The Irrawaddy River

[The Journal of Geology, 2007, volume 115, p. 629–640] � 2007 by The University of Chicago.All rights reserved. 0022-1376/2007/11506-0002$15.00. DOI: 10.1086/521607

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The Irrawaddy River Sediment Flux to the Indian Ocean:The Original Nineteenth-Century Data Revisited

R. A. J. Robinson, M. I. Bird, Nay Win Oo,1 T. B. Hoey,2 Maung Maung Aye,3

D. L. Higgitt,4 Lu X. X.,4 Aung Swe,5 Tin Tun,6 and Swe Lhaing Win3

School of Geography and Geosciences, University of St. Andrews,St. Andrews, Fife KY16 9AL, United Kingdom

(e-mail: [email protected])

A B S T R A C T

The Irrawaddy (Ayeyarwady) River of Myanmar is ranked as having the fifth-largest suspended load and the fourth-highest total dissolved load of the world’s rivers, and the combined Irrawaddy and Salween (Thanlwin) system isregarded as contributing 20% of the total flux of material from the Himalayan-Tibetan orogen. The estimates for theIrrawaddy are taken from published quotations of a nineteenth-century data set, and there are no available publisheddata for the Myanmar reaches of the Salween. Apart from our own field studies in 2005 and 2006, no recent researchdocumenting the sediment load of these important large rivers has been conducted, although their contribution tobiogeochemical cycles and ocean geochemistry is clearly significant. We present a reanalysis of the Irrawaddy datafrom the original 550-page report of Gordon covering 10 yr of discharge (1869–1879) and 1 yr of sediment concentrationmeasurements (1877–1878). We describe Gordon’s methodologies, evaluate his measurements and calculations andthe adjustments he made to his data set, and present our revised interpretation of nineteenth-century discharge andsediment load with an estimate of uncertainty. The 10-yr average of annual suspended sediment load currently citedin the literature is assessed as being underestimated by 27% on the basis of our sediment rating curve of the nineteenth-century data. On the basis of our sampling of suspended load, the nineteenth-century concentrations are interpretedto be missing about 18% of their total mass, which is the proportion of sediment recovered by a 0.45-mm filter. Thenew annual Irrawaddy suspended sediment load is MT. Our revised estimate of the annual sediment load364 � 60from the Irrawaddy-Salween system for the nineteenth century (600 MT) represents more than half the present-dayGanges-Brahmaputra flux to the Indian Ocean. Since major Chinese rivers have reduced their load due to damming,the Irrawaddy is likely the third-largest contributor of sediment load in the world.

Introduction

The eastern syntaxis of the Himalayas and the Ti-betan Plateau contains the most tectonically com-plex geology in Asia (Socquet and Pubellier 2005).This region is drained by the Red, Mekong, Sal-

Manuscript received January 26, 2007; accepted June 18,2007.

1 Department of Geography, University of Pyay, Pyay,Myanmar.

2 Department of Geographical and Earth Sciences, Universityof Glasgow, Glasgow G12 8QQ, United Kingdom.

3 Department of Geography, Yangon University of DistanceEducation, Yangon, Myanmar.

4 Department of Geography, National University of Singa-pore, Kent Ridge 117570, Singapore.

5 Department of Geography, University of Yangon, Yangon,Myanmar.

6 Department of Geography, University of Mawlamyine,Mawlamyine, Myanmar.

ween, Irrawaddy, Ganges, and Brahmaputra rivers,and two of these systems, the Ganges-Brahmaputra(G-B) and the Irrawaddy-Salween (I-S), debouch intothe Indian Ocean. We consider the Irrawaddy andSalween with the Sittoung and four smaller riverstogether because their catchments are adjacent andboth flow into the Gulf of Martaban (fig. 1) alonga similar length of coastline to the G-B river sys-tem. The headwaters of the I-S are undergoing someof the highest rates of landscape adjustment in aregion of active orogenesis (Zeitler et al. 2001).

