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JOURNALOF GEOPHYSICALRESEARCH,VOL. 99, NO. B7, PAGES13,871-13,883,ULY 10, 1994
Natural controlsof fluvial denudation ates in major world
drainage basins
M. A. Summerfield and N.J. HuRon
Macrogeomorphologyesearchroup, epartmentf Geography,niversityf Edinburgh,dinburgh
United Kingdom
Abstract. We present new compilation f estimates f modem atesof mechanical nd
chemical enudationor externally rained asins xceeding x 105 m2 n area. These
estimates re basedon sediment nd solute oad data selected n order o representnatural atesas
far aspossible.Chemical enudationateshavebeen alculatedy deductinghe
nondenudationalomponent f solute oad. Mechanical enudationates ange rom 1 mm kyr
for the St. Lawrence ndDneprbasins o 670 mm kyr x or the Brahmaputra asin. Chemical
denudationatesvary rom 1 mm kyr (Kolyma,Niger, Nile andRio Grandebasins) O27 mm
kyr4 (ChiangJiangbasin). The Kolymabasinhas he owest 4 mm kyr ), and the Bmhmaputra
basin he highest, verall ateof denudation688 mm kyr4 ). Relationshipsetweendenudation
ratesanda rangeof morphometric, ydrologic, ndclimaticvariables re nvestigatedhrough
correlation ndregression nalysis. Morphometric ariables,suchas mean ocal relief, are
accurately alculatedor largebasins or the first time by using he NationalGeophysical ata
Center 0-minuteopographicatabase. ariables xpressingasin eliefcharacteristicsnd
runoff are found o be moststrongly ssociated ith both mechanical ndchemicaldenudation
rates,with more than60% of the variance n total denudation eing accounted or by basin elief
ratioand unoff. Basinarea, unoffvariability,andmean emperature, owever, re only weakly
associated ith ratesof denudation.Althoughdirectcomparisonsannotbe made, t appearshat
ratesof basindenudation erived rom present-day ass lux estimatesre not,overall,
significantly ifferent rom estimates f long-term atesbasedon sediment olumeand
thermochronologicata. It therefore ppearshat he key factors dentifiedas controlling
denudationateshereare alsoapplicable o the geological ime spans elevant o the interaction
between ectonic nd denudational rocesses.
Introduction
A knowledgeof rates of denudation nd an understanding f
the factors that control them are important for a number of
reasons. Quantitativemodels of landscapeevolution [Ahnert,
1987] dependon estimates f the ratesat which andscape hange
occurs,while those nterested n the interactionbetween tectonic
and subaerialprocesses, oth n orogenic Molnar and England,
1990; Beaumont et al., 1992] and cratonic [Gilchrist and
SummerfieM,1990; Bishop and Brown, 1992] terrains need to
apply ealisticdenudationates n the calibration f their models.
Estimates f long-termdenudationatesare a vital component f
both geochemical Berner, 1991] and sediment fLeeder, 1991]
massbalancestudies,while the importance f understandinghe
factors hat controlspatial and temporalvariations n sediment
supply to sedimentarybasins is now becomingmore widely
appreciated Cross, 1990; Sinclair and Allen, 1992]. Recently,
the suggestionhat the creationof topography ssociatedwith
orogenesisan significantly ffect atesof chemicaldenudation
and therebyperturb the global carbonbudget and consequently
globalclimate Raymo t al., 1988;Raytooand Ruddiman,1992]
has further reinforced he need for a clear understanding f the
factorscontrolling enudationates.
Copyright 994by theAmericanGeophysical nion.
Papernumber 4JB00715.
0148-0227/94/94JB-00715505.00
Although range f approaches,ostnotablyhermo-
chronology nd calculations f offshoresedimentvolumes,can
provide stimatesf long-termenudationaies,onlydataon
present-dayatesderived rom sediment nd solutedischarges f
riversgenerally rovide he mostviablebasis or linking
variations n denudation ates to specificcontrollingvariables. A
numberf studiessinghis pproachave ocusedn he oleof
climate, r vegetationsmediatedyclimate, s hekeyvariable
determiningatesof denudationLangbein nd Schumm, 958;
Fournier, 1960; Corbel, 1964; Douglas, 1967; Wilson, 1973;
Jansenand Painter, 1974; Jansson,1982, 1988; Ohmori, 1983].
