Large woody debris (LWD) naturally accumulates in rivers and streams, where it provides important geomorphic functions like decreasing flow velocities, trapping sediment, and creating habitat. However, humans often remove LWD due to risks to infrastructure and recreation. Removal of LWD can significantly alter river geomorphology by increasing flow rates and bank erosion. Case studies show that reintroducing LWD or beaver dams can help restore river complexity and ecosystem health. Management strategies aim to balance ecosystem needs with human safety concerns.

1. SamanthaLasher
ESS 311 Final Paper
21 November2014
The Removal of Large Woody Debris and its Effect on Geomorphology in Rivers and Streams
Introduction
Large woodydebris(LWD) isa natural part of an ecosystem.Woodydebriscancome fromfallen
trees,landslidesandotherdisturbancesandmake theirhome inriversandstreams (Wohl 2013). Large
woodydebrisischaracterizedasanypiece of woodthat isat leasttencentimetersindiameterandat
leastone meterinlength(Faustinietal.2002).
LWD hasa majorinfluence onthe geomorphologyof riverandstreamsystems.The
presence of LWD promotespool andripple formation,stabilizesriversbanksandtrapssedimentthat
wouldotherwisedisappeardownstream(Boothetal.1996). Duringfloodevents,LWDcan slow down
floodwaters,decrease erosionandredirectfloodwaters(Gregoryetal.1992). Riversand streamstend
to have increasedgeomorphicandhydraulicdiversitywithLWDand consequently,thereisavailable
habitatfor organismsanda healthierecosystem(Krauseetal.2014).
Historically,there wasmuchmore LWD presentinriversandstreams,especiallyinforested
regions(Wohl 2014). Yet,despite the positive ecological impacts,LWDisbeingremovedfromour
freshwaterecosystems.LWDcanimpede navigationforcanoes,kayaksandotherformsof recreation,
blockwaterculvertsandbe a hazard forroads and bridgesandoverall humansafety(CDPE2014). These
actionscan have numerous,longtermeffectsonriverandstreamgeomorphology.RemovingLWDcan
increase flowrates,increase bankerosion,encourage scouringof channelsanddisrupthabitatfor
animal species(Boothetal.1996).
There needstobe a balance betweenecosystemhealthandhumansafety.There are various
managementstrategiesthatcanbe implementedincludingonlyremovingLWDif it presentsa
significantthreattoinfrastructure orisa navigational hazard(CDPE2014). ReintroducingLWD
structuresthroughoutstreamsandriversthatcontribute toa healthyecosystemaswell takinginto
account humansafetyiskey(Krause etal.2014).
The goal of thispaper isto gatherevidence of the geomorphicimpactsthatLWD has on river
ecosystems,andcompare tothe effectsof removingLWDfromthese ecosystems.Iwill firsttalk about
whatLWD bringsto a riveror streamecosystemandthe effectsithason streamgeomorphology.Iwill
thengo intowhyLWD isremoved,the effectsof removingLWDandhow geomorphiccharacteristicscan
transform.Next,Iwill discussspecificexamplesof riversinregardstoLWD and how theywere affected
as well ashowbeaverdamsare an importantcontribution.Iwill conclude bydiscussingvarious
managementstrategiesregionscantake to encourage riverhealthandkeeppeople safe.
Contributionsof Large WoodyDebris on Stream Geomorphology
Large woodydebris accumulatesnaturallyinriversandstreams. Thisecosystemcomponent can
transformstreamgeomorphology bydecreasingflowvelocity (Gregoryetal.1992). Decreasedvelocity
can decrease erosionof riverbanksand diminish sedimentbuild-upinthe flowingwaters. Riverswill
meanderthroughthe landscape ratherthancuttinga straightpath witha slowervelocity.Bermsalso
have the chance to form,whichprovidesabarrieralonga river’sedge,topreventsoil erosion (Douglas
et al.2001). Riparianvegetationisallowedtoflourishwithstabilizedbanksand canprovide foodand
habitatfor animal species(Gregoryet al.1992). The physical presence of LWDcreatesa roughnessand
heterogeneityinrivers andstreams. RoughnessfromLWDgeneratesslowerwaterflow adjacenttothe
debris,loweringthe pressure gradientandtherefore, preventsscouringof the channel bank (Wohl

