A Study Of Soatial Patterns In The East Branch Of The .
A Study of SoatialPatternsin the EastBranchof the Waterb Reservoir.in Waterb . Vennont.ResearchPaper PresentedBy:Kim DeMayoMay 2003In partial fulfillment of a Bachelorof Sciencein ty of Vermont\
ABSTRACT: This studylooks at the spatialpatternof sedimentdepositionin the EastBranchof the WaterburyReservoir. This researchis part of a larger project incorporatingthe entire Waterburyreservoirin a studyof land-usechangeandthe effect onsedimentation.Sedimentationinformation is usefulbecauseconstructionwithin thewatershedcould causesignificantimpactto the area. Reservoirscontaina recordof landusechangeand stormeventswithin the sediment.The studyof this sedimentcandetermine information about the area. 76 sedimentcoreswere takenin transectsalongthe EastBranchof the WaterburyReservoir. Using field information andcomputeranalysis,a patternof sedimentthickeningdownstreamwas seen. Bulk densityinformation was also takenfrom nine samplesitesalongthe reservoir. However,nopatternwasfound in the bulk densityinformation. Sandlayerswere evidentin a majorityof the cores,which meanstherewas a largehydrologic eventthat occurredafter theconstructionof the dam. The informationfound in this studycontainsinformation abouthow sedimentdepositionaffectsthe watershedof the WaterburyReservoir.2
INTRODucnON:The constructionof a damfor storage,power,or recreationcan causesignificanteffectsto the downstreamareaaswell asto the flow conditionsof natural streamsinsideand downstreamof the artificial lake (Johnson,2002). Flow regulationby damshasdisruptedthe naturalcun-entof mostrivers andhasalteredthe processesthat sustainbiodiversity (Johnson,2002). The constructionof damshasreducedthe transportofnutrientsresultingin severeaffectsto food web structures(Conley et aI, 2000). The damitself often blocks sediment,resultingin a diminishing storagecapacity. If the reservoirfills with sediment,flooding canoccur and further affect the area. The land-useandenvironmentalhistory of the drainagebasininfluencesthe rate of erosion,which leadstosedimentationin reservoirs. Severalmethods,suchascoring and stratigraphicanalysis,Cesium-127dating, and mineral magnetictechniquescanhelp datethe sedimentanddeterminehow fast the reservoiris filling. The reservoiritself actsasa sedimentyieldrecordfor determiningthe erosionratesof the area.Thousandsof damshavebeenconstructedallover the United Statesfor floodcontrol, storage,and power. The New England water resourcesregion has the highestdensity of dams in the country. which is a legacy of the regions long history of mill-dams(Graf, 1999). The reservoirs behind the dams provide a record of sediment yield for theimpounded basin. Sediment cores taken within semi-enclosed aquatic basins can providea historical record of land-use within the watershed (MeCray et aI, 2001). The sedimentrecord can be studied in order to gain an understanding of how sediment yield has variedover time (Ambers,2001).3
My researchproject exploredthe patternsof sedimentdepositionin a floodcontrol reservoirlocatedin northwesternVemlont. This paperreviewsrelevantliteratureon sedimentationstudiesin lakes,pondsandreservoirsandoutlinesthe methodologiesandresultsof my study.FAcroRS AFFEcrING EROSIONRATES:Sedimentdepositsin lakes,pondsor reservoirscan be usedto estimatesedimentyields andreconstructchangingpatternsof sedimentsourcesin responsetoenvironmentalchanges.Thereis a relationshipbetweenland-usechange,soil erosion,and sedimentyield. The sedimentdepositsaccumulatingin lakescontainboth thematerialthat originatedin the waterbody andthe materialderivedfrom externalsources,suchas sedimenterodingfrom the surroundingdrainagebasin. A variety of techniquescanbe usedto tracethe origins of the erodedmaterialsandthat information canbe usedto nbe usedto datethematerialto