SALINITYMOVEMENTWITHINDOGRIVER
LoganWheeler,DepartmentofEarthSciences,UniversityofSouthAlabama,
Mobile,AL36688.Email:lsw1221@jagmail.southalabama.edu
Saltwaterintrusionisanincreasingproblemincoastalareas.Asurban
developmentdepletesthewatertableandclimatechangecausessealevelstorise,
thedynamicsofDogRiverhavebecomethreatened.Thepurposeofthisresearchis
tobetterunderstandhowsalinitymovesintoDogRiver.Toaccomplishthis,salinity
measurementsweretakenusingaHydrolabMinsonde.Thesemeasurementswere
usedtolocateaboundarybetweenthefreshwaterandsaltwater,andunderstand
howsalinitychangeswithdepth.ResearchresultsshowthatsaltwaterfromMobile
Baycanmovewellinlanddependingontidalvariances,andsalinityvalueschange
withdepth.ThisstudywillhelptoshedlightonsaltwaterintrusioninDogRiver,
andprovideabasisforfuturestudies.
Keywords:Saltwater,intrusion,salinity,stratification
Introduction
Ascoastalcitiescontinuetodevelop,saltwaterintrusionhasbecomean
increasingproblem.Risingpopulationshavecreatedincreaseddemandfordrinking
water.Thesepopulationcenterspullwaterfromtheground,loweringthewater
table.Asthewatertableisdepleted,higherdensitysaltwatermovesintoreplaceit.
Riverscanprovideconduitsforsaltwater,allowingittoinfiltrateinland.As
saltwatermovesinland,ithasdrasticeffectsnotonlyonthewatertable,butthe
environmentaswell(Ningetal.,2010).InDogRiver,marshesandcypresstreesare
understressduetosalinityfarupstream.AsthedevelopmentofMobilecontinues,
salinitythreatenstocontinuemovingupstream.
Anestuaryisatransitionalzonewherefreshwaterfromariverorstreammeets
saltwater.Saltwaterismoredensethanfreshwater,sowhenthetwomeet,theydo
notalwaysmixeasily.Tides,riverdischarge,andwindareafewphysicalfactors
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thatcandeterminehowthewaterismixed(Steeleetal.,2010).Riversthat
dischargedirectlyintoabodyofsaltwaterandareweaklyinfluencedbytidal
currentscancreateasharpboundarybetweenwatermasses,creatingawedge
betweenthetwo(Ibanezetal.,1997).SeeninFigure1below,thelighterfreshwater
floatsontopofthesaltwaterwedge,whichextendstotheriverbottom.
Figure1:Salt‐wedge
Retrievedfromhttp://www.biodiversitybc.org/assets/Taking~Natures~Pulse/figure‐14.png
Theseestuariesarecalledsalt‐wedgeestuaries,orhighlystratifiedestuaries.A
smallamountofmixingdoesoccur,butisusuallylight(Sierraetal.,2003).The
mouthsoftheMississippiRiverandtheHudsonRiverareexamplesofsalt‐wedge
estuaries(Geyeretal.,2010).
Thelocationofthesalt‐wedgeisnotfixed.Tidesanddischargecauseawedgeto
moveupanddownariver.Atlowtide,therivercanpushthesaltwaterbacktoward
themouthoftheriver.Athightide,thesaltwaterpushesintotheestuary,moving
thewedgefartheruptheriver(Stanneetal.,1996).Periodsofheavyrainand
droughtalsogreatlyaffectthelocationofthesalt‐wedge.Heavyrainswillincrease
theamountoffreshwaterintoanestuary,pushingitdowntheriver.Droughtshave
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theoppositeaffect,reducingfreshwaterflowandallowingthewedgetoinfiltrate
upstream(Kurupetal.,1998).
ResearchQuestion
SaltwaterhasbeenfoundupstreaminDogRiver,butexactlywhereitmeetsthe
freshwater,nooneknows.InanestuarylikethatofDogRiver,asharplystratified
boundarycanbecreatedbetweenthetwomasses.Isthereastratifiedboundary
betweenthetwo,andifso,whereisitlocated?Thelocationofthisboundaryis
greatlyaffectingbythetidesandconstantlymovesupanddownstream.Whatisthe
locationofthesaltfrontatlowandhightide?
Methods
AHydrolabMinisondewasusedtotakesalinitymeasurementsbetweenthemouth
ofDogRiverandHallsMillCreek.IbeganattheDogRiverBridgeandpaddled
upstream.Everythirtyseconds,IwouldlowertheMinisondeintothewater,asin
Figure2below.
Figure2:MeasuringsurfacesalinitywiththeHydrolabMinisonde
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Thesemeasurementsweretakenatthesurface,threemeters,andfivemeters.At
eachtestsite,IwouldrecordtheGPScoordinates.
Results
Asharplystratifiedboundarywasnotfoundbetweenfreshwaterandsaltwater,
butstratificationwasfoundthroughoutthetestarea.Lowtidesawthegreatest
amountofstratification,withonesitemeasuring3.57pptatthesurfaceand4.12
pptatthreemeters.Thoughstratificationwasfoundthroughout,itwasnotuniform.
Thedifferenceinsalinityfromtoptobottomvariedwitheachtestsite.
Salinitydatashowthatthetideshavealargeeffectonthemovementofsaltwater
upstream.Atlowtide,thesaltfrontwaslocated0.74milesfromthemouthofthe
river.Athightide,thesaltfrontwasfound1.58milesupstream.Thedifference
betweenthetwocanbeseeninFigure3.
Figure3:Thelocationofthesaltfrontatlowtide(L)andhightide(H).
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Thoughwaterimmediatelyupstreamfromthefrontwasnotfresh,salinitylevels
droppedrapidly.
IntheFigures4and5below,weseeaveragesalinitylevelsastheyapproachthe
saltfront.
Figure4:SalinitylevelsatlowtideFigure5:Salinitylevelsathightide
Whensalinitydatawererecordedduringhightide,valuesactuallyincreasedfrom
themouthoftherivertothesaltfront.
Conclusion
AfterconductingresearchandspendingtimetestingonDogRiver,therearea
fewthingsthatIbelievewouldproducemorebeneficialresults.Firstly,itis
importanttoallowenoughtimetosurveythearea.Researchwasconductedona
kayak,meaningthattestingmultiplesiteswasverytimeconsuming.Themost
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importantchangewouldbethetimeofyearthatthistypeofstudyisconducted.
SpringcanbringhighamountsofraintoMobile,increasingthefreshwater
dischargeoftheriver.Higherdischargemeansthatsaltwatercannotintrudeinland
asfar.Thebesttimetoconductthisstudywouldbeduringthelatesummermonths
andintoearlyfall.Riverdischargewouldbediminished,andsaltwaterwouldbe
allowedtomovemuchfartherupstream.Evenduringthespring,salinityvaluesof
1.25pptwerefoundasfarupstreamasDogRiver(Luscher)Park.Overall,Ibelieve
thisresearchtobebeneficial.Assaltwaterintrusionbecomesanincreasing
problem,itisimportantthatwekeeptrackofitsmovementintotheriver.The
healthofDogRiverdependsuponit.
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