State of the Lake 2013

8/22/2019 State of the Lake 2013

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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. 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


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


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

[email protected]

(775)881-7560,ext.7563

INTRODUCTION


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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.

(CONTINUED ON NEXT PAGE)


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2.2

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).

(CONTINUED FROM PAGE 2.1)


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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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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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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.


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.


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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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).


The decline in the percentage of precipitation as snow in the next 100 years under the GFDL-A2 scenario.


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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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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.


Sugar pine seedlings at the Institute of Forest Genetics.

Sugar Pine

Western White Pine


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Real-time monitoring of water quality in Lake Tahoe


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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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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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.


21/80




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.


22/80

7

METEOROLOGY

TERC.UCDAVIS.EDU


23/80

7.1


TERC.UCDAVIS.EDU

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.


24/80

7.2


TERC.UCDAVIS.EDU

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


25/80

7.3


TERC.UCDAVIS.EDU

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


26/80

7.4


TERC.UCDAVIS.EDU

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


27/80

7.5


TERC.UCDAVIS.EDU

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)


28/80

7.6


TERC.UCDAVIS.EDU

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


29/80

7.7


TERC.UCDAVIS.EDU

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


30/80

7.8


TERC.UCDAVIS.EDU

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


31/80

PHYSICAL

PROPERTIES

TERC.UCDAVIS.EDU 8


32/80

TERC.UCDAVIS.EDU


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.


33/80

TERC.UCDAVIS.EDU


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.


34/80

TERC.UCDAVIS.EDU


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.


35/80

TERC.UCDAVIS.EDU


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.


36/80

TERC.UCDAVIS.EDU


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.


37/80

TERC.UCDAVIS.EDU


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.


38/80

TERC.UCDAVIS.EDU


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.


39/80

TERC.UCDAVIS.EDU


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.


40/80

TERC.UCDAVIS.EDU


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


41/80

TERC.UCDAVIS.EDU


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.


42/80

TERC.UCDAVIS.EDU


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)


43/80

TERC.UCDAVIS.EDU


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)


44/80

TERC.UCDAVIS.EDU


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)


45/80

TERC.UCDAVIS.EDU


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)


46/80

TERC.UCDAVIS.EDU


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)


47/80

TERC.UCDAVIS.EDU


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).


48/80

TERC.UCDAVIS.EDU


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).


49/80

TERC.UCDAVIS.EDU


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.


50/80

TERC.UCDAVIS.EDU


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.


51/80

NUTRIENTS AND

PARTICLES

TERC.UCDAVIS.EDU 9


52/80

TERC.UCDAVIS.EDU


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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TERC.UCDAVIS.EDU


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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TERC.UCDAVIS.EDU


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


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


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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TERC.UCDAVIS.EDU


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


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 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


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


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


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

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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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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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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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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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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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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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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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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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TERC.UCDAVIS.EDU


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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TERC.UCDAVIS.EDU


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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TERC.UCDAVIS.EDU


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


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

Documents

State of the Lake 2013