Suspended sediment fluxes in a high-Arctic glacierised...

www.elsevier.com/locate/sedgeo

Sedimentary Geology 162 (2003) 105–117

Suspended sediment fluxes in a high-Arctic glacierised catchment:

implications for fluvial sediment storage

Richard Hodgkinsa,*, Richard Cooperb,1, Jemma Wadhamc,2, Martyn Tranterc,2

aDepartment of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UKbThe Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland, UK

cBristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK

Abstract

Suspended sediment fluxes from the 68 km2 Finsterwalderbreen catchment in Svalbard were monitored intensively during

the 1999 and 2000 melt seasons, at proximal and distal ends of a 4.2 km2 proglacial area, which has been deglacierised during

the twentieth century. Measured distal sediment fluxes correspond to total catchment denudation rates of 2700F 710 t km� 2

year� 1 (1999) and 1800F 350 t km� 2 year� 1 (2000). Hourly net sediment flux time series (distal flux minus proximal flux,

isolating change within the proglacial area itself) reveal that the proglacial area serves as both a source and a sink of sediment

during different periods of the melt season, and that the majority of sediment evacuation from the area occurs during discrete

episodes of enhanced meltwater discharge. The mean net flux from the proglacial area itself was � 690F 230 t km� 2 year� 1

(1999) and + 3800F 1700 t km� 2 year� 1 (2000). Therefore, in 1999 there was a net increase in sediment storage in the

proglacial area (aggradation), and in 2000 there was a net decrease (denudation). The pattern of sediment storage change

appears to be driven by the runoff regime, with net storage occurring during a year of relatively episodic sediment transport in

which relative supply exhaustion occurs, and net release in a year of more sustained sediment transport when relative supply

exhaustion is absent. Many more years’ monitoring would be required for any trend to emerge from the large interannual

variability in sediment yield.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Glacial erosion; Denudation; Suspended sediment; Sediment budget; Sediment yield; Svalbard; Arctic

1. Sediment yields and storage in glacierised response to environmental change (Phillips, 1991).

catchments

Sediment storage may be the single most important

aspect of fluvial sediment systems for determining

0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0037-0738(03)00218-5

* Corresponding author. Tel.: +44-1784-443570.

E-mail addresses: [email protected] (R. Hodgkins),

[email protected] (R. Cooper), [email protected]

(J. Wadham), [email protected] (M. Tranter).1 Tel.: +44-1224-498200.2 Tel.: +44-117-928-8307.

Suspended sediment yields in particular are viewed

as a sensitive parameter of environmental change

(Walling, 1995), since suspended sediment is broadly

supply-controlled, while bed load is broadly hydrauli-

cally controlled; therefore, it is expected that suspended

sediment fluxes are more responsive than bed load

fluxes to climate-driven environmental change, other

factors being equal. The identification of spatial and

temporal patterns of sediment storage is therefore an

important task for understanding the interaction of

R. Hodgkins et al. / Sedimentary Geology 162 (2003) 105–117106

climate, glacier variations and landscape change. For

example, while sediment yield typically declines with

increasing catchment area, Church and Slaymaker

(1989) found that British Columbian catchments

exhibited a pattern of increasing specific sediment yield

at all spatial scales up to 3� 104 km2, resulting from the

contemporary erosion of Quaternary sediments; stor-

age of sediment during glacial periods in effect con-

founded the ‘normal’ pattern, with larger catchments

taking longer to evacuate this stored, Quaternary sed-

iment. The cycle of sediment production and storage

during episodes of glacial advance, and subsequent

sediment evacuation to ‘background’, nonglacial levels

during episodes of glacial retreat, has been termed the

paraglacial cycle (Church and Ryder, 1972).

Denudation estimates based on sediment yields are

only strictly valid if change in storage is negligible: it is

important to distinguish how much of the sediment

transport is derived directly from contemporary ero-

sion, and how much is reworked (Harbor and Warbur-

ton, 1993). Storage effects are particularly relevant to

the study of sediment transfer in glacierised catch-

ments, where sediment transport is often in large,

discontinuous events, and there are significant varia-

tions in sediment supply on diurnal and seasonal time

scales (e.g. Bogen, 1980; Fenn, 1989; Gurnell et al.,

1994; Hodgkins, 1996). Furthermore, the majority of

glaciers globally have probably been in retreat for

several decades or more, meaning that there may be

no meaningful equilibrium between contemporary gla-

cial and hydrological configurations and sediment

production, storage and availability for transport in

any given catchment. Warburton (1999) considers that

it is proglacial river systems, i.e. rivers immediately

downstream of glaciers that are influenced by fluxes of

glacial meltwater and sediment, which provide the key

link between glacial processes and the wider environ-

ment. For example, Maizels (1979) found that 16% of

fluvial sediment from Glacier des Bossons, France was

redeposited in the proglacial valley sandur; conversely,

Warburton (1990) found that 23% of the sediment yield

of Bas Glacier d’Arolla, Switzerland was eroded from a

similar location. However,Warburton (1999) notes that

few such studies are available from high-Arctic loca-

tions, by comparison with alpine locations. Hodson et

al. (1998) believed that the proglacial sandur at Austre

Brøggerbreen in Svalbard functioned as both a net

source and sink of suspended sediment during the melt

season, although they were unable precisely to quantify

these results. The lack of high-Arctic data is under-

standable, given the significant logistical constraints of

working in high latitudes, but remains a significant gap

in our understanding of fluvial sediment delivery in

glacierised catchments.

