state of the lake 2013
TRANSCRIPT
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TERC.UCDAVIS.EDU
TAHOE: STATE OF THE LAKE REPORT 2013
FUNDING USED TO ASSEMBLE AND DISTRIBUTE
THIS REPOR T WAS PROVIDED BY UC DAVIS AND
THE GENEROSITY OF SUPPORTERS WHO VALUETHE ROLE OF SCIENCE TO SAVE THE LAKE
THIS YEARS REPORT IS DEDICATED TO THE LEGACY OF PROFESSOR JOHN LECONTE, THE
FIRST PROFESSOR APPOINTED BY THE FLEDGLING UNIVERSITY OF CALIFORNIA, WHO
TOOK THE FIRST CLARITY MEASUREMENTS IN LAKE TAHOE 140 YEARS AGO.
John LeConte
1818-1891
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1
1. Introduction
2. Executive Summary
3. About Lake Tahoe
4. About the UC Davis Tahoe Environmental Research Center
5. Map of Tahoe Basin data collect ion sites
6. Recent Research Updates 6.1 Overview
6.2 UnderwaterGliderexperiment 6.3 UnderwaterGliderexperiment,continued
6.4 Projected21stcenturytrendsinTahoeshydroclimatology
6.5 Projected21stcenturytrendsinTahoeshydroclimatology,continued 6.6 Droughtadaptationbyforests
6.7 Real-timemonitoringofwaterqualityinLakeTahoe
6.8 Real-timemonitoringofwaterqualityinLakeTahoe,continued 6.9 MeasuringthebluenessofLakeTahoe
6.10 Shorelinechangeduetoprolongeddrought
7. Meteorology
7.1 Airtemperature(dailysince1911) 7.2 Below-freezingairtemperatures(yearlysince1910)
7.3 Monthlyairtemperature(since1998)
7.4 Dailysolarradiation(dailyin2012) 7.5 Annualprecipitation(yearlysince1910)
7.6 Monthlyprecipitat ion(2010,2011,2012and1910to2012average)
7.7 Snowasafractionofannualprecipitation(yearlysince1910) 7.8 Shiftinsnowmelttiming(yearlysince1961)
8. Physical properties 8.1 Lakesurfacelevel(dailysince1900)
8.2 Lakesurfacelevel,continued(dailysince2010)
8.3 Watertemperatureprofile(in2012)
8.4 Averagewatertemperature(since1970)
8.5 Annualaveragewatertemperature(since1970) 8.6 Surfacewatertemperature(yearlysince1968)
8.7 Maximumdailysurfacewatertemperature(every15minutessince1999) 8.8 Julyaveragesurfacewatertemperature(since1999)
8.9 Deepwatertemperature(since1970)
8.10 Depthofmixing(yearlysince1973) 8.11 Lakestability(since1968)
8.12 Stratifiedseasonlength(since1968) 8.13 Beginningofthestratificationseason(since1968) 8.14 Endofstratificationseason(since1968)
8.15 Peakofstratificationseason(since1968)
8.16 MeandailystreamflowofUpperTruckeevs.TruckeeRiver(wateryear2012) 8.17 TruckeeRiversummerdischargeandlakeelevation(since1980)
8.18 TruckeeRiversummerwatertemperatures(since1993) 8.19 AnnualdischargevolumeofUpperTruckeeandTruckeeRiver(since1993)
9. Nutrients and particles 9.1 Sourcesofclarity-reducingpollutants
9.2 Pollutantloadsfromsevenwatersheds
9.3 NitrogencontributionbyUpperTruckeeRiver(yearlysince1989) 9.4 PhosphoruscontributionbyUpperTruckeeRiver(yearlysince1989)
9.5 SuspendedsedimentcontributionbyUpperTruckeeRiver(yearlysince1989)
9.6 Nutrientconcentrationsinrainandsnow(yearlysince1981) 9.7 Nutrientloadsinrainandsnow(yearlysince1981)
9.8 Lakenitrateconcentration(yearlysince1980) 9.9 Lakephosphorusconcentration(yearlysince1980)
CONTENTS
TERC.UCDAVIS.EDU
TAHOE: STATE OF THE LAKE REPORT 2013
TABLE OF CONTENTS
(CONTINUED ON NEXT PAGE)
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10. Biology
10.1 Algaegrowth(primaryproductivity)(yearlysince1959)
10.2Algaeabundance(yearlysince1984) 10.3Annualdistributionofalgalgroups(yearlysince1982)
10.4Algalgroupsasafractionoftotalpopulation(monthlyin2012) 10.5Nutrientlimitationofalgalgrowth(2002-2012)
10.6PredominanceofCyclotellasp.(2002-2012)
10.7Shorelinealgaepopulations(yearlysince2000) 10.8 Shorelinealgaedistribution(in2012)
11. Clarity 11.1AnnualaverageSecchidepth(yearlysince1968)
11.2WinterSecchidepth(yearlysince1968)
11.3SummerSecchidepth(yearlysince1968) 11.4Lighttransmission(in2012)
12. Education and outreach
12.1 TERCeducationandoutreach(in2012)
12.2TERCeducationexhibits(in2012) 12.3TERCeducationprograms(in2012)
12.4 TERCeducationprograms,continued(in2012)
12.5TERCspecialevents(in2012)
CONTENTS, CONTINUED
TERC.UCDAVIS.EDU
TAHOE: STATE OF THE LAKE REPORT 2013
TABLE OF CONTENTS
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1.1
TheUniversityofCalifornia,Davis,
hasconductedcontinuousmonitoring
ofLakeTahoesince1968,amassingauniquerecordofchangeforoneofthe
worldsmostbeautifulandvulnerablelakes.
IntheUCDavisTahoe:Stateof
theLakeReport,wesummarizehownaturalvariability,longtermchangeand
humanactivityhaveaffectedthelakes
clarity,physics,chemistryandbiologyoverthatperiod.Wealsopresentthe
datacollectedin2012.Thedatashown
revealauniquerecordoftrendsandpatternstheresultofnaturalforces
andhumanactionsthatoperateattime
scalesrangingfromminutestodecades.ThesepatternsmakeclearthatLake
Tahoeisacomplexecosystem,behaving
inwayswedontalwaysexpect.Thiswasexemplifiedthisyearbythedecrease
intheabundanceofCyclotellainthelake,andthecorrespondingincrease
insummerclarity.WhileLakeTahoeisunique,theforcesandprocessesthatshapeitarethesameasthoseactinginall
naturalecosystems.Assuch,LakeTahoe
isananalogforothersystemsbothinthewesternU.S.andworldwide.
Ourroleistoexplorethiscomplexity
andtouseouradvancingknowledgetosuggestoptionsforecosystemrestoration
andmanagement.Choosingamong
thoseoptionsandimplementingthem
istheroleofthoseoutsidethescientific
communityandneedstotakeaccount
ofahostofotherconsiderations.Thisannualreportisintendedtoinformnon-
scientistsaboutsomeofthevariablesthataffectlakehealth.Untilrecently,onlyone
indicatorofLakeTahoeshealthstatus
waswidelyreported:theannualclarity(oftencalledtheSecchidepth,afterthe
instrumentusedtocollecttheclarity
data).Inthisreportwepublishmanyotherenvironmentalandwaterquality
factorsthatallprovideindicatorsofthe
lakescondition.
Thisreportsetsthecontextfor
understandingthechangesthatareseen
fromyeartoyearandthosethatareobservedoveratimescaleofdecades:
WasLakeTahoewarmerorcoolerthan
thehistoricalrecordlastyear?Aretheinputsofalgalnutrientstothelake
declining?HowmuchareinvasivespeciesaffectingLakeTahoe?And,ofcourse,
howdoallthesechangesaffectthelakesfamousclarity?
Thedatawepresentaretheresult
ofeffortsbyagreatmanyscientists,
engineers,studentsandtechnicianswhohaveworkedatLakeTahoethroughout
thedecadessincesamplingcommenced.
Iwould,however,liketoacknowledge(inalphabeticalorder)thecontributions
ofBrantAllen,VeronicaAlumbaugh,
NancyAlvarez,PattyArneson,Janet
Brewster,SudeepChandra,BobCoats,
MarizaCosta-Cabral,MichaelDettinger,
AngieElliot,KristinFauria,BillFleenor,AlexForrest,AllisonGamble,Alfredo
Gimenez,CharlesGoldman,GyemboGyeltshen,ScottHackley,TinaHammell,
BruceHargreaves,AlanHeyvaert,Simon
Hook,AndreaHoyer,DebbieHunter,PeterHunter,CamilleJensen,Anne
Liston,GeorgeMalyj,ParkerMartin,Tom
Mathis,KristinReardon,JohnReuter,BobRichards,JohnRiverson,DaveRizzo,
GolokaSahoo,HeatherSegale,Todd
Steissberg,RaphTownsend,AlisonToy,JoshViers,ShoheiWatanabe,KatieWebb,
andBrentWolfetothisyearsreport.