Althoughsome of these analysesalso considered opographic
controls n denudationa[es, he role of elevation nd relief has
beenemphasizedn relatively ew studies Ahnert,1970; Pinet
andSouriau,988; insele,992;Milliman ndSyvitski,992].
The aim here s to presenta new compilation f estimates f
rates of denudation or the world'smajor drainagebasinsand to
providean initial assessmentf the main controlsof denudation
rates at the regional to subcontinental calerelevant to the geo-
logical ime spans ppropriateor modellnghe interaction
between ectonicanddenudational rocesses.n order o produce
estimatesf denudationateswhichare moreappropi'iateo
modelingover geological imescaleswe have attempted o
exclude,s araspossible,nthropogenicffectshroughareful
assessment f the available sedimentand solute dischargedata.
In doing hiswe areaiming o provide stimatesf "natural"ates
of denudationand to assess he major "natural" controls hat
13,871
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13,872 SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES
determine them. All externally drained basins with an area of
more han x 105 m are ncluded,lthoughomprehensiveata
are not available for all of these (Figure 1). Together hese 33
basins over n areaof 5.28 x 107 m ,representingver35% of
the Earth's land area. Three internal basins above the area
threshold the Okavango, he Volga, and the Chari) have been
excluded rom the analysis s they do not contribute o an overall
reduction f continental levation. Our reason or focusing n the
very largestbasins s that we are interestedat this stage n estab-
lishing first-order effects rather than examining small-scale
controls on denudation rates.
The main constraintson our analysis are the quality of
available sediment and solute load data, the extent to which
appropriate llowances an be made for anthropogenicmpacts,
and the accuracywith which likely controllingvariablescan be
characterized. n order to minimize these potential imitations,
sedimentand solute oad data have been carefully selectedso as
to represent, s far as possible, onditions rior to major human
modification of the basins concerned. We have also been careful
in our estimation of chemical denudation rates to make an
adjustment or the nondenudational omponent f solute load
introduced y atmosphericnputs. The data set we present s our
best estimate, within the limitations of the available data, of
natural rates of mechanical and chemical denudation under
prevailing basin conditionsof topography, limate, vegetation,
and lithology. This data set in turn provides he basis for our
exploratory nalysisof the main factorsdeterminingworldwide
variationsn denudationates or very argebasins.
In addition to our new estimates of natural denudation rates for
major drainagebasins,anothernovel component f this analysis
is our use of National GeophysicalData Center (NGDC) 10-
minutedigital topographic ata (seeCurningand Hawkins [1980]
for full documentationf thisdatabase)o calculate ccuratelyor
the first time a rangeof morphometric ariables or thesebasins.
In particular, we produce he first calculationsof mean local
relief for large drainagebasins.
Data Sourcesand Quality
Potential Controlling Variables
Data are presented or 11 potential controllingvariables for
which measurement s possible at the interval or ratio scale
(Tables 1 and 2). (Full documentationof data sources s avail-
able on request rom the authors.) Variable selectionwas aimed
at including hose actors houghtmost ikely to play an mportant
role in controlling denudation rates and for which data of
adequate ualityare available. In order o use the NGDC digital
topographic atabase or the calculationof morphometricvari-
ables,all basinperimeterswere digitized. Where the locationof
basin perimeters s unambiguous ur basin area estimatesare
within 5% of thosegiven n existingpublished ources. Mean
local relief for large basinshas previouslybeen estimated rom
Dnepr
Tocantlns
Francisco
' -- 0 10 25 50 100 250
lOOO o lOOO
km Denudationatemmka1)
500
Figure 1. Locations ndestimatedotaldenudationates or major externallydrainedbasins.
