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Relative changes in sediment supply
and sediment transport capacity in a
bedrock-controlled river
Author
Young, W.J., Olley, Jon, Prosser, Ian P., Warner, R.F.
Published
2002
Journal Title
Water Resources Research
DOI
https://doi.org/10.1029/2001WR000341
Copyright Statement
Copyright 2002 American Geophysical Union. The attached file is reproduced here in accordance with
the copyright policy of the publisher. Please refer to the journal's website for access to the definitive,
published version.
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http://hdl.handle.net/10072/54847
WATER RESOURCES RESEARCH, VOL. 37, NO. 12, PAGES 3307-3320, DECEMBER
2001
Relative changesin sediment supply and sediment transport
capacity in a bedrock-controlledriver
W. J. Young, J. M. Olley, and I. P. Prosser
CSIRO Land and Water, Canberra, ACT, Australia
Departmentof Civil Engineering,CooperativeResearchCentrefor CatchmentHydrology,MonashUniversity
Melbourne, Victoria, Australia
R. F. Warner
Departmentof Geography,Universityof Sydney,Sydney,New SouthWales,Australia
Abstract. Riverscan be affectedby multiplenatural and human-inducedchangesto
sedimentsupplyand to sedimenttransportcapacity.Assessment
of the relativeimportance
of thesechangesenablesappropriateriver management.Here assessments
are madeof
the relativechangesin sedimentsupplyand sedimenttransportcapacityof an 83 km
section of the bedrock-controlled Coxs River, New South Wales, Australia. These relative
changesare estimated,in turn, for land degradation,historicalclimatevariations,and dam
closure.Measurementsof gullyerosionextentfrom aerial photographsindicatethat since
Europeansettlementthe averageannualsedimentsupplyto the river hasincreasedby a
factor of-20. Rainfall recordsand flow-gauging
recordsshowa climateshift in the mid1940s.On the basisof hydrologicmodelingthe increasedstreamflowin the period since
the mid-1940sis estimatedto have increasedsedimenttransportcapacityby a factor of
almost3. Closureof Lyell Dam in 1982 stoppedsedimentsupplyfrom the upper
catchment.On the basisof hydrologicmodelingthe flow regulationand abstractionat the
dam is estimatedto havereducedthe long-termaveragesedimenttransportcapacity
downstream
by 15%. The currentsedimenttransportcapacityafter the climateshift and
dam closureis 2.6 timeshigherthat in the first half of this century.In spiteof this net
increasein sedimenttransportcapacitythe hugevolumeof sedimentdeliveredto the
channelfrom gullyerosionhasled to widespreadsanddepositionalongmuchof the river.
Much of the river bed that waspreviouslydominatedby bedrockis now alluvialin
character.Only the steepestreachesremain bedrock-dominated.
1.
Introduction
River channelmorphologyis a result of past and present
flow regimesand patternsof sedimentsupplyand of the controls that catchmentgeologyand regional tectonicsplace on
long-profiledevelopment.The form of the channeladjustsin
responseto natural and human-induced
changesto the flow
regimeand/orsedimentsupply.In manystudiesof changesin
river form or processa singlecausalagenthasbeenexamined.
Singlecausalagentsmay influenceboth the flow regimeand
the sedimentsupply.For example, forest clearing increases
both catchmentrunoff and sedimentsupply[Calderand Maidmerit,1992],anddamclosuremodifiesthe flowregime(byflow
regulationand in somecasesdiversion)and cutsoff upstream
sedimentsupply[e.g.,Williamsand Wolman,1984].However,
manyriversare respondingto multiplechangesat anypoint in
time, and completeassessments
of river adjustmentneed to
take accountof the relativeimportanceof multipleinfluences
on sedimentsupplyand on the flow regime.
Changesto the flow regime and sedimentsupplymay be
gradualor suddenand may occurat a point or pointsin time
or spaceor be continuousalong a river length or througha
period of time. The temporal and spatialvariationsin the
Copyright2001 by the AmericanGeophysicalUnion.
Paper number2001WR000341.
0043-1397/01/2001WR000341509.00
balancebetween sedimentsupplyand transportcapacitydetermine the sedimentregime of the channel and nature of
benthic habitat. Where supply exceedscapacity, deposition
occursand the channel is alluvial. Where capacityexceeds
supply,the channelerodesto bedrockor other resistantsubstrate.By expressing
sedimentsupplyasa functionof drainage
area and sedimenttransportcapacityas a functionof drainage
area and channelslope,Montgomeryet al. [1996] showedthat
bedrock and alluvial channelsof Washington coastal rivers
(United States)couldbe distinguished
on an area-slopeplot
with bedrockreacheshavinghigher slopesthan alluvialchannels with comparabledrainageareas.In the formulation of
Montgomeryet al. [1996], bedrock and alluvial channelsare
separatedby a thresholdthat can be expressedas a critical
slopevalue for a givendrainagearea and for which sediment
supplyequalstransportcapacity.
Sedimenttransportcapacityis a functionof dischargeand
the local energygradientwhich can be approximatedby the
channelslope.Simple representationsof river form suggesta
monotonicdownstreamdecreasein slope and a uniform increasein dischargewith drainagearea [e.g., Church, 1996].
Suchrepresentations
leadto rapidincreases
in sedimenttransport capacitywith distancefrom the headwaterto a midcatchment maximum, followed by a slower reduction in capacity
with distanceto the catchmentexit [e.g.,Lawler, 1992;Abernethyand Rutherfurd,1998]. However, analysesof actualrivers
3307
3308
YOUNG
ET AL.: SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
suggest
thissimplepatternseldomoccurs[e.g.,Knighton,1999]
becauseof geologiccontrolson long-profiledevelopment,the
locationof majortributaryjunctions,andthe spatialvariability
in runoffgeneration.In manycatchments,
thereare alsostrong
spatialvariationsin erosionand hence sedimentsupply.In
catchments
wherethe downstreamvariationsin sedimentsupply and dischargeare poorlypredictedby drainagearea,drainage area and slopealonewill be an insufficientbasison which
to predict the occurrenceof bedrock and alluvial channels.
The two mostcommonlyreportedhuman-inducedcausesof
river adjustmentare dam closureand land clearing.Dam closure is a discreteand very obviousevent, to which the downstreamresponseshavebeen widely studied.For example,Wil-
liamsand Wolman[1984]reporton the downstream
changes
RIVER
Here we examinethe historicalchangesto sedimentsupply
and sedimenttransportcapacityin the CoxsRiver downstream
of Lyell Dam. There has been increasingconcernabout the
conditionof the river, especiallybecauseof widespreadsand
depositionbut alsobecauseof prolongedperiodsof low flow,
excessive
algalgrowth,andwidespreadinvasionof the channel
bywoodyweeds.The focusof recentinvestigations
hasbeenon
flow regulationand diversionsas the prime causeof river
degradation;in this paper, however,we considerthe relative
importanceof land degradation,historicalclimate variations,
and dam closureto changesin the sedimentsupplyand sediment transportcapacityof the CoxsRiver.
