RiverTerraceMappingofRushRiver,Sibley

County,MNBy

KylePinke

Athesissubmittedinpartialfulfillmentoftherequirementsforthedegreeof

BachelorofArts

(Geology)

At

GUSTAVUSADOLPHUSCOLLEGE

2017

RiverTerraceMappingofRushRiver,SibleyCounty,MN

ByKylePinke

UnderthesupervisionofProfessorLauraTriplett

Abstract

Approximately10,000yearsago,GlacialRiverWarrencarvedoutthemodern

MinnesotaRiverValley.WhentheMinnesotaRiverlateroccupiedthatvalley,it

workedtoachieveastateofequilibrium.Duringthisprocess,thefeaturesofthe

riverbegintoreflecttheamountofwaterandtheamountofsedimentbeing

transported.TheMinnesotaRiver’stributariesinturnrespondedtothenewbase

levelofthedeepenedMinnesotaRivervalley.Thetributariescarveddownthrough

thesubstrate,producingterracesthatwereleftbehindhighonthestreamvalley

walls.RushRiver,about3milessouthofHendersonMN,isonesuchtributary.In

thisstudy,threeseparateterracelevelswerelocatedandmappedwithintheRush

Riverravine.ThemappingoftheseterracesalongtheRushRiverValleycantellus

aboutpastevents,likethetimingofbaselevelchangesoftheMinnesotaRiverorthe

GlacialRiverWarren.Theterracescanalsoserveminingandexcavatingcompanies.

Bymappingtheseterracesandnotingtheirconsistency,wecanhelpcompanies

determineiftheareaisworthexcavatingforearthmaterialsforconstruction

purposes.

Acknowledgements Iwouldliketothankallthosewhocontributedtheirhelpandguidanceforthisproject.Dr.JamesWelshandDr.JulieBartleyforintroducingtheideatomaptheterracelevelsandtheirinputthroughoutthelengthoftheproject.ThankstoDr.LauraTriplettforassistancewithfieldworkandforherinputwithmappingandespeciallythewritingportionofthisproject.IwouldalsoliketothankRussKruegerforhisassistancewithArcGISprocesses.

IwouldliketothanktheGustavusAdolphusGeologyDepartmentforprovidingmewithadequatetoolsforfieldwork,andthetechnologyneededtocompletetheproject.

ThankyoutomyparentsforgrantingmethegiftoflifeandalloftheopportunitiesthatledmetodevelopintowhoIamtoday.

ContentsIntroduction…………………………………………………………………………………pg.6

GeologicSetting……………………………………………………………………………pg.10

WatershedCharacteristics……………………………………………………………pg.11

ResearchMethods………………………………………………………………………..pg.12

Results…………………………………………………………………………………………pg.13

Discussion……………………………………………………………………………………pg.21

Conclusion……………………………………………………………………………………pg.23

References……………………………………………………………………………………pg.25

FiguresandTablesFigure1………………………………………………………………………………….pg.6

Figure2………………………………………………………………………………….pg.7

Figure3………………………………………………………………………………….pg.9

Figure4………………………………………………………………………………….pg.14

Figure5………………………………………………………………………………….pg.16

Figure6………………………………………………………………………………….pg.17

Figure7………………………………………………………………………………….pg.19

Figure8………………………………………………………………………………….pg.20

Figure9………………………………………………………………………………….pg.20

Introduction

Attheendofthemostrecentglacialinterval,GlacialLakeAgassizcontained

enormousvolumesofglacialmeltwater,trappedagainsttheretreatingicesheetto

thenorthwest(OjakangasandMatsch,1982).Whenthemorainedamcontaining

thelakefailed,thereleaseofwatercarvedachannelandcreatedtheGlacialRiver

Warren(Figure1).GlacialRiverWarrenistheancestoroftheMinnesotaRiver.

OvertimeGlacialRiverWarrenanditstributariescarvedthroughtillintobedrock,

creatingawidechannelandassociatedfloodplains(Johnson,1998).