2. Aims of this study

Given the issues raised above, the aims of this study

are to determine, from hydrological monitoring, de-

tailed suspended sediment fluxes for a high-Arctic

glacierised catchment, in order to (1) partition denuda-

tion rates between the proglacial area, the glacierised

part of the catchment and the entire catchment; (2)

determine the suspended sediment budget of the pro-

glacial area, specifically to identify whether it consti-

tutes a net source or sink of sediment. Suspended

sediment is the focus of this study because its supply-

driven nature should reflect catchment-scale environ-

mental variations rather than reach-scale, hydraulic

controls; an example of environmental variations may

be storage changes within the proglacial area. Fluvial

sediment budget data of any kind are particularly sparse

in high-Arctic environments, where there are typically

complex histories of environmental change, reflected

in glacier variations, hydrological fluctuations and

sediment supply and storage changes.

3. Location of this study

Finsterwalderbreen is a 44 km2 polythermal glacier

occupying a 68 km2 catchment on the southern shore

of van Keulenfjorden at 77jN in the Norwegian high-

Arctic archipelago of Svalbard (Spitsbergen), with an

altitude range of ca. 50–1000 m a.s.l. (Fig. 1; Hagen

et al., 1993). Hodson and Ferguson (1999) indicate

that 96% of the glacier base along the centreline is

temperate, and Wadham et al. (2001b) show that a

significant subglacial drainage system is present. The

most recent maximum advance of the glacier followed

a surge between 1898 and 1910 (Liestøl, 1969), since

when there has been steady retreat of up to about 2

km, which has exposed a proglacial area of 4.2 km2

behind a 70 m-high terminal moraine complex. The

lithology of the catchment includes Precambrian car-

Fig. 1. (Clockwise from top left) Location of Finsterwalderbreen within the Svalbard archipelago (inset). Topographic map of the glacier terminus

and proglacial area, elevations in m a.s.l. 1995 aerial photograph of the glacier terminus and proglacial area (subset of aerial photograph S95 1113nNorwegian Polar Institute): stream monitoring locations are indicated (see text for further explanation; note that many of the stream courses

apparent on the map and photograph, e.g. X, are not currently active, and that all of the runoff from the catchment is channelled through the outlet).

Upstream views of the outlet stream on 24 June (discharge ca. 5m3 s� 1) and 21 July (discharge ca. 25m3 s� 1) 1999; the lighter colour of the stream

on 21 July is a result of the angle of the sun, rather than lower turbidity. High-elevation view of the Finsterwalderbreen proglacial area looking

northeast, showing stream monitoring locations (although the east stream location is to the right of the image).

R. Hodgkins et al. / Sedimentary Geology 162 (2003) 105–117 107

bonates, phyllite and quartzite, Permian sandstones,

dolomites and limestones, and Triassic to Cretaceous

siltstones, sandstones and shales (Dallmann et al.,

1990). The proglacial area, however, is covered with

recent, unconsolidated till and fluvial deposits. The

central part of the proglacial area consists of a


relatively flat basin (average gradient 0.014, shown

stippled in Fig. 1a), consisting of recently deposited

fluvial material (poorly sorted sandy mud and muddy

sand, D50 78 Am, near the glacier; poorly sorted mud

and sandy mud, D50 35 Am, near the fjord) whereas

the margins of this basin are characterised by a more

hummocky topography consisting of older, coarser till

deposits (very poorly sorted gravelly, muddy sand,

D50 240 Am). Meltwater issues from the glacier at its

east and west margins and flows across the proglacial

area, braiding extensively until it reaches a confluence

at the terminal moraine, which it subsequently

breaches before flowing a short distance across a

coastal plain and into the fjord.

4. Methods: stream monitoring

Fluvial sediment fluxes were measured at proximal

and distal ends of the proglacial area (Fig. 1). Prox-

imal fluxes represent glacial inputs to the proglacial

area, and are the sum of the fluxes in the east and west

glacier streams as these exit the glacier terminus; these

aggregate all sources of glacial sediment, and we do

not distinguish subglacial, ice-marginal, supraglacial

sediment sources, etc. Distal fluxes represent outputs

from the proglacial area, and represent the proximal

inputs plus or minus changes in proglacial sediment

storage. The monitoring period was 54 days during

the 1999 melt season (68% of the total melt season)

Table 1

Basic data sets

Season/stream Q range

(m3 s� 1)

Mean QF S

(m3 s� 1 (n)

1999 17:00 24/06–10:00 17/08

East (proximal) stream 1.4–18 3.7F 2.7 (

West (proximal) stream 1.2–43 6.8F 6.6 (

Outlet (distal) stream 2.7–61 10.4F 9.4 (

Total monitored distal meltwater flux in 1999: 4.8� 107 m3 in 54 daysuEstimated specific runoff (80-day melt season): 1.6 m year� 1

2000 12:00 28/06–13:00 13/08

East (proximal) stream 0.5–17 2.9F 2.2 (

West (proximal) stream 0.8–32 7.5F 6.7 (

Outlet (distal) stream 2.6–51 11F10 (

Total monitored distal meltwater flux in 2000: 4.2� 107 m3 in 46 daysuEstimated specific runoff (70-day melt season): 1.5 m year� 1

Q is discharge, SSC is suspended-sediment concentration, S.D. is standar

and 46 days during the 2000 melt season (65% of melt

season; the proportion of the melt season monitored is

determined by regressing discharge on air temperature

using data from an automatic weather station in the

Finsterwalderbreen catchment, and the relationship is

extrapolated by correlating the Finsterwalderbreen

data with meteorological records from Svalbard air-

port). ‘Melt season’ in this sense therefore corre-

sponds to the estimated period of significant runoff:

80 days in 1999, 70 days in 2000.

Stream stage was sampled every 20 s by a Druck

PDCR1830 pressure sensor, averaged and logged

every hour, and stored by a Campbell CR10X data

logger. Discharge time series were generated by

regressing discrete discharge measurements on stage;

discharge was measured by the velocity–area method,

using a Valeport current meter. Errors in discharge

range from 13% to 25%, varying with stream location

and period of the melt season. Errors are a probabi-

listic function of instrumental error in the current-

metering procedure, and forecasting uncertainty from

the stage–discharge relationship.