causedby dam closurefor 21 alluvialriversin North America. 2. Study Area
The Coxs River drains a catchmerit area of 2630 km 2 in
While bed degradationimmediatelydownstreamof a dam is
the mostobviousinitial response,this is onlythe first stagein eastern New South Wales, Australia, and flows into Lake Burchanneladjustmentto the imposedchangesin flow and sedi- ragorang,the major Sydneywater supplyreservoircreatedby
ment regime.Petts[1979]describes
the potentialfor bed deg- WarragambaDam on the Nepean River, whichhas inundated
radation,bed aggradation,andchannelmetamorphosis
follow- the lowest 20 km of the CoxsRiver valley (Figure 1). The
ing dam closure.Over a longerperiod the entire downstream upper catchmeritconsistsof broad, flat, mature valleysthat
river adjustsits physicaland ecologicalforms and processes havea generaloutlet to the MacquarieBasinto the west,while
[Petts,1980].The time lagsinvolvedin the downstreamchannel downstream
the broad,flat valleyshavebeentrenchedby deep,
adjustments,
the sequencing
of impacts,and the potentialfor juvenile gorges[Craft, 1928]. The easternboundaryof the
feedback means that river adjustmentto dam closureis a catchmentis an elevatedsandstone
blockwhichrisesin places
complexresponse.There havebeen severalstudiesof the ad- to 1100m abovesealevel.Betweenthe JenolanandKowmung
justment of coastal rivers in southeasternAustralia to dam Riversis the generallylevelJenolanPlateau(averageelevation
closure[e.g.,Erskine, 1985;Benn and Erskine, 1994;Erskine, •1200 m abovesea level) of soft Silurianrockstrenchedby
1996b;Erskineet al., 1999]. Adjustmentshave includedbed deepgorges[Craft,1928].The CoxsRiver itselfis underlainby
aggradation,bed degradation,and channel contraction,de- granites.
pendingon the effectson the floodregimeof dam closureand
The Coxscatchmenthas a cool temperateclimatewith a
operationand dependingon the input of sedimentfrom down- mean annual rainfall at Lithgow of 876 mm. Rainfall varies
stream tributaries.
stronglyacrossthe catchmeritlargelybecauseof the topograThe impactsof land usechangeon alluvialrivershavebeen phy. Stationson the easterncatchmentboundaryhave mean
widelyreported,includingthe impactsof deforestation[e.g., annualrainfallsin excessof 1400 mm, while the spatiallyavRai andSharma,1998],forestfires[e.g.,Whiteand Wells,1979], eraged mean annual rainfall for the entire catchment is 911
and mining [e.g., Graf, 1979]. In southeasternAustralia the mm basedon the climatedataandinterpolationroutinesof the
removal of native forest and the disturbance of the catchment
ANUCLIM software[Hutchinson,1989].
by introducedsheep,cattle,andrabbitsled to the development
The vegetationof the Coxscatchmentis dominatedby naof extensive
gullynetworksandthe entrenchment
of previously tiveforest,especiallythe southernhalfwhichis mostlynational
discontinuousstreams[Eyles,1977]. These changesthat re- park. However, extensiveareasof native and improvedpassultedfrom Europeansettlementaround150 yearsagohad a turesoccurin the middleanduppercatchment(Figure1). The
greaterimpact on gullyincisionthan any other changein the nativeriparianvegetationhasbeenclearedalongmuchof the
last 10,000years[Prosset
and Winchester,
1996] and delivered river, stock have accessto many parts of the channel, and
largevolumesof sedimentto uplandrivers.Significant
volumes introducedwillows(Salixspp),blackberry(Rubusfruticosus),
of sedimenthavebeendepositedin the lowerreachesof many and broom (Genistaspp) are commonin the channeland on
riversaffectedby upstreamgullyerosion:both gravelysandsin the river banks.
the channelitself and finer sedimentsacrossthe floodplain
The population of the catchmeritis •21,000, with 12,000
[Wasson et al., 1998]. As the postdisturbance sediment living in Lithgow and the remaindermostlyin the townsof
progresses
downstream,new river reachesare affectedby sed- Katoomba, Blackheath,and Mount Victoria. In the upper
iment deposition,alteringsubstratesizeand majorbed forms catchmentthe Coxs River Water SupplyScheme,which insuch as pools and riffles and degrading instream habitat cludes
Thompsons
Creek(27.5x 106m3),Wallerawang
(4.3x
[Prossetet al., 2000].
106m3),andLyell(33.5x 106m3) reservoirs,
wascommisDecadal and longer-term climate variationscan causeflow sionedin 1982to supplycoolingwaterto two coal-firedpower
regimechangesthat are substantialenoughto causedetectable stations(Figure1), althoughsubstantial
diversiondid not comchangesin sedimentregimesandriver morphology.In eastern menceuntil 1991.Most of the waterdivertedfor coolingis lost
New South Wales, Australia, a climate shift in the mid-1940s to evaporation,althoughsmallflowsare returned to the river
increasedflood magnitudesleadingto channelexpansionat between Wallerawang and Lyell Reservoirs.Pipelines and
the expenseof adjacentfloodplains[e.g., Erskine and Bell, pumpingstationsallowbothpowerstationsto be suppliedwith
1982;Erskineand Warner,1988;Erskine,1996a;Warner,1992] water from anyof the threereservoirs.There are no diversions,
and in manyriversthe removalof the fine top-stratumsedi- return flows,or regulationdownstream
of Lyell Dam, and so
ment from floodplainsurfaces(floodplainstripping)[Warner, the net effect of the schemeon the flow regime of the lower
1997;Nansonand Erskine,1988].
river is determinedby the flow releasesfrom Lyell Dam. The
YOUNG
ET AL.'
SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
3309
i
150o00'
Mount Piper Power Station
Thompsons Creek
Wallerawang Power Station
Reservoir
LOCATION
HAWKESBURY
MAP
- NEPEAN
CATCHMENT
33030 ' -
,,,,
Lyell Rese
coxs
RIVER
CATCHMENT
Mt Victoria
Blackheath
Katoomba
S0 u'TH :::.'•:i
....
.
PAC.
SCALE
tFtC
..
O (• E A AI
40km
N
34o00' --
SCALE
0
10
20km
I
Figure 1. CoxsRiver catchmentmap locatingthe storagesof the CoxsRiver Water SupplyScheme,power
stations,flow gaugingstations,and major towns.The shadingindicatesareasof remainingnative forest.
assessments
of river responsereported here are therefore for
the 83 km of river channelbetweenLyell Dam and the Kelpie
Point gaugingstation. Kelpie Point is a few kilometers upstreamof Lake Burragorangand immediatelyupstreamof the
junctionwith the KowmungRiver whichdrainsmainlynational
park (Figure 1).
3.
Hydrologic Regime
The hydrologyof the CoxsRiver hasbeen altered in historical times in at least three separateways. First, substantial
changes in mean annual rainfall have occurred; second,
changesin forest coverhave altered the patternsand amounts
3310
YOUNG
Table
1.
ET AL.: SEDIMENT
Measures of Flow Model
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
Performance
Coefficientof
Efficiencyof Sequential
Daily Flows
Coefficientof Efficiency
of Sorted,LogTransformedDaily Flows
Modeled:Gauged
Ratio of Total Flow
Volumes
Lithgow
0.77
0.91
1.015
Island Hill
0.64
0.84
1.023
Kelpie Point
0.63
0.95
0.937
Gauge
Station
of catchment runoff; and third, river flows have been diverted
and regulatedby the CoxsRiver Water SupplyScheme.The
magnitudeof the first and last of these changeshas been
assessed
usinga daily rainfall-runoff and streamflowmodel.
