0 0<1 <1 <1 <1 3 6 79 24 94 67 86 73 72 69 66 62
1 0c<1 <1 <1 <1 <1 <1 3 6 91 24 84 66 73 85 71 54 50 46
<1 <1 <1 <1 1 1 6 11 63 53 83 71 57 55 68 50 58
26 27 13 6 6 21 36 29 68 69 54 30 62
58 26 38 51 50 47 52
33 35 36 45 53
54 56 41 52
Figure 4-2 The numbers shown here, i/ihi/i the lake shapes in the bottom portion of the figure,
reflect the percentage of time that the lake's bottom sediments are disturbed by wind-driven waves
at each of the individual locations. Notice that in Lake Istokpoga (on the right) all the points on the
lakebed are frequently disturbed by the waves, while Lake Thonotosassa, a smaller and deeper lake,
has a large area i ith no sediment disturbance.
5 There is a shortcut method for estimating
S120 the impact of wave mixing on a whole lake.
First, we divide the square root of the lake
S' -- area in units of square kilometers, by lake mean
S\ depth in meters. The resulting number is called a
M 80 \
S\ dynamic ratio and was originally developed by a
So _ae Ispo lake scientist named Lars Hikanson for a
U\ different purpose. However, a recent study of
S40 Florida lakes found a relationship between the
\ dynamic ratio and the percent of the lakebed that
o 20 Lake Thonotosassa can be disturbed by waves.2
S0 2 Lakes with dynamic ratios above 0.8 were
L 0 20 40 .0 80 100 subject to wave disturbance at all areas of the
Percent of time lakebed at least some of the time.
Note: This calculation is made on the assumption
that there were not i.', iifi,, it amounts of aquatic
Figure 4-3 A comparison oflakebed disturbance for all plants in the lake. Ifplants are present in large
points in a lake. In this example, we've shown the lakebed quantities, the wind disturbance could be substantially
disturbance on two Florida lakes that are quite different in less than this calculation would indicate.
both size and mean depth. Notice the dilerevnce between the Lakes with dynamic ratio values below 0.8
lines for the two lakes. Lake Istokpoga is a rather large showed a linear decrease in areas disturbed at
shallow lake and Lake Thonotosassa is smaller and deeper
one time or another. If for example the ratio were
0.4, only about 50% of the lakebed would be
here in Figure 4-4 shows the calculated effects disturbed at one time or another.
that changes in water level might have on Lake
Apopka in central Florida. 12 Roger W. Bachmann, Mark V Hoyer and Daniel E.
Notice that changes in lake level are indicated Canfield, Jr 2000. The potential for wave disturbance in
by the numbers in the center of the graph. The shallow Florida lakes. Lake and Reservoir Management
"0" curve represents Lake Apopka at its mean
lake level and curves to the left of it represent
lake level increases in 1-foot increments. Curves to Mixing frequency curves for
the right of the "0" curve represent lake level Lake Apopka with elevation
decreases in one-foot increments. changes in feet
The graph clearly illustrates that lowered 0 s0
lake levels can increase the extent of the lakebed
..2 so -3
that is susceptible to wave action, while in- -2
creases in water levels can prevent waves from N o
reaching the lakebed in areas where they could 1
under normal lake levels. This is a good example M 40o
of how important water level changes can be in ,
shallow lakes. 20
0 t0 40 40 s0 100
Percent of time
o w e dit ime o Figure 4-4 This graph illustrates how changes in water
r s r 6 .t 00. level might o n. t Lake Apopka in Central Florida. The plus
signs indicate increases in water depth and minus signs
indicate decreases in water depth.
Bachmann, R.W., M.V Hoyer and D.E. Canfield, Jr. 2000. The potential for wave disturbance in
shallow Florida lakes. Lake and Reservoir Management. 16 (4):281-291.
Goldman, Charles R. and Alexander J. Home. 1983. Limnology. McGraw-Hill, Inc., New York.
Htkanson, Lars. 1981. A Manual of Lake Morphometry. Springer-Verlang, Berlin, Heidelberg, New York.
Hutchinson, Evelyn G. 1957. A Treatise on Limnology. Volume I Geography, Physics, and Chemistry.
Chapman and Hall, New York.
Lind, Owen T. 1979. Handbook of Common Methods in Limnology, second edition. The C.V Mosby
Company. St. Louis, Toronto, London.
Schiffer, Donna M. 1998. Hydrology of Central Florida Lakes A Primer: U.S. Geological Survey
Welch, Paul S. 1948. Limnological Methods. Country Life Press. Garden City, New York.