2. 2014). LWD can create variouspools, ripples andchannels alongthe riveraswell,whichprovidesfish
and invertebratehabitat(Boothetal.1996). The angle andsize of the LWD determinesthe size,depth
and placementof these poolsandripples. Thiscomplexnessalsotrapssedimentfromrushing
downstreamand allowsnutrientsto settle (Boothetal.1996). LWD notonlyhas majorimpressionson
riverecosystems,butonhumanpopulationsaswell.Floodwaterscanbe redirectedwiththe presence
of LWD (Gregoryetal. 1992). Withno barriers,floodwatershave ahighvelocityandcan impact
infrastructure andputpeople atrisk,especiallyif the floodisnearatown.LWD slowsdownflood waters
and containsthe water’senergy(Gregoryetal.1992). Ata highflow velocity,watercanact as a trigger
for masswastingevents (Gregoryetal.1992). These dangerousdownslopemovementsof soil androck
can be a hazard forcars, infrastructure andpeople’slives. Slow flow velocitycausedbyLWDcan lower
the riskof large mass wastingevents(Douglasetal.2001). LWD has copiouspositiveinfluences onour
riverecosystems andonhumanlives.
Removal ofLarge WoodyDebrisby Humans
Historically,LWDwasat a muchhigherlevel inrivers andwasseenasa natural occurrence in a
waterecosystem. Inthe pasttwo centuries,despitethe positive impactsof LWD on riversandstreams,
LWD hasbeenremovedbyhumansinriversaroundthe world (CDPEetal. 2014). Increasedrecreational
sportssuch as canoeing,kayakingandraftingrequire the removalof LWDas tonot impede navigation
(CDPEet al.2014). Toomuch debriscan cause injuriesandevendeathsforthe participants. LWDcan
alsoblock large waterculvertsina river,not allowingwaterto drainaccurately(CDPEetal.2014). Water
blockage can increase floodrisk, cause damage tothe culvertitselfandresultinexpensive repairs.
Similarly,LWDcan accumulate andimpede roadsandbridges (CDPEetal.2014). Cloggedtrafficand
accidentscan be inducedaswell asphysical damage tothe roads and bridges. The September2013
floodinColorado toppledmassive treesintothe PoudreRiverand causedvasterosionandextensive
damage to roads andhomes(Wohl etal. 2014). Stormengineersandthe publicsaw thisdebrisas
undesirableandwanteditremovedforhumansafety.Scientists,onthe otherhand,saw the LWD as a
natural feature of these riversystems thatsustainsecosystemhealth(Wohl etal.2014). Large flood
cases,like inSeptember2013, exhibitthe conflictoverchoosingriverecosystemhealthorhuman
safety.
Large Woody DebrisRemoval Effectson Geomorphology
AfterremovingLWDfromriversand streams,the geomorphologycanbe alteredcompletely.
WithoutLWD, there are no longeranyobstaclesandflow velocitywill increase dramatically (Gregoryet
al.1992). The increasedwaterflowwillresultinincreased downcuttingof the riverchannel (Gregoryet
al.1992). Riverswill meanderlessandrathercutstraightdownthe landscape erodingsoil layersand
looseningrocks. The channel bottomandsideswill slowly erodeaway. Withbankdestabilization,
riparianvegetationthatlinesriverswill be strippedawayandvaluable habitatwill be destroyed
(Gregoryetal. 1992). Loss of vegetationcan decrease waterquality, lowersoil retentionandleadto
increasedsusceptibilityof flooddamage (Gregoryetal.1992). Deadand decayingwoodydebris
introduce organicnutrientstothe systemand benefitaquaticfoodwebs(Boothetal.1996). Yet,these
nutrientsandsedimentsthatwouldnormallybe retainedinariverwill insteadbe carrieddownstream
(Boothetal. 1996). Homogeneityreplacesheterogeneitywiththe removal of LWDbecause of reduced
roughnessinthe waters.Pools,ripplesanddiversechannelsare severelyreducedoreliminated
(Gregoryetal. 1992). If poolsremain,theirdepthisreduced,ingeneral (Gregoryetal.1992).
Consequently,animal specieshabitatsare eradicated. Also,withoutLWD,the flow hasnochance to
meanderorsettle andrushesquicklydownstream,forminganarrow channel (Boothetal.1996). River
ecosystemhealthcansuffergreatly.