The model was developedby the New South Wales Department of Land and Water Conservationusingthe Integrated
QuantityQuality Model [Simonset al., 1996].Flow data from
the Lithgow(1960-1984), IslandHill (1981-1984),and Kelpie
Point (1962-1984)gaugeswere usedto calibratethe model.
Separateconceptualrainfall-runoffmodelswere calibratedto
predict the incrementalcatchmentinflows to these gauges,
usingthe AustralianWater BalanceModel [Boughton,1988].
Daily rainfall data from the Lithgowclimatestationwere used
to predictLithgowgaugedflows,and dailyrainfall data from
the Little Hartley climate station were used to predict the
incrementalinflowsbetweenLithgow and Island Hill and betweenIslandHill andKelpiePoint.While rainfallvolumesvary
stronglyacrossthe catchment,the temporal pattern is less
variable.The useof only two rainfall stationsprovedadequate
for capturingthesetemporalpatterns;the spatialvariationsin
rainfall volumesare adequatelycapturedby the flow records.
All threerainfall-runoffmodelsusedmonthlyevaporationdata
from the Bathurst Agricultural Research Station (latitude
33ø26', longitude149ø34'),the closestclimatestationto Lithgow that collectsevaporationdata. The monthlyevaporation
datawere usedby developinga relationshipbetweenthe number of rain daysin a month to the monthlyevaporationat the
Bathurststationand applyingthis relationshipto the Lithgow
and Little Hartley stationsto estimatemonthly evaporation
values.
The parametersof the rainfall modelswere calibratedusing
the shuffledcomplexevolutionalgorithmof Duan et al. [1992].
The objectivefunction used for the parameter optimization
was the coefficientof efficiencyof the log-transformedmodeled and gaugeddailyflow durationcurves(equation(1)):
modeled:gauged
ratio of total flow volumes(Table 1). These
measuresof model performanceindicatea goodto very good
matchingof gaugedand modeled daily flow duration curves
and a reasonablematchingof flow sequences.
By optimizingto
the flow duration curve, the best possiblepredictionof the
frequencydistributionof flows is obtained,with somelossof
model performancein flow sequencematching.This calibration method is appropriatefor this studywhere it is the frequency distributionof sedimenttransport capacitiesrather
thantheirsequence
thatisimportant.
The ratiosof totalflow
volumesindicatethat the modelslightlyoverpredicts
total flow
volumesat Lithgowand IslandHill and slightlyunderpredicts
total flowvolumesat KelpiePoint.The generallypoorermodel
performanceat Island Hill is a result of the short gauging
record. After calibrationthe model was run for an 85 year
(1900-1984) historicclimate sequenceto simulateboth the
long-termflow regimeunder schemeoperatingrulesfollowed
between1991 and 1999 (currentschemeoperation)and the
long-term flow regime without the regulationand diversion
influencesof any of the schemereservoirs(withoutscheme).
Analysisof the historicrecordsof annualrainfall at Lithgow
usingcumulativedeviationsfrom the long-termannualmean
showmean annualrainfall in the period sincethe mid-1940s
has been substantially
higherthan for the period prior to the
mid-1940s (Figure 2). This change is consistentwith the
changesin mean annualrainfall that occurredacrossmuchof
easternNew SouthWales [Pittock,1975;Cornish,1977].The
hydrologicmodeling(without scheme)indicatesthat the increases in rainfall
after the mid-1940s
translate
to increases in
total flowsof between69% (belowLyell Dam site) and 105%
(at IslandHill) (Table 2). Under the currentregimeof scheme
operationthe rainfall increaseswould have translatedto increasesin total flowsof between123% (at Kelpie Point) and
153% (belowLyell Dam site) (Table 2). Under schemeoperation the total flow belowthe Lyell Dam is less,sothe relative
increase in flows volumes
due to the rainfall
increases would
have been greater.The mid-1940sclimate shift affectedflows
Z (Gt- •)2_ Z (mi- Gt)2
of all magnitudes(e.g., Figure 3a, below Lyell Dam site);
i=1
i=1
E =
,
however,it is the increasesin the highflowsthat dominatedthe
changein total flow and the changesin sedimenttransport
capacity(discussed
in section4).
i=1
The constructionand operation of the Coxs River Water
whereGi arethesorted
log-transformed
gauged
dailyflows,0 SupplySchemehasgreatlyreducedflowsdownstreamof Lyell
Dam. Hydrologicmodelingusing the full climate sequence
is the mean of the log-transformed
gaugeddailyflows,andMi
are the sortedlog-transformedmodeleddaily flows.Muskin- (1900-1984) indicatesthat currentschemeoperationreduces
gum flow routing [Miller and Cunge,1975]was usedbetween mean daily flows(and hencetotal volumes)by between44%
stations,and routing parameterswere manuallyadjustedas (below Lyell Dam) and 11% (at Kelpie Point) and reduces
median daily flowsby between95% (below Lyell Dam) and
they had only minor influenceon the model calibration.
Model performancewas assessedusing the coefficientof 34% (at Kelpie Point) (Table 3). The operationof the scheme
efficiencyof the sortedlog-transformeddailyflows(the objec- has affectedall but the highestflows(e.g., Figure 3b, below
tivefunctionfor calibration,(1)), the coefficientof efficiencyof Lyell Dam). For the 1900-1945 climatethe simulatedreducthe sequential(unsorted)untransformeddaily flows,and the tion in total flow due to currentschemeoperationis between
YOUNG
ET AL.' SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
3311
2000
1000 -
-2000
-
-3000
,
1880
....
1900
1920
1940
1960
1980
2000
Year
Figure 2. Cumulativedeviationsfrom the long-termmean annualrainfall at Lithgowbetween1889 and
1997.
57% (belowLyellDam) and 16% (at KelpiePoint)(Table4),
and for the 1946-1984
climate the simulated reduction in total
ing into more openwoodlandand someareasof grasslandin
the widervalleys[Flannery,1998],suggesting
that, overall,the
flowdueto currentschemeoperationisbetween35% (at Lyell catchment was at least 90% forested. Between 1820 and 1840
Dam) and8% (at KelpiePoint)(Table4). The overallchange manylandgrantswere allocatedfor grazing,andin subsequent
in total flows since 1945 has been an increase of between 10%
decades,forestswere clearedto increasethe availablegrazing
land and to providetimberfor homesteads,
fences,and stockincreasein moderateto highflowsresultingfrom the mid-1940s yards[Barrett,1993].Usinga semiempirical
relationshipdevelclimateshiftpartiallyoffsetby the decrease
in lowflowsresulting oped by Zhang et al. [1999], this degreeof forest clearingis
from schemeoperation(e.g.,Figure3c,belowLyetlDam).
estimatedto have increasedannualstreamflowby >50%. The
Substantial reductions in forest cover have occurred in the
relationshipof Zhanget al. [1999]predictsannualevapotranscatchment,and becauseevapotranspirationfrom forests is pirationas a functionof annualrainfall and the percentageof
higherthanfrom grasslands
[Zhanget al., 1999],thiswill have forestcoverand wasparameterizedusingleastsquaresfits to
increasedrunoff volumes.The changesin daily streamflow, data sets from >250 catchments across 29 countries.
flood magnitude,and sedimenttransportcapacityresulting
This substantialincreasein annualrunoff doesnot imply an
from changesto forestcoverin the catchmenthavenot been
equivalentincreasein floodmagnitude.During extremerainquantifiedbecauseforestclearingwascompletebeforeflowterm in the short-termwater
gaugingrecordsbegan,preventingaccuratehydrologicsimu- fall eventsthe evapotranspiration
balance
is
small
relative
to
the
rainfall
input and runoff output.
lation of thesechanges.