Fundingfortheactualdatacollectionandanalysiscomesfrommanysources.
Whilemanyadditionalwaterquality
variablescouldbetracked,fundingultimatelylimitswhatwemeasure.
Currentfundingforthelong-termmonitoringandanalysisisprovidedby
theLahontanRegionalWaterQualityControlBoard,theTahoeRegionalPlanningAgency,theU.S.ForestService,
theU.S.GeologicalSurveyandUC
Davis.Ourmonitoringisfrequentlydoneincollaborationwithotherresearch
institutionsandagencies.Wearegrateful
fortheparticipationofourcolleaguesintheTahoeScienceConsortium.In
particularwewouldliketoacknowledgetheU.S.GeologicalSurvey(USGS),the
DesertResearchInstitute(DRI),the
UniversityofNevada,Reno(UNR),
theNationalAeronauticsandSpaceAdministration(NASA),andtheU.S.
ForestService.Somedataarealsocollectedaspartofresearchprojects
fundedthroughavarietyofsources.
Withoutthesedatatherearemanyquestionsthatcouldnotevenbeaskedlet
aloneanswered.
Thisyearwearepresentingupdatesonsomerecentresearch,aswellasproviding
updatesonthelakemonitoringefforts.
Thesenewresearchresultshighlightsomeofthemostexcitingfindingsof
workthatisstillinprogress,andwillbe
reportedonfullyinthemonthsandyearstocome.
Sincerely,
GeoffreySchladow,director
UCDavisTahoeEnvironmental
ResearchCenter
291CountryClubDriveInclineVillage,NV89451
(775)881-7560,ext.7563
INTRODUCTION
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TAHOE: STATE OF THE LAKE REPORT 2013
2.1
Thelong-termdatasetcollected
ontheLakeTahoeecosystembythe
UniversityofCalifornia,Davis,andits
researchcollaboratorsisaninvaluable
toolforunderstandingecosystem
functionandchange.Ithasbecome
essentialforresponsiblemanagement
byelectedofficialsandpublicagencies
taskedwithrestoringandmanaging
theTahoeecosystem.Thisisinlarge
partbecauseitprovidesabasisfor
monitoringoftheprogresstoward
attainmentofTahoesrestorationgoals
anddesiredconditions.
ThisannualTahoe:StateoftheLake
Reportpresentsdatafrom2012in
thecontextofthelong-termrecord.
Whilethefocusisondatacollected
aspartofourongoing,decades-long,
measurementprograms,thisyear
wehavealsoincludedsectionssummarizingcurrentresearchon
usingautonomousglidersfor
examiningthedetaileddistributionof
waterqualityacrossthelake;projected
21stcenturytrendsinTahoes
hydroclimatology;droughtadaptation
byforests;ouradditionofreal-time
waterqualitymonitoringindeep
wateroffthewestshore;measuring
thebluenessofLakeTahoe;andthe
possiblechangesinshorelineposition
onaccountofextendeddroughts.
TheUCDavisTahoeEnvironmental
ResearchCenter(TERC)hasdeveloped
sophisticatedcomputermodelsthat
helpscientistsbetterpredictand
understandhowLakeTahoeswater
movesandhowtheentireecosystem
behaves.Long-termdatasetsare
essentialtorefinetheaccuracyof
thosemodelsandtodevelopnew
modelsasknowledgeincreasesand
newchallengesarise.Intimesofrapid
change,reliablepredictivemodelsareindispensabletoolsforLakeTahoe
Basinresourcemanagers.
Withrespecttoweather2012was
notaparticularlyunusualyearfor
LakeTahoe.Thewinterof2011-2012
wasrelativelydry(seventyonepercent
ofthelongtermaverage).Thefraction
ofprecipitationthatfellassnow
continuedthedownwardtrendat41
percent.Temperatureswerecloserto
normal,withtheexceptionofthefall
monthsthatweredistinctlywarmer
thanthelong-termaverage.The
numberofdayswithbelowfreezing
temperaturesfellpreciselyonthelong-
termtrendlineofdecliningbelow-
freezingdays.Asaconsequence,the
peakinthetimingofthesnowmeltwas
againearlierthanhistoricalconditions,
occurringonMay4.
Lakelevelrosebyonly1.3feet
duringthesnowmelt,comparedwith
3.9feetthepreviousyear.Duringsummerandfall,lakelevelfellby
2.3feet,producinganetlossfor
theyear.Therateofincreaseofthe
volume-averagedlaketemperature
rosein2012,althoughthelongterm
rateofincreasehasslowedinrecent
years.Theannualaveragesurface
temperature(basedonmonthly
readings)was52.8degFin2012,the
highestvalueeverrecordedforLake
Tahoe.Mostofthisincreasecame
afterthesummer,asJulysurface
temperatureswererelativelycoolat
63.3degF.Otherconsequencesof
climatechangecouldalsobeseenin
therisingtemperatureofthedeep
watersofthelake.Inthelast37years
bottomtemperatureshaveincreased
byonedeg.F.
LakeTahoedidnotmixtoitsfull
depthin2012.Instead,themaximum
depthofmixingwasonly820feet,
reachedinMarch.Oxygenlevelsinthe
deepestpartofthelakearecurrentlybeingmonitoredtodeterminetherate
atwhichoxygenisbeinglostwhen
mixingdoesnotoccur.Thelackof
EXECUTIVE SUMMARY
1Previous year for some parameters means data collated in terms of the water year, which runs from October 1 through September 30; for other parameters, it means data forthe calendar year, January 1 through December 31. Therefore, water year data are from Oct. 1, 2011 through Sept. 30, 2012. Calendar year data are from Jan. 1, 2012 through
Dec. 31, 2012.
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EXECUTIVE SUMMARY
mixingwasduetothehighstability
indexthelakehadduring2012,
thehighestrecordedin45years.
Theupper330ftofthelakestayed
stratifiedfor203days,amonth
longerthanwhatwastypicalwhen
therecordbegan.
RiverreleasesfromLakeTahoeto
theTruckeeRiveroccuratthedam
inTahoeCity.Watertemperaturehas
beenmonitoredtherebytheUSGS
since1993.Thoughthedatasetis
incomplete,thereisevidencethatthe
summertimereleasetemperatures
haveincreasedsignificantlyoverthat
period,suggestingpotentialimpacts
ondownstreamfishspawning.
Theinputofstream-borne
nutrientstothelakedeclined
significantlyin2012duetothelower
precipitation.Onthewestsideof
thelake,thepollutantloadswere
reducedbyafactoroffourfrom2011.
Biologically,theprimaryproductivity
ofthelakecontinueditslong-term
increasein2012,withtheannual
averagevalueof243.98gramsof
carbonpersquaremeterbeingthe
highestvalueeverrecorded.The
reasonsforthisincreasearebelieved
tobelinkedtoalongtermshift
towardssmalleralgalspeciesthat
havetheabilitytoprocessnutrients
faster.Despitetheincreasein
productivity,theconcentrationof
chlorophyllinthelakehasremained
relativelyconstant.In2012,there
wasareductioninthespecies
Cyclotella.Alargeincreaseinthe
numbersofthisspeciesoverthe
lastfiveyearshasbeenlinkedto
climatechangeandhasresultedinsummertimeclarityreductions.This
yearsreductioncoincidedwithan
improvementinclarity.
Forthesecondstraightyear
clarityimprovedinLakeTahoe.The
annualaverageclarityimproved
by6.4feetoverthepreviousyear
to75.3feet.Thisvalueiswithin3
feetoftheinterimclaritytargetof
78feet.However,itisimportant
torecognizethatyear-to-year
fluctuationsarethenorm,andthe
targetmustbeseenasbeingavalue
thatcanbesustainedoverseveral
years.Thisimprovementoccurred
inbothsummerandwinter.The
reasonsfortheimprovementare
three-fold:itwasadryyearmeaning
watershedpollutantloadswerelow,
thelakedidnotundergodeepmixing
whichlimitsthetransportofdeep
storednitrogen,andthenumbersof
Cyclotelladecreasedtotheirlowest
levelsinfiveyears.
Thesummertimeclarityimproved
byover13feet.Whilecertainly
encouraging,examinationofthe
longtermtrendshowsthatthere
havebeenmanyperiodsofapparent
improvementonlytobeovertakenby
continueddecline.
Innewresearch,somevaluable
newtoolsarebeginningtoprovide
newinsightsintotheprocessesthat
drivechangeinLakeTahoe.An
underwatergliderthatoperatedin
thelakefor11daysinMayprovided
thefirsteversnapshotsofwater
qualityacrossaneast-westtransect.