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13,874 SUMMERFIELDNDHULTON: LUVIALDENUDATIONATES
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SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES 13,875
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13,876 SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES
partially, the effects of pollution by not incorporating ecent
analyses rom industrializedcatchments. They do, however,
include atmospheric nd recycled components. Previousesti-
matesof chemicaldenudation ave generallybeenbaseddirectly
on this raw data, but in order to assess orrectly he contribution
of dissolvedoad to denudationas opposedo total solute rans-
port) it is necessaryo deduct hesecomponents.Detailedmass
balancestudies re not available or large drainage asins,so we
have assumed,on the basis of the global mean estimatesof
Bernerand Berner 1987], that 4.5% of TDS is contributed y
precipitationnd hat64%of HCO 'originatesromatmospheric
CO . Values or HCO3' andTDS havebeenderivedixan
Meybeck [1979], except for the Amazon [Berner and Berner,
1987], Chiang Jiang [Hu et al., 1982] and Rio Grande
[Livingstone,963].Forbasinsorwhich odataonHCOz' re
available we have estimated that bicarbonate constitutes 52% of
(unpolluted)TDS (global mean value according o Meybeck
[1979]). Where not directly available, TDS values have been
calculated rom dissolved oad data. The denudational ompo-
nent for the Zaire Basin s a specific stimate y Nkounkou nd
Probst [ 1987].
Mean annual specific solid load and mean annual specific
denudational issolved oad were derived for each basin using
our digitizeddrainage reas. In all cases he lowestpoint n the
basin for which solid and solute load data are available have been
used, although his is not alwaysat the basin outlet. However,
the discrepancies etween the upstreamarea from these meas-
urementpointsand the digitizedbasin areasare small. Ratesof
mechanical, chemical, and total denudation were calculated
assuming mean ockdensity f 2700 kg m z (Table3). It is
important o note, however, that calculations f denudation ates
from mass ransport ata involve assumptionsbout he density
changeshat occurduring ock weatheringprior to the removalof
solidanddissolvedmaterial rom a drainage asin Surmneeld,
1991a]. In usingbedrockdensityas the volumetricconversion
factorwe are assuming steady tate egolith hickness.
Resultsand Analysis
The data or potential ontrollingariables enerally howat
least an order of magnitudevariation (Table 2). For instance,
maximum alues or relief ratio andmean ocal elief attained y
the Brahrnaputraasin 0.00554 and992 m, respectively) anbe
comparedwith the minimumvalues or the Dneprbasin 0.00039
and 30 m). Mean annual unoff s similarlyvariable, anging
from 5 mm for the Rio Grande to 1244 mm for the Orinico. The
Brahrnaputra asinhas he highest atesof both mechanical670
mm kyr ) and total denudation688 mm kyr ) (Table 3 and
Figure1), although recentsediment ischarge stimate f 540
Mt yr4 cited romunpublishedataby MillimanandSyvitski
[1992] indicates hat our estimate may be too high. The
Brahmaputra comes only second, however, in chemical
denudation, eingexceeded y the ChiangJiangbasinwith a rate
of 27 mm kyr4. The Dnepr and St. Lawrence asinshave he
lowest atesof mechanicalenudationt lmm kyr , while our
basins hareheminimum hemical enudationateof lmm kyr
- the Kolyma,Niger, Nile andRio Grande. The Kolymaalsohas
the lowest ate of total denudationt 4 mm kyr . As found n
previousstudies, he proportionof total denudation ontributed
by chemicaldenudationdecreases s total denudation ncreases
(Figure 2). This means that in basins experiencing ery high
total denudation ates chemicaldenudation s high in absolute
terms,but low in relative terms. In basinswith very ow overall
denudation ates, such as those of the major Siberian rivers,
chemical denudation can account for more than 50% of the total
(Table 3).
