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EnvironmentalAssessmentfortheRathbunLakeWatershed:SamplingDesign,MethodsandResultsbyJ.D.Opsomer1,Z.Wu1,R.Cruse1,T.Isenhart1,V.Sitzmann2,T.J.Jacobsen3,J.Y.Kim1,C.Botts11IowaStateUniversity2IowaDepartmentofAgricultureandLandStewardship3RathbunRegionalWaterAssociationJune11,2001IowaStateUniversityAmes,Iowa2EnvironmentalAssessmentfortheRathbunLakeWatershed:SamplingDesign,MethodsandResultsAbstractThisarticledescribesthesamplingdesign,surveymethodologyandfindingsofanaturalresourcessurveyconductedintheRathbunLakeWatershedinSouthernIowain1999-2000.Thegoalofthesurveywastoquantifytheerosionfromallsourcesonagriculturallandsandtheecologicalhealthofstreamsforeachof61subwatershedsinthearea.Atotalof183plotsofapproximately160acresinsizewereselectedbystratifiedrandomsamplingfortheerosionmeasurements,andthestreamslocationswereselectedbyatwo-stageprocedure.Themeasurementinstrumentsforallerosiontypes(stream,gully,ephemeralgully,sheetandrill),aswellasthatforthestreamhealthassessment,arebrieflydescribedinthearticle.Alltheselectedplotsandstreamlocationsweresampledduringfieldvisitsinthestudyperiod.Acompositesmallareaestimationprocedurewasusedtoproducethesubwatershed-levelerosionestimates.1.IntroductionTheRathbunLakeWatershed,locatedontheCharitonRiverinsouthernIowa,provideswatertomorethan50,000residentsinIowaandMissouri.Thewatershedincludesover365,000acres,includingpartofClarke,Lucas,Monroe,Decatur,WayneandAppanoosecounties.Numeroussmalllivestockfeedingoperationsarelocatedinthewatershed,andabout133,000acresinthewatershedarehighlyerodiblecropland,makingRathbunLakevulnerabletobothpointandnon-pointsourcesofwaterpollution.Themostimportantnon-pointsourcepollutionisthoughttobesoillossfromagriculturalland,causingfastersedimentationrate,highlevelofturbidityandelevatedherbicidelevelinthewater.DuetotheimportanceofRathbunLakeasawatersupply,wildliferesourceanditsrecreationalvalue,thewaterqualityinthelakeisofgreatconcern(RathbunLakeProject,1999).In1999,ateamofscientistsfromIowaStateUniversity,theCharitonValleyResourceConservationandDevelopmentofficeandtheU.S.DepartmentofAgricultureNaturalresourcesConservationServiceinitiatedaprojecttoassesstheenvironmentalhealthofthewatershedandtodevelopamanagementplanforimprovingitsfutureenvironmentalhealth.TheRathbunLakewatershedisdividedinto61sub-watershedsdefinedbythe14-digitHydrologicUnitCodes(HUC),withanaveragesizeof5,800acreseach.Estimation3ofthepollutionbysubwatershedsisoneofthemajorobjectivesoftheprojectandwillmakeitpossibletodeveloplocallytargetedmanagementplans.TheultimateobjectiveofthestudyistoachievetargetedlevelsofwaterqualityprotectionandimprovementinRathbunLake.Sinceerosionrepresentsoneofthemajorcomponentstowaterpollutioninthewatershed,asurveywasdesignedtoestimatethetotalamountoferosionthatisdeliveredtothestreamsinthewatershed.Erosionsourcesneedtobeidentifiedandtheamountsoferosiondeliveredintolocalwaterwaysfromeachsourcesneedtobequantified.Commonsourcesoferosionaresheetandrill,ephemeralgullies,gullies,streambanks,roads,constructionsites,andfeedlots.Aftersourceandquantityareidentified,asedimentdeliveryprocedurecanusedtodeterminedeliverytoaspecificlocation.Thissurvey,togetherwithacompleteenumerationofthemajorpointsourcesoferosionandotherwaterpollutants(e.g.livestockfeedlots),madeitpossibletodescribethemostimportantimpairmentstowaterqualityintheRathbunLakewatershed.Inadditiontoquantifyingthewaterpollutionimpacts,itwasalsoofinteresttoassessthecurrentecologicalhealthstatusofwaterwaysinthewatershed.Biologicalorecologicalintegrityofstreamsystemsiscommonlydefinedastheabilitytosupportandmaintainabalanced,integrated,andadaptivecommunitywithabiologicaldiversity,composition,andfunctionalorganizationcomparabletothoseofnaturalaquaticecosystemsintheregion(Frey1977).KarrandDudley(1981)listfourmainclassesofvariablesasdeterminantsoftheecologicalintegrityofrunningwatersystems:flowregime,habitatstructure,waterquality,andenergyrelationshipsandcommunitydynamics.Therefore,asecondconcurrentsurveywasusedtoperformanassessmentofthequalityofriparianandstreamhabitatThisdocumentdescribesthemethodologyandresultsofbothsurveysconductedaspartofthewatershedhealthassessment.Thefirstsurveymeasureserosionfromallmajorsourcesonagriculturallandon183randomlyselected160-acreplots.Thesecondsurveyisanassessmentofthequalityofriparianandstreamhabitat,measuredat110streamsitesinthewatershed.Thedocumentisstructuredasfollows.Section2describesthesamplingdesignofthesurveys.InSection3,themeasurementinstrumentsanddatacollectionmethodologiesarediscussed.Section4providessomebackgroundonthecalculationoftheestimates.Finally,Section5describestheresultsfromthesurveys.2.SamplingDesignWewillfirstdescribethesamplingdesignfortheerosionsurvey.Thesamplingunitisaplotofapproximately160acres,correspondingtoaquarterofasectionasdefinedbythePublicLandSurveySystem(PLSS).4Asinmanygeographicalsurveys,themainfactorfordecidingthesizeandnumberofsamplingunitsdependsonthetrade-offbetweencostandprecisionforthevariablesofinterest.Ifineachsub-watershed,thetotalareaonewouldliket