Suspended sediment concentration (SSC) was mea-

sured in water samples from each stream. The outlet

stream was sampled automatically with an ISCO

3700C portable sampler, while the other streams were

sampled by hand with spare 500 ml ISCO sampler

bottles. The frequency of sampling varies with each

stream location and from season to season: in 1999, the

outlet stream was sampled every 7 h on average (total

.D.

)

SSC range

(kg m� 3)

Mean SSCF S.D.

(kg m� 3 (n))

1335) 0.5–7.4 1.8F 1.8 (13)

1335) 0.2–7.0 0.80F 1.1 (60)

1335) 0.3–9.0 1.7F 1.8 (184)

9.0� 105 m3 day� 1

1175) 0.3–4.4 1.3F 1.0 (18)

1175) 0.2–4.0 1.5F 0.83 (44)

1175) 0.3–3.3 1.5F 0.88 (44)

9.1�105 m3 day� 1

d deviation. Further explanation is given in the text.


number of samples 184), the west stream every 22 h (60

samples), the east stream every 103 h (13 samples); in

2000, the outlet stream was sampled every 27 h on

average (total number of samples 44), the west stream

every 27 h (44 samples), the east stream every 65 h (18

samples). ISCO samples are acquired throughout the

24-h diurnal cycle; hand samples are acquired at

variable times between early morning and early even-

ing, and should therefore be broadly unbiased with

respect to diurnal cycling, sampling early morning low

flows, early-to-mid afternoon high flows, and late

afternoon to early-evening receding flows.

Each sample was pressure-filtered through pre-

weighed Whatman 8 Am fast-filtration paper and

returned to the laboratory so that the mass of retained

material could be determined gravimetrically. Simul-

taneous duplicate experiments verified that ISCO and

hand sampling did not yield statistically significantly

different results, nor did samples filtered through

Whatman 0.45 Am cellulose–nitrate membranes

instead of fast-filtration papers ( p>95% in a two-

tailed, nonparametric Mann–Whitney U-test). The

former probably results from the uniform mixing of

suspended sediment in the stream cross section

caused by high levels of turbulence in steep, hydrau-

Table 2

Statistical models used to synthesise sediment flux time series

Stream

location

Time interval Statistical model

(see text for further explanation)

1999

East 24/06 17:00–17/08 10:00 SSCt*=� 0.0028 + 0.41QtF 0.28

West 24/06 17:00–05/07 09:00 SSCt*= 0.63 + 0.15QtF 0.59

05/07 10:00–14/07 18:00 SSCt*= 0.0050 + 0.27QtF 0.49

14/07 19:00–17/08 10:00 SSCt*= 0.54 + et + 1.00SSCt�1 + 0

Outlet 24/06 17:00–17/08 10:00 SSCt*= 0.30 + et + 0.94SSCt�1 + 0

2000

East 28/06 12:00–18/07 08:00 log10SSCt*=� 0.38 + 0.67log10Qt

18/07 09:00–22/07 23:00 SSCt*= 0.35 + 0.18QtF 0.34

23/07 00:00–03/08 21:00 log10SSCt*=� 0.38 + 0.67log10Qt

03/08 22:00–13/08 13:00 SSCt*= 0.70(� 0.48 + 1.31Qt)F 0

West 28/06 12:00–10/07 17:00 SSCt*=� 1.03 + 0.82QtF 0.39

10/07 18:00–18/07 17:00 SSCt*= 1.07 + 0.11QtF 0.19

18/07 18:00–13/08 13:00 log10SSCt*=� 0.59 + 0.70log10Qt

Outlet 28/06 12:00–10/07 05:00 SSCt*=� 1.01 + 0.48QtF 0.43

10/07 06:00–19/07 20:00 SSCt*= 0.32 + 0.12QtF 0.22

19/07 21:00–03/08 14:00 log10SSCt*=� 0.84 + 0.85log10Qt

03/08 15:00–09/08 16:00 SSCt*=� 0.05 + 0.19QtF 0.39

09/08 17:00–13/08 13:00 SSCt*= 0.001Qt2.93F 0.17

lically rough streams, the latter from rapid clogging of

filter pores reducing the effective pore size (Gurnell et

al., 1992). Error in measured SSC is trivial compared

to the forecasting uncertainty associated with mod-

elled SSC (Hodgkins, 1999), discussed and quantified

below. Discharge and SSC data are summarised in

Table 1.

Like most hydrological monitoring studies in gla-

cierised catchments, which rely on temporary gauging

structures and labour-intensive hydrometry, the time

series do not extend to the precise onset and cessation

of runoff. As explained above, it is estimated that the

monitored time series represent 65–68% of the total

durations of the melt seasons; however, it is probable

that the amount of runoff monitored in each year is a

greater proportion of the total annual runoff, because

runoff is typically low before late June and from late

August in Svalbard (Hodgkins, 1997).

5. Methods: statistical modelling

Hourly interval time series of SSC were required to

determine suspended sediment flux (hereafter sedi-

ment flux) as the product of discharge and SSC.