In order to explore the relationships oth betweendenuda-
tional and potential controlling variables, and among the
controlling ariables hemselves, catterplots were produced or
all variablepairings. In addition,Pearsonian orrelation oeffi-
cientswere calculatedor all variablepairings or Y = mX + c,
and or selected ariablepairings or log Y = mX + c and og Y =
mlog X + c. Partial and multiple correlation oefficients ere
800 600 400 200 120
MECHANICAL CHEMICAL
100 80 60 40 20 0 20 40
Kolma
Dnepr
Ob
Niger
Yenisei
Nile
Murray
S.Lawrence
La Plato Parana)
Zambezi
Rio Grande
Shatt-E1-Arab
Orange
Columbia
Mackenzie
Yukon
Danube
HuangHe
Orinoco
Mississippi
Amazon
Colorado
Mekong
Chiang iang
Indus
Ganges
Brahmaputra
DENUDATIONATEmm a1)
Figure 2. Histogramsomparingmechanicalndchemical enudationates or majorexternally rained asins.
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SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES 13,877
also calculated for selected variable pairings. Unlike in the
recentanalysisof Milliman and Syvitski 1992], all data points
were included in the calculation of correlation coefficients and
regression quations inceusingan arbitrarycutoffpoint n terms
of standarddeviation would have masked the real degree of
scatter in the data.
For mostvariablepairings his degreeof scatterwas high, an
unsurprisinginding given the wide range of factors ikely to
control rates of mechanical and chemical denudation. However,
an importantadditional eason or high data scatter s probably
low data quality, since there is no reason to suspect that
inaccuraciesn the data would be systematically iased so as to
enhance any correlationsbetween controlling and denudation
variables. On the contrary, data errors are likely to be
approximately andom, and therefore the true strength of
statistical associations between controlling and denudation
variablesdetermined rom the data presented ere is likely to be
underestimated. In other words, our interpretations of the
strengthof associations etween controllingand denudational
variablesare nherentlyconservative.
The relatively low amount of variance accounted or among
most of the pair-wise associations ean that a range of best fit
regression elationshipsdescribe them about equally well.
Nevertheless, close examination of the data indicates that
associations etween the potential controllingvariables are best
described n terms of linear relationships Table 4), whereas
associations between denudation rates and basin variables are
moreadequately escribed y the form og Y = mX + c (Table 5).
Among the potential controllingvariables there is a relatively
high degree of correlation between those expressing various
aspects f topography,hat s, mean runkchannelgradient, asin
relief, relief ratio, mean local relief, and mean modal elevation
(Table 4). However, basin hypsometry, s representedby the
hypsometricntegral, is weakly correlatedwith the other basin
variables xcept or basin area and meanmodalelevationwhere
there is a moderatedegreeof association.None of the climatic
variables (including ranoff and ranoff variability) is strongly
associated with the other basin variables.
Table 5 andFigure3 illustrate he relativelystrong orrelation
between he topographic ariables,exceptingbasin hypsometry,
and rates of both mechanical and total denudation. Of
considerablenterest s the fact that mean local relief appears o
be no better as a predictorof denudation ate than basin relief
ratio, in spite of the former providing a much more detailed
representation f averagebasin relief and slope characteristics.
Given that mean local relief is a time-consuming ariable to
quantify, his s a helpful finding or those equitinga simplebut
effectivemeansof characterizing asin elief in modelingstudies.
However, this result may be scale-specificand does not
necessarilymean hat relief ratio providesan equally appropriate
representation f basin relief characteristicsor smaller basins
than those considered here. Chemical denudation rates are more
weakly associated with these topographic variables than
mechanical enudationates. Interestingly, iven he findingsof
some other studies [Milliman and Syvitski, 1992], only an
extremely weak negative correlationwas found between basin
area and denudation rates. In terms of the "climatic" variables
only mean annual ranoff and to a lesser extent mean annual
precipitation re at all stronglyassociated ith denudation ates
(Figure 3). Runoff variability, at least as defined n this study,
shows no association with denudation rates.