Collision, uplift, and shear zone developmentsince the early Tertiary have resulted in a complexpattern of river capture involving the Irrawaddy,Salween, Red, Yangtze, Mekong, and Brahmaputrarivers (Clark et al. 2004). These large rivers delivermaterial from the Himalayas to the ocean, and

630 R . A . J . R O B I N S O N E T A L .

Figure 1. Location of the original Irrawaddy study siteof Gordon (1879–1880, 1885) at Seiktha and the riversincluded in this article. Our measurement sites for mod-ern discharge and material fluxes are at Seiktha and Pyayon the Irrawaddy and at Hpa-An on the Salween.

knowledge of the magnitude of fluxes, as well asthe mineralogical and chemical characteristics ofthe transported material, is a prerequisite for un-derstanding the impact of Himalayan uplift onglobal biogeochemical cycles (Raymo et al. 1988)and the impact of human activities on recent-mod-ern sediment yields (Wilkinson 2005). An under-standing of denudation rates in areas of active tec-tonism is critical in assessing the extent to whichclimate-driven erosion influences large-scale tec-tonic behavior (An et al. 2001; Zeitler et al. 2001).

Together, the G-B and I-S are thought to deliver

close to half of the current total flux of water, sed-iment, and dissolved load from the Himalayas andTibet to the ocean, with ∼20% of this attributed tothe I-S (Milliman and Meade 1983). The IrrawaddyRiver ranks fifth in the world in terms of suspendedload (265 MT yr�1) and fourth in terms of dissolvedload (Milliman and Meade 1983, their table 2);these published sediment and water fluxes are de-rived from a nineteenth-century data set (Gordon1885), and the dissolved loads were published in anarticle by Meybeck and Ragu (1995). Stamp (1940)states that 48 MT yr�1 should be added as dissolvedload for the Irrawaddy but gives no source for thisestimate. Values published by Meybeck and Ragu(1995), Meade (1996), Gaillardet et al. (1999), andMeybeck et al. (2003) equate to combined I-S an-nual averages of 697 km3 of water, 365 MT of sus-pended material, and 162 MT of dissolved material.Although figures for these rivers are widely quotedand used for global flux calculations of carbon (e.g.,Subramanian and Ittekkot 1991), there are no mea-surements of suspended load for the Salween inMyanmar, and the values quoted by Meade (1996,his table 1) are estimates based on the measuredsuspended load values of the Irrawaddy and Me-kong. The increase in damming along the Yangtze(Changjiang) and Yellow (Huanghe) rivers has sub-stantially reduced the sediment load of these rivers(Walling 2006), such that the Irrawaddy may nowrank as having as high as the third-largest sedimentload in the world after the Amazon and the G-B.

We have reanalyzed the original nineteenth-century Irrawaddy data and subsequent early twen-tieth-century engineering reports that identifyproblems with the data collection. In their seminalarticle, Milliman and Meade (1983) use the Irra-waddy as an example of the potential error that canarise when compiling global sediment budgets us-ing data recycled from previous reports rather thanoriginal, often inaccessible, data. Given the verysignificant contribution of the Irrawaddy River toglobal sediment budgets, we have begun to comparethe revised nineteenth-century water and sedimentload values to our own field measurements col-lected during the onset of the 2005 and 2006 mon-soon seasons. We use our preliminary data set heresimply to compare the efficiency of Gordon’s andour filtering methods; we do not present any com-parison of nineteenth-century and modern sedi-ment load values because this is the focus of on-going research.

Study Area and Methods

The Irrawaddy and Salween rivers are ∼2000 and∼2800 km long and have drainage areas of

Journal of Geology I R R A W A D D Y S E D I M E N T F L U X 631

and km2, respectively (Nyi6 60.413 # 10 0.272 # 101967; Bender 1983). The Irrawaddy catchment rocktypes include Cretaceous to mid-Cenozoic flyschof the western Indo-Burman ranges, Eocene-Miocene and Quaternary sediments of the Myan-mar Central Basin, and the Late Precambrian andCretaceous-Eocene metamorphic, basic, and ultra-basic rocks of the eastern syntaxis of the Himala-yas. The Salween and eastern tributaries of the Ir-rawaddy drain Precambrian, Oligocene-Tertiarysedimentary, and acidic and metamorphic rocks ofthe eastern Shan Plateau (Bender 1983; Curray2005; Najman 2005; Socquet and Pubellier 2005).