Figure1.GlacialLakeAgassizwithaSouthernoutletcreatedbythelossofanaturalicedamdrainingwaterwhichcreatedtheGlacialRiverWarrenwhichisthepresentdayMinnesotaRiver.(TheInstituteforMinnesotaArchaeology)

RushRiverislocatedinSibleyCounty(Figure2.)about3milessouthofHenderson,

Minnesota(Figure3).Theriverisestimatedtohaveformed10,000yearsagoafter

meltwatersfromtheLaurentideIceSheetformedLakeAgassiz.Whenthelake

brokethroughanaturaldam,amassive

outflowofwater–TheGlacialRiverWarren–

carvedthelandscapeofsouthernMinnesota

creatingahugevalleythatwasultimately

occupiedbytheMinnesotaRiver(Fisher

2003).Whenwateraccumulatesinuplands,

itwillflowtothelowestspotpossibleorbase

levelduetogravity.Thebaselevelofariver

isthelowestpointtowhichwatercanflow.

Forexample,iftherewereaperiodofmassive

amountsofflowintheMinnesotaRiver,more

andmoresedimentwouldbemoved

downstreamcausingmoreincision.Inturn,tributariestotheMinnesotaRiver

wouldalsobegintodown-cutandinciseinresponsetothebaselevelchangeofthe

MinnesotaRiver.Thisperiodofincreasedincisioncouldthenberecordedby

terraces,whereinformationaboutpasteventscanbeobtained.

Terracescomposedofsandandgravelareremnantsofformerfloodplains;

theymanifestgeomorphicallyasstep-likefeaturesattheedgesofrivervalleys.

Becausebaseleveldroppedastheriverinciseditsvalley,highterracesaretypically

Figure2.SibleyCountyhighlightedinred.

olderandtheyoungestterraceshavelowerelevationandsitclosetothemodern

riverchannel.Overtimeaswaterfloweddowntributarystreams,headward

erosionlengthenedtheravinesupstream,eventuallydevelopingnewtributaries

andravines.WhenGlacialRiverWarrencausedlocalbaseleveltodropthese

tributarieswouldsimilarlyhavecutdownward,leavingterracesattheedgesof

theirpaleochannels(Bock,2010).

OneexampleofatributarywithterracesisSevenMileCreek,atributaryto

theMinnesotaRiverthatislocatedabout5milessouthofSaintPeter,Minnesota.

Thatcreekanditsravinesystemhavebeenextensivelystudiedinthepastand

presentinhopestobetterunderstandissuesofhydrologyandpollution.Thisarea

ofMinnesotaisheavilyusedinagriculturesoissuesofsedimentsuppliesandrunoff

fromfertilizersmakethistributaryafitcandidateforpollutioncontrolstudiesin

ordertolearnmoreaboutwhypollutionandsedimentsuppliesaresoheavyinthe

Minnesota

River,MississippiRiver,andultimatelyintotheGulfofMexico.RushRiver,located

inSibleyCountyabout3milessouthofHenderson,Minnesota,isananalogous

systemtoSevenMileCreekalsoleadingintotheMinnesotaRiver.

TheRushRiverareahasnotyetbeenstudiedasextensivelyasSevenMile

Creek.LocatingandmappingriverterracesatRushRiverwillopenawindowinto

Figure3.ShowsthelocationofHenderson,MNinrelationtoRushRiverandtheMinnesotaRiver

thepastandgiveusknowledgeoftheevolutionofthetributaryravinesofthepost

MinnesotaRiver.Riverterracesareimportantlandformsbecausetheycanprovide

depositsofaggregateusedforconstructionpurposesaswellassanddeposits.They

canprovideinformationabouthydrologyforfloodmanagementandplanning

purposesaswellaslonger-termenvironmentalchangesdrivenbycombinationsof

climateandanthropogenicchanges.

InBock’sthesisaboutSevenMileCreek,hewrotethatweneedtoadddata

fromotherravinesmuchlikeSevenMileCreek(2010).Iwishtoaddtothedata

collectedbymimickinghisstudyatRushRiver.Then,thetwotributariescanbe

compared.

Overthecourseofthisstudy,usingmethodsfrompreviousstudiesofriver

terraces,ImadevarioustripstoRushRivertolocateterracesandconfirmtheir

compositionsinordertomapthevariouslevelsofterraceswithintheRushRiver

Ravine.