Mean SSC (kg m� 3,

measured/predicted)

Goodness of fit

1.79/1.37 R2 = 0.98

1.90/1.90 R2 = 0.92

1.02/1.04 R2 = 0.79

.14QtF 0.06 0.74/1.32 log-likelihood =� 34.44

.12QtF 0.33 1.71/1.46 log-likelihood =� 309.60

F 0.14 0.88/0.79 R2 = 0.75

1.73/1.65 R2 = 0.71

F 0.14 0.92/0.57 R2 = 0.75

.23 2.62/1.78 R2 = 0.99

1.54/1.21 R2 = 0.80

1.68/1.56 R2 = 0.78

F 0.20 1.50/1.21 R2 = 0.61

1.41/1.09 R2 = 0.82

1.70/1.38 R2 = 0.90

F 0.13 1.69/1.56 R2 = 0.88

1.25/1.19 R2 = 0.76

1.42/0.97 R2 = 0.87


Sediment fluxes are often regressed on discharge for

this purpose, but this is statistically spurious, as a

result of the colinearity associated with regressing a

product of discharge on discharge itself. Therefore,

SSC models were determined to synthesise continu-

ous SSC time series. This approach also yields a more

realistic assessment of error, as the forecasting uncer-

tainty of statistical models can be, in the absence of

colinearity, reliably quantified. The preferred model is

a maximum-likelihood autoregression model, which is

essentially a lagged regression of a series on itself, and

is particularly useful for modelling series where first-

order, autoregressive pattern is present in the residuals

from linear regression (Hodgkins, 1999); this takes the

form:

SSCt ¼ k þ aSSCt�1 þ bQt þ et

where k is a constant, a and b are empirically derived

coefficients and e is a white-noise disturbance, at time

steps t and t� 1 as denoted by subscripts. This form of

model is ideally suited to time series with autocorre-

lation arising from hysteresis, which is typical for SSC,

as a result of relative sediment supply exhaustion.

Where autoregression models cannot be estimated

satisfactorily, least-squares linear regression models

are estimated. Because of changing discharge–SSC

responses, between one and five regression models

were used to fit an individual SSC time series; all the

models are summarised in Table 2. In order to achieve

the closest possible fit to observed data, observed

values of SSC are substituted back into the modelled

SSC series, and cubic spline fitting (Press et al., 1988)

is used to adjust the modelled curve where there are

significant discrepancies. Such a procedure has no

value for forecasting, but achieves the closest possible

fit to the observed series, which is the aim here. The

standard error of the estimate of the models ranges

from 0.14 to 0.59 kg m� 3; this is expressed as a

percentage of the mean SSC during the estimation

period to determine error limits. Percentage errors in

Fig. 2. Time series from 1999: (top) discharge and suspended-sediment co

stream) sediment fluxes, plus the integrals of these fluxes; (bottom) proglac

fluxes. Probable minimum and maximum values of proximal and distal sed

forecasting errors, as described in the text, and these provide percentage

calculated net flux is a probabilistic function of the distal and proximal flux

hour and integrating, a realistic total net flux, with a realistic error term,

minimum and maximum error limits, and therefore it is believed that ther

discharge and SSC are combined probabilistically as

the root of the sum of the squares, to determine

realistic (nonadditive) flux errors: these are used to

define probable minimum and maximum fluxes, in

order to determine net change in the total flux from the

proglacial area, and particularly whether this flux is

significantly different from zero (see below).

6. Results: sediment flux time series

Figs. 2 and 3 show time series of discharge and

SSC for all three streams in both melt seasons, 1999

and 2000. Runoff is dominated by meltwater dis-

charge, as there are no significant rain storms during

the monitoring periods: during the 1999 time series,

for example, only 29.4 mm of rain fell in total, with

the maximum rate being 1.4 mm h� 1 (Hodgkins,

unpublished data). The 1999 time series is, however,

dominated by two episodes of enhanced meltwater

discharge, around 29 June and 19 July, which are

reflected at all three stream locations. Sediment trans-

port during the first discharge peak is relatively poorly

constrained, with east and west stream sampling not

commencing until the falling limb of the flood hydro-

graph; it is assumed that the statistical models dis-

cussed above provide a reasonable representation of

sediment inputs in the earliest days of the time series.

However, sediment transport in the outlet stream is

well-constrained throughout, and a distinctive pattern

of relative sediment supply exhaustion appears to

occur in the outlet stream, with maximum SSC during

the second discharge peak being little higher than

during the first peak, despite significantly higher

discharge; this corresponds to a seasonal pattern of

clockwise hysteresis. The second discharge peak in

mid-July is thought to be driven by a reorganisation of

the subglacial drainage system, marked by the release

of subglacially stored meltwater (Wadham et al.,

2001a,b). Similarly to this study, Hodson et al.

(1997) found high suspended sediment concentrations

ncentration (SSC); (middle) distal (outlet) and proximal (west + east

ial (distal–proximal) net sediment fluxes, plus the integrals of these

iment fluxes have been calculated from measurement and statistical

error terms for the respective fluxes. The percentage error in the

errors. By determining the probable range of net fluxes from hour to

is derived: this is significantly different from zero, as shown by the

e is net storage change (in both years: see also Fig. 3).


during early-season low flows at Finsterwalderbreen,

which subsequently declined slowly, except for peri-

odic disturbances within the subglacial drainage sys-

tem associated with episodes of high discharge.

There is a single major discharge peak in 2000

(around 23 July), although there are rising discharges

towards the end of the monitoring period when dis-

charges in 1999 were steadily declining. In 1999, the

total (distal or outlet) flux of meltwater during the

monitoring period was 4.8� 107 m3, or 9.0� 105 m3

day� 1; in 2000, the equivalent figures were 4.2� 107

m3 and 9.1�105 m3 day� 1. While the 2000 melt

season lacks the dramatic peaks of 1999, it therefore

exhibits more sustained levels of discharge. SSC peaks

in 2000 were again not as pronounced as in 1999, but

west and outlet streams had generally higher SSC than

in 1999 overall, when assessed by the median. Relative

sediment supply exhaustion does not appear to occur

in the outlet stream in 2000: SSC rises rapidly at

moderate discharges in early August. The sediment

transport regimes of the 1999 and 2000 melt seasons

are therefore, respectively, relatively episodic and

relatively sustained in character.