Gordon (1885) presented monthly discharge datacollected between 1869 and 1879 and 1 yr of sed-iment load data (1877–1878) to the Royal Geo-graphical Society (RGS) without details of his fieldmethods or sampling locations. Gordon’s originalreport, containing his full daily discharge, rainfall,and sediment concentration data set, survey maps,channel cross sections, and a detailed descriptionof sampling techniques (Gordon 1879–1880), waslocated in the archives of the RGS in 2005; anothercopy is located in the British Library in London.These data have been digitally reconstructed usingOCR software, and every number has been checkedagainst the original report. The data are of remark-able quality, particularly because there is a fullrange of flows, including monsoon peak discharge.Sediment concentration was measured at threedepths within the water column, including thelower part of the water column. Milliman andMeade (1983, p. 2) note that inadequate samplingat peak flows and from throughout the water col-umn are among the most important sources of errorin sediment load data.

Gordon, a civil engineer in charge of river worksin Myanmar during the late nineteenth century,investigated the magnitude and duration of floodevents on the Irrawaddy for the government of In-dia, in order to calculate the required size andheight of flood embankments. Gordon selectedSeiktha (fig. 1) as the main measurement site forvelocity and sediment concentration because it isthe farthest downstream site above the delta thatis constrained by bedrock highs on both its westernand its eastern banks. Seiktha is 16 mi upstreamof Myanaung (fig. 1), where daily stage was recordedfrom 1869 to 1879 but where the width of the rivermade multiple cross-sectional measurements ofsediment load, velocity, and depth too difficult.

Nineteenth-Century Cross-Sectional Area and FlowVelocity Measurements. Gordon’s (1879–1880)hydrological sampling strategy was based on theflotation methods published in an article byHumphreys and Abbot (1861), as used by Caleb

Goldsmith Forshey at hydrological stations on theMississippi River between 1848 and 1855 duringthe U.S. government’s Mississippi Delta Survey.Gordon made daily measurements of flow velocityand depth and recorded stage during 1872–1873 andagain in 1875, missing only Sundays, holidays, anddays of very bad weather. Channel width at Seiktharanges from ca. 700 m at low flow to ca. 1600 mat peak flow. Flow depth was measured along a linenormal to the main flow from a boat using a leadand line, and the boat position was surveyed bytriangulation from the bank with theodolites. Flowvelocity was estimated from the time taken forfloats to travel between two cross-channel base-lines spaced about 60 m apart, and theodolites po-sitioned at the end of each baseline were used totrack float trajectories. The starting and final po-sitions of the floats were drafted as scaled drawings,and the downstream component of velocity wasresolved trigonometrically from the time of travel.

Surface floats, suspended up to ∼1 m below thesurface to avoid wind and waves, were used ini-tially. Gordon recognized the need for vertical ve-locity profiles and so sank floats on increasinglengths of line in ∼1-m intervals down through thewater column; the depth of the river at Seikthanever exceeded 25 m (Gordon 1879–1880). The ve-locity measurements were usually reported as theaverage of two or three readings. The float (calleda double float) was a solid cylinder of wood (1 ft[30 cm] in height and 6 in [15 cm] in diameter) witha -in ( -cm) hole on the outside into4 # 4 10 # 10which was inserted the amount of wet clay ballastneeded to sink the cylinder to the required depth(Gordon 1879–1880). Floats were suspended from athin cord ( in [ca. 1.5 mm] diameter) attached1/16to a smaller wooden cylinder (6 in [15 cm] diameterand 1 in [25 mm] thick) that floated just below thewater surface. Gordon grouped similar surface ve-locity values together to divide the channel crosssection into 10 subsections (the sections were al-tered slightly for overbank floods) and calculatedthe area of each subsection. He calculated the ar-ithmetic mean velocity for each vertical profileacross the channel and mean surface velocity foreach subsection. Some subsections had more thanone vertical velocity profile (and some appear tohave none), and for days of continuous stage height,he calculated the mean of the average vertical ve-locities for each profile. Gordon (1879–1880) notesthat his measurements at any location remainedthe same on consecutive days with constant stage(and differed with stage) and varied both within asubsection if there were multiple profiles andacross the whole cross section. About 10,000 ver-tical profiles of float velocity were recorded by the


Figure 2. Annual hydrographs at Seiktha, using Gordon’s original (1879–1880) data. Daily discharge values for 1869–1874 (A) and 1875–1879 (B). The timing, duration, and peak value of the monsoon seasons produce total annualdischarges that vary by �11% from the mean (inset in A).

end of 1875, and each channel cross section (1 d ofmeasurements) was covered by about 10 profiles(Gordon 1879–1880). He then calculated dischargeby summing the velocity-area product of each sub-section. He used 2 yr of daily data (excluding Sun-days and holidays) and the stage readings to developstage-discharge rating relationships for rising and

falling flows at Seiktha (fig. 2); he presented boththe measured and the predicted discharge valuesand states that in only very few cases do the dif-ferences exceed 10% (Gordon 1879–1880, p. 22).