Once
the
soil
moisture
store
is
full,
nearly
all rainfall is transAssessments of the reductions in forest cover do allow the
lated
into
streamflow,
and
land
use
effects
are negligible.
changein total annualflowto be estimated.The currentextent
Hence,
while
minor
increases
in
flood
magnitude
may have reof nativeforestis 55% abovethe Lithgowgaugestation,49%
are
betweenthe Lithgowand IslandHill gaugestations,and 98% sultedfrom forestclearing,mostof the streamflowincreases
in moderateflows.Nonethebetweenthe IslandHill and Kelpie Point gaugestations(Fig- likelyto haveoccurredasincreases
ure 1). Thejournalsof explorers
in theearly1800sdescribe
the less,the affectof forestclearingon dailyflowsand on sediment
in thisanalysis.
presettlement
vegetationas heavilyforestedhillslopes,open- transportcapacityremainsunquantified
(belowLyellDam) and86% (at KelpiePoint)(Table5), withthe
Table 2. RatiosComparingthe 1946-1984Periodto the 1900-1945Periodfor Total
Flow and SedimentTransport Capacity
Total
Location
Flow Ratio
Without Scheme
SedimentTransportCapacityRatioa
Current
Without Scheme a
Current b
Lyell Dam
1.69
2.53
2.04
2.80
Island Hill
2.05
2.50
3.01
3.57
KelpiePoint
2.03
2.23
2.71
2.89
•Averageby reachfor sedimenttransportcapacityis 2.93for without-scheme
flowconditions.
bAverage
byreachfor sediment
transport
capacity
is 3.40for currentflowconditions.
3312
YOUNG
ET AL.: SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
lOO
I A
Without scherm,190•1945 clirmte
Without scherre,194fv1984clirmte
lO
o.1
0.01
10
20
30
40
50
60
70
80
90
100
Percent Exceedance
lOO
Without
scherre,
1900-1984
clirmte
--
IO
Oamnt scherreoperation,190•1984clirmte
o.I
0.01
0
10
20
30
40
i
i
i
i
i
50
60
70
80
90
100
Percent Exceedance
lOO
C
Without
scherre,
1900-1•
clirmte
ß
.
lO
o.1
O.Ol
o
lO
20
30
40
50
60
Percent Exceedance
70
80
90
lOO
YOUNG
ET AL.: SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
3313
Table 3. ModeledMean and Median Daily FlowsUnder Without-scheme
(WS) and Current (C) Flow Conditionsfor the
Period
1900-1984
a
Mean Daily Flow
Median Daily Flow
Sediment
WS
C
Ratio
C/WS
Lyell Dam
1.61
0.90
0.56
0.61
0.03
0.05
Island Hill
3.46
2.75
0.79
1.07
0.46
0.43
0.83
Kelpie Point
6.35
5.65
0.89
1.87
1.23
0.66
0.91
WS
C
Ratio
C/WS
TransportCapacity
Ratio C/WSb
0.62
aTheeffectsof landusechange
havenotbeenmodeled.
Flowsaregivenin m3 s-•.
bAverage
byreachsediment
transport
capacity
ratio(C/WS)is 0.85.
4.
Sediment Transport Capacity
Sedimenttransportcapacityper unit channelwidth(q•) can
be modeled by the following function of dischargeper unit
channelwidth (q) and channelslope(S) (to approximatethe
hydraulicgradient)[Julienand Simons,1985]:
qs= kq•S•,
(2)
where the k includesparametersdescribinghydraulicroughnessand the bed sediments(e.g., sizeand density)and/3 and
3' are empiricallyderived exponents.From a review of publishedsedimenttransportdata from rivers,overlandflow, and
laboratoryflumes,Prossetand Rustomji[2000]foundmedian
values of both /3 and 3' to be 1.4. Total sedimenttransport
capacity(Q s) can therefore be expressedas the following
functionof channelslope,discharge(Q), and channelwidth
(w):
Lyell Dam is very difficultto quantifyand sohasbeenomitted
from the analysis.This simplification
will meanthat for current
conditionsthe sediment transport capacityin this sectionof
river will be overestimated.
The surveyedcross sectionsshow only a small variation
(+_20%)in total channelwidth (includingflood channelsand
lateral benches)and no downstreamtrend in channelwidth.
Crosssectionshave not been surveyedin the lower 50 km of
channel,but approximatemeasurementsfrom l:25,000-scale
topographicmapsand from aerial video of the entire channel
do not reveal any substantialdownstreamincreasein total
channelwidth over the 83 km river sectionand reveal only
minor spatialvariationin total channelwidth. In alluvialrivers,
width typicallycovarieswith slope,and the lack of substantial
variation
in width in the lower Coxs River is attributed
to the
Exoticwillows(mainly,Salixfragilis)
haveinvadedthe muchof
the first 50 km of channel(and lateral benches)downstream
extent of bedrock and the degree of valley confinementthat
generallylimit channelwidening.The smallspatialvariationin
width and relativelyweak dependenceof sedimenttransport
capacityon width (equation(3)) meanstreatingwidth as spatially constantis a reasonablesimplificationto estimatingrelative changesin sedimenttransportcapacity.
With width and k treated as constant,relative changesin
sedimenttransportcapacitywill be determinedby changesin
dischargeand slope. Reach-averagedchannel slopes were
measuredfrom l:25,000-scalemaps.BetweenLyell Dam and
Kelpie Point the river drops by 590 m, and channel slopes
rangefrom 0.0016to 0.1 (Figure4). This sixty-foldvariationin
slopeand the strongdependenceof sedimenttransportcapacity on slopemean the downstreampatternsin sedimenttransport capacityare likely to be dominatedby slopevariations.
Daily flowsweremodeledat Lyell Dam, IslandHill, andKelpie
Point. Incrementalincreasesin drainagearea betweengauge
stationswere usedto estimatethe downstreampattern of daily
dischargeincrease.Daily flowsare of a sufficientresolutionto
distinguishindividualrainfall eventsin this catchmentand so
are sufficientto capturethe differencesin both flow magnitude
and variabilitybetweendifferent climateor reservoirmanage-
from Lyell Dam, increasinghydraulicroughness.
The lower 33
km of river is largelyfree of willow invasion,and aerial video
of the river revealedonly minor variationsin channelform.
Hydraulic roughnessin this lower sectionis therefore treated
as spatiallyand temporallyinvariable.The hydrauliceffect of
Table 4. Ratios Comparingthe Current Regime to the
Without-SchemeRegime for Total Flow and Sediment
Transport Capacity
Qs= kw-ø'4Q
1'451'4.