Figs. 2 and 3 also show time series of proximal

(west stream plus east stream) and distal (outlet

stream) sediment fluxes. There are no large, sustained

departures in the relative levels of proximal and distal

sediment flux in either season, although cumulative

differences do emerge (see below). The impact of the

two discharge peaks on total sediment transport in

1999 is great: both proximal and distal sediment flux

integrals rise sharply during the two discharge peaks,

with the distal rising faster than the proximal, partic-

ularly during the first peak. Between the two peaks

and following the second, the proximal flux integral

rises faster than the distal, such that towards the end of

the monitoring period, the proximal flux exceeds the

distal. This overall pattern is consistent with the

relatively episodic sediment transport regime identi-

fied above. There is a much simpler pattern in 2000,

with the proximal and distal flux integrals diverging

significantly during the major discharge peak: the

distal flux integral rises faster than the proximal, then

Fig. 3. Time series from 2000: (top) discharge and suspended-sediment co

stream) sediment fluxes, plus the integrals of these fluxes; (bottom) proglac

fluxes. See the comments in Fig. 2 caption for further explanation. The ax

year comparison.

following the peak the rates of increase appear to

stabilise and the distal flux integral remains greater to

the end of the monitoring period, consistent with the

relatively sustained sediment transport regime identi-

fied above.

Figs. 2 and 3 also show time series of proglacial

net sediment fluxes (distal flux minus proximal flux,

in order to isolate change within the proglacial area

itself). The treatment of errors is particularly impor-

tant for these time series. Probable minimum and

maximum values of proximal and distal sediment

fluxes have been calculated, as described above, from

measurement error and from forecasting error associ-

ated with the statistical modelling procedure, and

these provide percentage error terms for the respective

fluxes. The percentage error in the calculated net flux

is then redetermined as a probabilistic function of the

distal flux and proximal flux errors used in the

calculation. The benefits of this procedure are (1)

the central tendency is preserved of values (i.e. the

net proglacial fluxes) which are calculated from other

values (i.e. the distal and proximal fluxes) which have

wide error limits; (2) the use of percentage errors

allows the error magnitude to vary realistically when

there are order-of-magnitude variations in the values

of the variables; (3) the use of percentage errors

allows errors from different sources (e.g. the east

and west streams which constitute the proximal sed-

iment source) to be compared and combined realisti-

cally; (4) probabilistic combination of different

sources of error (e.g. error in each of the distal and

proximal fluxes, which are used to calculate the net

proglacial flux) gives realistic, nonadditive overall

errors. By determining the probable range of net

fluxes from hour to hour and integrating over the

length of the monitoring interval, a realistic total net

flux, with its own error term, is derived: this is

significantly different from zero, as shown by the

minimum and maximum error limits in Figs. 2 and

3, and therefore it is believed that there is net storage

change. It is assumed that we start at zero net storage:

this seems reasonable, given that early season progla-

cial conditions (discussed further below) are dominat-

ncentration (SSC); (middle) distal (outlet) and proximal (west + east

ial (distal–proximal) net sediment fluxes, plus the integrals of these

es in these plots have the same ranges as in Fig. 2, to allow year-to-

Table 3

Total sediment fluxes and equivalent denudation rates

Proximal (t) Distal (t) Net (t)

1999 128,000F 48,600 126,000F 32,700 � 1900F 660

2000 68,700F 26,300 79,300F 15,400 10,500F 4800

Equivalent annual denudation ratesa

(t km� 2 year� 1) (mm ky� 1)b

Glacierised part of catchment (44 km2)c

1999 4300F 1600 1900F 710

2000 2400F 910 1000F 400

Entire catchment (68 km2)

1999 2700F 710 1200F 310

2000 1800F 350 780F 150

Proglacial area only (4.2 km2)

1999 � 690F 230 � 300F 100

2000 3800F 1700 1700F 760

Total proximal, distal and net proglacial sediment fluxes during

monitoring periods (Figs. 2 and 3 show the equivalent time series).a Adjusted for the length of melt season (148% and 153% of

monitoring period in 1999 and 2000, respectively).b All rates in mm ky� 1 assume a rock density of 2300 kg m� 3,

representative for crustal sediments.c Calculated from proximal flux over glacier-covered area;

excludes the influence of the proglacial area, and also assumes

contribution of fine sediment from extraglacial headwalls at higher

elevations is negligible.


ed by the surviving snowpack, yielding diffuse snow-

melt runoff with restricted access to sediment sources.

By the time monitoring starts in both years, the

transient snowline is close to the glacier terminus

and the proglacial area is snow-free.

Following the proximal and distal flux time series,

there is a more complex pattern of net proglacial

sediment fluxes in 1999 than in 2000, with the

proglacial area functioning as both a source (e.g. 29

June and 17 July) and a sink (e.g. 20 July) of sediment

at different times during the melt season. The net

proglacial flux integral is incremented during both

discharge peaks but declines steadily at other times

and is negative at the end of the monitoring period

(Fig. 2). In other words, there are two episodes of

sediment evacuation during periods of high meltwater

discharge, outside of which sediment is stored during

longer episodes of low meltwater discharge. There is a

single, significant increment to the net proglacial

sediment flux integral during the major discharge

peak in 2000, following which there is little net

change (Fig. 3). In other words, there is an episode

of sediment evacuation followed by approximate

input–output balance. Relative sediment supply ex-

haustion therefore appears to occur in the outlet

stream in a year of net proglacial sediment storage

(1999) but not in a year of net proglacial sediment

release (2000). Proglacial sediment storage therefore

appears to occur during relatively sustained periods of

low discharge following episodes of sediment release,

and appears to be linked with the occurrence of

relative sediment supply exhaustion in the distal

stream.