GeologicSetting

WhenglacierscamethroughthisareaofMinnesotaforthelasttime,ending

around10,000yearsago,theyleftthickdepositsofglacialtillontopofPaleozoic

sedimentarybedrock.Thetillisspatiallyheterogeneous,withvariousstratigraphic

layersandunitsasdescribedbyLusardietal.(2012).Becausetheglacialtillisso

clayrichitisabletoholdtogethermuchstrongerthanifitwererichinsand-sized

particles.Sinceitissoclayrich,theRushRiverravineisabletokeepitssteepvalley

wallsintact.Thisisimportantasitalsohelpskeepterracesintactandwell

preserved.AlsolocatedwithintheRushRivervalleyareunitsofquaternary

alluvium,quaternarycolluviumandalluvialterracedeposits.Alluviumisasanda

gravelunitwithsiltandclaythathavebeendepositedbythemodernRushRiverin

channelsandfloodplains.Colluviumisaunitthatconsistsofclaytoboulders.This

unitisdepositedonsteepslopesbyweatheringandgravitationalprocesses

(Jenningsetal.2012).Thealluvialterracedepositsarecomprisedofasandand

gravellysandunitwithsiltandclay.Thisunitiswellsortedandtheyformanearly

levelsurfacewithsomeareasofstreamlinedbarsandshallowchannels(Jenningset

al.2012).Theunitliesabovethemodernfloodplain.

WatershedCharacteristics

SincetheGlacialRiverWarrencamethroughthisarea,RushRiverandits

tributarieshavebeenslowlyincisingthroughtheglacialtillandunderlyingrock

layers,tryingtoreachbaselevel.Inmorerecentyearsinadvancementsof

agricultureanddrainagetechniques,floodplainsandwetlandshavebeenalteredor

losttheirabilitytostorefloodwaters.Becausedrainagetilingspeedsuptheprocess

ofdrainingwaterflushedoutintodrainageditchesorriversatahighrateofspeed

causingincreasedflooding,erosionandsedimentloadedwater(Fennessyetal).

ResearchMethods

Thedatacollectedcamefromamixtureoffieldworkandobservationsmade

atoutcropsandlandformswithinthepark.Informationwasgatheredinafield

notebookandphotographsweretakenforvisualization.Informationfromthefield

suchasthecompositionoftheterracesaswellasgatheredfrommapsfromthe

MinnesotaGeologicSurveyforSibleyCounty(Jenningsetal.2012)wasusedto

verifylocationsofterraces.Thisinformationwasrecordedonafieldbasemap.

ThesemapsarecontourmapsthatarederivedfromLightDetectionandRanging

(LiDAR)takenfromtheMinnesotaGeospatialCommonsforSibleyCounty

(MNDNR).Themapgivesdetailsofdifferentelevations;terracesatdifferent

elevationscandistinguishseparateunits.ThroughoutRushRiverCountyPark,

terraceswereidentifiedandtheirlocationwasrecordedonthebasemaps.Sand

distributionmodelsfromtheMinnesotaGeospatialCommonsdeterminedtheir

extentforSibleyCounty(Lusardietal.2012).Oncetheinitialfielddatawasinput

intoArcMap,additionalterraceregionswereinterpretedbasedontheirsimilar

appearancetoknownterracestructuresinArcMapaswellasusinginformationof

differentunitsfromtheSurficialGeologyofSibleyCountyMapfromtheMinnesota

GeologicSurvey(Jenningsetal.2012)Fromthedatacollected,mapsofterraces

alongRushRiverweremadeusingArcGIStovisuallyshowthelocationandwhat

uniteachterracebelongsto.Becausethereisnotimescaleoftheeventsthat

happenedtocreatetheterraces,theywerebrokendownbylocationandelevation.

Intervalsofaround50ft.werenotedforeachterracelevel.FurtherstudiesofRush

Riverarerequiredinordertobetterdatetheterracelevels.Hypothesesofevents

thatoccurredovertimearethebestdatingmethodsusedinthisstudy.

Results

ByusingacombinationofArcGIS,DEMdata,contourelevationmaps,the

SurficialGeologymapfromtheMinnesotaGeologicalSurvey(Jenningsetal.2012),

andfieldwork,terraceswerelocatedandtheirextentwasmapped.Threeterrace

levelswereidentifiedalongtheRushRiverravine.Thethreeterracelevelsare

distinctfromeachotherfromelevationsof970-740ft.abovesealevel.

Figure4.ThecoloredsectionsarethelocationsofterraceswithintheRushRiverravine.Terracelevelsaremeasuredinelevationabovesealevel.

