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This is to certify that the
thesis entitled

SUBGLACIAL EROSION AND THE FATE OF COARSE
MATERIAL IN THE SUBGLACIAL DRAINAGE SYSTEM OF
THE MATANUSKA GLACIER. ALASKA

presented by

NICHOLAS J. WATERSON

has been accepted towards fumllment
of the requirements for the

Master‘s degree in ¥ Ged_ogi£al Sciences

 

 

 

 

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6/01 cJCIRC/DateDuepGSHJ. 1 5

SUBGLACIAL EROSION AND THE FATE OF COARSE MATERIAL IN THE
SUBGLACIAL DRAINAGE SYSTEM OF THE MATANUSKA GLACIER,
ALASKA
By

Nicholas J. Waterson

A THESIS
Submitted to
Michigan State University
In partial fulﬁllment of the requirements
For the degree of
MASTER OF SCIENCE
Department of Geologiwl Sciences

2003

ABSTRACT
SUBGLACIAL EROSION AND THE FATE OF COARSE MATERIAL IN THE
SUBGLACIAL DRAINAGE SYSTEM OF THE MATANUSKA GLACIER,
ALASKA

By
Nicholas J. Waterson

Melt-water streams draining temperate glaciers are known to transport
considerable sediment via suspended and bed load. However, recent studies
of sediment in melt-water streams discharging from vents along the margin of
the Matanuska Glacier in south-central Alaska show bed load to be <1% of the
total load despite the availability of coarse material at and near the glacier bed.
A possible explanation for this is that much of the coarse material is being
trapped within the subglacial drainage system due to an adverse slope of the
glacier bed near the terminus. An estimate of the mass of coarse material
potentially being trapped was calculated by determining the grain-size
distribution of the debris-rich basal ice and subglacial sediment at the
Matanuska and measuring the suspended sediment ﬂux in melt-water streams
and calculating an average minimum effective erosion rate for six consecutive
ablation seasons. The results show that 2.85E+08kg to 8.61 E+08kg of coarse
material could by trapped at the glacier bed each year. If deposited within 5km2
of the glacier terminus this would be equivalent to a layer 2.9 to 9.3 cm thick.
How the coarse sediment might be distributed, however, would depend on the
geometry of the adverse(s) and the type and pattern of the subglacial drainage
system. The average minimum effective erosion rate for the six consecutive

ablation seasons of 1997-2002 is 1.13 mmlyr.

ACKNOWLEDGEMENTS

Like any type of research, one person can seldom do it alone. I would
like to take this Opportunity to thank the people that made this research
possible. First, I would like to thank my advisor, Grahame J. Larson for passing
on his knowledge and for his patience and guidance with my many revisions.
The knowledge that l have learned has been invaluable from subjects to glacial
geology, writing, and surﬁcial mapping. Secondly, I would like to thank my
committee members, David T. Long and Gary S. Weissmann, for taking time to
critique the thesis and suggest the proper changes. Third, I would like to thank
the Matanuska Maﬁa for the opportunity to work in Alaska at the Matanuska
Glacier and for the experience of working with so many great scientist in the
ﬁeld. Forth, I would like to thank the Camp Crew, including, John Linker,
Michiel Kramer, James Cassionne, for help setting up camp and collecting
samples throughout the summer. Lastly, I would like to thank my family for their

support during my time here at MSU.

iii

TABLE OF CONTENTS

LIST OF FIGURES

METHODS OF BED LOAD ESTIMATION................. ..........

METHODOLOGY ..

Discharge Measurements: '.'.'.'.I'.'..'
Suspended SedIment

POTENTIAL SEDIMENT SOURCES ..
SubglacialSediment...

Basal Ice... ..

Supraglacial debris. ..
Proglacial Sediment...

GRAIN-SIZE ANALYSIS OF SUSPENDED SEDIMENT AND POTENTIAL

SEDIMENT SOURCES...

DISCHARGE CHARACTERISTICS... ..

SUSPENDED-SEDIMENT FLUX CHARACTERISTICS... ..

MINIMUM EFFECTIVE EROSION RATE.

....vi

...vii

boobs

...10
...10
...14
...14
.. 16

...21

...25

DISCUSSION...

Annual Suspended-Sediment Flux vs. DIscharge
Comparison of Glacier Erosion Rates”

Role of an Overdeepening in Sediment transport

CONCLUSION

REFERENCES CITED.....

......