In addition to daily sampling at Seiktha for theperiods mentioned above, Gordon used a simpletransfer function to match the Myanaung stage rec-


Figure 3. Original discharge and suspended sediment concentrations for 1877–1878. A, Concentrations are basedon an average of surface, middle, and near-bottom depth suspended sediment sample measurements. B, Individualconcentrations for each water depth; inset shows the ratio of the average of surface and middle sediment concentrationsto bottom concentration.

ord (fig. 1) to that at Seiktha and then calculateddaily discharge for the remaining years between1869 and 1879 at Seiktha from the Myanaung stagemeasurements (fig. 2). Gordon used the Myanaungrecord instead of the Seiktha stage record becausea sand bar developed downstream of Seiktha be-tween 1873 and 1875, and it raised the water levelat the gauging location “by 2–3 feet” (Gordon 1879–1880, p. 22) and because Myanaung had the longer

stage record. Gordon (1879–1880, p. 22) comparedthe Seiktha and Myanaung records for 1872–1873and stated that the Myanaung record was “withsome uniformity about 1/2 foot lower” than atSeiktha and the difference did not vary over a rangeof flows.

Gordon (1879–1880) discussed possible errorswith the double float method, including drag as-sociated with the cord, the accuracy of calculated


Figure 4. A, Relationship between sediment concen-tration in surface (CS) and middle depth (CM) levels of thewater column for the 1877–1878 data, shown with ourdata from 2005 and 2006. B, Average of CS and CM (CSM)compared with near-bottom concentration (CB), shown

with our 2006 data set. C, Relationship between totalmass measured in 2005 and 2006 and settled mass beforesieving with 10.45-mm nucleopore mesh. Average under-estimate of the historical Irrawaddy suspended concen-tration values inferred from this data is 18%.

time over the short distance, and eye fatigue of theobservers. In 1882, Gordon cross-checked the dou-ble float method with an electrical Deacon currentmeter and found that the water velocities fromlower parts of the water column were less than hisfirst float measurements and that the difference in-creased with flood level. He consequently sug-gested reducing his calculated discharges (Gordon1879–1880) by 10%, 5%, and 0% for high, medium,and low discharge ranges, respectively, and appliedthese corrections to the discharge data set pre-sented to the RGS in 1885. He did not discuss thenumerical values of high, medium, and low dis-charges.

Subsequent engineers reevaluated Gordon’s data,particularly Samuelson (1913), who took over theEmbankment Division in 1913. Samuelson notedseveral problems with Gordon’s data, in particularthat the Myanaung and Seiktha stage records variedwith changing channel geometry at Myanaung, pro-ducing systematic errors in the transfer functionbetween the two locations. One of the features ofGordon’s data set (fig. 2) is an apparent increase indischarge for the years after 1875 when the stagerecords were merged. We discuss this source of un-certainty in “Review of Potential Sources of Error.”

Nineteenth-Century Sediment Load Measurements.Gordon measured sediment load for 6 d each week(excluding Sundays, holidays, and storms) for theyear of 1877–1878 (52 wk) at three positions acrossthe river: ∼90 m from the right bank, in midstream,and ∼1220 m from the right bank. At the first twopositions, samples were taken at three depths: sur-face (∼1 m depth), middle, and near-bottom (0.5–1m above the riverbed). The left bank position hadonly surface and lower measurements as a resultof the shallow water depths there. He used a sed-iment sampler that was based on the device de-signed for the Mississippi Delta Survey (Hum-phreys and Abbot 1861). Gordon collected waterusing a hollow barrel (1 ft [30 cm] long and 6 in [15cm] diameter) loaded with lead rings to make itsink. The barrel had two large valves on the topand bottom, and its lids were made of leather andlead, and they freely opened upward; as the barreldescended through the water column, the lids re-


Table 1. Drainage Areas, Water Discharge, and Suspended Sediment Loads for Rivers Entering the Gulf of Martaban,Myanmar

Drainage area(#106 km2)

Drainagelength(km)

Water flux(km3 yr�1)

Suspended sediment flux(#106 T yr�1)