Whatthedataconfi rmedwasthe
presenceofgiantinternalwaves
deepinthelake,thatcouldmove
algaeandpollutantsverticallyover
150feet.Possiblymoreimportant
wasthesuccessfulinstallationofa
waterqualitymonitoringstationin
360feetofwateroffthewestshore.
Connectedtoshorebyanunderwater
cable,thisstationprovidesdatafrom
toptobottomevery30seconds.Thisisthefirstsuchstationinanylake
worldwide.
ThisreportisavailableontheUC
DavisTahoeEnvironmentalResearch
Centerwebsite(http://terc.ucdavis.edu).
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TAHOE: STATE OF THE LAKE REPORT 2013
3.1
Maximumdepth:1,645feet(501
meters),makingitthe11thdeepest
lakeintheworldand2nddeepestlakeintheUnitedStates
Averagedepth:1,000feet(305
meters)
Lakesurfacearea:191squaremiles
(495squarekilometers)
Watershedarea:312squaremiles
(800squarekilometers)
Length:22miles(35kilometers)
Width:12miles(19kilometers)
Lengthofshoreline:72miles(116
kilometers)
Volumeofwater:39trilliongallons
Numberofinflowingstreams:63,
thelargestbeingtheUpperTruckee
River
Numberoflargelakesworldwide
withannualclarityexceeding
Tahoes:0
Numberofoutflowingstreams:one,
theTruckeeRiver,whichleaves
thelakeatTahoeCity,Calif.,flowsthroughTruckeeandReno,andter-
minatesinPyramidLake,Nev.
Lengthoftimeitwouldtaketorefillthelake:about600years
Averageelevationoflakesurface:
6,225feet(1,897meters)
Highestpeakinbasin:FreelPeak,10,891feet(3,320meters)
Latitude:39degreesNorth
Longitude:120degreesWest
Ageofthelake:about2million
years
Permanentpopulation:55,000
(2010Census)
Numberofvisitors:3,000,000annually
ABOUT LAKE TA HOE AND THE TA HOE BASIN
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4.1
TheUCDavisTahoe
EnvironmentalResearchCenter
(TERC)isaworldleaderinresearch,educationandpublicoutreachon
lakes,theirsurroundingwatershedsandairsheds,andthehuman
systemsthatbothdependonthem
andimpactthem.TERCprovidescriticalscientificinformationtohelp
understand,restoreandsustainthe
LakeTahoeBasinandotherlakesystemsworldwide.Wepartner
closelywithotherinstitutions,
organizationsandagenciestodeliversolutionsthathelpprotectLake
Tahoeandotherlakesaroundtheworld.
TERCsactivitiesarebasedat
permanentresearchfacilitiesintheTahoeBasinandattheUniversitys
maincampusinDavis,California,
about90mileswestofthelake.
Ourmainlaboratoriesandoffices
areinInclineVillage,Nevada,onthe
thirdflooroftheTahoeCenterfor
EnvironmentalSciencesbuilding.Onthefirstfloor,weoperatetheThomas
J.LongFoundationEducationCenter,alearningresourcethatis
freeandopentothepublic.
InTahoeCity,Calif.,weoperateafieldstation(housedinafully
renovated,formerstatefishhatchery)andtheErikssonEducationCenter.TahoeCityisalsothemooringsite
forourresearchvessels,theJohn
LeConteandtheBobRichards.
Oursecondarylaboratoriesand
officesarelocatedontheUCDavis
campusattheCenterforWatershedSciencesandinWicksonHall.
Ourwebsite(terc.ucdavis.edu)
hasmoreinformationaboutourprograms,including:
Informationforpotentialstudents,
staff,faculty,andresearch
collaborators;
Accesstonear-real-timedatagatheredbyourgrowingnetwork
ofsensors;
Alistofpublications;
Exhibitsandeventsatthe
EducationCenters;and
Informationaboutsupportingourresearchandlearningprograms.
ABOUT THE UC DAVIS TA HOE ENVIRONMENTA L RESEARCH CENTER (TERC)
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5.1
MAP OF TAHOE BASIN
DATA COLLECTION SITES
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RECENT RESEARCH
UPDATES
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TERC.UCDAVIS.EDU 6.1
TAHOE: STATE OF THE LAKE REPORT 2013
Overview
While the State of the Lake Reportis primarily intended to focus on
the trends emerging from long term
data collection efforts, this sectionpresents the results of some current
and short term projects. In some cases
the projects are complete, but in mostcases they are a preview of some new
and exciting research directions.
This year we focus on five areas:an underwater Glider experiment,
the projected 21st century trends in
Tahoes hydroclimatology, droughtadaptation by forests, new efforts on
collecting real-time data on Tahoes
water quality and measuring theblueness of Lake Tahoe.
RECENT RESEARCH UPDATES
The University of Minnesota, Duluths Glider just prior
to descending into Lake Tahoe for the start of its 11
day mission to traverse Lake Tahoe 17 times.
Research divers Brant Allen and Katie Webb preparing
to lay the first section of the underwater cable for
Tahoes first real-time water quality monitoring station.
Cleaning the underwater radiometer at Buoy TB3,
a joint project between TERC and NASA/JPL to
continuously measure the changing light conditions inthe lake.
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TAHOE: STATE OF THE LAKE REPORT 2013
Underwater Glider experimentUnderwater Gliders are ultra-low poweredinstrument packages that can measure arange of water quaIity variables in oceans
and lakes. They operate by repeatedlydiving and rising while traveling along
a pre-determined path, and taking
measurements while they do so. Theyhave the ability to do this for months
at a time under conditions that make
conventional sampling dangerous.
In a collaboration with Dr Jay Austin of
the Great Lakes Observatory, Universityof Minnesota, Duluth, TERC conducted
a Glider experiment in Lake Tahoefrom May 11- May 22, 2013. The Glider
conducted 17 measurements transects on
the same east-west line, diving repeatedlyto a depth of over 500 feet.
The panels show the data collected
during the transect that commenced at
2:02 PM on May 21, and concluded at5:30 AM on May 22. The west shore is
at 0 miles on the horizontal scale and
the east shore is at 12 miles. The firstpanel shows the raw temperature data,
superimposed on the trajectory lines
taken by the Glider. The Glider was setto only collect data while descending.
RECENT RESEARCH UPDATES
Temperature contours along a transect from west (left)
to east (right) across Lake Tahoe. The severe tilt is
due to a wind-driven upwelling. This is a commonspringtime event in Lake Tahoe.
Raw temperature data collected by the Glider along
each of its 29 dives along the east-west transect.
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Underwater Glider experiment, continuedThis transect took place under extremelywindy conditions, with the wind from t he
south west. This caused an upwelling,
where water from as deep as 150 feet onthe west shore is raised up to the surface
in a giant internal wave. Warm surfacewater is pushed eastward toward Nevada.
This transport impacts all other waterquality parameters. Chlorophyll, a
measure of algal abundance, that has its
highest values in a deep layer centeredat about 200 ft, also rises in the west
and sinks in the east in response to theinduced wave motion. Dissolved oxygen,
which tends to have higher concentrations
near the surface and lower concentrationsat depth, is simil arly distorted.
These results highlight the extremenatural variability that is present in Lake
Tahoe at short timescales. This variabilityneeds to be understood in order to analyze
the long term trends in water quality.
Similarly, the lake motions themselves areresponsible for transporting pollutants
within the l ake and need further attention.
RECENT RESEARCH UPDATES
Dissolved Oxygen (percent saturation) contours along a
transect from west (left) to east (right) across Lake Tahoe.
Chlorophyll fluorescence contours along a transect from
west(left) to east (right) across Lake Tahoe.
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TAHOE: STATE OF THE LAKE REPORT 2013
Projected 21st century trends in Tahoes hydroclimatologyTERC, in collaboration with HydroikosAssociates; the US Geological Survey/
Scripps Institute of Oceanography;
Tetra Tech, Inc.; Northwest HydraulicConsultants; and Hydrology Futures,
LLC., completed a SNPLMA fundedstudy to provide a first assessment of the
extent to which climate change needsto be considered in ongoing efforts to
protect and manage the unique waters of
Lake Tahoe. The results are publishedas a series of five papers in the journal
Climatic Change (copies available uponrequest).
RECENT RESEARCH UPDATES
The decline in the percentage of precipitation as snow in the next 100 years under the GFDL-A2 scenario.
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TAHOE: STATE OF THE LAKE REPORT 2013
RECENT RESEARCH UPDATES
Projected 21st century trends in Tahoes hydroclimatology, continued
Estimated snow pack depths at different time intervals under theGFDL-A2 model scenario.
Estimated dissolved oxygen concentration in Lake Tahoe overthe next 100 years under the GFDL-A2 model scenario. Note
the complete loss of oxygen below 200 m after 2065.