...40

...42

List of Tables

Table 1. Average effective erosion rates recorded for several glaciers
around the world, including the Matanuksa Glacier ................ 34

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

List of Figures
(Images in this thesis are presented in color)

Location map for the Matanuska Glacier, modiﬁed from Lawson,

Map showing margin of the Matanuska Glacier with sample
locations... . .......7

Melt—out till exposure approximately .5 km from the terminus.
Pointing to contact between buried basal ice on bottom and melt-
out till on top........ ..11

Lodgement till exposure under an ice ramp along the active
terminus. The pick of the ice axe is resting on the ramp of the ice
and the sediment hanging down are till curls. The red arrow is

pointing to the lodgementtill... .....12

Exposure of an overridden moraine along the active terminus...13

Exposure of basal ice along the active terminus. The notebook is
sitting at the base of stratiﬁed basal ice and the red arrow is
pointing to the contact between englacial ice and basal ice ....... 15

Ternary plots for the average grain-size analysis distribution of
suspended sediment and potential sediment sources .............. 18

Cumulative percent curves of average grain-size distribution of
suspended sediment and potential sediment sources... .20

Plots showing suspended sediment and discharge recorded at
South Branch, Matanuska River “during.” the 1997-2002 ablation
seasons” .. . . ...22

Total annual sediment ﬂux of the Matanuska Glacier during the
period of 1997-2002... .. .. .. ...27

Average minimum effective erosion rate of the
Matanuska Glacier... ....29

Plots of annual suspended sediment vs. discharge for
A. Matanuska Glacier, 1997-2002, B. Nigardsbreen
Glacier, Norway, 1983-1988 (Bogen, 1996), C.
Engabreen Glacier, Norway, 1983-1988 (Bogen, 1996)

vii

Figure 13.

D. Gomera Glacier, Switzerland, 1987-1992.
(Collins, 1991)30

Overdeepening Model, Mechanism for trapping coarse

material at the ice-bedrock interface.

Condition A Glacier lacking supercooling

Condition 8. Glacier far into the supercooling phase

(Modiﬁed by Alley et al., in review)... .37

Introduction

Melt-water streams draining mountain glaciers are characterized by
large and rapid discharge ﬂuctuations that transport major amounts of
sediment and with analysis of the sediment yield, an erosion rate for the
glacierised basin can be determined (Ostrem, 1973; Hammer and Smith, 1983;
Gurnell, 1987; Bezinge, et al. 1989; Hallet et al., 1996; Collins, 1998; Linker,
2001 ). For example, Hallet et al., (1996) studied sediment yields from melt-
water streams draining the Nigardsbreen Glacier, Norway, from the past 25
years and calculated an erosion rate of 0.15 mm year". In this study, he also
complied erosion rates calculated by Benzinge (1987), Andrews et al., (1994),
and Hunter (1994) for different glaciers. These ranged from 0.01 mm year" for
polar glaciers (Andrews et al., 1994) to 1.0 mm year" for small temperate
glaciers in the Swiss Alps (Benzinge, 1987), and 10 to 100 mm year" for large
temperate valley glaciers in southeast Alaska (Hunter, 1994). Subsequently,
Collins (1998) studied the sediment yield from melt-water streams draining the
Batura Glacier, India during the melt season of 1990 and calculated erosion
rate 7.7 mm year". More recently, Linker (2001) studied the sediment yield
from melt-water streams draining the Matanuska Glacier, Alaska for the years
of 1997 to 2000 and calculated an average erosion rate of 1.6 mm year", a
value that is in the middle of the rates compiled by Hallet et al., (1996).

Many difﬁculties exist, however, when calculating erosion rates for
glacierised basins. One difﬁculty is obtaining representative samples of

sediment from melt-water streams (Fenn et al., 1985; Collins, 1998; Linker,

2001). Another difﬁculty is approximating the inﬂux of sediment from subaerial
erosion of adjacent nonglacierized areas (Fenn et al., 1985; Collins, 1998;
Linker 2001). Still another problem is the collection of long-term, annual data
sets of sediment yield, which is needed because of the annual variability of
sediment production caused by temporary storage of sediment within and
beneath the subsole of the glacier and changes within the subglacial plumbing
(Ostrem, 1975; Hammer and Smith, 1983; Gurnell, 1987; Bezinge et al., 1989;
Hallet et al., 1996; Collins, 1998; Linker, 2001). Lastly, and probably the
greatest difﬁculty, is estimating the bedload component of the total sediment
output for the melt-water streams because of the 1) logistical problems
associated with monitoring bed load in remote, high energy, melt-water
streams and 2) because of the great disparity between published values of bed
load ranging from <1 % of the total clastic load at the Matanuska Glacier
(Pierce et al., 2003) to >75% of the total clastic load for glaciers in southeast
Alaska (Ostrem, 1975; Hammer and Smith, 1983; Hallet et al.,1996; Linker,
2001)

The objective of this research is to calculate the average minimum
effective erosion rate for the Matanuska Glacier during six consecutive ablation
seasons and to determine the fate of coarse material at the glacier bed by

analyzing the grain-size characteristics of the available sediment supply.