Locatedwithinalloftheterraceunitsinthepreviouspage(Figure4.)are

sandandgravellysandunitswithsiltandclay.Theseunitsarewellsorted,fining

upward.Theterracelevelsformanearlylevelsurfacewithsomeareasof

streamlinedbarsandshallowchannels.Therearelocationsfilledwithfine-grained

sediment,whichlieabovethemodernfloodplain.

TerraceLevel1-thisterracelevelismappedyellowincolor.Theelevationatwhich

theseterracesarelocatedis970ft.inelevationabovesealevel.Thisterracelevel

sitsatornearthetopoftheRushRiverravine

TerraceLevel2-thisterracelevelismappedblueincolor.Theelevationofthe

surfaceoftheterraceis860ft.inelevationabovesealevel.

TerraceLevel3-thisterracelevelismappedgreenincolor.Theelevationofthe

surfaceterraceis790ft.abovesealevel.

Althoughwedonothavedatesofwhentheseterraceswereformed,wedo

havegeologicinformationofwhattypesofmaterialsthatwaterfromRushRiverhas

hadtocarvethroughforthousandsofyears.BelowisaroughsketchofaCross

SectionofRushRiver.

Figure5.AroughsketchofthecrosssectionoftheRushRiverRavine.(Notdrawntoscale)

ThecrosssectionsketchofRushRiver(Figure5.)givesageneraldescription

ofthematerialsthatRushRiverhaserodedordepositedduringits~10,000year

evolution.

Figure6.Threedistinctterracelevelsshownwith10ft.contourlines.Terracelevelsaremeasuredatelevationabovesealevelinfeet.

Theelevationcontoursareshownat10ft.intervals(Figure6).Fromthetop

oftheoldestterracelevelstothebottomoftheyoungestterraceslevelsthereisa

distanceofabout230feet,withinthat230feetterracesweremappedbasedon

pointsofequalelevation.Basedontheseelevationdifferencesbetweenterrace

levels,itispossibletobegintointerpretpreviousenvironments,suchaswhen

periodsofhigherorloweramountsofdowncuttingandincisionwereoccurring

withinthearea.

Figure7.AlluviumandColluviummappedwiththe3distinctterracelevelsoftheRushRiverravine.Showingrelationofterracelevelsandunitsofalluviumandcolluvium.

Alluvium(Figure8.)isaunitconsistingofsandandgravelwithsiltandclay.

Fine-grainedsediments,whicharedepositedbymodernstreamsinchannelsand

floodplains.

Colluvium(Figure9.)isaunitthatconsistsofclaytoboulders.Thisunitis

mostlyfine-grainedsedimenttosandandgravel.Depositedonsteepslopesby

weatheringprocesses.

Figure8.ExampleofquaternaryalluviumdepositsinthefloodplainofRushRiver.RushRiverislocatedontheleftinthispicture.

Figure9.ExampleofquaternarycolluviumwithintheRushRiverravine.Thereissoilalongwithotherfinegrainedsedimentsandlargergravelsizeparticlesdepositedonaslopeoftheravineduetoweatheringandgravitationalprocesses.

Levelsurfaceslocatedbelow740ft.andslopinghillswithcolluviumdeposits

arenotconsideredterracesinthisstudy(Figure7.).Althoughtheyhavesimilar

characteristicsofterraces,locationsbelow740ft.withintheravineareconsidered

tobewithinthemodernfloodplainorhaveendedupintheareaduetoweathering

processes.

Discussion

ThenumerousterracespresentwithintheRushRivervalleysuggestthatthe

streamhasgonethroughmanyperiodsofdowncutting.Whendidtheseperiodsof

downcuttingoccurandwhatfactorscontrolledthem?Onehypothesisofmineis

thatterracelevel1isdifferentfromtheothers.Thisterracelevelisinterpretedto

betheoldestterracelevelwithintheRushRiverRavine.Ibelievethatthisterrace

levelwasformedduringperiodsofpost-glacialmelt,butpriortotheGlacialRiver

Warrencarvingthroughthisarea.BecausetheseterracelevelslieabovetheRush

Riverravine,itispossiblethattherewerepre-GlacialRiverWarrenflowsofwater

inthisarea.Duringthistime,therewaslittletonovegetationduetotheglaciers

thathadjustreshapedthelandscape.Aftertheglaciersmeltedaway,theyleft

massivedepositsofglacialtill.Thisloosesedimentwithnovegetationtoholdit

togetherwouldhavebeeneasilyshapedbyfluvialprocesses.Elevationsdecrease

fromthewesttotheeast,whichisthesamedirection,thatRushRiverdrainsinto

theMinnesotaRiver,leadingonetobelievethatthenaturalflowofwaterpriorto

RushRiverbeingformedisthesimilarwesttoeastdirection.Theseterracelevels

werepreservedwhileRushRiverorothererosionalprocesseswashedoutother

terraceswithinthislevelduringthedrainingofGlacialLakeAgassiz.