Wetzel, Robert G. and Gene E. Likens. 1979. Limnological Analyses., second edition.
Springer-Verlang, Berlin, Heidelberg, New York.
White, William A. 1970. The Geomorphology of the Florida Peninsula. Geological Bulletin No. 51.
Bureau of Geology. Florida Department of Natural Resources. pp 102-104. Tallahassee.
Lake surface area can be measured Digital tablets or computer scanners can
with a bathymetric map using any of also be used to trace or scan a bathymetric
the following techniques: map image. Once the image is digitally memorized
(i.e., traced or scanned), computer mapping
SOne of the most accurate methods is to use software can be used to calculate surface area.
a planimeter to trace the shoreline contour
of a lake. This hand-held instrument is designed 3 Another method involves placing a grid
for measuring the area of a shape as drawn on a pattern over a lake map and counting the
two-dimensional plane. squares (of a known dimension) from the grid,
Using the tracer point of a planimeter, you to determine lake surface area.
can carefully follow the outermost contour of a Step 1: Trace the lake map on a piece of graph
bathymetric map. The planimeter calculates the paper or draw a square grid on top of a copy of
area of the shape inplanimeter units (PU) while the map, as illustrated in Figure A-i on page 30.
tracing its outline. Once you have the area in
nitit with the Step 2: Count all the squares that fall within
planimeter units you can compare it with the .
animer unit y cn c re the shoreline of the lake. At the shoreline, count
scale of the map to convert the PU to the lake's
actual surface area. only those squares that are more than half
actual surface area. .
inside the lake shoreline area. Do not count
Note: For detailed information about how to do this, squares that are more than one-half outside the
refer to a limnology methods manual. lake boundary.
A planimeter can be used to trace the shoreline contour of a lake, in planimeter units.
Step 3: Using the map scale, determine the area
represented by one square. For example, suppose
the map scale shows that 1 inch represents 1000
feet and the squares of the grid are one-half inch
on a side. Using this information, we can see that 5
each square represents a measurement of 500 3.0
feet per side [ 0.5 x 1000 = 500 ft.]. Therefore, 4
the area of one square would equal 250,000
square feet [ 500 x 500 = 250,000 sq ft ].
Step 4: The area of the lake, in square feet,
would be equal to the number of squares counted i 4.
from the grid (N) X 250,000. To convert the area
from square feet to acres, divide by 43,560.*
N X 250,000 = lake surface area _3.
43,560 in acres 1.
O 43,560 is the conversion factor for conv7'rting
square feet to acres. 1000 ft
4 There is another way to calculate surface
area that is relatively simple, but it does Step 2: Using the same piece of paper that you
require a weight scale that is sensitive enough to cut the lake shape from, but
weigh a piece of paper. from an area outside of the
lake shape cutout, measure
Step 1: Lightly trace an outline / and c ut a square of
of the lake (from a bathymetric non din sand sqar
map or satellite map, for instance) 1
.. .. ,, \' w eigh that too. -
onto a heavy grade of paper such as weigh tha too
construction paper. Cut out your Note: The square cutout should be similar in size to
newly drawn lake shape and weigh it. the lake shape cutout. For this example, we 'll use a
3-inch square cutout.
Note: The lake shape example shown .
Step 3: Once weights are obtained for the lake
here is much smaller than you should use. Your cutout Step 3: Once wei are obtained for the lake
should be closer in size to the lake map in Fig,, A-1 shape and the square cutout, use the equation
(or larger) for obtaining a weight on most laboratory below to find the area of the lake in square feet.
scales. Note: To convert the area from square feet to acres,
divide by 43,560.
Find the AREA of the square paper cutout (to scale). Multiply by the WEIGHT AREA
For instance, if the map's scale equates 1 inch with 1000 feet, then one of the lake shape of a lake,
side of the 3-inch cutout square represents 3000 feet. Consequently, that 3-inch paper cutout. as drawn
square piece of paper represents 9,000,000 sq ft of surface area. (See below.) If the lake shape on a map.