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13,878 SUMMERFIELDAND HULTON:FLUVIAL DENUDATIONRATES
Table 5. Pearsonian orrelationMatrix for Denudational ersusMorphometric,Hydrologic,and
Climatic Variables
Log. Mechanical Log. Chemical Log.Total
Denudation Rate Denudation Rate Denudation Rate
Area -0.11 -0.16 -0.18
Mean tnmk channel radient 0.67 0.36 0.66
Basin relief 0.80 0.51 0.79
Relief ratio 0.78 0.50 0.79
Mean modal elevation 0.66 0.36 0.66
Mean local relief 0.68 0.54 0.71
Hypsometricntegral -0.03 -0.06 0.03
Mean annual ranoff 0.45 0.52 0.54
Runoff ariability -0.04 0.08 -0.05
Meanannualemperature 0.41 0.09 0.34
Meanannual recipitation 0.42 0.32 0.52
Interpretationand Discussion
Mechanical Denudation Rates
The importance f basin topography, nd to a lesserextent
rimoff, in influencing ates of mechanicaldenudation s sup-
ported by the relatively strongstatisticalassociation etween
these variables. On the other hand, the very weak correlation
between mechanicaldenudation ate and rimoff variability does
not support he importance f "storminess"n influencing enu-
dation ates [Molnar and England, 1990] for the scaleof basins
consideredere. The strong tatistical ssociationetween elief
and mechanical enudationate may, itself, be partly a function
of other factors elated to relief, suchas high levels of seismicity
and the prevalenceof fractured ock in high-relief orogenic
terrains Milliman and Syvitski,1992].
The significant ata scatter, ven n the relationships etween
mechanical denudation rate and relief and ranoff, could be due to
a number of factors n addition to errors in specific sediment
yield andwaterdischargestimates.One s the ackof anydirect
assessmentf variations rt erodibility,suchas thoseassociated
with lithology,but for very large basinserodibilityvariations
might be expectedo average ut and thusnot play a majorrole
in contolling asin-wide enudationates. A secondactorwhich
is potentiallymoresignificant, speciallyor thevery argebasins
considered ere, is variablestorage ffects. In very large basins
sediment may be "temporarily"stored in floodplains for
thousands f years or more. Clearly, for such river systems,
sediment ampling t thebasinmouth,evenoverseveral ecades,
mayprovidean unrepresentativenapshot f the ong-termmean
sediment lux. Given theseeffects, t is perhapssurprising hat
the datado not, in fact, showan evengreaterdegreeof scatter.
The very weak negative orrelation etweenmechanical enu-
dation rate and basin area observedhere contrastswith findings
in a number f previous tudies,most ecentlyhat of Milliman
and Syvitski 1992]. This maybe because nly a relativelysmall
size range of basinswas considered, panningonly about one
order of magnitude. However,basin area itself can have no
causal link with denudation rates since area itself cannot be a
determining actor. The associationdentified in previous
studies, herefore, robablyarises rom the fact that basin area s
negatively orrelatedwith otherpotentialcausalvariables,such
asrelief ratio andmean ocal relief (Table 4). This is a complex
issue,however, as demonstrated y data from westernCanada
which shows that denudation rates are lowest in the smallest
basins100,000 km2) were found to have intermediate enudation
rates.
The idea that mechanical, and indeed total, denudation rates
vary as a function of mean elevation would appear to be
supportedy the datapresentedere Table5). This association
has been applied n a numberof studiesmodeling nteractions
between tectonics and denudation Lainbeck and Stephenson,
1986;Slingerland nd Furlong, 1989;Pitman and Golovchenko,
1991; Lorenzo and Vera, t992] either through a
misunderstandingf the studies y Ahnert 1970] andRuxton nd
McDougall [1967] (whichdemonstrated relationshipwith local
relief rather than elevation), for mathematical convenience,or
from an assumed causal link between elevation and denudation
rate. But clearly,as with basinarea,elevation tself cannot e a
direct determinant of denudation rate. The flux of sediment at a
specific ocationwill be a functionof the gradientat that point,
irrespective f its elevation bovemeansea evel [Summerfield,
1991b].