Key findings of the study till the end of the 21st Centuryincluded, for the range of scenarios tested:
1. Air temperature increases as high as 10 F
2. The fraction of snow to rain could fall to 0.1-0.2, leading to
reduced water storage in the spring snow pack and increasesin drought severity
3. Changes in stream low-flow conditions could render the
lower reaches of some streams completely dry more often
4. Dramatic increases in flood magnitude
5. Sediment and nutrient loading to Lake Tahoe from streams
should not increase substantially
6. Overall fine sediment load reductions should still be
achievable if storm water treatment facilities are properly
sized
7. Lake Tahoe could cease to mix to the bottom for extendedperiods, resulting in complete oxygen depletion in the deep
waters with loss of habitat and an increase in sedimentnutrient release,
8. Lake surface level is more likely to drop below the natural
rim for extended time periods.
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TAHOE: STATE OF THE LAKE REPORT 2013
Drought adaptation by forests
Experiments have been establishedby researchers from UC Davis and
the USDA Forest Service Pacific
Southwest Research Station to evaluateecologically important plant traits in
sugar pine, western white pine, and
white bark pine, three co-dominantand dominant tree species in the Lake
Tahoe Basin. Determining water-use
efficiency by measuring the carbon stableisotope ratio, d13C, may improve our
understanding of drought adaptation
and provide information about howpopulations of forest tree species will
respond to global climatic change (e.g.
adaptation from standing variation). Theless negative value ofd13C corresponds
with higher water use efficiency.
RECENT RESEARCH UPDATES
Sugar pine seedlings at the Institute of Forest Genetics.
Sugar Pine
Western White Pine
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TAHOE: STATE OF THE LAKE REPORT 2013
Real-time monitoring of water quality in Lake Tahoe
RECENT RESEARCH UPDATES
A vertical array of temperature, pressure and dissolved
oxygen measuring instruments are installed at a depth of
120 m off the west shore of Lake Tahoe, adjacent to TERCs
long-term Index station. A 600 m long underwater cable
from the shore connects to the array and provides power and
transmits data to the internet every 30 seconds.
The temperature acts as a tracer for mixing processes in the
lake, allowing us to understand the mechanisms by which
dissolved oxygen is transferred to the deep water. This is a
critical question that needs to be addressed in order to better
understand climate change impacts on Lake Tahoe. The
pressure data are being used to study the long period waves
at the surface of the lake (seiches) and to also relate their
impact on internal processes in the lake.
This project is made possible through a collaboration withObexers Marina, who are providing the terminus land site
for the underwater cable. Funding to launch the project was
provided by the UC CITRIS program and private donors.
The sub-surface buoy atop the instrument array at Homewood.
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TAHOE: STATE OF THE LAKE REPORT 2013
RECENT RESEARCH UPDATES
The left panel represents nineteen days of
temperature traces that show the gradualwarming of the surface water through June
12. The peak temperature was 15 C (59F).Intense winds for the next three days
cooled the surface layer of the lake by 4 C
(8 F) in two days. Note that the bottom
temperatures are virtually unchanged.
The right panel shows thirty six hours of datafrom the real time station. The green trace
shows the variations in the dissolved oxygen
concentration at the lake bottom. The black
trace is the f luctuation of the lake sur face.The red and blue lines are the surface and
bottom temperatures, respectively.
Real-time monitoring of water quality in Lake Tahoe, continued
All 16 temperature traces for 19 days. Note the rapid cooling at the
end of the record.High winds forced the mixing of the lake surface.
Oxygen fluctuations at the bottom of the lake were not expected.
They appear to vary in the opposite direction to the lake level.
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TAHOE: STATE OF THE LAKE REPORT 2013
RECENT RESEARCH UPDATES
Measuring the blueness of Lake Tahoe
Most people are familiar with the slogan
Keep Tahoe Blue, yet the goal forLake Tahoe has always been phrased in
terms of clarity rather than color. Using
hyperspectral radiometric measurementsthat measure the light upwelling from
the lake, it is possible to quantify theactual color of the lake water. This is
being done using instruments deployed
upon a buoy in the center of Lake Tahoe
(TB3) in collaboration with NASA/JPL.Measurements are being taken every
30 minutes during daylight hours. It isplanned to start transmitting data in real
time in 2013.
The importance of this data is that it can
help understand what drives changes in
lake color at a range of time scales, fromhours to years. These measurements
are unique and are part of what is being
done to better understand the linkagesbetween water quality contaminants and
the spectral response of lakes. A long
term goal of the project is to use remotelysensed imagery from satellites to provide
similar data for lakes world-wide.
Quantification of water color for a 7 month period in 2012. The values
shown are the b* coordinate values of the CIE L*a*b* color space. Higher
values indicate increased blueness. The decrease in blueness is evidentin fall and winter. Short term fluctuations in blueness are seen in May and
June. Data gap around September was due to inst rument maintenance.
Radiometric profiles are taken regularly to calibrate
the buoy sensors.
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TAHOE: STATE OF THE LAKE REPORT 2013
RECENT RESEARCH UPDATES
Shoreline change due to prolonged drought
In a dry year lake Tahoe typically loses
1.5 feet of water level. Predictionsof future climate conditions in the
basin point to more extreme events
in the future. A 5-year drought couldconceivably reduce the lake level to
6215.5 feet, or 5 feet below the record
1992 levels. How would the shorelinelook under those conditions. The images
below show the shoreline positionscorresponding to the natural rim of the
lake (without the dam), the 1992 drought
shoreline, the 5 year drought shoreline,
and the shoreline corresponding tothe ancient drought 5000 years ago
when lake level was lower by 40 feet forhundreds of years.
Four shorelines positions for Incline Village, NV. Green thenatural rim, blue- the 1992 drought, orange a possible 5 year
drought, and red the ancient drought. Note the dock positions.
Three shorelines positions for McKinney Bay, CA. Green thenatural rim, blue- the 1992 drought, and orange a possible 5 year
drought. Note the dock positions.
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7
METEOROLOGY
TERC.UCDAVIS.EDU
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TAHOE: STATE OF THE LAKE REPORT 2013
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Air temperatureDaily since 1911
METEOROLOGY
Daily air temperatures have increased
over the 100 years measured at TahoeCity. The long-term trend in daily
minimum temperature has increased
by more than 4 F (2.2 C), and thelong-term trend in daily maximum
temperature has risen by less than
2 F (1.1 C). The trend line for t he
minimum air temperature nowexceeds the freezing temperature of
water, which points to more rain and
less snow, as well as earlier snowmelt.These data have been smoothed by
using a two-year running average
to remove daily and seasonal
fluctuations. 2012 was warmer thanthe previous year, which came at the
end of a decade-long cooling trend.
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TAHOE: STATE OF THE LAKE REPORT 2013
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Below-freezing air temperaturesYear ly s ince 1910
METEOROLOGY
The method used for this analysis
sums the number of days with dailyaverage temperatures below freezing
between Dec 1 and March 31 for each
Water Year. Although year-to-year
variability is high, the number of dayswhen air temperatures averaged below
freezing has declined by about 25 days
since 1911. In 2012, the number of
freezing days fell directly on thelong-term trendline.
0
20
40
60
80
100
120
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Num
bero
fdays
Water year
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Monthly air temperatureSince 1998
METEOROLOGY
In 2012, monthly air temperatures
were generally similar to values in theprevious year and to the long term
average. A notable exception was
that 2012 was warmer than the longterm average in the fall months of
September through November. Any
month with more than 25 percent ofthe daily data missing was not plotted.
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC20
30
40
50
60
70
7
0
5
10
15
2019982012 average
2011 average
2012 average
Temperature
(oF)
T
emperature
(oC)
5
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Daily solar radiationDaily in 2012
METEOROLOGY
Solar radiation showed the typical
annual pattern of increasing then
decreasing sunlight, peaking at thesummer solstice on June 21 or 22.
Dips in daily solar radiation are
due primarily to clouds. Smoke and
other atmospheric constituents play
a smaller role. It is noteworthy thatsolar radiation on a clear day in mid-
winter can exceed that of a cloudy day
in mid-summer. Data for August are
missing due to instr ument calibration.
The station where these data arecollected is located on the U.S. Coast
Guard dock at Tahoe City.
0
2
4
6
8
10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Kilowatth
ourspersquarem
eter
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TAHOE: STATE OF THE LAKE REPORT 2013
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Annual precipitationYear ly s ince 1910
METEOROLOGY
From 1910 to 2011, average annual
precipitation (water equivalent of rainand snow) at Tahoe City was 31.59
inches. The maximum was 69.2 inches
in 1982. The minimum was 9.2 inches
in 1977. 2012 was well below average,
with 22.48 inches of precipitation.Generally there is a gradient in
precipitation from west to east across
Lake Tahoe, with almost twice as much
precipitation falling on the west side of
the lake. (Precipitation is summed overthe Water Year, which extends from
October 1 through September 30.)