Methods of Bed Load Estimation

There have been only a few studies that attempt to deal with bed load
transport in glacial melt-water streams (Ostrem, 1975; Hammer and Smith,
1983; Gurnell, 1988; Hallet et al., 1996; Pierce et al., 2003). Ostrem (1975)
studied the suspended sediment yield from several Norwegian glaciers, but at
the Nigardsbreen Glacier, he estimated the bed load component by building a
steel mesh fence across a stream draining the glacier that trapped bed load.
After several weeks, he estimated the bed load to be about 25% of the total
load. Hammer and Smith (1983) also studied bed load in a melt-water stream
draining a small cirque glacier in northern Banff National Park, Alberta during
the melt season of 1977 and 1978. They measured bed load by using a steel
mesh fence extended across the melt-water stream that trapped bed load.
Periodically, they excavated and measured the accumulated bed load and
estimated bed load to be about 57% of the total load in 1977 and 54% in 1978.

At two small glaciers in the Swiss Alps, Gurnell et al., (1988) studied
bed load in melt-water streams draining Glacier de Tsijiore Nouve and Bas
Glacier d’ Arolla. They measured bed load from these melt-water streams
using accumulation traps in hydroelectric power adduction structures. For each
glacier, Gurnell et al., (1988) estimated that the bed load to be about 60% of
the total load.

At the Nigardbreen Glacier, Hallet et al., (1996) estimated bed load from
annual measurements of deltaic growth in a lake close to the terminus of the

glacier. He estimated bed load to be about 41% of the total load.

Other studies have also attempted to estimate bed load from
measurements done at other glaciers. For example, Collins (1998) referred to
ratios determined by Ostrem (1975) for the Nigarsbreen Glacier (75/25), by
Gurnell et al. (1988) for Glacier de Tsijiore Nouve (60/40), and for Bas Glacier
d’ Arolla (60/40) to estimate bed load in streams draining the Batura Glacier,
India. He estimated bed load in those streams to be about 50 % of the total
load.

However, Pierce et al., (2003) studied bed load from melt-water streams
discharging from vents along the margin of the Matanuska Glacier and found
bed load to be < 1% of the total load despite the availability of coarse material
at and near the glacier bed. They sampled bed load by using two Helly—Smith
hand-sampling devices of different sizes and sampled the same location

throughout the 2002 ablation season.

Study Area

The Matanuska Glacier is a large valley glacier that ﬂows north from ice
ﬁelds of the Chugach Mountains and terminates in the upper Matanuska Valley
in south-central, Alaska (61° 47’N, 147° 45’W) (Figure 1). The regional
bedrock of the basin consists of sedimentary and low to mid grade meta-
sedimentary rocks such as phyllites, slates, and argillites (Winkler, 1992). The
glacier is approximately 45 km long and ranges In width from approximately 2.5
km near the equilibrium line to about 5 km at the terminus and occupies

approximately 57% of an elongate drainage basin that covers an area of 665

 

 

 

 

Figure 1. Location map for the Matanuska
Glacier, Alaska (modiﬁed from Lawson, 1979).

km2 (Lawson et al., 1998; Strasser et al., 1996). The glacier ranges in
elevation from approximately 3500 m at the highest source on Mount Marcus
Baker to about 500m at the terminus (Lawson et al., 1998). For the last 200
years, the terminus has remained in a stable position (Williams and Ferrains,
1961 ), but unpublished data from Lawson suggests the glacier terminus has
retreated at a rate of approximately 10-30 m yr" over the past decade.

The glacial drainage system near the terminus consists of a complex
network of subglacial conduits that feed individual discharge vents that emerge
from the glacier margin (Lawson, 1993; Lawson et al., 1998). Several of these
vents occur along the southwestern edge of the terminus to form a shallow
perennial proglacial lake and are the major source for the headwaters of the
South Branch, Matanuska River (Figure 2). A second main discharge vent
emerges from the western edge of the terminus and is the Source for Little
River (Figure 2), which ﬂows west approximately 500m and joins the South
Branch, Matanuska River. A third large vent emerges from the northern edge
of the terminus and is the primary source of water for the North Branch of the
Matanuska River (Figure 2). There are several minor vents occurring on the
north-northeastern edge of the glacier but the discharges are <1 % of the total

discharge and are insigniﬁcant (Denner, personal communication).