The reasonwhy mean basin elevation s stronglyassociated
with denudation ate is that elevation s itself stronglycorrelated
with other topographicactors which are causally elated to
specific ediment ield. This is evident rom Table 4 where t
can be seen hat trunk channelgradient, elief, relief ratio, and
mean ocal relief are all either moderately r strongly elated o
mean modal elevation. These relationships,however, must be
examinedwith care. Although here is a clear relationship
between mean modal elevation and mean local relief for basins as
a whole (Figure 4), when intrabasinpatterns are examined
important ifferences merge. For a numberof basins,suchas
the Yukon, the intrabasin pattern replicates the interbasin
associationetween levation nd ocalrelief (Figure5). But for
basinssuch as the Ganges he data points cluster around a
distinctcurvilinear rend causedby a decline n local relief at
high elevations>3500 m). This kind of pattern s also ound n
other basins, such as the Indus, Brahmaputra, and La Plata
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SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES 13,879
1000.
1000'
E
E
,,, 100'
z
0
lO-
z
lOOO.
1oo.
lO.
z
o
lOOO
E
E
Lul O0
z
o
1o
z
o
0.0 d.4 0:8 1:2 1:6 l.0 .2:4 0.000
MEAN GRADIENTOF TRUNK CHANNEL(mkm")
4(0 8(0 12'00 1100 2(00 2,100 0
MEAN MODAL ELEVATION m)
,
* .
* , *
,
coo
1,1% *
0 2(0 4b0 66o 86o 1(oo 12bo 0
MEAN ANNUAL RUNOFF (mm)
,
, *
,
, , ,
,
t
, *
*
,
MEAN ANNUALTEMPERATURE C)
o.D01 0.602 o.b03 o.b04
RELIEF. RATIO
,
.
,
,
OIiO,
*
,0.
*
200 400 600
MEAN LOCAL RELIEF (m)
0305
800
lb 2'o o ,{o o go io 8b
RUNOFF VARIABILITY %
Q ,
,
,
8
e
e
, * * ,
,
1 ooo
0 260 4bo 6bo 8b0 ld0012'00 1,001001100 2600
MEAN ANNUAL PRECIPITATION mm)
Figure 3. Scatter lotsof totaldenudationatesversus ariousmorph,metric,hydrologic ndclimaticvariables.
(Parana), which have part of their upper catchments n high
mountain plateaus. Another kind of intrabasin pattern is
characteristic f basins which drain to high elevation passive
margins, such as the Orange and Zambezi. Here there is a
clusteringof very low local relief values at moderateelevations
of around 1000 m. This pattern s also suggested y the high
hypsometric ntegrals for these basins (Table 2). The
significance f thesecontrastingelationshipss that local relief
is stronglycorrelatedwith local slopegradients Ahnert, 1970],
so such intrabasin variations in local relief are likely to be
mirroredby contrastsn denudationates [Summerfield, 991b].
In basinswhere there s a very low correlationbetweenelevation
and ocalrelief, suchas the Orangeand Zambezi (Figure 5), ele-
vation may provide a very poor basis for predictingdenudation
rates.
lOOO,
a: OOO,
40O,
'OO,
C
0 4 8 12:X) 130 2(:30 200 X)
MEAN MODAL ELEVATION m)
Figure 4. Scatterplot of mean modal elevationversusmean
local relief.
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13,880 SUMMERFIELDAND HULTON:FLUVIAL DENUDATIONRATES
4000
3500
3000
'-' 2500
,,, 2000
1500
1000
5OO
YUKON
GANGES '
50001
45001
4000'
. 3500,
u_ 3000'
,,, 2500'
< 2000
8
_a 1500
1000 ::: ; . '.. .
'.i'.:::..::: . : . .
500 ..:::.' '"-:-:::.:'
0 500 1000 1500 2000 2500 3000 3500 4000
MEANMODALELEVATION (rn)
..... : : ' : .- . . ....
; :.... : : : . . . : i ..: :-..
.
'' ':' ': '" : ';'3' : :.. : -
..
.....
..