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
0
10
20
30
40
50
60
70
0
25
50
75
100
125
150
175
Inches
Centimeters
Water Year (Oct. 1 Sep. 30)
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TAHOE: STATE OF THE LAKE REPORT 2013
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Monthly precipitation2010, 2011, 2012 and 1910 to 2012 Average
METEOROLOGY
2012 was well below average in total
precipitation, and this is clearly ev identin the comparison of the monthly
precipitation with the previous two years
and the long-term average. The monthlyprecipitation for Jul-2010, Aug-2011, and
Dec-2011 was 0 inches. The 2012 Water
Year extended from October 1, 2011, throughSeptember 30, 2012.
OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP0
2
4
6
8
10
12
14
0
10
20
30
40
50
Inches
Centimeters
WY 1910 WY 2012 Average
WY 2010
WY 2011
WY 2012
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TAHOE: STATE OF THE LAKE REPORT 2013
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Snow as a fraction of annual precipitationYear ly s ince 1910
METEOROLOGY
Snow has declined as a fraction of
total precipitation, from an averageof 51 percent in 1910 to 36 percent
in present times according to the
line of best fit. In Tahoe City, snow
represented 41 percent of the 2012
total precipitation, slightly above thelong-term trend. These data are based
on the assumption that precipitation
falls as snow whenever the average
daily temperature is below freezing.
(Precipitation is summed over theWater Year, which extends from
October 1 through September 30.)
0
10
20
30
40
50
60
70
80
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Percent
Water Year
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TAHOE: STATE OF THE LAKE REPORT 2013
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Shift in snowmelt timingYear ly s ince 1961
METEOROLOGY
Although the date on which peak
snowmelt occurs varies from year to
year, since 1961 it has shifted earlieran average of 2 weeks (16.3 days). This
shift is statistically significant and is one
effect of climate change on Lake Tahoe.
In 2012, peak discharge was one of the
earliest recorded, occurring around May
4. Peak snowmelt is defined as the datewhen daily stream flows reach their
yearly maximum. Daily stream flows
increase throughout spring as the snow
melts because of rising air temperatures,
increasing solar radiation and longer
days. The data here are based on theaverage from the Upper Truckee River,
Trout Creek, Blackwood Creek, WardCreek, and Third Creek.
1960 1970 1980 1990 2000 2010
Water Year
Dateo
fPea
kSnowme
lt
Disc
hare
15APR
25APR
5MAY
15MAY
25MAY
4JUN
14JUN
24JUN
4JUL
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PHYSICAL
PROPERTIES
TERC.UCDAVIS.EDU 8
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8.1
6220
6222
6224
6226
6228
6230
6232
18961896
1897
1898
1899
1900 1920 1940 1960 1980 2000 2020
Feet
Meters
Natura l r im6223 fee t
Maximum lega l l imi t 6229.1 feet
Lake surface levelDaily since 1900
PHYSICAL PROPERTIES
Lake surface level varies throughout
the year. It rises due to high streaminflow, groundwater inflow and
precipitation directly onto the lake
surface. It falls due to evaporation,
in-basin water withdrawals,
groundwater outflows, and outflowvia the Truckee River at Tahoe City.
Overall, lake level fell during 2012. The
highest lake level was 6227.68 feet on
June 5, and the lowest was 6225.37 feet
on November 28. In 2012, the lake levelrose by only 1.3 feet during snowmelt,
compared with 3.9 feet in 2011.
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8.2
6220
6222
6224
6226
6228
6230
6232
18961896
1897
1898
1899
2010 2011 2012 2013
Feet
Meters
Natura l r im6223 f ee t
Maximum lega l l imi t 6229.1 fee t
Lake surface level, continuedDaily since 2010
PHYSICAL PROPERTIES
Identical data as used on page 8.1
except the period displayed is
shortened to 2010-2012. This moretime resolved presentation of recent
lake level data allows us to see t he
seasonal patterns in higher definition.
Data clearly show the lake level below
the natural rim at the beginning of2010 as well as the timing of highest
yearly lake levels in late spring
following snowmelt. The effects of the
very early snowmelt in 2012 on lake
level are clearly evident.
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8.3
Water temperature profileIn 2012
PHYSICAL PROPERTIES
Water temperature profile s are
measured in the lake using a SeabirdCTD at the times indicated by the
dots along the top of the figure. The
temperature is accurate to within0.005 F. Here the temperature in
the upper 330 feet is displayed as
a color contour plot. In 2012, the
lake temperature followed a typicalseasonal pattern. In late March, the
lake surface was at its coldest but
complete vertical mixing down to1625 feet did not occur. The beginning
of the 2012-2013 winter mixing is
evident at the end of the plot, with
the surface layer both cooling anddeepening. By the end of 2012, mixing
had proceeded to only 80 feet, a
relatively shallow amount. Arrowsindicate days on which profiles were
measured.
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8.4
Temperature
(oF)
Temperature
(oC)
41.0
41.5
42.0
42.5
43.0
43.5
44.0
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
Jan1970
Jan1980
Jan1990
Jan2000
Jan2010
44.5
Average water temperatureSince 1970
PHYSICAL PROPERTIES
The trend in the volume-averaged
temperature of Lake Tahoe hasincreased by approximately 0.7 F
since 1970. The monthly temperature
profile data from the lake has been
smoothed and deseasonalized tobest show the long-term trend. Up
till the late 1990s the warming rate
was considerably greater, but a high
number of deep mixing years since1997 have slowed the warming rate.
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8.5
Tem
perature
(oF)
Tem
erature
(oC)
1970
1975
1980
1985
1990
1995
2000
2005
2010
41.0
41.5
42.0
42.5
43.0
43.5
44.0
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
6.6
Annual average water temperatureSince 1970
PHYSICAL PROPERTIES
The volume-averaged temperature of
the lake for each year since 1970 isshown. In 2012 the volume-averaged
temperature increased by 0.4 F over
the previous year. The years with
the largest decreases in temperaturegenerally correspond to those years in
which deep mixing occurred.
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8.6
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
48
49
50
51
52
53
8.99.0
9.5
10.0
10.5
11.0
11.5
T
emperature(oF)
T
em
erature
oC
Surface water temperatureYear ly s ince 1968
PHYSICAL PROPERTIES
Surface water temperatures have been
recorded monthly at the mid-lake stationsince 1968 from the R/V John LeConte.
Despite year-to-year variability, water
temperatures show an increasing
trend. The average temperaturein 1968 was 50.3 F. For 2012, the
average surface water temperature
was 52.8 F, an increase of 1.6 F over
2011, making it the warmest year yetrecorded.
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8.7
40
45
50
55
60
65
70
75
80
4.445
10
15
20
25
1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
Tem
erature
(oF)
Tem
erature
(oC)
Maximum daily surface water temperatureSurface tempera ture measured since 1999 every 15 minutes
PHYSICAL PROPERTIES
Maximum daily surface water
temperatures were slightly higherin 2012 than the previous year. The
highest maximum daily surface water
temperature was 75.65 F, which was
recorded on August 14, 2012. Thelowest maximum daily surface water
temperature was 41.41 F, which was
recorded on March, 18, 2012. These data
are collected in real-time by NASA andUC Davis from 4 buoys located over the
deepest parts of the lake.
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8.8
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
60
61
62
63
64
65
66
67
68
15.56
16
17
18
19
20
Tem
perature
(oF)
Tem
erature
(oC)
July average surface water temperatureMeasured since 1999 every 2 minutes
PHYSICAL PROPERTIES
Since 1999, surface water temperature
has been recorded every twominutes from four NASA/UC Davis
buoys. Shown here are 14 years of
average surface water temperatures
in the month of July when water
temperatures are typically warmest. In2012, July surface water temperature
averaged 63.3 F, compared with 62.7
F in 2011. This increase is most likely
attributable to the absence of deep lake
mixing in 2012, an event that cools thesurface layers of the lake. The average
for the 14 year period is 64.7 F.
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8.9
Deep water temperatureSince 1970
PHYSICAL PROPERTIES
The water temperature at a depth of 1320
feet (400 m) is indicative of conditions
in the deeper waters (hypolimnion)of Lake Tahoe. Since 1970 the deep
water temperature has increased by
approximately 1 F. This increase has
not been steady but is punctuated by
occasional drops in temperature. These
coincide with times when the lake mixesall the way to the bottom, an event
which allows a huge amount of heat to
escape from the lake. The short spikes
of temperature increase are temporary
effects caused by sloshing of internal
waves.