 

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7

Methodology

Located on each of the three main meltwater streams (South Branch,
North Branch, Little River) draining the Matanuska Glacier are a suspended
sediment sampling (SSS) station and a gauging station. At South Branch, the
SSS station is located approximately 200m from the western terminus of the
glacier and is adjacent to the gauging station (Figure 2). The SSS station at
North Branch is located approximately 25 m upstream from the gauging station
at the large vent that forms the headwaters for the North Branch stream (Figure
2). At Little River the SSS station is located approximately 150m from the edge
of the glacier terminus and adjacent to the gauging station (Figure 2).
Discharge Measurements

During the ablation seasons of 1995-2001, the US. Army Cold Regions
Research and Engineering Laboratory (CRREL) maintained the gauging
station on the South Branch and the North Branch, Matanuska River and
generated stage/discharge rating curves for both streams. CRREL also
maintained the Little River gauging station during the ablations seasons of
2000-2001 and generated a rating curve for that stream. At each station is a
datalogger and a nitrogen gas bubbler system, which continuously collects
stage data throughout most of the year and allows for hourly calculation of
discharge through the entire melt season with a measurement accuracy of

approximately 93% (Linker, 2001: Denner, personal communication).

Suspended Sediment

Suspended sediment samples of 1000 ml were obtained at two-hour
intervals at each of the SSS stations, using an automated water sampler (ISCO
3700). At SSS station at South Branch, samples were continuously collected
from early June through late August, 1997-2000. At the SSS station along
North Branch, samples were continuously collected from late July through late
August, 1999, and early June through late August, 2000-20001. Little River
was not sampled during 1997-1999 melt seasons because it was not a
signiﬁcant meltwater stream but became a signiﬁcant melt-water stream from
early June though late August, 2000-20001, samples were collected
continuously (Denner, personnel communication).

Each ISCO water sampler was equipped with an intake connected to
approximately 15 m of 1.5 cm diameter latex tubing. The intake was anchored
10-30 cm above the channel bed or in the case of North Branch, within the
opening of the large vent. Each sample was processed by vacuum ﬁltration
through individually pre-weighed numbered 90mm borosilicate microﬁber ﬁlter
circles. Sediment ﬁlled ﬁlter circles were then dried in an oven at 110°C and
gravimetrically weighed to a precision of 1 1 mg. Suspended sediment
concentrations in terms of g/L (equivalent to kg/m3) were calculated by dividing
the mass of the dried samples by the volume of the water sample.
Concentrations of suspended sediment recorded at South Branch, North

Branch, and Bridge Stream ranged from about .3g/L to 329/L.

Potential Sediment Sources

Sediment supplied to the three main melt-water streams draining the
Matanuska Glacier is derived from four principal sources: 1) subglacial
sediment, 2) debris within basal ice, 3) superglacial debris, and 4) proglacial
sediment. Each source has a characteristic grainsize distribution.

Subglacial Sediment

Subglacial sediment is composed mainly of melt-out till, some
lodgement till, and some interstratiﬁed material composed of debris ﬂows, and
ﬂuvial lacustrine sediments that have been overridden by ice (Lawson, 1979;
Lawson, 1982). Melt-out till is locally exposed along the terminus of the
glacier, generally below debris ﬂow deposits (Figure 3). Where as lodgement
till is exposed sporadically along the western part of the terminus along active
shear planes associated with the override of stagnant ice (Figure 4) (Lawson,
1979). lnterstratiﬁed debris ﬂows and ﬂuvial and lacustrine sediments are
visible along the western part of the terminus where ice is overriding what
appears to be a former moraine (Figure 5).

Lawson (1981) characterized three types of melt-out till at the
Matanusaka Glacier, which vary from 1) structureless pebbly silts to 2) pebbly
sandy silt containing disturbed, discontinuous laminae to 3) lenses of sorted
and stratiﬁed sediment of variable grain size or composition. In melt-out till,
subangular to rounded clasts of pebble to cobbles are randomly dispersed in a
loose matrix. Depostis of melt-out till occur as sheets to discontinuous sheets

with a variable thickness of one to three meters (Lawson, 1979). Lawson

10

 

Figure 3. Melt-out till exposure mproximately 0.5km from the
terminus. Pointing to contact between buried basal ice on bottom
and melt-out till on top.

 

Figure 4. Lodgement Till exposure under an ice ramp along the
active terminus. The pick of the ice axe (1m) is resting on the ramp of
ice and the sediment hanging down are till curls. The red arrow is
pointing to the lodgement till.

6358.9 0286 05 9.6.6 0599: 563.63 cm Co 053qu .m 239”.