7. Discussion

The total proximal and distal sediment fluxes for

both melt seasons can be used to determine rates of

denudation for the glacierised part of the catchment

and the entire catchment, respectively, while the

proximal fluxes can be subtracted from the distal

fluxes to determine the net change in the proglacial

area itself (Table 3). The net proglacial flux in 1999,

� 1900 t, is the equivalent of 1.5% of the total

proximal (input) flux (note that it is not claimed that

this represents the level of accuracy of the calculated

fluxes; errors are discussed in detail above). An

uncertainty of F 660 t (defined above; Table 3) shows

that this net proglacial flux is significantly different

from zero, and negative. There is therefore an increase

in proglacial sediment storage in 1999 (proximal

inputs exceed distal outputs), with net aggradation

of the proglacial area. The net proglacial flux in 2000,

+ 11,000 t, is the equivalent of 15% of the total

proximal (input) flux. An uncertainty of F 4800 t

(defined above; Table 3) shows that this net proglacial

flux is significantly different from zero, and positive.

There is therefore a decrease in proglacial sediment

storage in 2000 (distal outputs exceed proximal

inputs), with net denudation of the proglacial area.

Hence, the proglacial area functions in opposite ways

in successive years: as a sediment sink in 1999, and as

a sediment source in 2000.

The occurrence of net proglacial sediment storage

or release appears to be linked to the pattern of

meltwater discharge: a highly episodic transport re-

gime in 1999 led to relative exhaustion of the sedi-

ment supply, and storage occurred at low discharge


following episodes of sediment release generated by

two major peaks in discharge; a more sustained

transport regime in 2000 did not lead to noticeable

sediment exhaustion, and there appeared to be ap-

proximate balance between sediment inputs and out-

puts following the less pronounced, single discharge

peak. The occurrence of relative sediment supply

exhaustion at Finsterwalderbreen is in agreement with

results from the same glacier by Hodson and Ferguson

(1999), but is in contrast with the results of Gurnell et

al. (1994), Hodgkins (1996) and Hodson et al. (1998),

who both found evidence for steadily increasing

sediment supply at the smaller, non-temperate glaciers

Austre Brøggerbreen and Scott Turnerbreen; Finster-

walderbreen is more like an alpine glacier in this

respect, which can be ascribed to its thermal regime

and the presence of significant subglacial drainage.

It was noted above that an estimated 26 days

(1999) and 24 days (2000) of the melt season are

not monitored. It is not believed that the unmonitored

periods have a significant impact on the results

obtained here, for the following reasons: in the early

melt season, access to sediment sources is limited

subaerially by the existence of a significant snow

cover (which can easily remain at sea level even into

late June in Svalbard), and by the spatially restricted

glacial drainage system; in the late melt season,

declining solar radiation receipts lead to diminishing

runoff, and stream energy and sediment transport

correspondingly decline. In 1999, it is observed that

low flow conditions are associated with proglacial

sediment storage (discussed above), so the unmoni-

tored late-season period should reinforce the aggrada-

tion pattern recorded over the season. A similar effect

may occur after the cessation of monitoring in 2000,

but there is insufficient time for low-flow storage to

have anything other than a minor impact on the

recorded net proglacial denudation, which is an order

of magnitude greater than in 1999.

Entire catchment denudation rates of up to 2700F710 t km� 2 year� 1 are, as would be expected, sig-

nificantly in excess of the global mean suspended

solids rate of 91 t km� 2 year� 1 (Milliman and Meade,

1983), emphasising the general efficacy of glacial

denudation. Measured sediment yields from other

catchments in the Svalbard archipelago with varying

glacier covers of varying thermal regimes range from

24 to 2900 t km� 2 year� 1 (Barsch et al., 1994; Bogen,

1993; Hodgkins et al., 1997; Hodson et al., 1997;

Kostrzewski et al., 1989; Sollid et al., 1994). Hodson

et al. (1997) measured the equivalent of this study’s

west proximal sediment fluxes at Finsterwalderbreen

in 1994 and 1995, expressing these as a yield for the

entire catchment (here calculated from the distal flux,

to allow for the influence of the proglacial area). The

yields obtained were 2900 t km� 2 year� 1 (1994) and

710 t km� 2 year� 1 (1995), further underlining the

significant interannual variability of sediment yields at

this as at other glacierised catchments (e.g. Bogen,

1989; Fenn, 1989; Repp, 1988; Hodson et al., 1998).

Hodson et al. (1997) compared sediment yields from

the 12 km2, mainly non-temperate Austre Brøg-

gerbreen, with those from Finsterwalderbreen, which

is mainly temperate (see above). Sediment yields were

significantly higher at Finsterwalderbreen (710–2900

t km� 2 year� 1, as opposed to 81–110 t km� 2

year� 1), which was, like the seasonal trends in sedi-

ment supply discussed above, ascribed to the influence

of thermal regime on the glaciers’ drainage system

structures and the main sources of sediment: mainly

ice-marginal at Austre Brøggerbreen, mainly subgla-

cial at Finsterwalderbreen.

The magnitude and variability of bed load transport

is unknown in this catchment, as it is in almost all

high-Arctic catchments. However, solute transport in

the catchment has been studied by Wadham et al.

(2001a), who determined the chemical denudation rate

for the 44 km2 glacier-covered area from the non-

snowpack-derived solute flux at the glacier terminus;

in 1999, this rate was 61 t km� 2 year� 1, or about

1.4% of the equivalent suspended solids denudation

rate. However, the chemical denudation rate in the

proglacial area was about 3.3 times that of the

glacier-covered area, or a much more significant

26% of the proglacial suspended solids denudation

rate (though this was an order of magnitude smaller

than the 2000 suspended solids denudation rate, as

discussed above).

The calculated rate of surface lowering is of the

order of 1 mm year� 1 (Table 3), which Hallet et al.

(1996), in their review of global glacial erosion rates,

considered typical of small, temperate glaciers on

diverse bedrock types, significantly greater than rates

for polar glaciers or temperate glaciers on resistant,

crystalline bedrock (0.1 mm year� 1 or less), but well

below rates for large, fast-moving, temperate Alaskan


glaciers (10 mm year� 1 or more; however, note that

the Alaskan glaciers in Hallet et al.’s data set are

mainly tidewater glaciers much greater than 100 km2

in area). The denudation rates recorded for Finster-

walderbreen in this study and in that of Hodson et al.