Temperature
(oF)
Temperature
(oC)
39.8
40.0
40.2
40.4
40.6
40.8
41.0
41.2
41.4
41.6
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5.0
5.1
5.2
Jan1970
Jan1980
Jan1990
Jan2000
Jan2010
5.3
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8.10
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
16401600
1400
1200
1000
800
600
400
200
0
500
400
300
200
100
0
Dep
th
(feet)
Depth
(meters)
Depth of mixingYear ly s ince 1973
PHYSICAL PROPERTIES
Lake Tahoe mixes each winter as surfacewaters cool and sink downward. In a
lake as deep as Tahoe, the wind energy
and intense cooling of winter stormshelps to determine how deeply the lake
mixes. Mixing depth has profound
impacts on lake ecology and waterquality. Deep mixing brings nutrients
to the surface, where they promote
algae growth. It also moves oxygento deep waters, promoting aquatic
life throughout the water column.
The deepest mixing typically occursin February to March. In 2012, Lake
Tahoe mixed to a depth of only 820 feet
(250m). This lack of deep mixing mostlikely contributed to the warmer surface
temperature and the improved clarity.
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8.11
Lake stabilitySince 1968
PHYSICAL PROPERTIES
When the lake has a vertical distribution
of temperature, it has a corresponding
density distribution, with warm andlighter water at the sur face, and colder,
denser water at depth. The stability
index is a measure of the energy required
to fully mix the lake when its density
is stratif ied. Plotted here is the average
stability index for the upper 100 meters(330 feet) of Lake Tahoe for the period
of May through October each year. The
values are derived from temperature
profiles taken at the Index Station at
approximately 10-20 day intervals. There
has been an overall increase in lakestability in the last 45 years. In 2012, the
lake stability was at an all time high.
900
950
1000
1050
1100
1150
1200
1250
1300
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
MaytoOctaveragesta
bility
index
(kilog
ramspersquaremeter)
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8.12
Stratified season lengthSince 1968
PHYSICAL PROPERTIES
The stability index, a measure of the
energy required to fully mix the lake,
can be evaluated for every day ofthe year. We define the stratification
season as the length of time when the
stratification index exceeds a value of
600 kilograms per square meter. Since
1968 the length of the stratificationseason has lengthened, albeit with
considerable year-to-year variation.
Overall the stratification season has
lengthened by approximately three
weeks.
130
140
150
160
170
180
190
200
210
220
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
Length
ofstratification
sea
son
(days)
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8.13
Beginning of the stratification seasonSince 1968
PHYSICAL PROPERTIES
The length of time that Lake Tahoe is
stratified has lengthened since 1968by approximately three weeks. The
commencement of stratification appears to
occur earlier in the year by approximatelythree days on average.
120
130
140
150
160
170
180
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
Beginnin
g
ofstratification
season
(Ju
lian
day)
(days)
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8.14
End of stratification seasonSince 1968
PHYSICAL PROPERTIES
The length of time that Lake Tahoe is
stratified has lengthened since 1968 byapproximately three weeks. The end
of stratification appears to have been
extended by approximately 18 days on
average. In other words, the fall season forthe lake has been considerably extended.
280
290
300
310
320
330
340
350
360
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
En
d
ofstratification
season
(Ju
lian
day)
(days)
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8.15
Peak stratification valueSince 1968
PHYSICAL PROPERTIES
The maximum value that the stability
index obtains for each year has beenplotted. As can be seen, the strength of the
stratification has not changed significantly
since 1968. However, as the previous
figures indicate, the length of time forwhich the lake remains density stratified
has increased.
190
200
210
220
230
240
250
260
270
280
1965 1975 1985 1995 2005 2015
Pea
k
of
stratification
seaso
n
(Ju
lian
day)
(days)
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8.16
0
100
200
300
400
500
600
0
4
8
12
16
Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep
(cu
bicmete
rspersecon
d)
Truckee River at Tahoe City
Upper Truckee River
Mean
dailystream
flow
(cu
bicfee
tpersecon
d)
Mean daily streamflow of Upper Truckee River vs.
Truckee RiverWater Year 2012
PHYSICAL PROPERTIES
The Upper Truckee River, the largest
inflow into Lake Tahoe, has a naturalannual hydrograph for a snow-fed stream.
The small peaks in the hydrograph
represent rain events or short warmperiods in winter or spring. The major
peak in the hydrograph represents themaximum spring snowmelt. The peak
in 2012 was 678 cubic feet per second
on April 26, two-thirds of the previousyears peak. The Truckee River is the only
outflow from Lake Tahoe. It is a regulatedflow, with release quantity controlled
by the Federal water master. The release
rates are set according to downstreamdemands for water and concerns for
flooding. The maximum discharge in 2012was 368 cubic feet per second on June 26
(50% higher than the previous year), and
the peak temperature of the dischargewas 72.7 F on August 15. Streamflow
data are collected by the US Geological
Survey under the Lake Tahoe InteragencyMonitoring Program (LTIMP).
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8.17
0
10
20
30
40
0.0
0.2
0.4
0.6
0.8
1.0
1980 1985 1990 1995 2000 2005 2010
(billioncu
bicmeters)
Water year
Truckee River at Tahoe City
Upper Truckee River
Annua
lstream
flow
(billioncu
bicfeet)
Truckee River summer discharge and lake elevationSince 1980
PHYSICAL PROPERTIES
Flow into Lake Tahoe (e.g. UpperTruckee River) and discharge out of
Lake Tahoe (Truckee River at TahoeCity) have shown considerable variation
since 1980. The large peaks in discharge
from the lake correspond to years whenprecipitation (and therefore total inflow)
was the greatest, e.g. 1982-1983, 1986,
1995-1999. Similarly, the drought-like
conditions in the early 1990s and the
low precipitation years in the beginningof the 2000s also stand out. Since many
of the pollutants of concern for Lake
Tahoes clarity enter along with surfaceflow, year-to-year changes in clarity are
influenced by precipitation and runoff.
In 2012 discharges into and out of the
lake were well below the long-term
averages. The Upper Truckee Riverinflow volume was 1.80 billion cubic feet
(long-term average 3.09). The TruckeeRiver discharge was 5.65 billion cubic
feet (long-term average 7.29).
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8.18
Truckee River Summer Water TemperaturesSince 1993
PHYSICAL PROPERTIES
56
58
60
62
64
66
68
13
14
15
16
17
18
19
20
1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012
Temp
erature
(degrees
C)
Tem
erature
de
rees
F
Truckee R iverwate r tempe rature
Tahoe Cityair tempe rature
Water temperature of the Truckee River
as it departs Lake Tahoe in the summer
months (July-September) is measuredby the US Geological Survey. Data gaps
prevent a complete pattern, but themeasurements suggest that a 4-5 F rise
in the average temperature has occurred
during the period since 1993. Average air
temperatures from Lake Tahoe for the
same period also suggest a temperaturerise but at a lower rate. Rising river
temperatures impact downstream fishspawning.
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8.19
Annual Discharge Volume for Upper Truckee River andTruckee RiverSince 1980
PHYSICAL PROPERTIES
0
50
100
150
200
250
300
350
400
0
1
2
3
4
5
6
7
8
9
1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012
Truckee Rivermean daily discharge
Lake Tahoegage height
Disc
harge(cu
bicfeetpersecond)
G
age
height(feet)
Flow rate of the Truckee River as it
departs Lake Tahoe in the summermonths (July-September) and lake level
is measured by the US Geological Survey.
Here the relationship between these twovariables is evident, with mean daily
river discharge typically showing a one
year lag from the mean lake elevation.
Gage height is measured relative to adatum of 6,220 feet. Release of water
from Lake Tahoe is controlled by theFederal Water Master.
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NUTRIENTS AND
PARTICLES
TERC.UCDAVIS.EDU 9
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9.1
16%
15.5%
55%
0.5%0.5%
12.5%
Total Nitrogen
39%
26%
15%
1%
4%
15%
Urban Watershed
Nonurban Watershed
Atmospheric Deposition
Stream Channel Erosion
Shoreline Erosion
Groundwater
Total Phosphorus
72%
9%
15%4%
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9.2
Pollutant loads from seven watershedsIn 2012
NUTRIENTS AND PARTICLES
The Lake Tahoe InteragencyMonitoring Program (LTIMP)
measures nutrient and sediment
input from seven of the 63 watershedstreams a reduction of three streams
since 2011. Most of the suspendedsediment contained in the 7 LTIMP
streams is from the Upper Truckee
River, Blackwood Creek, Trout Creek
and Ward Creek. Over 75 percentof the phosphorus and nitrogen
comes from the Upper Truckee River,Trout Creek and Blackwood Creek.
Pollutant loads from the west-side
streams were a factor of four lowerin 2012, compared with 2011. This
was largely due to the drier year that
the basin experienced. Blackwood
Creek suspended sediment loadshave exceeded those of the Upper
Truckee River for the last five years.For the eastside streams, Incline Creek
pollutant loads fell relative to Third
Creek particularly for nitrogen.
N = NitrogenP = PhosphorusDIN = Dissolved Inorganic NitrogenSRP = Soluble Reactive PhosphorusTON = Total Organic NitrogenSS = Suspended Sediment
The LTIMP stream water quality
program is supported by the U.S.