coincidewith environmentalchanges(FosterandWalling, 1994).The land-usehistory of an area,suchasagriculture,grazing,or forestry practicescan affect the rate of erosionwithin the watershedandquantity of sedimentdepositedin alake,pond or reservoir(FosterandWalling, 1994). In a studyin England,FosterandWalling (1994)found that surfacematerialfrom pastureareasrepresentedthe dominantsourceof depositedsedimentin a reservoir. They attributedsedimentdepositionto anincreasein livestock and grazingintensity after World War II. Becauseof this, a patternof sedimentyield for the life of the reservoircanbe constructedand a clear trend ofincreasingsedimentyield is apparentin the reservoir. It wasfound that the sedimentwaspredominantlyfrom the slopesof the basinandhadbeenmobilized from pastureareas
(FosterandWalling, 1994). Walling (1999)determinedthat forest clearingafter 1968accountedfor a 1.8 fold increasein the sedimentload of a reservoir. In small basinstudies,it hasbeenfound that thereis an increasein sedimentproductionafterclearcutting,and road building. Unpavedlogging roadsare alsosignificant sedimentsources(Ambers,2001). The relationshipbetweenland-useand sedimentationcansignificantly impact the storagecapacityof a basin.Sedimentdatamay alsobe usedto confirm evidenceof pastdisturbancesandtodeterminewhetherthe rate of erosionis "normal" or "accelerated."Sedimentqualitiescan be linked to specific eventsin the basinsuchasdeforestationor early farming(Dearing,1991). "Accelerated"erosioncan occurbecauseof the way the surroundingland wasusedin the past. For example,increasedlivestock and grazingintensity canincreaseerosionandrunoff into the reservoir(FosterandWalling, 1994). Presentdaysedimentloadsare21.6 times higher thanin the recentgeologicalpastandhumanactivity is one explanationfor this increase(Walling 1999). A studyby OwensandWalling (2002)concludedthat land clearance,cultivation, populationgrowth, andtechnologicaladvanceshaveacceleratederosion. The ratesof soil erosionon agriculturalland in the United Kingdom haveincreasedover the past 100 years. This reflectsthegrowth in the amountof land undercultivation andthe intensificationof farmingpractices(OwensandWalling, 2002). In a studyby McIntyre (1993), long term linked. In this case,reservoirsedimentationhad decreasedover time becauseof the conversionof fields to perennialpastures.When the abandonedfields with baresoil areaswerereducedby naturalvegetationit causeda decreasein sedimentyield. 70% of the sedimentwas depositedin
the first 20 yearperiod of the reservoirandthenrapidly decreasedover time asthe landusechanged(Mcintyre, 1993)Lake sedimentspreservea recordnot only of land-usehistory but alsoofhydrologicalevents. Severalstudieshaveshownthat sedimentyield changesrecordedinlake sedimentswererelatedprimarily to precipitationhistory, ratherthan gradualenvironmentalchange(Royall, 2000; Ambers,2001). Lake sedimentshave also beenusedto interpretlong-term stormhistory of a region. Severalstudiesin New Englandhaveindicatedthat the condition of a watershedat the time of a stormmay stronglyinfluencethe amountof erosionthat occurs(Brown et ai, 2000). It was determinedin astudyby Brown et a1(2000)that the causefor depositionof the coarser,more teITestrialinputs to a basin in northernVermont is runoff triggeredby hydrologic events.APPROACHFSTO STUDYING DEPOS110NIN RESERVOIRS:Thereare severaldifferent methodsto documentand analyzesedimentdepositionin lakes,ponds,andreservoirs. Coring methodsare widely employedto study lakesediments.Coreswere takenfrom basinsandvisually studiedin projectsby Ambers(2001) andPanis (2001). In a studyby Ambers(2001),eight sedimentcoreswerecollectedand subjectedto detailedstudyin orderto obtain information aboutvariationsin the sedimentyield of the watershed.Eachcore representedthe total thicknessof thelake sedimentat its samplinglocation, asdeterminedby the presenceof either pre-lakesoil or an impenetrablerock layer at the baseof the cores. The stratigraphywasdescribedin detail. Another40 coresweretakenfrom a variety of locationsto detenninethe averagedensityand organicmattercontent. A studyby Parris (2001) used23sedimentcoresfrom small basinsin orderto determinethe history of stormsandhillslope6