(1997) are the fastest so far recorded for a terrestrial

glacier in Svalbard, and comparable with the 189 km2

tidewater-terminating glacier Kongsvegen (Elverhøi et

al., 1980); other estimates of surface lowering rates in

Svalbard are significantly lower, and include 0.27 mm

year� 1 at Brøggerbreen (Bogen, 1993), and 0.08 and

0.19 mm year� 1, respectively, at Hannabreen and

Erikbreen (Sollid et al., 1994).

8. Conclusion

Detailed hydrological monitoring has allowed sus-

pended solids denudation rates to be partitioned

between the entire Finsterwalderbreen catchment,

the glacierised area, and the proglacial area. The

proglacial area is of particular interest here as the part

of the catchment that has been deglacierised over the

past century or so, and therefore presumably under-

going paraglacial adjustment. Catchment denudation

rates (Table 3) of 2700F 710 t km� 2 year� 1 (1999)

and 1800F 350 t km� 2 year� 1 (2000) are among the

highest recorded for the Svalbard archipelago, and

comparable to alpine glacier systems.

Attempts to infer long-term change in sediment

yields at remote glacierised catchments are typically

confounded by an absence of historic time series.

Etzelmuller (2000) determined a mean rate of net

denudation of the Finsterwalderbreen proglacial area

of + 460 t km� 2 year� 1 over the period 1970–1990,

by quantitative comparison of digital elevation mod-

els. It was considered that this denudation was attrib-

utable to fluvial erosion of only a limited part of the

proglacial area. This 20-year mean rate is bracketed by

the net proglacial denudation rates determined here by

monitoring of fluvial fluxes (Table 3): � 690F 230 t

km� 2 year� 1 (1999) and + 3800F 1700 t km� 2

year� 1 (2000). A mean calculated from these two

years’ data would clearly not be very meaningful,

and several more years’ monitoring would be required

to determine a realistic mean, and probably several

decades to determine net change with confidence.

However, if the proglacial area can act as a net sink

and a net source of sediment in successive years, one

interpretation may be that it is not far from overall

sediment-budget equilibrium: if the proglacial area

were instead far from equilibrium, large negative or

positive changes might be expected year-on-year,

assuming that interannual variability in sediment yield

is small compared to multi-decadal variability arising

from paraglacial adjustment (Church and Ryder, 1972;

Church and Slaymaker, 1989). Sediment-budget equi-

librium would not imply that nonglacially driven

processes dominate the proglacial area, but that there

is an input–output balance with respect to the current

glacier configuration (though with very large interan-

nual variability). That the pattern of proglacial storage

appears to be driven by the runoff regime could

perhaps also be interpreted as evidence for proglacial

sediment-budget equilibrium, permitting sensitive and

rapid responses to interannual hydrological variations,

reinforcing the importance of the suspended fraction of

the sediment budget for environmental change studies.

It is notable that total catchment denudation is

actually greater in the year when the proglacial area

acts as a sediment sink (1999) than when it acts as a

sediment source (2000), suggesting that there is no

simple relationship between proglacial and total-

catchment processes. Net proglacial change is cur-

rently a small but order-of-magnitude variable fraction

of total sediment input. The changing role of progla-

cial storage through time (closely related to the

sediment delivery ratio and a critical modulator be-

tween glacial erosion and marine sedimentation)

remains essentially unknown, and therefore an intrigu-

ing aspect of landscape development.

Acknowledgements

Margaret Onwu conducted the laboratory analyses,

and Deborah Jenkins and Elizabeth Farmer provided

field assistance. Norsk Polarinstitutt gave permission

to reproduce aerial photograph S95 1113. Adrian Fox

(British Antarctic Survey) produced the topographic

map of Finsterwalderbreen used in Fig. 1. Financial

support was provided by the U.K. Natural Environ-

ment Research Council Thematic Grant GST/02/2204

and the University of London Central Research Fund.

The comments of two anonymous referees led to

significant improvements of the manuscript.


References

Barsch, D., Gude, M., Mausbacher, R., Schukraft, G., Schulte, A.,

1994. Recent fluvial sediment budgets in glacial and periglacial

environments, NW Spitsbergen. Zeitschrift fur Geomorpholo-

gie. Supplementband 97, 111–222.

Bogen, J., 1980. The hysteresis effect of sediment transport sys-

tems. Norsk Geografisk Tidsskrift 34, 45–54.

Bogen, J., 1989. Glacial sediment production and development of

hydro-electric power in glacierized areas. Annals of Glaciology

13, 6–11.

Bogen, J., 1993. Erosion og sediment transport i Bayelva og

Londonelva. SINTEF-NHL Rapport STF60 A93081.

Church, M.A., Ryder, J.M., 1972. Paraglacial sedimentation: a con-

sideration of fluvial processes conditioned by glaciation. Geo-

logical Society of America Bulletin 83, 3059–3072.

Church, M., Slaymaker, O., 1989. Disequilibrium of Holocene

sediment yield in glaciated British Columbia. Nature 337,

452–454.

Dallmann, W.K., Hjelle, A., Ohta, Y., Salvigsen, O., Bjornerud

M.G., Hauser, E.C., Maher, H.D., Craddock, C., 1990. Geo-

logical Map, Svalbard, Van Keulenfjorden B11G. Norsk Po-

larinstitutt, Oslo.

Elverhøi, A., Liestøl, O., Nagy, J., 1980. Glacial erosion, sedimen-

tation and microfauna in the inner part of Kongsfjorden, Spits-

bergen. Norsk Polarinstitutt Skrifter 172, 33–62.

Etzelmuller, B., 2000. Quantification of thermo-erosion in pro-gla-

cial areas—examples from Svalbard. Zeitschrift fur Geomor-

phologie. Supplementband 44, 343–361.