Geological Survey in Carson City,Nevada, UC Davis TERC and the
Tahoe Regional Planning Agency.Additional funding was provided
by the USFS Lake Tahoe Basin
Management Unit.
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9.3
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
0
10
20
30
40
50
60
Metrictons
Total organic nitrogen
Dissolved inorganic nitrogen
Water year
Nitrogen contribution by Upper Truckee RiverYear ly s ince 1989
NUTRIENTS AND PARTICLES
Nitrogen (N) is important because it,
along with phosphorus (P), stimulatesalgal growth (Fig. 9.1 shows the major
sources of N and P to Lake Tahoe). The
Upper Truckee River is the largest of the63 streams that flow into Lake Tahoe,
contributing about 25 percent of the
inflowing water. The rivers contribution
of dissolved inorganic nitrogen (nitrate
and ammonium) and total organicnitrogen loads are shown here. The
year-to-year variations primar ily reflect
changes in precipitation. For example,1994 had 16.6 inches of precipitation
and a low nitrogen load, while 1995had 60.8 inches of precipitation and a
very high nitrogen load. Similarly 2012
had 22.48 inches of precipitation and2011 had 51.78 inches of precipitation.
This below-average precipitation in
2012 resulted in a nitrogen load thatwas almost one quarter of the previous
years. (One metric ton = 2,205 pounds.)
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9.4
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
0
2
4
6
8
10
Metrictons
Other phosphorus
Soluble reactive phosphorus
Water year
Phosphorus contribution by Upper Truckee RiverYear ly s ince 1989
NUTRIENTS AND PARTICLES
Soluble reactive phosphorus (SRP)
is that fraction of phosphorusimmediately available for algal growth.
As with nitrogen (Fig. 9.3), the
year-to-year variation in load largely
reflects the changes in precipitation.
Below average precipitation in 2012resulted in a factor of three reduction
of the phosphorus load over the
previous year. Total phosphorus is
the sum of SRP and other phosphorus,
which includes organic phosphorusand phosphorus associated w ith
particles. (One metric ton = 2,205
pounds.)
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9.5
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Metrictons
Suspended sediment
Water ear
Suspended sediment contribution by Upper Truckee RiverYear ly s ince 1989
NUTRIENTS AND PARTICLES
The load of suspended sediment
delivered to the lake by the UpperTruckee is related to landscape
condition and erosion as well asto precipitation and stream flow.
Certainly, inter-annual vari ation in
sediment load over shorter time scalesis more related to the latter. Below
average precipitation in 2012 resulted
in a factor of three decrease of thesuspended sediment load compared
with the previous years. This andthe previous two figures illustrate
how greatly changes in hydrological
conditions affect pollutant loads.Plans to restore lake clarity emphasize
reducing loads of very fine suspendedsediment (less than 20 microns in
diameter). Efforts to restore natural
stream function and watershedcondition focus on reducing loads of
total sediment regardless of size.
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9.6
0
50
100
150
200
250
300
350
400
0
1
2
3
4
5
6
1980 1984 1988 1992 1996 2000 2004 2008 2012
DIN Avg. Conc. SRP Avg. Conc.
Dissolv
edinorganicnitrogen
(microgramsper
liter)
So
lublereactivep
hosp
hor
us
(microgramsper
liter)
Nutrient concentrations in rain and snowYear ly s ince 1981
NUTRIENTS AND PARTICLES
Nutrients in rainwater and snow
(called wet deposition) contributelarge amounts of nitrogen, but also
significant phosphorus, to Lake
Tahoe. Nutrients in precipitationhave been measured near Ward Creek
since 1981, and show no consistent
upward or downward trend. Annualconcentrations in precipitation of
dissolved inorganic nitrogen (DIN)
and soluble reactive phosphorus(SRP) vary from year to year. In
2012, concentrations of DIN and
SRP increased significantly over theprevious year. This may be due to the
lower precipitation in 2012.
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9.7
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
0
500
1000
1500
2000
2500
0
10
20
30
40
50
60
70
80
Precipitation
(inc
hes)
Water YearDIN Load SRP Load Precipitation
0
50
100
150
200
250
D
isso
lve
dinorganicnitrogen
(gramsper
hectare)
S
olublereactivep
hosp
horus
(gramsper
hectare)
Nutrient loads in rain and snowYear ly s ince 1981
NUTRIENTS AND PARTICLES
The annual load for wet depositionis calculated by multiplying the
concentration of dissolved inorganic
nitrogen (nitrate and ammonium)and soluble reactive phosphorus (in
the previous graph) by total annualprecipitation. While nitrogen and
phosphorus loads from precipitation
have varied from year to year at theWard Creek monitoring site, no
obvious long-term trend has emerged.In 2012, the nitrogen and phosphorus
loads were within the range seen inprevious years.
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9.8
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
0
5
10
15
20
25
N
itratenitrogen
(mic
rogramsper
liter)
Lake nitrate concentrationYear ly s ince 1980
NUTRIENTS AND PARTICLES
Since 1980, the volume-weighted
annual average concentration of nitrate-nitrogen has remained relatively
constant, ranging between 13 and
19 micrograms per liter. In 2012,the volume-weighted annual average
concentration of nitrate-nitrogen
was 18 micrograms per liter. Thesemeasurements are taken at the MLTP
(mid-lake) station. Water samples could
not be collected in March or September2012 due to weather conditions.
However, the two collections in
October (10/2/12 and 10/18/12) wererepresentative of the Sept-Oct 2012
period.
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9.9
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Totalhyd
rolyza
blep
hosp
horus
(mic
rogramsper
liter)
Lake phosphorus concentrationYear ly s ince 1980
NUTRIENTS AND PARTICLES
Phosphorus naturally occurs in
Tahoe Basin soils and enters the lakefrom soil disturbance and erosion.
Total hydrolyzable phosphorus, or
THP, is a measure of the fractionof phosphorus algae can use to
grow. It is similar to the SRP that
is measured in the streams. Since
1980, THP has tended to decline. In2012, the volume-weighted annual
average concentration of THP was
approximately 1.8 microgramsper liter, a slight decrease over
the previous year. Water samples
could not be collected in March
or September 2012 due to weatherconditions. However, the two
collections in October (10/2/12 and
10/18/12) were representative of theSept-Oct period.
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BIOLOGY
10TERC.UCDAVIS.EDU
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10.1TERC.UCDAVIS.EDU
0
50
100
150
200
250
1960
1970
1980
1990
2000
2010
Carbon,gr
amspersquarem
eter
Algae growth (primary productivity)Year ly s ince 1959
BIOLOGY
Primary productivity is a measure
of the rate at which algae producebiomass through photosynthesis.
It was first measured at Lake Tahoe
in 1959 and has been continuously
measured since 1968. Primary
productivity has generally increasedover that time, promoted by nutrient
loading to the lake, changes in the
underwater light environment and a
succession of algae species. In 2012,
primary productivity was 243.8 gramsof carbon per square meter.
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10.2TERC.UCDAVIS.EDU
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2009
2010
2011
2012
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Chlorop
hy
ll
(mic
rogramsper
liter)
Algae abundanceYear ly s ince 1984
BIOLOGY
The amount or biomass of free-f loating
algae (phytoplankton) in the water isdetermined by extracting and measuring
the concentration of chlorophyll a, a
photosynthetic pigment that allows plants
to absorb energy from light. Though the
value varies annually, it has not shown asignificant increase since measurements
began in 1984. The annual average value
for 2012 was 0.68 micrograms per liter.
The average annual chlorophyll a level
in Lake Tahoe has remained relativelyuniform since 1996. For the period of
1984-2012 the average value was 0.71
micrograms per liter.
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10.3TERC.UCDAVIS.EDU
Annual distribution of algal groupsYear ly s ince 1982
BIOLOGY
The amount of algal cells from different
groups varies from year to year. Diatomsare the most common type of alga,
comprising 40 to 60 percent of the
total abundance of algal cells each year.
Chrysophytes and cryptophytes are
next, comprising 10 to 30 percent ofthe total. While the proportion of the
major algal groups show a degree of
consistency from year-to-year, TERC
research has shown that the composition
of individual species within t he majorgroups is changing, both seasonally and
annually, in response to lake condition.
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10.4TERC.UCDAVIS.EDU
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC0
20
40
60
80
100
120
140
160
180
2012
Chlorophytes
Chrysophytes
Cryptophytes
Diatoms
Dinoflagellates
Cyanophytes
Phycomycetes
Biovo
lume
(cu
bicm
illimeterspercu
bic
meter)
Algal groups as a fraction of total populationMonthly in 2012
BIOLOGY
Algae populations vary month to
month, as well as year to year. In2012, diatoms again dominated the
phytoplankton community, especially
in the first six months of the year.