 

13

(1979) also characterized Iodgment till at the Matanuska Glacier as a mixture
of gravel-sand-silt with mostly subrounded clasts of pebble to cobbles
randomly dispersed to clustered in a dense matrix. Deposits of lodgement till
generally occur as discontinuous pockets or sheets (Lawson, 1979).
Observations beneath ramps of overriding ice near the Matanuska Glacier
terminus during the winter of 98 Show deposits of lodgement till up to 10-200m
thick. No information, however, exists about the thickness of lodgement till
beneath active ice further up glacier.
Basal Ice

Basal ice is exposed along the active western terminus and occurs as a
discontinual layer ranging in thickness from about 0.2 to 15 meters (Lawson,
1979). It is stratiﬁed, with debris distributed in alternating .01 to 1.7m thick
layers of debris rich and debris poor ice (Figure 6)(Lawson, 1979; 1981 ).
Sediment within the debris rich layer consists of clay to sand with sedimentary
structures occasionally preserved (Lawson, 1979; 1981). Isolated angular to
rounded clasts ranging from gravel to cobbles also occur randomly within the
basal ice (Lawson, 1979).
Supraglacial Debn’s

Supraglacial debris is exposed all along the glacier margin but mainly on
the eastern part of the terminus where it is associated with a medial moraine
and ranges in thickness from <1cm to > 3 meters (Lawson, 1979; 1981).
Along the western part of the terminus it consists as isolated cobbles and

boulders (Figure 2). It is composed chieﬂy of very angular to angular, sandy

14

 

Figure 6. Exposure of basal ice along the active terminus. The ﬁeld
notebook is sitting at the base of stratiﬁed basal ice and the red
arrow is pointing to the contact between englacial ice and basal ice.

gravel and more than half is composed of cobbles and boulders (Lawson,
1979)
Proglacial Sediment

Proglacial sediment is exposed mainly in an ice-cored zone along the
western terminus of the glacier and is composed mainly of subaerial gravity
ﬂow deposits and some interstratiﬁed material including melt-out till, spall,
ﬂuvial and lacustrine sediments (Lawson, 1979; 1982). It occurs mainly in an
ice-cored zone along the western terminus of the glacier (Figure 2) (Lawson,
1982). Lawson (1979; 1981; and 1982) has classiﬁed the subaerial ﬂow
deposits into four types, primarily as a function of water content. These ﬂows
range in thickness from .01 to approximately 2 meters (Lawson, 1979; 1982).
Type I and Type II ﬂows are matrix dominated with subangular to rounded
cobble and boulder clasts dispersed in a matrix of pebbly sandy silt (Lawson,
1979). Type III ﬂows are matrix to clast dominated with subangular to rounded
cobble and boulder clasts in a matrix of gravelly sand to sandy silt (Lawson,
1979). Type IV ﬂows are relatively ﬁne grained with a homogenous texture of
sandy silt or a coarse silty sand with coarse subangular to rounded clasts in

traction at the bed of the ﬂow (Lawson, 1979).

Grain-Size Analysis of Suspended Sediment and Potential Sediment
Sources
Samples of supraglacial debris (n=10) were collected for grain-size

analysis from debris-covered ice along the western margin. Also, samples of

16

subglacial debris (n=10) were collected from an exposure located along the
active western terminus of the glacier, and samples of basal-ice debris (n=10)
and melt-out till (n=10) were collected from an exposure of buried, stagnant
basal-ice approximately 0.5 km from the western terminus. Samples of
lodgement till (n=10) were collected from an exposure located along the active
western terminus. Proglacial lacustrine sediment (n=10) was sampled along a
transect in front of the western terminus and suspended sediment was
sampled from a discharge vent (Mega Vent) at the western terminus and also
from the Matanuska River at the South Branch S.S.S. station. All samples
were analyzed by dry sieving at 1/2 phi intervals for the grain-size range of -2.0
to 4.0 phi (4mm to .063mm). A Micromeritics Sedigraph 5100 was used for the
grain-size range of 4.25 to 10 phi (.053mm to .001 mm).

The grain-size ternary diagram presented in ﬁgure 7 Show that
supraglacial, subglacial, and basal-ice debris, melt-out till, lacustrine sediment,
and suspended sediment has different textural characteristics. Samples of
supraglacial debris plot mainly in the gravel and sand region of the ternary
diagram and have an average gravel, sand, silt/clay content of approximately
55%, 36%, and 9%. The samples of subglacial debris plot mainly in the middle
region and have an average gravel, sand, silt/clay content of approximately
29%, 40%, and 31%. Basal-ice debris and melt-out till samples plot between
the silt/clay and sand region and have an average gravel, sand, silt/clay

content of 11%, 38%, and 51% and 11%, 35%, and 54%, respectively.

17

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Debris in stagnant basal ice Meltout till associated with
associated with meltout till stagnant basal ice n=10.
n=10. Mean-O Mean-

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A I; A A A A A A A

Overridden morainal sedimem Lacustrine sedimerd n=10.