erosionin New England. The corescontaineddiscreet,terrestrially derived,inorganicdepositsidentified by physicalchangesin the sedimentincluding visual observation,losson-ignition, and grain sizeanalysis.In a studyby Souchand Sloymaker(1986),methodsfor the assessmentof thevariability of sedimentaccumulationin small pondsweredonewithout measuringchemicalattributesof the cores. The coreswereextractedandthe stratigraphywasstudied. The volume of the nof core,probe,and surveydata. Total sedimentvolume in the pondswascalculatedby using different grid sedeterminedfor the 2x2 metergrid. Assumptionswere alsomadeaboutthegeometricshapeof the accumulatedsedimentbody. This methodboth under andoverestimatedthe volume in orderto fall somewherein the middle. It also avoidserrorbecausethe shallow,elliptical natureof the sedimentbody in the lake hasno regularpatternof accumulationabouta singlecenterof deposition(Souchand Sloymaker,1986).This methodshowsthe importanceof the determinationof the threedimensionalgeometry of lake sediments,volume,density,and masscalculation. A studyby Dearing(1991)hasalsoemployeda different methodto determinethe amountof accumulationofsedimentin reservoirs. ndmicrofossil propertiesto detenninethe quality of sedimentin orderto infer the sourceofthe depositedmaterials. After doing this, sedimentqualitiescanbe linked to particulareventsin the basin suchasdeforestationor early fanning (Dearing 1991).Oneof the most widely usedmethodsis Cesium-13?dating to measuresoilerosionand sedimentaccumulationrates. Cs-137is a uniquetracerfor studyingerosion7
and sedimentation.Radioactivefallout in the form ofCs-137 hasbeendepositedacrossthe landscapefrom atmosphericnucleartestsandhasabsorbedonto soil particles(RitchieandMcHenry 1990). However,the Cs-137techniquecan only be usedwith sedimentdepositedsince 1954,the yearthat fallout wasfirst depositedin measurableamounts.Dating with Cesiumis alsouseful becauseperiodsof higher fallout can be relatedtodatesduring abovegroundtestingandperiodsof lower fallout canbe relatedtomoratoriumson testingand the Test Ban Treaty (Ritchie andMcHenry 1990).Magnetic susceptibilityis anothercommonmethodusedfor dating. It is ameasureof the concentrationof magneticmineralsin a substance.Thesecotrelatesedimentsequencesfrom different partsof the lake by visual comparisonof the magneticprofiles (Royall 2000). Mineral magneticsignaturescan also be usedto distinguishtopsoil and subsoilmaterial (FosterandWalling 1994).Collectively, thesestudiesshowthat lakes,pondsandreservoirscan be usedasarecordof basinsedimentyield. This recordcandetectland-usechangeover time or thepresenceof any pasthydrologic events. The purposeof this project is to documentthedepositsof sedimentsin the EastBranchof the WaterburyReservoir. My analysisexploresthe stratigraphyand spatialpatternsevidentin the depositsand looks forevidenceof storm eventsin the deposits. A companionstudy (Janow,2003) estimatesbasin sedimentyield from the samedataset. This researchis part of a larger projectincorporatingthe entire Waterburyreservoirin a studyof land-usechangeand the effecton sedimentation.:MElliODS:STUDY AREA:8