Terracelevel2reflectsahigherbaselevelthatexistedastheGlacialRiver

Warrenwasstillcarvingthevalleyshortlyafterdeglaciation.Aterraceheightof

around50ft.isasignificantdistance.Geologistsdonotknowhowlongittook

GlacialRiverWarrentocarvetheentiredepthoftheMinnesotaRiverValley.There

mayhavebeenmultiplephasesofoutburstfloodsanddowncuttingwithassociated

pausesbetweenfloodevents.Duringthosepauses,tributarieslikeRushRiver

wouldhavehadtimetodevelopfloodplainsthatwouldlaterbecometerraces.Thus,

thisterracelevelmayindicateaperiod,howevershort,duringwhichGlacialRiver

Warren–and,thus,thelocalbaslevelforRushRiver–wasstableatahigher

elevation.

Terracelevel3wasformedinmorerecentyears,sometimeafterthefinal

drainingofLakeAgassiz.Thisterracelevelisinterpretedtobetheyoungestof

terracelevelswithintheravine.Olderterraceswerecontrolledbynaturalfactors

butyoungerterracedevelopmentmayhavebeeninfluencedbyanthropogenic

events.Onehypothesiswouldbewithmoreandmoredraintilebeingplaced

underground,waterdrainingfromthesetilereachtherivermuchfastercausing

higherratesoferosionandflooding.RelativetonormalflowofRushRiver,thereis

evidenceoflargescalefloodingeventsthatcovertheselowareas.Thisterracelevel

couldtellusmoreabouterosionratesaswellastheeffectsthatthesepracticeshave

beenhavingonourriversystemsandtheamountofwaterbeingdrainedinthe

area.

Althoughwedonothaveagesoftheseterracesandarethuslimitedinwhat

wecaninterpret,wecancomparetheseterraceswiththoseofSevenMileCreek.

Bockalsolocatedthreeterracelevels.And,issimilarinbothtributaryvalleys,one

terracelevelofaround59feetinSevenMileCreekissimilartoTerrace2inthe

RushRivervalley,whichisatabout50feet.Havingacorrelationinterracelevelsin

bothSevenMileandinRushRiverisstrongevidencethattherewasaperiodof

stabilityoftheGlacialRiverWarrenatthisterraceelevation:thesetwostreamshad

timetodevelopfloodplainsatthatelevation.

Becausetheagesoftheseterracelevelsaswellasthefactorscontrolling

theirdevelopmentareunknownitisdifficulttopinpointthetimesthatthese

terraceswereformed.However,IbelievemyinterpretationscorrelatewithBock’s,

physicallyexploringtheareatoidentifyterracelevelswashelpfulinensuring

accuracy.Furtherinvestigationcouldhelpdetermineifhumaninteractionhasin

factbeenaffectingterracedevelopment,andavaluablefutureeffortwouldbeto

collectdatesoftheterracematerialstodetermineifhumandevelopmentaffected

terraceprocesses.

Conclusion

BymappingtheterracelevelswithintheRushRivervalleyabetter

understandingofthecurrentandhistorichydrologyoftheareahasbeenobtained.

Themappingshowsevidenceofpost-glaciationbutpre-GlacialRiverWarrenflows

wherethereisalluvialsedimentlyingabovetheravine.Thereisalsoevidenceof

theperiodwhereRushRiverwastryingtoreachbaselevelwiththeMinnesota

RiverduringthedrainingofGlacialLakeAgassizwhentheGlacialRiverWarren

carvedthroughthearea.Finally,Ibelievethereisevidenceofanthroprogenic

influencetoyoungerterracedevelopmentwithintheravineduetohigheramounts

ofwaterbeingdischargedfromdrainagetiling.

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