Fenn, C.R., 1989. Quantifying the errors involved in transferring

suspended sediment rating equations across ablation seasons.

Annals of Glaciology 13, 64–68.

Gurnell, A.M., Clark, M.J., Hill, C.T., Greenhalgh, J., 1992.

Reliability and representativeness of a suspended sediment

monitoring programme for a remote alpine proglacial river.

Proceedings of the Oslo Symposium on Erosion and Sedi-

ment Transport Monitoring Programmes in River Basins. In-

ternational Association of Hydrological Sciences Publication,

vol. 213, pp. 191–200.

Gurnell, A.M., Hodson, A.J., Clark, M.J., Bogen, J., Hagen, J.O.,

Tranter, M., 1994. Water and sediment discharge from glacier

basins: an arctic and alpine comparison. Proceedings of the

Canberra Symposium on Variability in Stream Erosion and

Sediment Transport. International Association of Hydrological

Sciences Publication, vol. 224, pp. 325–334.

Hagen, J.O., Liestøl, O., Roland, E., Jørgensen, T., 1993. Glacier

Atlas of Svalbard and Jan Mayen. Norsk Polarinstitutt Medde-

lelser 129.

Hallet, B., Hunter, L., Bogen, J., 1996. Rates of erosion and sedi-

ment evacuation by glaciers: a review of field data and their

implications. Global and Planetary Change 12, 213–235.

Harbor, J., Warburton, J., 1993. Relative rates of glacial and non-

glacial erosion in alpine environments. Arctic and Alpine Re-

search 25, 1–7.

Hodgkins, R., 1996. Seasonal trends in suspended-sediment trans-

port at an Arctic glacier, and their implications for drainage

system structure. Annals of Glaciology 22, 147–151.

Hodgkins, R., 1997. Glacier hydrology in Svalbard, Norwegian

High Arctic. Quaternary Science Reviews 16, 957–973.

Hodgkins, R., 1999. Controls on suspended-sediment transfer at a

High-Arctic glacier, determined from statistical modelling. Earth

Surface Processes and Landforms 24, 1–21.

Hodgkins, R., Tranter, M., Dowdeswell, J.A., 1997. Solute prove-

nance, transport and denudation in a High-Arctic glacierised

catchment. Hydrological Processes 11, 1813–1832.

Hodson, A.J., Ferguson, R.I., 1999. Fluvial suspended sediment

transport from cold and warm-based glaciers in Svalbard. Earth

Surface Processes and Landforms 24, 957–974.

Hodson, A.J., Tranter, M., Dowdeswell, J.A., Gurnell, A.M., Ha-

gen, J.O., 1997. Glacier thermal regime and suspended-sediment

yield: a comparison of two high-Arctic glaciers. Annals of

Glaciology 24, 33–37.

Hodson, A., Gurnell, A., Tranter, M., Bogen, J., Hagen, J.O., Clark,

M., 1998. Suspended sediment yield and transfer processes in a

small High-Arctic glacier basin, Svalbard. Hydrological Pro-

cesses 12, 73–86.

Kostrzewski, A., Kaniecki, A., Kapuscinski, J., Klimczak, R.,

Stach, A., Zwolinski, Z., 1989. The dynamics and rate of denu-

dation of glaciated and non-glaciated catchments, central Spits-

bergen. Polish Polar Research 10, 317–367.

Liestøl, O., 1969. Glacier surges in West Spitsbergen. Canadian

Journal of Earth Sciences 6, 895–898.

Maizels, J.K., 1979. Proglacial aggradation and changes in braided

channel patterns during a period of glacier advance: an Alpine

example. Geografiska Annaler 61A, 87–101.

Milliman, J.D., Meade, R.H., 1983. World-wide delivery of river

sediment to the oceans. Journal of Geology 91, 1–21.

Phillips, J.D., 1991. Fluvial sediment budgets in the North Carolina

Piedmont. Geomorphology 4, 231–241.

Press, W.H., Flannery, B.P., Teukolsky, S.A., Vetterling, W.T., 1988.

Numerical Recipes in C. Cambridge Univ. Press, New York.

Repp, K., 1988. The hydrology of Bayelva, Spitsbergen. Nordic

Hydrology 19, 259–268.

Sollid, J.L., Etzelmuller, B., Vatne, G., Ødegard, R.S., 1994. Glacial

dynamics, material transfer and sedimentation of Erikbreen and

Hannabreen, Liefdefjorden, northern Spitsbergen. Zeitschrift fur

Geomorphologie. Supplementband 97, 123–144.

Wadham, J.L., Cooper, R.J., Tranter, M., Hodgkins, R., 2001a.

Enhancement of glacial solute fluxes in the proglacial zone of

a polythermal glacier. Journal of Glaciology 47, 378–386.

Wadham, J.L., Hodgkins, R., Cooper, R.J., Tranter, M., 2001b.

Evidence for seasonal subglacial outburst events at a polyther-

mal-based glacier, Finsterwalderbreen, Svalbard. Hydrological

Processes 15, 2259–2280.

Walling, D.E., 1995. Suspended sediment yields in a changing

environment. In: Gurnell, A.M., Petts, G.E. (Eds.), Changing

River Channels. Wiley, Chichester, pp. 149–176.

Warburton, J., 1990. An alpine proglacial sediment budget. Geo-

grafiska Annaler 72A, 261–272.

Warburton, J., 1999. Environmental change and sediment yield

from glacierised basins: the role of fluvial processes and

sediment storage. In: Brown, A.G., Quine, T.A. (Eds.), Flu-

vial Processes and Environmental Change. Wiley, Chichester,

pp. 363–384.

Suspended sediment fluxes in a high-Arctic glacierised...

Documents

Transcript of Suspended sediment fluxes in a high-Arctic glacierised...