The study areaconsistedof the EastBranchof the WaterburyReservoirinWaterbury,Vemlont. It is locateda quartermile westof StateRoute 100. TheWaterburyReservoiris the ninth largestwaterbody in the stateof Vernlont (WaterburyReservoir,2003) with a drainageareaof 16.9km"3 (GreenMountain PowerCorporation,1999). The reservoirwas formedby an earthfill damcompletedby the United StatesArmy Corpsof Engineersin the summerof 1937for flood control and storageof waterfor power (USGS, 1997). From late springto early fall the reservoiris maintainedto asurfaceareaof 860 acresand a maximumdepthof 100feet. The surfaceareais reducedto between250 and 300 acresin the winter to preparefor spring snowmelt(WaterburyReservoir,2003). During the collection of data(summerandfall 2002),the reservoirwasdrainedto allow work to be doneon the dam. The bottom was exposedand a streamwasrunning throughit. The reservoirsedimentwas a deepbrown, saturated,clayey material.FlEW SAMPUNG AND LAB ANALYSIS:Sedimentcoresweretakenfrom 76 sitesin the EastBranchof the WaterburyReservoir. Eachof the 76 corestakenwere visually studiedand logged. Eachcorerepresenteda completethicknessof lake sediment,either by hitting pre-reservoirsedimentor the presenceof the old streambed.Coresweretakenfrom the approximatecenterof the reservoiroff ReservoirRoadin transectsrangingfrom 1 to 295 meters. Thecoring processinvolved using a one-metergaugeaugerwith a one-meterhandleandpushingit down into the sediment.The core was openon one sidein order to seethematerial. A GPSwas usedto recordthe location of eachsite. Oncethe core wasextracted,the stratigraphyof the sedimentwas visually describedand logged. To the bestof our knowledge,evidenceof significantsandlayerswere noted. The smallestwas9
approximately4cm so anythingthicker wasconsideredsignificant.Eachcore representeda completethicknessof the reservoirsediment,asdeterminedby the presenceof pre-lakematerialor a rock layer. The thicknessof the post reservoirsedimentwasdetenninedbyvisual characterandnotedNine sedimentsamplesfrom randomsiteswere preservedin bagsandtakento thelab in order to perform a bulk densityanalysis. The sampleswere bakedat 105degreesCelsiusovernightand then weighedin a pre-taredpan to detenninedry mass(m) of thesample. Core volume (v) was determinedby multiplying length of the post-reservoirsedimentsin the coreby the cross-sectionalareaof the auger. Bulk density (p) of eachsamplewascalculatedas p m/v.COMPUTER ANALYSIS:A geographicinformation systemandstatisticalanalysissoftwarewere usedtoanalyzethe sedimentdata. A location mapof the studyareawascreatedon ArcMap, aPC-basedGIS software,usingorthophotosasa basemap.Pointsfrom an exce]spreadsheetcontainingthe locationof the coreswereimportedinto ArcMap to createapoint layer of samplinglocations. Maps of core locationsandcoreswith sandlayersweremadeusing attributedatacontainedin the point layer. Streamflowdatafrom anearbygaugingstationon the White River, in WestHartford, Vennont wasusedforinformation on annualpeakflows from 1938(the constructionof the dam) to the present(peakStreamflowfor Vermont, 2003). This stationwas usedbecauseit containsthelargestdatarecordin Vermont on an undamedriver. Pointsfrom an excel spreadsheetcontainingthe streamflowdatawere usedto createa graphshowingannualpeakflow. Athree-dimensionalmap showingthe thicknessof the post reservoirsedimentwas10
constructedto visually examineany patternof thickeningsediment.MiniTab, astatisticalprogram.wasusedin orderto determineif therewas a relationshipbetweendistanceand sedimentthicknessor bulk densityusinga regressionanalysisRFBUL TS:This surveycoveredan areain the EastBranchof the WaterburyReservoirapproximately12,000meterslong; using23 transects(Table 1) that rangedfrom 1 meterto 295 meterswide (Figure 1). Thicknessof the postreservoirsedimentvaried from 8cmto 162cm. Most corescontainedthe reservoirsedimentalongwith sandclay-coupletsofvarying sizes(Figure 2a-c). Thesesignify the seasonalchangesin the movementofwater within the reservoirand are usuallyattributedto depositionfrom turbidity currents(Ambers,2001). Significant sandlayerswerefound