(3)
Relative spatialor temporalchangescan be investigatedeven
when all of the termscannotbe evaluated,providedthe terms
that are not includedcan be reasonablyshownto be spatially
or temporallyconstant.The most difficultparameterto evaluate in (3) is the hydraulicroughnesscomponentof k. Hydraulicroughness
cannotbe measureddirectly,and evenindirect estimates are difficult to obtain. At transporting
discharges,hydraulicroughnessis likely to be dominatedby
channelshapeand the extentof woodyvegetationin the channel and on lateral
benches.
Sediment
texture
will be a minor
componentof hydraulicroughness
at highflows.Detailed surveyshavebeenmadeof 10 channelcrosssectionsin the first 33
km downstreamof Lyell Dam, and these show little downstream variation in channel cross-sectionshape. Substantial
downstreamchangesin the extentof woodyvegetationboth in
the channel and on lateral benches were observed in the field.
the willows
in the first 50 km of channel
downstream
from
Total
Figure 3. (opposite)Flow durationcurvesfor modeleddaily
flowsimmediatelybelowLyell Dam showing(a) the effectof
the climateshift, (b) the effect of the schemeoperation,and
(c) the combinedeffectsof the climate shift and the dam
operation.
1900-1945
Flow Ratio
1946-1984
SedimentTransport
CapacityRatio
1900-1945
1946-1984
Lyell Dam
0.43
0.65
0.50
0.69
Island Hill
0.70
0.85
0.73
0.87
Kelpie.Point
0.84
0.92
0.87
0.93
3314
YOUNG
ET AL.: SEDIMENT
CHANGES
Table 5. Ratios Comparingthe Current Regime Under
1946-1984 Climate to the Without-SchemeRegime Under
1900-1945 Climate for Total Flow and SedimentTransport
Capacity
Total
SedimentTransport
CapacityRatioa
Flow Ratio
Lyell Dam
1.10
1.41
Island Hill
1.74
2.62
Kelpie Point
1.86
2.51
aAveragereach sedimenttransportcapacityis 2.56.
ment regimes.Higher temporalresolutionflow data wouldbe
likely to give higher absolutevalues of sedimenttransport
capacitybut would not affect the relative differencesinvestigated in this study.
Using(3) and the simplifications
describedabove,the relative downstreamchangesand relativetemporalchanges(current flowregimeto without-scheme
regime)in sedimenttransport capacityin the CoxsRiver were investigated(Figure5).
The sedimenttransportcapacityratios are expressedrelative
to the mean sedimenttransportcapacityof all reachesunder
long-term(1900-1984) without-scheme
flow conditions.Figure 5 showsthat sedimenttransportcapacityin the lowerCoxs
River doesnot simplydecreasewith distancedownstreamasis
the casein simplemodelsof river form. Sedimenttransport
capacityvariesspatiallyby a factorof nearly500, largelyas a
result of slopevariations.The averagetotal annual discharge
onlyvariesby a factorof 2 overthe 83 km, and sothe discharge
increasewith distanceand the spacingof tributarystreamsdo
not greatly influencethe downstreampatternsin transport
capacity.The reacheswith the lowestsedimenttransportcapacity are, first, the 6 km immediatelydownstreamof Lyell
Dam; second,two short reaches(2.3 km) before the 20 km
markandonelongerreach(4.5 km) afterthe 20 km mark;and,
finally, the 12 km reach from 38 km to 50 km (Figure 5).
IN A BEDROCK-CONTROLLED
RIVER
Althoughthe slopebetween70 km and76 km is againverylow,
there are significantadditionsof flowby thispoint,sothat the
sedimenttransportcapacityhere is higher.The highestcapacity reachesare thosebetween50 km and 60 km downstream
from Lyell Dam.
The mid-1940sclimate shift increasedthe averagesediment
transport capacityof the without-schemeflow regime by a
factorof 2.9 (Table 2 and Figure5a). Under currentscheme
operationthe climate changewould have increasedthe average sedimenttransportcapacityby a factorof 3.4 (Table 2).
Dam closurecausedan averagereductionin sedimenttransport capacityof only 15% relativeto long-term(1900-1984)
without-scheme
conditions(Table 3 and Figure 5b). The reduction in sedimenttransportcapacitydue to dam closure
reduces with distance downstream
as the relative
reduction
effect of the mid-1940s
climate shift and dam closure has been
to increasethe averagesedimenttransportcapacityby a factor
of 2.6 (Table 5 and Figure5c).
5.
Sediment Supply
In the last 150yearsthe sedimentsupplyto the lower Coxs
River is likely to havebeenmodifiedby widespreadgullyerosion in the catchment,by the mid-1940sclimate shift that
remobilizedfloodplaindeposits,and by closureof Lyell Dam
that cut off sedimentsupplyfrom the upper catchment.
The extent and severityof gully networksacrossthe catchment hasbeen mappedby the New SouthWales Department
of Land and Water Conservationusing aerial photographs
from the early 1980sand limited field surveys.The gulliesare
mostlylessthan 1 km in length,havesmallcrosssections,and
havedrainageareas<3 km2.The networks
aredensest
in the
catchments
of midcatchment
tributaries,suchas the 51 km2
8OO
0.12
700
- 0.10
600
- 0.08
500
400
300
- 0.04
200
- 0.02
100
0
I
0
10
20
30
40
in
flow becomesless, even though dischargevariationsdo not
dominatethe downstreamchangesin sedimenttransportcapacity(Table 3 and Figure5b). At Lyell Dam the reductionin
sedimenttransportcapacitydue to dam closureis 38%, but at
KelpiePointthe reductionis only9% (Table3). The combined
I
I
I
I
50
60
70
80
0.00
90
Distancefrom dam(km)
Figure 4. Channelelevationand slopein the lower CoxsRiver as a functionof distancedownstream
from
Lyell Dam.
YOUNG
ET AL.: SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
100
Without scherm,19004•
clirmte
A
Without scherm,1946-1984clirmte
0.01
0
I
I
I
I
I
10
20
30
40
50
6O
70
80
Distance from dam(kin)
100
Without scherre,1900•1• clirra•
B
Cunentschermoperation,1900•1984clirmte
0.01
I
0
10
[
,
I
I
I
I
l
20
30
40
50
60
70
80
90
Distance from dam(km)
100
Without scherm,190(•1945ctkmte
c
Cunentschermoperation,
1946-1984
c]kmte
0.01
0
i
i
,
,
i
,
10
20
30
40
50
60
Distance hom dam(kin)
Figure 5. Modeled relative sedimenttransportcapacityin the lower CoxsRiver as a functionof distance
downstream
from Lyell Dam showing(a) the effectof the climateshift,(b) the effectof the schemeoperation,
and (c) the combinedeffectsof the climateshiftand the dam operation.Transportcapacitiesare relativeto
the modeledlong-term(1900-1984) averagetransportcapacityfor the without-scheme
flow regime.
3315
3316
YOUNG ET AL.' SEDIMENT
CHANGES IN A BEDROCK-CONTROLLED
Table 6. Length and Densityof Gully Erosionin the Coxs
RIVER
Catchment
Area
Total Gully
Length,km
Gully Density,
km km-2
ter [1996]in the Bredocatchment.
