EVPP 550Waterscape Ecology and

Management

Professor R. Christian

JonesFall 2007

Origins of Lakes

• Glacial• Tectonic• Volcanic• Solution• Fluviatile• Impoundments

Origin of Lakes - Tectonic• Epeirogenesis or overall

crustal uplifting• More complex than graben• Entire section of the crust

is uplifted– Caspian Sea: formerly part

of the ocean, cut off by crustal uplift

– Lake Okeechobee, FL: similar origin, partially maintained by daming with plant material

– Lake Titicaca, Peru

Origin of Lakes – Tectonic• Earthquake Lakes

– Reelfoot Lake, TN-KY

– Major earthquake (8 on Richter scale)

– Caused surface to uplift in some areas and subside in others

– Mississippi R was diverted into a subsidence region for several days forming Reelfoot Lake

Origin of Lakes - Tectonic

• Landslide Lakes– Mountain Lake, VA

• One of two natural lakes in Virginia

• Formed when landslide dammed a mountain valley

• The lake is estimated to be about 6,000 years old and geologists believe it must have been formed by rock slides and damming

Origin of Lakes - Volcanic

• Crater/caldera Lakes– Lake occupies a

caldera or collapsed volcanic crater/cone

– If cone blows out the side like Mt. St. Helens, no basin left

– Ex. Crater Lake, OR

Origin of Lakes – Volcanic Lakes

• Lava dams– Lava flow dams

an existing valley– Lake Kivu, Africa– Meromictic Lake,

contains high conc of CO2

– Could cause suffocation if overturned

Origin of Lakes – Solution Lakes

• Carbonate areas– Basin created by

dissolution of removal by groundwater of CaCO3 and MgCO3 rocks

– Overlying ground eventually collapses: “sinkhole”

– May lead to lakes or, if there are seams of carbonate, to a “karst” landscape

– Lakes of Central Florida

Origin of Lakes – Solution Lakes

• Salt collapse basins– Underground

seepage dissolves salt lenses, ground collapses and basin fills

– Montezuma Well, AZ

Origin of Lakes – Fluviatile (river-made)

• Ponding by deltas– Lake Pepin: WI-MN

• Oxbow Lakes– Isolated meanders

of an alluvial river– Lake Chicot, AR

• Pothole Lakes– Excavated by

streambed erosion– Grand Coulee

Lakes, WA

Origin of Lakes – Animals

• Humans– Intentional reservoirs– Incidental flooding of

basins constructed for other purposes

• Quarries• Peat diggings

• Other agents– Beavers– Alligators

Origins of Lakes - Reservoirs• Purposes

– Water supply• Human• Livestock

– Irrigation– Flood control– Sediment control– Recreational– Power generation– Navigation

Origin of Lakes – Lake Districts

• Because most of the factors responsible for lake origins or localized or regional, lakes tend to be clustered in “districts”– Glacial Lakes: MN, WI,

Ontario, NY, New England– Oxbow Lakes: lower

Mississippi Valley (AR, MS, LA, TN)

– “English Lake District”– Even reservoirs are

clustered due to favorable geology, physiography, demand

Morphology of Lakes

• Parameters related to surface dimensions– Maximum length

• Distance across water between two most separated points on shoreline

• Most significant when this corresponds with direction of prevailing winds

• Less clear in curved lakes

– Maximum width or breadth• Greatest distance across water perpendicular to axis

of maximum length

Morphology of Lakes

• Parameters related to surface dimensions– Surface area

• Can be derived from map by planimetry, weighing or counting squares

• Determines the amount of solar energy entering the lake and the interface available for heat and gas exchange with the atmosphere

– Mean width• Surface area/maximum length

Morphology of Lakes• Parameters related to surface dimensions

– Shoreline length• Related to the amount of shallow water available for

littoral organisms as well as the degree of interaction with adjacent terrestrial system (leaffall)

– Shoreline development index, DL• Compares the lakes actual shoreline length with that of a

circular lake of the same surface area• Allows comparison among lakes• High DL, elongate latkes, river impoundments• Low DL, calderas, solution basins, simple kettle lakes

Morphology of Lakes• Parameters requiring

bathymetric or subsurface dimensions– Maximum depth, zmax

• Popular and oft-cited datum• Some ecological significance

– Relative depth, zr• Ratio of maximum depth to

diameter of a circular lake with the same area

• Provides a way of comparing large and small lakes

Morphology of Lakes

• Volume– Total amount of water in the lake– Most easily derived from hypsographic curve– Hypsographic curve: Plot of Area vs. Depth