3. Various Case Studiesand Effects of BeaverDams on Geomorphology
Numerousriversaroundthe countryhave experienced the impactof LWD removal,but some
have alsoseena positive change fromthe reintroductionof LWD. The QueetsRiverbasininnorthwest
WashingtonState is1164 square kilometersandissurroundedbyrichtimberforests (Abbe etal.1996).
One type of LWD studied wasa Bar Apex Jam(BAJ) (Abbe etal.1996). A BAJ ischaracterizedby a
crescent-shapedpool,anarchedupstreambarand downstreambar(Abbe etal.1996). Scientists
discovered thatriverlocationswithBAJs, alsocontained pools,islandsandbars(Abbe etal.1996). These
geomorphicformationsproduce complexity. The size of these featuresdepends onthe size,placement
and numberof BAJs.In one 25 kilometersection,the deepestpoolswere found inlocationswithLWD,
(Abbe etal.1996). Poolsare vital physical featuresfor aquaticspecies,specificallyandromousfishinthe
QueetsRiver(Abbe etal.1996). In 1931, there were noevidentBAJsinone sectionof the riverandthe
channel wasnarrowwithlittle riparianvegetation(Abbe etal.1996). A BAJwas placedinthe river in
1939 to observe effectsonthe geomorphology(Abbeetal.1996). A few more BAJstructureswere
insertedupstreamfromthe original barbetween1939 and 1985 resultinginreducedchanneldepth as
well asthe formationof one large bar by 1985 (Abbe etal.1996). By 1993, multipleislands were
witnessedaswell asriparianvegetation(Abbeetal.1996). Between1985 and 1992, the riverhad
sufferednumerousfloodsandthe BAJssuccessfullyalleviatedwaterpressure onthe banks (Abbe etal.
1996). In a short 54 years,a once simple,narrow channel wasrenovatedintoanintricate,vibrant
ecosystem.
A similarexperimentwasdone in2000 on a 2 kilometerstretchinthe Little Topashaw Creekin
Mississippi (DouglasShieldsetal.2001). Large woodystructureswere introducedtothe creekfor
streamrehabilitationpurposes. Thissectionof the creekhadincisedchannelsanderodedbanks
(DouglasShieldsetal.2001). Surveysof thisstretchoccur annuallyandresearchersfoundthat water
velocitieshadbeenreducedbecause of the woodystructures(DouglasShieldsetal.2001). The highest
velocitiesoccurredonthe outside of the riverbend,butwiththe LWD structures,waterslowedandwas
expelledfurtherawayfromthe bendpreventingsoil erosion(DouglasShieldsetal.2001). Researchers
concludedthatthe LWD structures were effectiveinreducingflowvelocity,preventingmasswasting
and stabilizingbanks (DouglasShieldsetal.2001).
A rarelythoughtof LWD source isbeaverdams. Beaversare keystone speciesmeaningthey
have a disproportionate effectonanecosystemrelativetotheirnumber.Beaversare amajor
geomorphicplayerwiththe buildingof damsandcanals (Gurnell 1998). Beaverspreferfirsttofourth
orderstreamsthat are dominatedbysandandgravel,adjacenttooxbowsandin flat,non-rocky areas
(Gurnell 1998). Beaverdamshave shownto create diverse flow velocities,withlow velocitiesnearthe
structure and highervelocitiesfurtheraway(Gurnell1998). Water depthaltersbetweenhighandlow
levelsdependingonwhere the damisbuiltwithinthe catchment(Gurnell1998). Flow isdivertedaround
the dams and can establishmultiple stablechannelswithinthe river(Gurnell 1998).Multiple channels
can betterhandle floodsorunseasonablyhighflows(Gurnel1998). Downcuttingisdecreased,channel
widthanddepthare more variable,and riparianvegetationcanflourish aswell (Gurnell1998). Beavers
have beenpurposefullyintroducedtoriversinordertostabilize banksandtrapsedimentsinsome areas
of NorthAmerica(Gurnell 1998). Beaversmake upa small percentage of the worldpopulation,buthave
a significanteffectonstream ecosystemsand are beneficialforrestoringstreamhealth.
Future Management Strategies
LWD isan unavoidable feature of riverecosystems. Understandinghow tomanage LWD with
refinedskillsandinsighttobenefitbothpeople andthe environmentisessential. Rivermanagerscan
start out bycreatinga checklistof whetherto immediatelyremove the woodydebrisortoconsider
otheroptions(Wohl etal.2014). LWD shouldonlybe removedif itisasubstantial floodinghazard,a
navigational hazardora directendangermentto humanlives(CDPEetal.2014). If managersdecide to

4. leave the LWD inthe river,they can performa quantitative assessmentof how likelythe debrisisto
move downstreamandcause harm(Wohl et al.2014). This all dependson seasonal flow levelsand
floodingevents.A tool calledLarge WoodStructure StabilityAnalysiscanaidmanagersindefiantly
decidingwhetherornotthe LWD is a large enoughhazardto remove (Wohl etal.2014). Assessingall
risksand benefitsof LWD ineach unique case anddecidingonthe bestdecisionisthe laststep(Wohl et
al.2014). Takingintoaccount locationof the woodydebrisandproximitytoinfrastructure,recreational
sports,streamgeomorphology,riparianvegetationandaquatichabitatare all important(Wohl etal.
2014). Likewise,simplyrelocatingthe debrisfurtherupstreamordownstreamorreplacinglarge woody
debriswithsmallerwoodpiecescouldaccountforhumansafetyaswell asmaintainingarobust
ecosystem.
Conclusion
Eventhe smallestof processescanaffectanentire ecosysteminabigway. Fallentrees, lost
woodsticks and beaverdams can accumulate ina riverand endup alteringthe entire geomorphic
characteristicsof a riveror stream.Water velocity,riverchannel andbanks,pool,barandripple
formationandwaterdepthare all affected. The complexityLWDintroducestoa riveror stream benefits
speciesdevelopmentandallowsnature’sprocessestofunctionproperly. Humansafetyisjustas
importantas ecosystemhealth andalsoneedstobe takenintoconsideration. LWDcanbe a menace
duringlarge floodeventsortothe recreational rafter. Makingsound,well researcheddecisionson
whethertoremove LWD or notfrom riverswill begintoinspire abalancedrelationshipbetweenpeople
and nature and leadtoa more sustainable future.

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