3,000 ft X 3,000 ft = 9,000,000 sq ft cutout weighs 0.35 oz, Using the
X then you would numbers
Divide the numberfrom above (i.e., 9,000,000 sq ft) by the multiply 0.35 with the provided here,
WEIGHT of that same 3-inch square piece of paper. number from the we found the
If the weight of the square piece of paper was 0.25 oz, then your bottommost portion of the area to be:
answer for this part of the equation would be 36,000,000. (See below.) equation at left:
0.35 oz X 36,000,000 sq fto 12,600,000
9,000,000 sq ft + 0.25 oz = 36,000,000 sq ft/oz 0 oz 3000000 s oz square feet
There are several ways to calculate x-Ax
and/or estimate the volume of a lake: Lake Area
The simplest way involves using basic 0 I 0 lake surface -50 10--
algebraic equations for determining 0S ---
volume. To do this, one has to have the ., g---- -
approximate dimensions of the waterbody --
such as average depth, length and width. -
Note: This method is used as a quick way to deter- 24--
mine volume for ponds or smaller lakes and is 27 -
generally less accurate than thefollowing methods. 30-
\ ..- iQuii Itll' Ifo. Eitnuingi thle I 'oltnl Step 2: The lake volume represented by each
o. a Luak in .4cre-Feelt 1' ivn 32 square can then be found, and the total of all of
these volumes will give us the lake's volume. To
do this, multiply the area represented by one square
For lake basins that are almost conical in along the x-axis times the depth represented by one
shape and structure (i.e., some solution square on the y-axis. This product is then multiplied
lakes), a rough estimate can be made using the by the number of squares counted within the
same equation used for determining the hypsographic curve.
volume of a cone:
Volume Bathymetric maps can be used to determine
Vol (V) 1.047 rh lake volume and it's done like this:
r is the radius of the top (surface) of the cone (lake) Step 1: Using the same technique of counting
h is the height (maximum depth) of the cone (lake) squares described in Appendix APart 3 (see Figure
A-i), place a grid of small squares on a bathymetric
Hypsographic curves can also be used. map of your lake and calculate the area found
As you can see in Figure B-l, volume is within the various individual contour lines.
proportional to the area between the x-axis, Note: Using Figure A-], you should have four separate
the y-axis and the curve itself. Based on this area measurements: one for the surface area of the entire
knowledge, we can determine the lake's lake, one for the area within the 1.5 contour line, one for
volume using the following method: the 3.0 contour line and onefor the 4.5 line.)
Step 2: The next step is to calculate the volume
Step 1: First, draw the hypsographic curve of water layer by layer, starting with the top layer.
onto lined graph paper, as shown in Figure B-1. (See Figure B-1.) You can do this by finding the area at
Then count the number of squares found
between the x-axis, the y-axis and the curve
itself. Note: squares that are more than half-way lume
inside this area are to be counted and squares Volume
that are more than half outside the area should Volume
not be counted.
the top of the first layer (Aop ) and the area at the bottom
of the first layer (Abottom). Plug both numbers into the
equation provided in Figure B-2. Note: See Appendix A V = h (At + Aboom+ Atop xA )
Part 3for more on c ,,,,,,h tlu:g a lake surface area
using a l',,, t,,, ti map.
Step 3: After finding the volume of the top layer, Let: Atop = the area of the top of the layer
calculate the volume of the second deepest layer,
using the same technique, and continue on down for Aotto = the area of the bottom of the layer
each layer of the lake. (See Figure B-1.) h = the distance between contour lines
Step 4: Add the volumes of the respective layers to
V = the volume of one layer
find the total volume for the lake.
Step 5: If the areas are in units of square feet and the
depth interval is in feet also, the volume would be in
cubic feet. If the areas are in acres and the depth interval A a i o
Note: An acre-foot is one acre covered with onefoot of water
in feet, then the area would be in units of acre-feet.
*I X X X X X'X X X X X
XXXX X X XXX KXX
s V radius .
Step 1: First determine the average depth of the lake. This is also referred to as its mean depth.
You can find this number by collecting a series of water depth measurements at various locations in
the waterbody, and then averaging them. An electronic fathometer, or something as simple as a
weighted line, marked in increments of feet or meters, can be used to collect water depth measure-
ments. Collect these measurements every 10 to 20 feet for both the long and short axis of the
waterbody. Add all of these numbers together and then divide by the total number of readings that
were taken to obtain the average depth.
Step 2: Once you've determined the average depth of the lake, you can use this number, along with
the waterbody's length and width to solve the following equations. Notice that the equations are different,
depending on the general shape of the waterbody:
If your lake or pond is rectangular in shape multiply the lake's
length, width, and average depth and divide by 43,560 to find its volume in acre-feet.
length x width x average depth = acre-feet
43,560 is the conversion factor used to convert cubic feet into acre-feet.
If your lake is circular in shape use its radius, pi (rT or 3.14), and
average depth in the following equation:
3.14 x r2 x average depth = acre-feet
43,560 is the conversion factor used to convert cubic feet into acre-feet.
As always, we welcome your questions or comments.