Average surfaceand middle

depth

Average surface,middle, and

bottom depth

Milliman and Meade’s (1983) data for I .43 … 428 … 265 (261a)Reanalysis of I data from Gordon (1879–1880) .413b 1985 440 � 48 (1j) … …

Corrected data after velocity reductionc … … 422 � 41 (1j) 191 � 32 309 � 49Addition due to unmeasured fine fraction (18%) … … … 226 � 39 364 � 60

Salween (Thanlwin) .272b 2800 211b 110c 180c

Sittoung, Bago, Bilin, Attaran, and Gyaning .06b … 119b 30c 50c

Total for I-S system .745 … 752 370 600G-B system 1.59a 2700 971 1060a 1060a

Percentage of estimated I-S relative to G-B (%) 47 … 77 35 57

Note. ; ; .I p Irrawaddy I-S p Irrawaddy-Salween G-B p Ganges-Brahmaputraa From Milliman and Syvitski (1992). Note that the sediment flux is used for the G-B in calculating the ratios of sediment flux forthese systems because this is the only value available.b Drainage areas, river lengths, and Salween and Sittoung and smaller river discharge values from Meybeck and Ragu (1995), Bender(1983), and Nyi (1967).c The sediment loads of the Salween, Sittoung, and smaller rivers are estimated proportionally using discharge scaling and theIrrawaddy sediment load values.

mained open, and water entered and passed throughit, but the lids closed once the barrel was beinglifted out of the water. The sampler thus collectedwater from the lowest depth to which the barreldescended. Gordon combined 100 g of water andsediment from every bottle collected over 6 d foreach of the three water depths to produce 52 weeklymeasurements of sediment concentration for eachdepth. He filtered the combined samples (600 g ofwater) through a double thickness of weighed filterpaper, and although he does not mention the qual-ity or grade of paper, it seems unlikely that it wasa very small pore size because filtering such largevolumes of water and sediment would have beenextremely slow. The dried filter papers and sedi-ment were weighed carefully using a Fortin bal-ance; the filter paper alone was reweighed, com-pared to its original weight, and the sediment masswas calculated. Only the combined weekly con-centrations for each depth were reported by Gordon(1879–1880). Measurements were reported in gramsof sediment per 100 g of water (1 g per 100 g beingequivalent to 104 mg L�1) for each depth and weekof measurement. The suspended sediment concen-tration and discharge data are presented in figure 3and will be discussed in “Reanalysis of Gordon’sOriginal Data.” Gordon developed a concentration-discharge relationship for the 1877–1878 data butdid not discuss how it was derived. This relation-ship was combined with the 1869–1879 dischargeestimates to calculate annual sediment loads thatrange from 248 to 352 MT yr�1; the directly mea-sured load for the year of measurement (1877–1878)

is 340 MT yr�1. These sediment load values (Gor-don 1879–1880, his table 16) are based on the av-erage sediment concentrations from all waterdepths (fig. 3). Gordon (1879–1880) originally re-ported that the 10-yr average sediment load for1869–1879 was 286 MT yr�1; this is close to thenumber quoted in table 1 of Milliman and Meade(1983). However, as discussed earlier, by 1885, Gor-don had reduced the discharge measurements forthe monsoon months, and the most frequentlyquoted annual sediment load value of 261 MT yr�1

(i.e., Stamp 1940; Milliman and Syvitski 1992)comes from the 1885 article presented to the RGS(table 1 in Gordon 1885).

Modern Discharge and Sediment Load Measure-ments. In June 2005, data were collected at Gor-don’s original site at Seiktha, including 10 bathy-metric cross-profiles, 10 vertical velocity profiles(measured using a Valeport velocimeter), and 18suspended sediment load samples collected usinga horizontally deployed, cylindrical 2-L Van Dornwater sampler with openings at each end. We useda sediment sampling protocol similar to that ofGordon (1879), involving collection of water andsediment from surface and middle depths (but notbottom depth) and then settling sediment from 2L of water for 24 h. We decanted the water andpassed it through a 0.45-mm nucleopore filter todetermine the likely recovery efficiency of Gor-don’s protocol. Separate quantification of the set-tled and filtered components suggests that 18% ofthe sediment load remains suspended after 24 h ofsettling and is recovered with a 0.45-mm filter (fig.