The gullymappingin the
Coxscatchment
indicates
gullydrainageareasof <3 km2, and
measurements
by Prossetand Winchester
[1996]showthe averagewidth of gulliesin the Bredocatchmentwith drainage
Upstream of Lyell Dam
Lyell Dam to Island Hill
IslandHill to Kelpie Point
KowmungRiver
68
129
18
3
0.18
0.22
0.04
<0.01
matedgullydepthof 2.0 m and an assumedaveragewidth of
2.0 m, it wasestimatedthat a total of 860,000m3 of sediment
hasbeenerodedfromgulliesin the catchment
upstreamfrom
River Catchment
areasof <3 km2 is 2.0m (range0.5-5.5m). Usingtheesti-
Kelpie Point.
To determineaverageannualsedimentyields,the sediment
volumeswereaveraged
overan estimated140yearsof delivery.
Ganbenang
Creeksubcatchment
wherethe densityof thegully This deliveryperiodwas assumedfrom the knowledgethat
network is 0.56 km km -2. Because extensive forested areas gully initiationusuallybeginssoonafter forestclearinghas
remain,the averagedensityof gullyerosionin the CoxsRiver becomewidespread[Eyles,1977],that forestclearingin the
catchmentis low (Table 6). From the gully mapping,gully CoxsRiver catchmentbeganin the middle of the nineteenth
lengthswere determinedfor each tributarycatchmentand century,and that long-term flood recordsfor farther downintertributary area and totaled to show the accumulationof streamin the Hawkesbury-Nepean
River systemindicatethat
gullylengthwith distancedownthe CoxsRiver (Figure6). between1860and 1900morefloodswere experienced
thanin
There are 215 km of gulliesin the catchmentaboveKelpie the 60 yearsbeforeor after [Hall, 1927;Riley,1980].As forest
Point,197km of whichareupstream
of IslandHill (Table6). clearingin Coxscatchment
wasalreadywidespread
by 1860,it
The strongspatialpatternsin land degradationmean that islikelythatit wasduringtheperiod1860to 1900thatthegully
postdisturbance
sedimentsupplyin the CoxsRiver cannotbe networkbeganto develop.The period 1860-2000was therepredictedas a simplefunctionof increasing
drainagearea.
fore selectedfor averagingsedimentdelivery.The gulliesare
To assess
the relativechangein sedimentsupply,gullyero- all well connectedto the streamnetwork,and so complete
sionrateswere comparedto background
surfaceerosionrates deliveryof eroded materialwas assumed.
in the catchment.This requiredconvertinggully lengthsto
The averageannualyield for the catchmentaboveKelpie
sedimentvolumesusingaveragegullydimensions.
The gully Pointis6143m3yr-•, andtheaverage
specific
yieldis4.2m3
mappingindicatesthe depthclass(0.5-1.5 m, 1.5-3 m, 3-6 m, km-2 yr-•. Thespecific
yieldfor Ganbenang
Creek,themost
or >6 m) of individualgullies,and distance
weightingby the gulliedsubcatchment,
is 16.1m3 km-2 yr-•. Because
of small
averagedepthvaluefor eachclass(andusing7.5m for the top gullycrosssections
thisisnot a highlong-termyieldfromgully
depthclass)providedanestimate
of 2.0m for theaverage
gully erosion.
A specific
annualgullyerosionyieldof 72 m3 km-2
depthin the Coxscatchment.
Thisfigureis comparable
to the yr-• wascalculated
for a 102yearperiodin the45km2 Wanaveragedepth of-2.5 m (range0.5-4 m) for 44 gulliesat garah Creek catchmentin southeastNew South Wales by
Bredboin southernNew SouthWales,wheregullydepthwas Prosset
et al. [1994],anda specificannualgullyerosionyieldof
found to be unrelatedto drainagearea but, rather, reflected 118m3km-2 yr-1 wascalculated
fora 115yearperiodin the
the depth of alluviumor colluvium[Prossetand Winchester, 80 km2 Fernances Creek catchment in eastern New South
1996].Gully widthsin the CoxsRiver catchmenthavenot been WalesbyMelvilleandErskine[1986].In thesecases,although
surveyed;
however,field observations
suggest
they are of sim- the gullynetworkdensitieswere comparableto thosein the
ilar dimensions
to the gulliessurveyed
byProssetand Winches- CoxsRiver catchment,the gullycrosssectionswere consider-
12000
25O
10000
200
8000
150 •
6000
4000
Average gully erosion
Average surfaceerosion
Cumulativegully length
2OOO
0
10
20
30
40
50
60
70
80
90
Distancefrom dam (kin)
Figure 6. Cumulativelengthof gullyerosionandcumulativeaveragegullyandsurfaceerosionratesin the
catchment
of the lowerCoxsRiverasa functionof distance
downstream
fromLyellDam.
YOUNG
ET AL.: SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
RIVER
3317
30
0
10
20
30
40
50
60
70
80
90
D•tanceffomdam(km)
Figure 7. Spatiallyand temporallyaveragedrelativesedimentsupplyin the lower CoxsRiver as a function
of distancedownstreamfrom Lyell Dam. Sedimentsupplyis relative to the averagepredisturbancesurface
erosionrate. The top curveshowsthe ratio of the sumof gullyandsurfaceerosionloadsto the surfaceerosion
load;it indicatesthe relativeincreasedue to land degradationignoringsedimenttrappingby Lyell Dam. The
middlecurveshowsthe relativesedimentsupplyassuming
surfaceerosiononlyis trappedby Lyell Dam, that
is, assumingall gully-derivedsedimentfrom upstreamof the dam passedthe dam site before closure.The
bottomcurveshowsthe relativesedimentsupplyassuming
all upstreamsurfaceandgullyerosionsedimenthas
been trappedby Lyell Dam sinceclosure.
ablylarger:---15m2 and20 m2 in WangarahCreekandFer- many
eastern
NewSouth
Wales
catchments,
including
rivers
in
nancesCreek, respectively.
For comparisonwith background the Hawkesbury-Nepean
Basin,havebeen linked to the midsurfaceerosionrates the volumetricyieldswere convertedto 1940sclimateshiftby comparisons
of historicalchannelcrossmassyieldsusinga bulksoildensityof 1.5t m-3.
sectionsurveys[e.g.,Erskine,1986;Warner,1991],photogramThe predisturbanceand ongoingsurfaceerosionratesin the metry of historical aerial photographs[e.g., Departmentof
CoxsRiver catchment(Figure 6) were estimatedusinga re- PublicWorks,1987],andfield observations
[e.g.,Warner,1995].
gression
relationship
betweenyield (t yr-•) and catchment Historicalchannelsurveysare not availablefor the CoxsRiver
area(km2) derivedbyWasson
[1994].Thisrelationship
(yield downstreamof Lyell Dam, but comparisonsbetween aerial
= 1.6(drainage
areaø'79),/,2
_ 0.85) isbasedonpresettlementphotographsfrom the 1930sto 1950sand field observations
average annual sediment yields estimated by stratigraphic andaerialvideoof the currentchannelrevealthe development
methodsin 11 catchments
(all <500 km2) on the southern of parallel and meanderchutesespeciallyupstreamof Island
tablelandsof New SouthWales.The averagespecificsurface Hill. Field observations also indicate that most of the fine
erosionyield for the Coxscatchmentupstreamof Kelpie Point alluviumhasbeen removedfrom floodplains,strippingfloodwasestimated
as0.35t km-2 yr-•.
plain surfacesdownto cobbleand boulderbases.Small areas
Comparingthe sum of the gully and surfaceerosionyields of finer sedimentremain, held in placeby the roots of native
(averagesedimentsupplyafter catchmentdisturbance)to the river oaks (Casuarinacunninghamiana).