Morphology of Lakes• Volume

– Hypsographic curve: Plot of Area vs. Depth

– Can derive total water volume or volume of specific strata

Morphology of Lakes

• Mean Depth, z bar– zbar = V/A– One of the most important and meaningful

morphometric parameters– A general index of lake productivity

• ↑ zbar ↑ volume/area, dilution of incoming solar energy, ↑ volume unlit

• ↓ zbar ↓ volume/area, concentration of incoming solar energy, ↓ volume unlit

Morphology of Lakes• Deepest lakes are grabens; calderas and some

glacial lakes can also be deep• Grabens have the greatest volume

Morphology of Lakes• Glacial scour lakes can be large, but not necessarily

deep• Note that drift basins are neither large nor deep, but are

very numerous

Morphology of Lakes

• Hydraulic retention time, Tr

– Average time spent by water in the lake– “residence time”– Tr = Volume/Outflow rate– Varies greatly, some lakes have no outlet– Superior 184 yrs– Tahoe 700 yrs– Some reservoirs have Tr of only a few days or

even hours

Morphology of Lakes

• Elements also have retention times– If very soluble and not biologically active (Cl),

elemental retention time ≈ hydraulic retention time

– If associated with particles or biologically reactive (P), elemental retention time >> hydraulic retention time

Light in Lakes

• Sun is virtually the only source of enerby in natural aquatic habitat: photosynthesis and heat

• Solar constant– Rate at which radiation arrives at edge of

Earth’s atmosphere– ≈ 2 cal/cm2/min– More than half of this is lost coming through

the atmosphere

Light in Lakes• Absorption by

different chemicals in atmosphere

• Water and ozone (O3) are especially important

• Ozone is the most important in the UV range

Light in Lakes• Spectrum of light,

wavelength, λ– Ultraviolet: < 400 nm– Visible: 400-750 nm– Infrared: > 750 nm

• Light waves may also be characterized by their frequency, ν– ν = c/λ, where c =

speed of light

Light in Lakes

• Light may be considered to be made up as particles called photons

• Energy (E) content of a photon is related to its frequency

• E = hν where h=Planck’s constant• Therefore higher frequency (shorted wavelenth)

radiation has more energy per photon• Light is often quantified as photon flux density• Moles/m2/sec; 1 mole of photons = 1 Einstein

Light in Lakes

• Losses of Radiant Energy– Absorptive

compounds in atmosphere

– Cloud cover– Reflection at

Lake’s surface

Light in Lakes

• Scattering and Absorption– Physically different processes, but usually

hard to separate– Scattering

• deflection of photons by particles• Includes both side scattering and back scattering• Best measured by “turbidity”

– Absorption• Conversion of photon to another form of energy• Usually heat, but sometimes chemical (ex psyn)

Light in Lakes

• Attenuation– Disappearance of water with depth in a lake– Due to a combination of scattering and

absorption– Approximated by the Beer-Bouguer Law

• In a homogeneous medium a constant proportion of photons and their energy is absorbed (disappears) with each linear unit of medium

Light in Lakes

• Attenuation– Mathematical statement of Beer-Bougher Law

• I(z) = I(0) x e-kz

• where– I(z) is Irradiance (light) at depth z– I(0) is Irradiance (light) at the surface minus reflection– k is the coefficient of attenuation

• The rate of light attenuation for each unit of depth is e-k

Light in Lakes

• K, the rate of light attenuation is due to– Water, kw

• Not very large • Greatest for longer

wavelengths (red)• Least for short

wavelengths (blue)• Explains why in clear

water objects have a bluish cast

Light in Lakes• K, the rate of light

attenuation is due to– Dissolved material– Particulate material

• Net result is to shift wavelength of max penetration from blue toward green as attenuation increases

Light in Lakes• K, the rate of light attenuation is determined by

plotting ln I(z) vs z• Slope is –k, in this case -3.78 m-1

Light in Lakes• Light attenuation in

lakes is also approximated by determining Secchi disc depth, zSD

• Secchi disc depth has been shown to be related inversely to light attenuation coefficient

• One equation commonly used is:

• K = 1.7/zSD

Light in Lakes

• Photic zone– Lower limit defined by 1% of surface light– Depth at which I(z)/I(0) = 0.01– zPZ = - ln 0.01 / k

– zPZ = 2.7 zSD