Diatom concentrations peaked in April(the bloom) and stayed high through
June. In the previous year (2011), the
spring bloom occurred two to three
months later, highlighting the naturalvariability in t he lakes biota.
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10.5TERC.UCDAVIS.EDU
Nutrient limitation of algal growthFor 2002 - 2012
BIOLOGY
JanApr MaySep OctDec0%
20%
40%
60%
80%
100%
N P N+PN P N+P N P N+P
Percento
fexperimentswith
increase
da
lga
lgrowth
Bioassays determine the nutrient
requirements of phytoplankton.In these experiments, nutrients
are added to samples of lake water
and the change of algal biomass ismeasured. Phytoplankton response
to nutrient addition for the period
2002-2012 is summarized in the
panels below. Between January and
April, algal growth was limited largelyby phosphorus (P). From May to
September, Nitrogen (N) added by
itself was more stimulatory, but thelake was co-limited, as shown by
the greater response to adding bothnutrients. Phosphorus is slightly
more stimulatory from October to
December, but co-limitation wasagain the dominant condition. These
results highlight the role of nutrients
in controlling algal growth. They alsounderscore the synergistic effect when
both are available.
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10.6TERC.UCDAVIS.EDU
Predominance of Cyclotella sp.From 2002 through 2012
BIOLOGY
0x100
2x106
4x106
6x106
8x106
10x106
2002 2004 2006 2008 2010 2012
Cellsper
liter
5 m 20 m
0
10
20
30
40
50
Secchidepth
(m)
Secchi depth
1x107
0
In 2008, one species of algae, Cyclotella,
started to dominate the make up of algaeat Lake Tahoe. The cells range in size
from 4 - 30 microns in diameter. During
the summer, the smallest cells, 4 - 5microns control the community in the
upper euphotic. This size range is ideal
for light scattering, and the growing
numbers of Cyclotella in 2008-2011 were
believed to be in large part responsiblefor the major decline in summer clarity
in those years. In 2012 the concentration
of Cyclotella cells decreased to thelowest level in five years, and summer
clarity showed an improvement over theprevious four years. The red and blue
lines below indicate the concentrations
of Cyclotella at depths of 20 m (66 ft) and5 m (16.5 ft) respectively. The black lines
indicate the individual Secchi depths
taken since 2002. The summer values ofSecchi depth coincide perfectly with the
changes in Cyclotella concentration.
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10.7TERC.UCDAVIS.EDU
C
hlorop
hy
ll
(milligrams
persquaremeter)
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
0
50
100
150
200
250
300
SugarPine Pt.
Zephyr Pt.
Pineland
Tahoe City
Shoreline algae populationsYear ly s ince 20 00
BIOLOGY
Periphyton, or attached algae, makes
rocks around the shoreline of LakeTahoe green and slimy, or sometimes
like a very plush white carpet.
Periphyton is measured eight timeseach year, and this graph shows the
maximum biomass measured at four
of the sites. In 2012, concentrations at
Sugar Pine Pt. (no urban inf luence) andZephyr Pt. (low urban influence) were
below the long-term average. The site
with the most periphyton (Pineland)is close to an urban area, and was
relatively high this year. The Tahoe
City value was about average for that
site. To date, no statistically significantlong-term trend in maximum
periphyton biomass has been detected
at any of these individual locations.Monitoring periphyton is an important
indicator of near-shore health.
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10.8TERC.UCDAVIS.EDU
Shoreline algae distributionIn 2012
BIOLOGY
Periphyton biomass was surveyed
around the lake during the springof 2012, when it was at its annual
maximum. Nearly 45 locations
were surveyed by snorkel in 1.5feet of water. A Periphyton Biomass
Index (PBI) is used as an indicator
to reflect what the casual observerwould visually detect looking into
the lake from the shoreline. The PBIis defined as the percent of the local
bottom area covered by periphyton
multiplied by the average length ofthe algal filaments (cm). Zones of
elevated PBI are clearly seen. (The
width of the colored band does notrepresent the actual dimension of
the onshore-offshore distribution.
Similarly its length does not representthe longitudinal extent.) Overall
conditions in 2012 were similar to2011.
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CLARITY
TERC.UCDAVIS.EDU 11
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11.1
Annual average Secchi depthYearly since 1968
CLARITY
In 2012 the annual average Secchi
depth was 75.3 feet, an improvementof 6.4 feet over the previous year.
This was the second consecutive yearof improved annual average clarity.
The annual average clarity in the
past decade has been better than in
recent decades. In 1997-1998, annualclarity reached an all-time average
low of 65.1 feet. From 2003-2012
the average clarity was 70.1 feet.The clarity level is the average of 22
individual readings taken throughout
the year. The highest individual value
recorded in 2012 was 107 feet, andthe lowest was 57 feet. It is important
to understand the causes of clarity
change and to evaluate past actionsand future investments. Some critical
knowledge gaps are in the monitoring
of urban stormwater flows.
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11.2
Winter Secchi depthYearly since 1968
CLARITY
Annual winter (December-March)
Secchi depth measurements from1968 to the present indicate that
winter clarity at Lake Tahoe isshowing definite improvement. In
2012, the winter clarity increased to
88.3 feet, well above the worst pointseen in 1997. The reasons behind the
continued improvement in winterclarity are not fully understood, but
possibly tied to reductions in the
quantity of fine particles f rom urbanstormwater. Dry conditions in 2012
contributed to the trend. Urban
stormwater is the largest source offine particles to Lake Tahoe, and
generally enters the lake in winter.A comprehensive, regional urban
stormwater monitoring plan is
needed to determine if recent capitalinvestments in stormwater projects
have indeed reduced these loads.
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11.3
Summer Secchi depthYearly since 1968
CLARITY
Summer (June-September) clarity
in Lake Tahoe in 2012 was 64.4 feet,an improvement of over 13 feet from
2011. This coincided with a decline inthe concentration of small algal cells
in 2012. Despite this improvement,
the summer trend is dominated by aconsistent long-term decline but with a
noticeable 10-15 year cyclic pattern. The
red dashed lines are l inear regressionsfor the per iods: a) 1976 to 1983, b)
1987-1998, and c) 2001 to 2011. The
most recent improvement may be acontinuation of this cyclical trend. The
reasons behind this periodicity are
being investigated.
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11.4
9Jan2012
De
th
feet
18Oct20122Mar2012
Light transmission (%)
1500
1250
1000
750
500
250
088 92 96 88 92 96
10Dec2012
88 92 96
457.2
550
500
450
400
350
300
250
200
150
100
50
088 92 96
Depth
(meters)
Light transmissionIn 2012
CLARITY
A light transmissometer measures
what percentage of a given wavelengthof light is transmitted over a 10
inch path length. Here, the lighttransmission through the full depth
of the lake is shown at four times
a year. It is evident that the lowestlight transmission is in the surface
layers where typically less than 93
percent of light is transmitted. Thehighest light transmission is in the
very deepest parts of the lake where
as much as 96 percent of the light canbe transmitted. The reason is that fine
particles are believed to aggregate into
larger particles that rapidly settle outin the deep water.
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EDUCATION AND
OUTREACH
TERC.UCDAVIS.EDU 12
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12.1
Education center tours
Student education
Workshops
Community presentationsResearc vesse
oca organ zat ons
TOTAL NUMBER OF CONTACTS: 11,817
TERC education and outreachIn 2012
EDUCATION AND OUTREACH
Part of TERCs mission is education
and outreach. During 2012, TERC
recorded 11,817 individual visitorcontacts. The majority represented
student field trips and visitors tothe Thomas J. Long Foundation
Education Center at Incline Village.
In addition, TERC hosts monthlypublic lectures and workshops, makes
presentations to local organizations
and takes a limited number of visitorsout on our research vessels. TERC
organizes and hosts annual events
and programs including ChildrensEnvironmental Science Day, Science
Expo, Youth Science Institute, Trout
in the Classroom program, ProjectWET workshops, Summer Tahoe
Teacher Institute and a volunteer
docent training program. TERC alsopartners with numerous groups to
deliver education in the Tahoe basin.
In 2012, these included AmeriCorps,COSMOS, Sierra Watershed Education
Partnerships (SWEP), Space Sciencefor Schools, Young Scholars and many
others.
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TERC.UCDAVIS.EDU
TAHOE: STATE OF THE LAKE REPORT 2013
12.2
TERC educational exhibitsIn 2012
EDUCATION AND OUTREACH
Hands-on science exhibits such
as the new Shaping Watersheds
Interactive Sandbox will join theresearch vessel, laboratory and award-
winning 3D movie Lake Tahoe inDepth to provide guests with the
latest Lake Tahoe research. Several
new 3D visuali zations, includingLakes of the World and Following
a Drop of Water, are also being
developed as part of a larger informalscience education project funded by the
National Science Foundation.
Two new aquariums join the exhibit
area p