While there is suffisurfaceerosionyield (averagesedimentsupplybefore catch- cient evidencefrom the historicalaerial photographsto reament disturbance)providesan indicationof the relative in- sonablyattributethesechangesto the mid-1940sclimateshift,
creasein sedimentsupplydue to land degradationalone (top there is insufficientquantitativeinformationto accuratelydecurvein Figure7), that is, withoutanyreductiondue to Lyell termine volumes of sediment mobilized. Field observations
Reservoir.The value at 0 km on Figure 7 is the relative in- indicatethat chutesare ---3-4 m wide and 1 m deep,andthere
creasein sediment
supply
for theupper381klTl
2 of thecatch- are estimatedto be manykilometersof chutesbetweenLyell
ment (aboveLyell Dam). The relativeincreasevariesbetween Dam and Island Hill. The strippingof 1-4 m depth of fine
•--16 and 27; the highestrelative increaseoccursbetween38 alluviumhas been reportedfor floodplainsurfacesin other
and 47 km downstreamfrom Lyell Dam and is coincidentwith coastalNew SouthWales locations[Warner,1997], and crude
the lowest sedimenttransport capacities.On average,land calculations
basedon thesevaluessuggestthat the volumeof
degradationhasincreasedthe erosionrate by a factor of 19.5. sedimentderivedfrom chuteformationand floodplainsurface
In comparison,Neil and Fogany [1991] used data from 12 strippingin the CoxsRiver may be a similarorder of magnigulliedcatchmentsand four undisturbedcatchments(all <5 tude to the volume of sedimentderived from gully erosion.
km2) to estimate
thatdiscontinuous
gullynetworks
increased However,becauseof the high uncertaintyin the estimatesof
sediment yield l 1-fold, and continuousgully networks in- alluvium mobilized since the mid-1940s climate shift, these
creasedsedimentyield 64-fold.
volumeshave not been included in this analysisof relative
Channel expansionand strippingof floodplainalluviumin changesin sedimentsupply.
3318
YOUNG
ET AL.:
SEDIMENT
CHANGES
IN A BEDROCK-CONTROLLED
The closureof Lyell Dam in 1982 stoppedall transportof
coarsesedimentfrom the upper catchmentto the lower river.
The sedimentsupplyto the lower river is now from the midcatchmenttributariesand from the reworkingof main channel
alluvium.The proportionof the sedimentderivedfrom gully
erosionin the upper catchmentthat was transportedpast the
dam site before dam closure is unknown; hence the exact
impactof the dam on sedimentsupplyis unknown.The smallest effect the dam can have had on sedimentsupplyis to have
cut off delivery of all coarsesedimentderived from surface
erosionin the upper catchment(middle curvein Figure 7).
This assumes
that all of the coarsesedimentderivedfrom gully
erosionin the upper catchmentwas transportedpastthe dam
sitebefore closure.This scenariorepresentsa 5% reductionin
sedimentsupplybelowthe dam, reducingto <2% by Kelpie
Point. The greatesteffect the dam can have had on sediment
supplyis to have cut off the deliveryof the coarsesediment
derived both from surfaceerosion and from gully erosion in
the uppercatchment(bottomcurvein Figure7). This assumes
that in the period betweenthe initiation of gully erosionand
the constructionof the dam, no sedimentfrom upper catchment gullieswastransportedpastthe dam site.This represents
a 100% reductionin sedimentsupplybelowthe dam, reducing
to 31% by Kelpie Point. Even under this scenarioof complete
sedimenttrappingby the dam, the currentsedimentsupplyto
the river by Kelpie Point is still over 13 times higher than
before catchment
disturbance.
The actual effect of the dam on
RIVER
pacity dependshow close they are to the alluvial-bedrock
threshold.To comparecapacityto supplyrequiresfull dimensionalevaluationof (3), and ask cannotbe measureddirectly,
it mustbe evaluatedby measurements
of transportrates.Without transportmeasurementsfor the Coxs River an indication
of susceptibility
was obtainedusinga value for k derivedby
Yang[1972]from existingdata setsusinghis unit streampower
approach.For (3) in SI units(kilograms,meters,andseconds),
the value of k used was 2.5 x 104 and indicated that for the
long-term averagenatural transportcapacitythe increasesin
sedimentsupplyfrom gullyerosioncouldhave causedasmuch
ashalf the lengthof the lowerCoxsRiver to crossthe threshold
from bedrockto alluvial.Field observationsby the authorsand
aerial video have revealedthat sanddepositionis widespread
alongmuchof the lower CoxsRiver, includingmanyreachesin
the national park and downstreamof the Kowmungjunction.
Many reachesthat were previouslybedrock are now alluvial.
The volume of sandthat appearsto still be storedin channel
depositssuggests
that many decadeswill be requiredfor the
channelto fully adjustto the pulseof sedimentdeliveredfrom
gully erosion.
The analysesof supplyand capacitychangesin the Coxs
River demonstratea simpleapproachfor assessing
the likely
impactsof land degradation,climatevariability,and flow regulation on river channelcharacter.Suchanalysescan be used
to identify the major causesof channelchangeand to guide
remedialactions.In the CoxsRiver, for example,it is clearthat
sedimentsupplyis expectedto be closerto the smallerreduc- environmental flows are not an effective means of rehabilitattion in sedimentsupply;that is, a substantialfraction of the ing the riverbed.Rather, controlof sedimentsourcesand the
gully-derivedsedimentfrom abovethe dam site is expectedto removalof in-channeldepositsand/orstabilizationwith native
havebeentransportedpastthe dam site.Over 15% of the gully vegetationare preferred approachesto rehabilitation.Analyerosion above the dam site has occurred in Farmers Creek
sesof this type could be extendedby detailed surveyingof the
which now flowsinto Lyell Reservoir.At least this fraction of river channel to determine the volumes and locations of stored
the gully-derivedsedimentfrom abovethe dam site can rea- sediment.Comparingthe volumesof sedimentstoredto the
sonablybe expectedto have been transportedpast the dam volumessuppliedto the river would enable constructionof a
site.
sedimentbudget,allowingdimensionalquantificationof average transportrates.This would providemore certaintyto rehabilitationefforts,asit would allow an estimateof the period
6.
Discussion
requiredto flushstoredsedimentfrom the channel.QuantifiIn the last 150 yearsthe sedimentregime of the CoxsRiver cationof a sedimentbudgetwould alsoprovideimprovedeshaschangedas a resultof forestclearingand consequentgully timatesof k in (3) for reconnaissance
studiesof other rivers.