. . .
o6 ' "'ioO0 260036004600 5000" 6007600
MEAN MODALELEVATION (m)
32o
2800
2400
2000
--n 16oo
LIJ
< 12002
8OO
400
ORANGE
::iiiiiiii:.i i i : '
- '500 1600 1500 2000 2500
MEANMODALELEVATION (rn)
Frequencysarea,km )
1000
4000
9000
* 16000
25000
" 36000
3000
2000:
1800
1600'
1400
E
'"1200,
-hi000
' 800
0
..j 600
4O0
ZAMBEZI
.;::.::. :.:: .:. .: .
o ....................
.
: :' :: ::::.': :':::;:::;:::' :
0 ..... eelee ....
O - ' ;40'0' ' -800' ' i200' ' 1600 ' 2000 ' 24'00
MEANMODALELEVATtON (m)
Figure5. Scatter lotof intrabasinariationsf local eliefandarea requencyeanmodal levationorthe
Yukon,Ganges, range, ndZambezi asins.
Chemical Denudation Rates
'rhe statisticalassociationsecorded n Table 5 support he
conclusion that chemical denudation rates, like those for
mechanical enudation, re more strongly nfluencedby relief
factors han by climatic controls Summerfield, 991a;Raytoo
and Ruddiman, 992]. In particular, t the scaleof this study,
temperature ppears o play no role in controllingates of
chemicaldenudation.This supportshe idea that the efficient
removal of weathered egolith and the resulting continuous
advectionof bedrock nto the weatheringzone is the critical
determinant f the rate of chemicalweathering. n fact, our data
suggest hat chemical denudation ates are more clearly
influencedby relief variables han (indirectly)by elevation.
Thick weatheringmantlesdevelopedn areasof minimal ocal
relief are likely to be inimical to high rates of chemical
denudation,rrespective f elevation. The correlationof
chemicaldenudation ates with runoff suggestshat maximum
rateswill occurwherehigh runoff s coupledwith high relief, a
conclusionupported y the rates observedn basinsdraining
A factor not considered n the correlation and regression
analysispresented ere is that of the lithologiccontrol of
chemical denudation rates. A detailed study of very small
(median area 7.8 km ) ,artpollutedatchments f uniform
lithology by Meybeck [1987] has demonstrated ignificant
variations n chemicaldenudationate as a functionof rock type.
It is probable hat the otherwise nomolouslyigh chemical
denduationate for the ChiangJiang Table 3) canbe explained
by the extensive utcrop f carbonateockswithin ts catchment
area. The potential ole of lithology aises he question f the
extent to which high rates of chemicaldenudationn orogenic
belts are a function of the exposureof readily weathered
sedimentarytrata ather han relief itself [Bernerand Berner,
1987,pp.225-226;Raytoo ndRuddiman, 992].
Irrespective f the specific ontrols n chemical enudation
rates t is clear hat for basinswith high overalldenudationates
chemicaldenudation onstitutes nly a small proportionof the
total (Figure2 and Table 3). Both mechanical nd chemical
denudationatesare positively ssociated ith relief variables,
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SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES 13,881
topography. This is dramatically llustrated by comparing he
Brahmaputraand Ob basins. Although chemical denudation
accounts or only 2.6% of the total in the former basin, the
absoluteateof 18 mm kyr1 s the second ighestn the basins
consideredere. Nearly 65% of the denudationn the Ob basin,
however, is attributable to solute loss, but the absolute rate is
less hana quarterof thatof theBrahmaputra.
Interaction of Relief and Runoff
The individual ole of two, at leastpartly, ndependentactors
(relief and runoff) in influencing denudation ates raises the
question s o how stronglyheir combined ffectcontrols enu-
dation ates. Taking relief ratio to characterizeasin opography,
the linear partial correlationcoefficient or denudation ate and
relief ratiokeeping unoffconstants 0.732. This is only slightly
lower than the correlation coefficient for relief ratio and denuda-
tion rate (r = 0.757) indicating hat the latter relationships not
significantlynflated by the impact of interbasindifferences n
runoff. The joint effect of relief and runoff can be clearly seen
through he linear multiple correlation oefficient or denudation
rate against he relief ratio and ranoff (r = 0.792). Thus these
two variables alone account for over 62% of the variance in
denudationate. This is a remarkably igh amountof explained
variance given the low data quality and the fact that only
erosivityvariablesare ncluded.