(debris ﬂow deposits?) n=10. Mean-L.

Mean-

Gravel>2mrn Gravel >2mm

‘sﬁﬁcAAAAAAsz‘ "AG‘AAAAAAT

Suspended sediment from Average grain-size

Matanuska River discharge distribtution of suspended

(SGS) n=10. Mean- sediment and sediment

sources.

Figure 7. Ternary plots for the average grain-size distribution of
suspended sediment and potential sediment sources.

Lodgement till samples plot between the silt/clay and sand region and have an
average gravel, sand, and silt/clay content of approximately 24%, 21%, and
55%. Samples of lacustrine sediment plot mainly in the silt/clay region and
have an average gravel, sand, silt/clay content of approximately 3%, 27%, and
70%. Samples of suspended sediment from the discharge vent and the
Matanuska River plot mainly in the extreme silt/clay region. The samples are
generally void of any gravel and sand and have an average silt/clay content of
approximately 99%.

The average cumulative-percent curves of grain-size for supraglacial,
subglacial, and basal-ice debris, melt-out till, lacustrine sediment, and
suspended sediment presented in ﬁgure 8 also show that the samples have
different textural characteristics distinct. The curve for supraglacial debris
shows a grain-size distribution similar to that described by Lawson (1979) for
supraglacial debris at the Matanuska Glacier which he attributes to material
weathered from valley walls and nunataks and mass transported onto the
glacier surface. The curve for subglacial debris is within the range of curves
published by Lawson (1979) for recent debris-ﬂow deposits at the Matanuska
Glacier. The similarity suggests that the subglacial sediment originated as an
assemblage of debris-ﬂow deposits and was subsequently overridden by
advancing ice. The curve for basal-ice debris is also comparable to curves
published by Lawson' (1979) for basal-ice debris from the Matanuska Glacier.
Strasser et al., (1996), Alley et al., (1998), Lawson et al., (1998), Evenson et

al., (1998) attribute the origin of the basal-ice to "glaciohydraulic supercooling",

19

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20

a mechanism creating debris rich basal-ice by entraining sediment during ice
growth and accretion process at the basal zone of the glacier. The curve for
melt-out till is similar to curve for basal-ice debris. This is to be expected since
the melt-out till is derived from the in Situ melting of buried basal ice (Boulton,
71; Lawson, 81). The curve for lacustrine sediment shows a lack of coarse
sediment. The two curves for the suspended sediment Show that the
suspended sediment from the discharge vent is texturally equivalent to

suspended sediment in the Matanuska River.

Discharge Characteristics

The discharge recorded for South Branch S.S.S. station during the
1997-2000 ablation seasons (Linker 2001) and during the 2001-2002 ablation
seasons (this study) is presented in ﬁgure 9a., b., c. In general, the record
shows increasing discharge between early to mid June, peak discharge
occurring between mid to late July or early August, and decreasing discharge
throughout the remainder of the melt season (Figure 9 a., b., c). Total
discharge recorded for the period June through August 1997-2002 is
approximately 6.54E+08 m3, 5.06E+08 m3, 6.42E+08 m3, 4.39E+08 m3,
5.01 E+08m3 and 4.97E+08m3.

The record Shows substantial variability in daily discharge that can be
attributed to rainfall events (Linker, 2001), air temperature changes that effect
rate of ice melt (Collins, 1998), rapid changes in ice-surface albedo due to

snow fall (Lawson,1993) and periods of sunny cloudless weather where melt

21

 

 

 

 
 
   

 
  
    

 

 

 

1997 -—— Discharge
Sediment
I.2E+08 I L 1.2E+07
L
>, 8.0E+07 ~ » 8.0E+06
(U
'2
O) 4.0E+07 « l 4.0E+06
x \ A
-l-' " AI
m 0.0E+m T T T ‘T T T T T T T T 0.0E+m
E 04-Jun 18-Jun 02qu 16-Jul 30qu 13-Aug
‘O
:93
“U 1.60E+08 — 1998 —— Discharge I 1.6E+07
q) -- Suspended
'0 Sediment
c 1.20E+08 - — 1.2E+07
(D
Q.
(I)
3 8.00E+07 « L 8.0E+06
:| 531
4.00E 07 . . .
+ {I 1. /L "k 40E+06
M)! V m
..1"
o.mE+oo l T T T T 1 T T T l T 0.0E+00

 

04-Jun 18-Jun 02-Jul 16—Jul 30-Jul 13-Aug

Date

Figure 9. Plots showing suspended sediment and discharge
recorded at South Branch, Matanuska River during the 1997 and
1998 ablation seasons.