This studyof the CoxsRiver illustratesthe needto assess
the
erosion,historicalclimatevariations,and dam closure.By far
the largestrelativechangein the sedimentregimeof the Coxs relative importanceof multiple influenceson sedimentsupply
resultingspatial
River hasbeen the nearly20-fold increasein sedimentsupply and sedimenttransportcapacityand to assess
dueto gullyerosion.Spatialpatternsin gullyerosionacrossthe and temporal patternsin a river'ssediment.The requirement
multipleinfluenceson sedimentsupplyand transport
catchmentmeanthat sedimentsupplycannotbe predictedasa to assess
simplefunctionof drainagearea. The mid-1940sclimateshift capacityoccursin many rivers,and the better known Snowy
is estimatedto haveincreasedthe sedimenttransportcapacity River in southeasternAustralia is another obviousexample.
by a factorof 2.9,while damclosurehasonlyreducedtransport Interbasin transfersdivert 99% of the flow from the upper
Authority,1993]
capacityby 15% and sedimentsupplyby between 10% and SnowyRiver [SnowyMountainHydro-Electric
25%. Future climatevariabilityis thereforeexpectedto have a suchthat the only flowsthat passJindabyneDam to the lower
greaterinfluenceon the sedimentregimeof the river than any Snowy River are controlled low-flow releases.Immediately
changesto the amountsand patternsof water diversionfrom below the dam, the capacityto transportcoarsesedimenthas
the dam. Overall, channelresponses
will havebeen dominated been reducedto zero, and the overall sedimenttrap efficiency
by the increasein sedimentsupplydueto gullyerosion,rather of the dam hasbeen estimatedas 99.5% [Erskineet al., 1999].
than the increasein transportcapacitydue to the mid-1940s Severe land degradation on the Monaro Tablelands downclimate shift or the relatively minor changesin supply and streamof the dam has deliveredlarge volumesof sandto the
channel[Caitcheonet al., 1991],and the patternsof sanddepcapacitydue to the flow regulationand dam closure.
The actual impact of the changesin sedimentsupplyand osition along the channel are well documented[Brizgaand
transportcapacitydependson the relative balancebetween Finlayson,1992, 1994]. Although a detailed analysisof the
capacityand supply.The CoxsRiver is bedrock-dominated
but downstreampatternsof relativechangein sedimentsupplyand
has an alluvialbed in the flatter reaches.The susceptibility
of sedimenttransportcapacityhas not been made, it has been
[Erskineet al., 1999;Brizgaand Finlayson,1994]
the bedrockreachesto increasedsupplyand/or decreasedca- acknowledged
YOUNG
ET AL.: SEDIMENT
CHANGES
that the current sedimentregime and bed condition of the
lower SnowyRiver are a result of both land degradationand
dam closure.
IN A BEDROCK-CONTROLLED
RIVER
3319
the hydrologicmodeling,and Andy Marr from SnowyMountainsEngineeringCorporation assistedwith interpretation of the modeling
results.AnthonyScottandAndrewHughesassisted
with data analysis,
aerial photographinterpretation,and figurepreparation.Heinz Buettikofer alsoassisted
with figurepreparation.Constructivecommentsby
Peter Hairsine and Ian Rutherfurd and reviewsby John Buffington
and Robert Ryan improvedthe manuscript.
The changesinfluencingthe sedimentregime of the Coxs
River occurin many rivers.Gully erosionof similaror greater
intensityto that of the CoxsRiver catchmenthas occurredin
many southeasternAustralian catchments[Prosserand Winchester,1996] and in other partsof the world [e.g.,Cookeand
References
Reeves,1976;Nachtergaele
and Poesen,1999]. The mid 1940s
climate
shift that occurred
in the Coxs River
catchment
has
been noted for most of eastern New South Wales [Pittock,
1975;Cornish,1977],and other historicalclimatevariationsof
hydrologicalsignificance
havebeen describedby Knox [1993]
andBallingand Wells[1990].The impactsof damson sediment
regimeshave been widely studied [e.g., Petts, 1979; Williams
and Wolman,1984] and dependon the flood mitigationability
of the dam. Dams that havea large storagevolumerelativeto
the inflowvolumeand damsthat are operatedto provideflood
storage,reduceflood peaksto a greaterextentand hencehave
a greaterimpacton the downstreamsedimenttransportcapacity. Lyell Dam has a capacityof lessthan 70% of the mean
annualinflowvolume, and the minor flood mitigationeffect it
providesis reflected in the small reductionin downstream
sediment transport capacity.Many dams have much larger
relative storagecapacityand hence a far greater impact on
sedimenttransportcapacity.While manyriverswill havebeen
influencedby land degradation,climate variations,and dam
closure,the relative importanceof these changeswill vary
between
7.
catchments.
Abernethy,B., and I.D. Rutherfurd,Where alonga river'slengthwill
vegetationmosteffectivelystabilisestreambanks?,Geomorphology,
23, 55-75, 1998.
Balling,R. C., and S. G. Wells, Historicalrainfall patternsand arroyo
activitywithin the Zuni River drainagebasin, New Mexico, Ann.
Assoc.Am. Geogr.,80, 603-617, 1990.
Barrett, J., CoxsRiver:Discovery,Historyand Development,Graphic
Workshop,Armadale,Victoria, Australia,1993.
Benn, P. C., and W. D. Erskine, Complexchannelresponseto flow
regulation:CudgegongRiver below Windamere Dam, Australia,
Appl. Geogr.,14, 153-168, 1994.
Boughton,W. C., Modelling the rainfall-runoffprocessat the catchment scale,Civ. Eng. Trans.Inst. Eng.Aust.,30(5), 153-162, 1988.
Brizga,S. O., and B. L. Finlayson,The SnowyRiver sedimentstudy:
Investigationinto the distribution,transportand sourcesof sandin
the SnowyRiver betweenLake Jindabyneand Jarrahmond,Rep.81,
Dep. of Water Resour.,Melbourne,Victoria, Australia,1992.
Brizga,S. O., and B. L. Finlayson,Interactionsbetweenuplandcatchmentsand lowlandrivers:An appliedAustraliancasestudy,Geomorphology,
9, ! 89-201, 1994.
Caitcheon,G. G., A. S. Murray, and R. J. Wasson,The SnowyRiver
sedimentstudy: Sourcingsedimentsusing environmentaltracers,
Rep. 80, Dep. of Water Resour., Melbourne, Victoria, Australia,
1991.
Calder, I. R., and D. R. Maidment, Hydrologiceffectsof land use
change, in Handbook of Hydrology,edited by D. R. Maidment,
McGraw-Hill, New York, 1992.
Conclusions
Bedrock-controlledriversseldomhave simpleconcavelong
profiles,and large variationsin channelsloperesult in large
variationsin sedimenttransportcapacitywith distancedownstream.Land degradation,climatevariation, and dam closure
can influenceriver sedimentregimes,and where land degradationis heterogeneous
acrossa catchment,spatialpatternsin
sedimentsupply result. In the Coxs River, Australia, gully
erosionin the upperandmidcatchment
from the middleto late
nineteenthcentury increasedsedimentsupplyby varying degreesalongthe river, on average,by a factor of 20. A climate
shift in the mid 1940s led to an almost threefold
increase in
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