Implications
The datapresented bove ndicate he degreeof variabilityof
denudation ates for very large drainagebasins and the major
factors hatappear o controlsuchvariations. A critical question,
however, s to what extent these ates, and the interpretationof
the factors that control them, can be extrapolated to the
geological imescales elevant o the interactionof denudation
and tectonics. A common view is that modem denudation rates
have limited applicability to these long timescalesbecause
sediment ields since he beginning f farming are probably ar
in excessof preagricultureates (see Milliman and Syvitski
[1992] for discussion). Although this is undeniably rue in
specific nstances,t is less certain hat such a generalisations
valid at the global scale. Our reason or suggestinghis is that
presentdenudationates for major basinsare in some cases
comparableo long-term ates estimated rom sedimentvolume
and thermochronologic ata. For instance, preliminary
calculationsor the Orangebasin rom offshore ediment olumes
indicate rates of denudation since the formation of the South
Atlantic ranging rom 7 to 114 mm kyr1 (depending n
assumptions ade aboutchangesn drainagearea through ime)
[Rustand Summerfield, 990]. These ates, which have varied
both temporally nd spatially,are compatiblewith estimates f
post-riftingdepthsof denudation f up to 3-4 km from apatite
fission track analysis Brown et al., 1990], but they are also
comparableo the modern stimate f 28 mm kyr for the
Orangebasin. The greatextentof the basinsconsideredn this
studyprecludes irectcomparisons ith thermochronologicata
which provide denudation estimates for limited areas.
Nevertheless, vailabledata certainly ndicate denudation ates
which are at least of the same order of magnitude as modern
rates. For instance, hermochronologicata from the Alps and
Himalayasndicate enudationates anging p to 1000mm yr1
and more over the past few million years Brownet al., 1994;
Clark andJiiger, 1969;Hurford, 1991].
A more specificcomparison an be made for the Gangesand
Brahrnaputra asinssince the accumulation f sediment n the
Himalayan molasse oredeep and alluvial plains, the subaerial
part of the GangesDelta and the Bengal Fan can be used to
estimate atchmentmechanical enudationatesover the past20
m.y. Assumingno change in catchmentboundaries, his
indicates a mean combined denudation rate for these two basins
of 435 mm kyr 1 from20 to 7.7 Myr ago,and300 mm kyr 1 from
7.7 Myr ago to the present [Einsele, 1992]. These figures
comparewith the presentcombined ate estimatedhere for the
BrahrnaputrandGanges f 420 mm kyr 1. The dataof Milliman
and Syvitski 1992] yield a similarly comparable igure of 242
mmkyr 1
If the grossvariations demonstrated ere at the regional to
subcontinental calebetweendenudation ates n drainagebasins
of contrasting elief and runoff characteristics re acceptedas
broadly valid over geological timescales, then present-day
denudation ates and the factors hat appear o control hem can
be used as a basis for modeling nteractions etween tectonics
and denudation. The careful applicationof such data should
greafiy mprove he calibrationof tectonicmodels ncorporating
the effectsof subaerialprocesses, lthough he scaledependence
of the factorscontrollingdenudationmustbe considered.
Two main avenuesof future researchare indicated by the
conclusionsrom this preliminarysurveyof the world'smajor
drainagebasins. One is the need for an application f the kinds
of morphometric ata presented ere. In particular, t would be
interesting o see to what extent variations n denudation ates n
smallerbasinscouldbe "predicted"rom a combination f basin
morphometry ndrtmoffdata. The otherpotentially ruitful line
of enquiry would be a more detailed comparisonof modem
denudation ates with estimatesof long-term rates culled from
thermochronologicndsediment olumedata.
Acknowledgments. Texaco nc. providedprimary funding or this
research. Computingwas carried out in the GIS Laboratoryof the
Department f Geography, niversity f Edinburgh, nd we are very
grateful o S. Dowers or hisassistancen datamanagement.
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