Introduction to Lake Surveys Field Techniques

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Introduction to Lake SurveysField Techniques

• Unit 3 Module 8 Part A Lake Morphometry

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Objectives

• Students will be able to
• determine common morphometric characteristics of
  lakes
• identify characteristics of a bathymetric map.
• describe methods used to create bathymetric maps.
• use bathymetric maps to determine areal
  characteristics for lakes.
• determine the importance of lake volume and mean
  depth in lakes.
• calculate lake volume and mean depth.
• interpret hypsographic and volumetric curves of
  lakes.
• explain the hydraulic residence time of a lake.
• calculate the hydraulic residence time of a lake.

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Basic water quality assessment

• These slides focus on learning basic field
  techniques used by limnologists
• Morphometry - estimating critical lake basin
  measurements
• Field Profiles - physical and chemical parameters
  measured from top to bottom of the water column
• Sampling collecting water, sediments, and
  aquatic organisms

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Part A Lake morphometry
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Part A Lake morphometry

• Goal
• This module will help you
• Learn how to determine common morphometric
  characteristics of lakes

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Lake morphometry

• Morphometry defines a lakes physical
  dimensionsand involves the quantification and
  measurement of lake basin shape.
• These dimensions influence the lakes water
  quality and productivity levels.

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Determining lake morphometry

• First Step is to obtain or develop a bathymetric
  map-a bathymetric map is essentially a
  topographic map of the lake bottom that shows
  depth contours within the lake basin-used to
  estimate morphometric and many hydrological
  parameters-the reliability of your morphometric
  data will depend on the accuracy of the
  bathymetric map

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Working with a bathymetric map

• Many lakes have already been mapped and a good
  map will contain the following
• name, county, and geographic location of the
  water body
• an outline of the shoreline drawn to a known
  scale
• depth contours drawn to a known interval
• geographic orientation (which way is north?)
• date of map and data collectors

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Creating a bathymetric map

• Making your own map requires measuring depths at
  precise locations
• Sounding weight (through the ice works well)
• A secchi disk will work if weighted
• Fish finder, echo sounders ( sonar)

Lake mapping prior to 1950-MN DNR photo
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Creating a bathymetric map

• Bathymetric maps can be made by-Drawing a
  general outline of the lake or finding an aerial
  photo or map-Measuring and recording water
  depths at a number of locations-Then connecting
  the depth dots to develop simple contour lines

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Creating a bathymetric map

• The most commonly used method today involves
  using precise echo sounders, on board computers
  and GPS systems.There are several components to
  lake bathymetric mapping -the GPS equipment
  which will work in tandem with -the depth
  sounding equipment, and -the data collection
  process

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Creating a bathymetric map

• Soundings are taken during as the boat follows
  transects across the lake surface
• Location of transects and frequency of sounding
  measurements depends on
• Map scale
• Basin complexity

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Drawing the map

• Transect location and direction is recorded on a
  hardshell which is a drawing of the lake outline
  and surrounding features.
• Hardshells are drawn from orthorectified aerial
  photos or USGS quadrangle maps

MN DNR photo
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A bathymetric map allows determination of these
areal characteristics
Areal characteristics
characteristic units
elevation metersfeet
surface area(Ao) hectares (ha) acres (ac)
maximum depth (z max) metersfeet
Shoreline length(L) metersfeet
Shoreline development(DL) metersfeet
FetchMax widthMax length metersfeet
Littoral Area
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Areal characteristics

• ExampleMorphometric (and watershed)
  characteristics for Ice Lake

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Areal characteristics
Maximum length (fetch)
Maximum width

• Several areal characteristics and measurements
  can be taken directly off the bathymetric map

Z max
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Areal characteristics surface area

• Other measurements, such as lake surface area,
  require more work
• There are several methods for determining lake
  surface area
• Cut and weigh method
• Planimetry
• Grid method
• Digitized map

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Surface area cut and weigh method

• Youll need
• A photocopy of bathymetric map (as large as
  possible and be sure to include map scale)
• An analytical balance

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Surface area planimetry method

• Youll need
• -a lake map
• -polar compensating planimeter (200-600)

Image from http//lakewatch.ifas.ufl.edu
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Surface area grid method

• Youll need
• Bathymetric map
• Grid paper
• Method
• Count up the number of squares that fall within
  the shoreline of the lake
• Use the map scale to determine area represented
  by each square