Figure 5. Sediment rating curves for 1877–1878 measurements. A, Regression of data derived from log-log plot usingaverage of surface, middle, and bottom depths (CSMB). B, Linear plot showing interpretation of positive hysteresis forearly monsoon period and negative hysteresis for late monsoon period. C, Cumulative annual and seasonal ratingcurves based on concentrations averaged over surface and middle depths (CSM) and CSMB compared with Gordon’smeasured values. Modeled rating curve values of sediment concentration deviate from measured values for the latemonsoon period.

4). When we used average velocities and bathym-etry for each of our profiles, the velocity-areamethod gives a discharge of 5058 m3 s�1 on June26, 2005, carrying 252 mg L�1 of suspended sedi-ments with a median grain size of 10.5 mm.

In May 2006, we remeasured discharge with anacoustic Doppler current profiler and collected wa-ter and sediment samples using the same protocolas in 2005, including sampling near-bottom waterdepths (0.5–1 m above the riverbed) from Seikthaand Pyay (40 mi upstream) and at Hpa-An on theSalween. Measurements of water discharge (andsediment concentration) made between May 23 andMay 30, 2006, were 5370 m3 s�1 ( mg L�1)271 � 16at Seiktha, 5030 m3 s�1 ( mg L�1) at Pyay,210 � 9and 2270 m3 s�1 ( mg L�1) at Hpa-An. Our127 � 11total sediment concentrations (settled and filteredfractions) for June 2005 and May 2006 are plottedtogether with Gordon’s data in figure 4. This articlemakes no statements about possible changes in sed-iment load between the nineteenth century and to-day because we have insufficient data on whichsuch comparisons can be based. We do, however,discuss how sampling and filtering methods canaffect measured sediment loads in “Review of Po-tential Sources of Error.”

Reanalysis of Gordon’s Original Data

We have recalculated discharge (fig. 2) and sus-pended sediment load from the data in Gordon’s1879–1880 report (fig. 3; table 1). Our 10-yr averageof total annual discharge is calculated as 440 �

km3 yr�1 (table 1), which is higher than Gordon’s48(1879–1880) value of 425 km3 yr�1, as a result ofarithmetic errors in his calculations. However,Gordon’s (1885) later publication includes a reduc-tion of the original discharge measurements. Gor-don does not discuss exactly how he adjusted hisdischarge measurements, and the data are not com-plete enough to allow a correction factor to be cal-culated. He states that high, medium, and low flowmeasurements were reduced by 10%, 5%, and 0%,respectively, and by comparing the 1879 and 1885publications, we estimate that “low flows” are be-

low 10,000 m3 s�1, “medium flows” are between10,000 and 35,000 m3 s�1, and “high flows” aregreater than 35,000 m3 s�1. This reduces our cal-culation of mean annual water flux by ∼4%, from

to km3 yr�1 (table 1); Gordon’s440 � 48 422 � 411885 reduced discharge value is 400 km3 yr�1,which represents an ∼6% reduction. We intend toquantify the uncertainty that should be attributedto Gordon’s double float method in future field sea-sons in order to objectively correct the originalnineteenth-century discharge values.

The daily concentration measurements for 1877–1878 are presented as an average of surface, middle,and near-bottom depth concentrations (CSMB; fig.3A); as individual surface (CS), middle depth (CM),and near-bottom depth measurements (CB; fig. 3B);and as ratios of CSM to CB (fig. 3B). Figure 4 dem-onstrates that the values of CS are ∼80% of thosemeasured at CM and that the values of CB are be-tween two and four times higher than those in theupper parts of the water column and display morevariability. Gordon suspected that some of his mea-surements of CB were overestimated because of beddisturbance. Our small data set collected in May2006 during the onset of monsoon (days 144–151)has ratios of in the range of 0.86–1.1, whichC /CSM B

may suggest that Gordon’s CB measurements aretoo high (fig. 4A). However, Gordon’s data set alsoshows that the ratio of approaches 1 duringC /CSM B

the onset of monsoon (May–June), which is whenwe sampled (fig. 3B). In the absence of a modernsediment concentration data set that covers the fullrange of flows, which would allow us to evaluatewhether Gordon overestimated near-bottom sus-pended concentrations, CSM will be used as a lowerlimit for the mean sediment concentration andCSMB will be used as an upper limit. It is worthwhilerepeating here that Gordon’s (1885) quoted sedi-ment load values are the average of the three depthconcentration measurements.