in 45 out of the 76 cores(Figure 3)Any core containinga coarsesandlayer thicker than4cm wasconsideredsignificant.Theselayersrangedin thicknessandcolor but stoodout amongthe fine-grainedreservoirsediment.More than half of thesesamplescontainedsandlayers,which were spreadoutin different locationsalongthe study area. After the constructionof the reservoir,severallarge stonn eventsoccurredbut the most influential one occunedin 1974and canbecontributedto the ubiquitoussandlayer seenin 59% of the cores(fable 2) (Figure4).Visual inspectionof the sampledpoints showsa distinct patternof thickeningreservoirdepositsin the downstreamdirection (Figure 5). This patternis non-linearandstatisticallysignificant (p O. XX S)(Figure6). Core thicknesswas highly variable,particularly amongsamplestakendownstream.Someof this variability is likely duetovarying core thicknessacrossindividual transects(Figure 2a-c). 35.6%of the variabilityof thicknesscanbe explainedby the cubic relationshipbetweendistanceand thickness.11
This is a weakregressionequation,which showsthat theremust be somethingelseaffecting the variability (Figure 6).Bulk densities of the nine samples analyzed ranged from 0.3 to 1.5 g/cmJ\3,with amean of 0.55 g/cmJ\3 (552.9 kg/mJ\3) (Table 3). Only one sample had a bulk densitygreater than 1 g/cmI\3, and bulk density of the nine samples taken from East Branchdiffered little from two samples collected near Stevenson Brook, located on the westernbank of the Waterbury reservoir. There was no spatial pattern in bulk density of thesamples with distance upstream (p O.376) (Figure 7).DISCUSSION:A spatialpatternof sedimentdepositionwasfound in the EastBranchof theWaterbury Reservoir. The thicknessof reservoirsedimentwas shownto be increasingdownstream(Figure4 and Figure 5). Although a clear patternwasvisualized,aregressionanalysisof the dataprovedthat only 35.6%of the variability of thicknesscould be explainedby the cubic relationshipbetweendistanceandthickness. This is aweakregressionequationandmeansthat thereareother variablesaffecting therelationship. Therewas no relationshipbetweendistanceandbulk density,which meansthat thereis no spatialpatternbasedon the data. Sincesandlayerswere found in amajority of the samples,a largehydrologic eventmust haveaffectedthe areasincetheconstruction of the dam. This stormeventcauseda flux of sedimentto be depositedinthe area. Basedon the annualpeakstreamflowdatafor the White River, severallargestormeventsoccurredafter the constructionof the dam. 1973containedthe largestdischarge(42,300cfs) during the life of the reservoir. A goodinterpretationwould bethat this stOmlcausedthe sandlayersfound in a majority of the cores. This warrants12
further researchto determineif the dateof the sandlayerscorrelateswith the 1973storm.If major constructionprojectsbegin within the watershedof the WaterburyReservoir,increasedsedimentationmight be expected.Further studyon this subjectwould beextremelyuseful to uncovermore information aboutpossibleeffectson the WaterburyReservoir.13
-,iJWn 11491MfeI4"1W6 I- -.:::'- - l- - 37.4 '- r-"&i I,; I . . ;;,,-. ; !- "-I;-.:'- ! 12.1 '13.4"-r221-rTable 1: Table of point names,coordinates,andthicknessesof reservoirsedimentfromvariouslocationsat the WaterburyReservoirin Waterbury,Vermont. Presenceof a sandlayer in the core is indicatedby an asterisk.
NASampling Locations.cores.cores bulk densityWaterbury Reservoir01,000 2,0004,0006,0008,000
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Branch of the Waterbury Reservoir. This research is part of a larger project incorporating the entire Waterbury reservoir in a study of land-use change and the effect on sedimentation. Sedimentation information is useful because construction within the watershed could cause significant impact
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