22

Discharge m3/day

1999 — Discharge

 

 

 

 

 

1.6E+08 - — 1.6E+07
— Suspended
Sediment

1.2E+08 — - 1.2E+O7
>4 8.0E+07 — - 8.0E+06
(D
g)

4.0E+07 - - 4.0E+%
x
q—a
C
0) 0.0900 I I . . . I I . I I I 0.0900
E 04-Jun 18-Jun O2-Jul 16—Jul ISO-Jul 13—Aug
"C
3
.0 1.6E+08 — I 1.6E+07
q) 2000 — Discharge
'C — Suspended
0:) 1.2E+08 1 Sediment - 1.2907
0. . _
(I)
(3 8.0907 - — 8.0906

4.0907 - . 4.0E+06
0.0900 -—’>:‘;\// I I I 0.0900

 

Mun 18~Jun 02-Jul 16—Jul 30alul 13-Aug

Figure 9 (continued). Plots showing suspended sediment and
discharge recorded at South Branch, Matanuska River during the
1999 and 2000 ablation seasons.

Discharge m3/day

1 .6E+08 I

1.2E+08 4

8.0E+O7 4

4.0E+07 *

 

2001 -— Discharge

—— Suspended
Sediment

 

r 1.6E+07

I 1.2E+07

— 8.0E+06

- 4.0E+06

0.0E+OO

 

’ 0.0E+OO

1.60E+08 -

1 2051-08 a

8.00E+07 .

Suspended Sediment kg/day

4.00E+07 ~

T

2002

 

T l T T l j T T T

O4-Jun 18-Jun 02-Jul 16-Jul 30-JuI 13-Aug

— Discharge
-— Suspended
Sediment

 

“2.437%??ka

— 1.60E-I-07
F 12OE+07
- 8.00E+O6

- 4.00E+06

 

0.00E+OO

 

0.00E+OO

T

T T T T

04-Jun 18-Jun 02-Jul 16—Jul 30-Jul 13-Aug

T

Figure 9 (continued). Plots showing suspended sediment and
discharge recorded at South Branch, Matanuska River during the
2001 and 2002 ablation seasons.

24

Discharge m3/day

from solar radiation is excessive. For example, Linker (2001) attributes peak
discharges recorded 1 and 8 August and 12 and 20 August, 1999 to rainfall
events. On the other hand, the high discharge recorded 24 June to 3 July,

2001 can be attributed to a period of high solar radiation receipts.

Suspended-Sediment Flux Characteristics

The suspended-sediment ﬂux recorded for South Branch S.S.S. station
during the 1997-2000 ablation seasons (Linker, 2001) and during the 2001-
2002 ablation (this study) is also shown in ﬁgure 9a., b., c. The daily-
suspended- sediment ﬂux was calculated as a product of the measured
suspended-sediment concentrations and the average discharge measured at
the station summed over a 24-hour period (Linker, 2001). Total suspended-
sediment concentration for the period June through August for 1997-2002 is
approximately 9.82E+08 kg, 1.47E+09kg, 1.55E+09kg, 6.96E+08kg,
8.62E+08kg, and 1.04E+09kg.

The record in ﬁgure 9a, b., c. also shows substantial variability in daily
suspended-sediment ﬂux. This variation can be attributed to meltwater
outbursts that transport large volumes of suspended sediment and the
migration of channels that erode unworked sources of sediment stored at the
ice-bed interface (Gurnell, 1987). The variability can also be attributed to
increasing discharge from rainfall events, which transport additional suspended
sediment from inwash into the glacier basin, and due to increasing melt from

high solar radiation receipts that increase subsequent erosion by subglacial

25

streams. For example, Linker (2001) attributes peak suspended sediment ﬂux
recorded 1 and 8 August and 12 and 20 August, 1999 to rainfall events. On
the other hand, the high suspended sediment recorded 24 June to 3 July, 2001

can be attributed to increase melt due to high solar radiation.

Minimum Effective Erosion Rate

The total suspended-sediment ﬂux shown in ﬁgure 10 includes data
from all three melt-water streams draining the Matanuska Glacier. According
to Linker (2001), South Branch contributed approximately 96% of the total
sediment ﬂux during the 1997, 1998, and 1999 ablation seasons, whereas
North Branch contributed approximately 4% of the total sediment ﬂux for those
years. He also found South Branch contributed approximately 95% of the total
sediment ﬂux during the 2000 ablation season, whereas North Branch
contributed approximately 3.6% of the total sediment ﬂux and Little River which
developed that year, only about 1.4% of the total sediment ﬂux for that year. In
this study, it was found that South Branch contributed approximately 93% of
the total sediment ﬂux during the 2001 and 2002 ablation seasons while North
Branch contributed approximately 5% of the total sediment ﬂux and Little River
only about 2% of the total sediment ﬂux during those years.