We have calculated new sediment rating curvesfor the adjusted discharge and sediment concentra-tion values for 1877–1878 (fig. 5) based on a re-gression of the log-log plot of discharge and sedi-ment concentration and including a bias correction


factor to account for the inherent underestimationin total load that comes from using a log-log datatransformation; this implicitly makes allowancefor the poor fit of the regression to sediment con-centrations at low discharges (Ferguson 1986). Thesediment-discharge relationships are different forearly and late monsoon periods; the former exhibits“first flush” positive hysteresis behavior with asteeper rating curve, while negative hysteresis isdisplayed for the late monsoon period (fig. 5B). Wecompared sediment concentrations calculated us-ing one annual rating curve (fig. 5A) with thosecalculated using two seasonal curves (fig. 5C). De-spite the clear visual differences in the sedimentconcentration-discharge behavior between earlyand late monsoon periods, a seasonal (two-curve)relationship overpredicts sediment concentrationsfor both CSM and CSMB, compared with the measuredvalues (fig. 5C). We have therefore used the annualcurve to calculate sediment concentrations for theremaining 9 yr of the discharge records. Notably,our single annual sediment rating curve predicts asediment load of 337 MT for 1877–1878, which isthe same as the measured value of 340 MT (fig. 5C)reported by Gordon (1879–1880).

Our filtered sediment concentration data from2005 and 2006 suggest that the concentrations cal-culated by Gordon did not include the 10.45-mmparticulate fraction. We therefore use an increaseof 18%, derived from our data set, as an estimatedincrease to sediment load (a filtered correction).The raw and adjusted Gordon data, using both CSM

and CSMB, are presented in table 1. With no correc-tion for the missing particulate fraction, the Irra-waddy transported km3 yr�1 of water and422 � 41

MT yr�1 of sediment load (10-yr averages).309 � 49With our 18% increase for the missing fine partic-ulate fraction, the sediment load becomes 364 �

MT yr�1.60In summary, the recalculated sediment load data

differ from Gordon’s (1885) values because (1) therewere minor arithmetic errors in the original nine-teenth-century discharge calculations (accountingfor an ∼4% decrease in discharge); (2) our sedimentrating curve produces a 27% increase in the 10-yraverage annual sediment load measurement, eventhough our rating curve correctly estimates themeasured value for 1877–1878; and (3) our filteredcorrection that accounts for a missing particulatefraction in Gordon’s concentration measurementsincreases the loads by 18% (table 1).

Review of Potential Sources of Error

There are four sources of potential uncertainty inGordon’s Irrawaddy data set: (1) the velocity mea-

surement technique, (2) the missing 10.45-mm fil-tered fraction in the sediment concentration data,(3) the merging of two stage-discharge relationshipsdeveloped at two different locations, and (4) thecontribution of CB to the daily depth-averaged con-centration values. We have adjusted the raw dis-charge values to account for the uncertainty due tothe velocity measurement technique (an ∼4% re-duction in the 10-yr average of total annual dis-charge) but recognize that Gordon’s method of mea-suring velocity needs to be independently verified.Sediment concentrations have been increasedbased on our estimation of the missing particulatefraction (an 18% increase). Our analysis of the stagerecords from Seiktha and Myanaung suggests thatthe merging of two stage records from different lo-cations introduces an uncertainty of about 8%. Fi-nally, the largest uncertainty in the data set is thevalues for CB. It is possible that some of the CB

measured by Gordon overrepresents the true bot-tom suspended sediment concentration. The barrelused in Gordon’s sampling probably had a largerinternal diameter than the openings that let theflow move through: therefore, as the barrel de-scended through the column, sand-sized particlesin suspension may have been trapped inside, thusresulting in oversampling of the suspended sand-sized material. The amount of this oversamplingwould increase with depth because the barrel col-lects sand for a longer period of time in deeper flowsand because the sand concentration increases to-ward the bed. Another possibility is that duringsampling, the barrel could have disturbed the bed,producing bottom concentration values that are notentirely due to suspended load. We intend to ad-dress all these sampling issues in future field cam-paigns by comparing various devices for measuringsuspended load concentrations, including a barrelwith a design similar to that of Gordon’s (i.e., thatof Humphreys and Abbot [1861]) and the Van Dornsampler.