Area squares counted X area of one square
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Surface area digitized lake maps

• Youll need
• Bathymetric map
• Digitizing software (e.g., ArcPad)
• Digitizing pad
• Method
• Software dependent

www.remetrix.com
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Areal characteristics shoreline length

• Shoreline length (L) circumference or perimeter
  of lake
• The linear measurement of the lakes entire
  perimeter at a given water level
• Provides a measurement of the amount of interface
  between the lake and surrounding land

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Areal characteristics shoreline development

• Shoreline development (SLD) a measure of how
  much the lakes surface shape deviates from being
  a perfect circle.
• Important is assessing the potential habitat
  available
• For a lake that is a perfect circle the SLD 1
• A reservoir that impounds water in valleys may
  have an SLD gt 4.

Calculating SLD
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Areal characteristics littoral area

• The littoral (shallow near shore) zone is the
  portion of a lake where sufficient light can
  penetrate to the lake bottom.
• It is also sometimes defined as that portion of
  the lake that is less than 15 feet in depth.
• The littoral zone is where the majority of the
  aquatic plants are found and is a primary area
  used by young fish.

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Volumetric characteristics
characteristic units
Volume (V) Cubic metersacre feet
Stratum volume Cubic metersacre feet
Mean depth(z mean or z) metersfeet
Hypsographic curve Graph (depth vs area)
Volumetric curve Graph (depth vs volume)
Hydraulic retention time (HRT) years

• Bathymetry also allows determination of several
  volumetric characteristics

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Volumetric characteristics volume

• Importance
• Total lake volume can influence a lakes dilution
  capacity.
• Allows the determination of mixed layer
  (epilimnion) volume.
• Or hypolimnion e.g. determining available trout
  habitat with temperatures from 4 to 25 oC and DO
  gt 5 mg/L.

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Volumetric characteristics volume
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Calculating Lake Volume

• Atop the area at the top of the layer
• Abottom the area at the bottom of the layer
• z the distance between contour lines
• V the volume of one layer

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Mean depth (z)

• Mean depth (z) volume ? surface area
• Mean depth is important for the following
  reasons
• Shallow lakes are generally more productive than
  deep lakes and mean depth is a quick way of
  assessing overall depth
• Also indicates the potential for waves and mixing
  events to disrupt bottom sediments
• If volume is not available you could collect
  numerous lake depth measurements and average
  them. Of course this is not as accurate and only
  practical for small lakes.

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Hypsographic curves

• Hypsographic curve Area as a function of depth
• To estimate the amount of potential bottom
  spawning habitat for brook trout or bass (perhaps
  defined by a range of temperature and dissolved
  oxygen)
• To estimate the littoral zone area potentially
  available for macrophyte growth (limnologically
  defined as the depth to 1 of surface light).
  Often the epilimnion volume is used as an
  approximation. Often related to secchi depth by
  fisheries folks.
• To estimate the area of sediments exposed to low
  oxygen. This allows you to predict internal
  phosphorus release (Nurenberg 1985).

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Volumetric Curve

• Volumetric Curve volume as a function of depth
• When used in conjunction with temperature and DO
  profiles, this curve can be used to estimate
  fisheries habitat.

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Hydraulic residence time (HRT)

• HRT is the time required to refill an empty lake
  with its natural inflow.
• A large deep lake with a moderate inflow will
  have a longer HRT than a small, shallow lake with
  the same inflow.

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HRT - importance

• HRT is needed to determine annual lake budgets
  for water, nutrients, heat, oxygen contaminants,
  and herbicides.
• It also provides an estimate of the turnover time
  for water in a lake, or flushing time

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Calculating HRT

• A lakes residence time is calculated by dividing
  the lakes volume by its average annual water
  outflow.
• Lake managers calculate outflow on an annual
  basis so that seasonal variation doesnt unduly
  influence results.
• Volume (V) is usually expressed in acre-feet, and
  mean outflow is expressed as acre-feet/year.

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Calculating HRT cont.

• So the formula looks like this
• HRT (years) lake volume (acre-ft) / mean
  outflow (acre-ft/yr)

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Calculating SLD
BACK
If A lake area, then a circle with area A has
a perimeter
(1)
(2)
(Formula for area based on radius)
(6)
(3)
(collect terms)
(4)
(7)
Substituting eq.(3) into eq. (1)
(By definition)
(5)
(8)
Substituting eq(6) into eq. (7)
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