Implications

In order to estimate the suspended sediment loaddebouched into the Gulf of Martaban, we havecombined the new Irrawaddy load values with es-timates for the Salween and Sittoung, as well asfour smaller rivers (Bilin, Bago, Attaran, andGyaing) that terminate in the gulf (fig. 1). Theirestimates are based on a scaling of the Irrawaddyannual sediment load values in proportion to theirannual discharge relative to the Irrawaddy, and werecognize that this is a crude estimate in the ab-sence of more data; we therefore present these val-ues, and the total combined flux, as rounded to the


nearest two significant digits (table 1). Our lowervalue of suspended sediment load for the Salweenis 110 MT yr�1, similar to the 100–MT yr�1 estimateof Meade (1996), which was based on scaling valuesfor the Irrawaddy and Mekong. Our upper value of180 MT yr�1 is similar to an annual suspended loadof 164 MT, calculated from the yield value pre-sented by Meybeck et al. (2003) for the Salween inThailand. The total annual suspended sedimentload to the eastern Indian Ocean for the Irrawaddy-Salween-Sittoung system is potentially as large as600 MT, representing 57% of the modern G-B sed-iment flux (table 1); this represents sediment loadscalculated from three depths (average of CS, CM, andCB) for the Irrawaddy. In comparison, because of theuncertainty surrounding the near-bottom concen-tration measurements, total annual sediment loadfor the Irrawaddy based on averaging just the upperand middle depths is 370 MT and represents 35%of the G-B flux.

Over the past century or more, many riversworldwide have undergone substantial declines insediment export as a result of damming and irri-gation, but the results of Syvitski et al. (2005) sug-gest that the Irrawaddy has undergone the largestrelative increase in sediment delivery to the oceanof any major river as a result of land use and pop-ulation changes and the absence of dams on itsmainstem or major tributaries. The majority of theIrrawaddy load was thought by Stamp (1940) tohave been derived from below Mandalay from theunconsolidated and rapidly eroding “dry zone”(!800 mm annual rainfall) of the Central Basin ofMyanmar (fig. 1), where recent population and ag-ricultural changes have produced significant landuse changes. We are currently developing modernmass flux records for the Irrawaddy and Salween,as well as methods that allow us to hindcast sed-iment loads back to the nineteenth century, in or-der to address how land use and climate have in-fluenced sediment loads. The sparsely inhabitednature of the Salween catchment means that landuse change has had less of an impact on this river,but an extensive damming and hydroelectricalscheme is scheduled to begin in 2007 (MizzimaNews 2007), which will have a significant effect onfuture sediment flux. Measuring the sediment loadof the, as yet, longest nondammed river remaining

in southeast Asia at the present time is absolutelycritical.

Conclusions

The existing sediment flux data for the IrrawaddyRiver, one of six major systems draining the easternHimalaya, come from a comprehensive nineteenth-century hydrological study, but these original datahave not been reevaluated since recognition of thesignificance of these river systems to global climatechange and the evolution of the Himalayan orogen.To date, only recycled values from early twentieth-century summaries of the work have been incor-porated in global river sediment load and dischargedatabases. We have evaluated the sampling strat-egies, reanalyzed the original data, and recalculateddischarges and sediment loads and find that thetypically quoted sediment flux values (e.g., Milli-man and Meade 1983; Meade 1996) are underesti-mated.

We therefore believe that the significance of theI-S contribution to “natural” Himalayan denuda-tion and land-ocean fluxes has not been fully ap-preciated because (1) reanalysis of the original nine-teenth-century data (Gordon 1879–1880) for theIrrawaddy suggests that the often-quoted flux of261 MT yr�1 may be a significant underestimate,with the true value as high as MT yr�1,364 � 60which would place the Irrawaddy as third-largestcontributor of sediment load after the Amazon andG-B (due to reduced loads on the major dammedChinese rivers), and (2) the I-S system may con-tribute 35%–57% (370–600 MT yr�1) of the G-Bload.

A C K N O W L E D G M E N T S

We would like to thank the Royal GeographicalSociety and the Carnegie Trust for the Universitiesof Scotland for funding this research, including thepurchase and installation of logging equipment andthe costs of fieldwork. We are very grateful for thecareful and thorough review of Bob Meade and toJohn Milliman and an anonymous reviewer, whosecomments also helped to improve the article. Thispublication is a contribution from the Scottish Al-liance for Geoscience, Environment, and Society(http://www.sages.ac.uk).

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The Irrawaddy River