In this study, the annual, minimum effective erosion rate is deﬁned as
the total annual suspended-sediment ﬂux recorded in the three melt-water
streams draining the glacier (difference between what is being eroded and

deposited at the bed of the glacier) and assumes no change in sediment

26

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27

storage within the ice. It is also assumed that input of sediment by subaerial
erosion from nonglacierized areas within the basin is minimal and does not
signiﬁcantly contribute to the total sediment ﬂux. A third assumption is that the
annual sediment ﬂux recorded in the melt-water streams during the ablation
season approximates the total annual sediment ﬂux from the glacier, or at least
the minimum, total annual sediment ﬂux.

Annual, minimum effective erosion rates for the Matanuska Glacier were
calculated for the period of 1997-2002 by dividing the minimum total annual
suspended-sediment ﬂux by the total area of the glacierized basin,
approximately 380 km2, and subsequently dividing by an estimated bedrock
density of 2.65 g/cm3. The calculated annual, minimum effective erosion rates
are presented in ﬁgure 11 and show rates ranging from 0.75mmyr'1 for 2000
ablation season to 1.56 mmyr’1 for 1999 ablation season, or an average
minimum effective erosion rate of approximately 1.13 mm yr’1 over the six year

penod.

Discussion
Annual Suspended Sediment Flux vs. Discharge

A plot of suspended-sediment ﬂux vs. discharge at South Branch for the
1997-2002 ablation seasons is presented in ﬁgure 12a and shows an unstable
relationship between the two parameters. For example, the highest recorded
suspended-sediment ﬂux occurred in the 1999 ablation season, whereas the

highest recorded discharge occurred in the 1997 ablation season. On the

28

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29

8.8. t km2 W1

Sus ended Sediment vs. Discharge for

  

 

 

 

 

 

 

tanuska Glacier, 1997-2002 A,

“'1
‘L m: 1999 Where: Alaska
> 3500
at 3000 Area: 380 km2
35 ’3‘ 1997 Mean s.s. Yield:
05 1500 2000 2885 tkmi’yl‘1
(0’ “ID-l

500

0350 II» 45:0 550 550 sec ago 750
Q x 106 m3 yl'1
Suspended Sediment vs. Discharge for

m - Nigardsbreen 1987-1993 B.

350 ~ Where: Norway

32:: Area: 48 km2

zoo ~ Mean 8.8. Yield

1'50 ' 211 tkm2yr1

1m ‘

so, 1993

O I I I I I
0 50 100 150 200 250
Q x 106 m3 yr-1

Figure 12. Plots of annual suspended sediment vs. discharge. (A)
Plot of Matanuska Glacier 1997-2002. (B) Plot of Nigardsbreen
Glacier, Norway, 1987-1993.

30

8.8. t km2 yr'1

8.8. tonnes 103

Suspended Sediment vs. Discharge for

 

 

 

 

 

Engabreen 1987-1993 C,
800 . 1991
700- Wlere: Norway
‘ 1990 Area: 36 km2

400- 1992 Mean s.s. Yield:

2°° ‘ 1993

100 <

O I I I I I I I I I
0 25 50 75 100 125 150 175 200 225
Q x 106 m3 yr1
Suspended Sediment vs. Discharge for Gomera,
1983-1988 D.

1G) 1
140 l 198 1987 Where: Switzerland
mi 5 Area: 82 km?
100 1 1984 1986

801 \ Mean 38. Yleld:

801 1985 1988 1O4tonnes103

40 .

20 1

0 I I I
75 100 125 150

Q x106 rn3 yl'1

Figure 12 (continued). Plots of annual suspended sediment vs.
discharge. (C) Plot of Engabreen Glacier, Norway 1987-1993. (8)
Plot of Gomera Glacier, Switzerland, 19823-1988.

31

other hand, the lowest suspended-sediment ﬂux and lowest melt-water
discharge recorded occurred in the 2000 ablation season. The plot also shows
that during the 1997-1999 ablation seasons, recorded annual suspended-
sediment fluxes and melt-water discharges were relatively high, whereas that
during the 2000-2002 ablation seasons, recorded annual suspended-sediment
ﬂuxes and melt-water discharges were relatively low. When compared to
records of annual suspended sediment ﬂux vs. discharge recorded for other
glaciers of varying sizes from around the world (Collins, 1990; Bogen,
1996)(ﬁgure 12, b,c,d), the instability relationship observed at the Matanuska
Glacier is not unusual and can be attributed to several factors: 1) seasonal
development of the subglacial drainage system (Collins, 1989; 1998), 2) the
scale and occurrence of seasonal discharge and suspended-sediment ﬂux
pulses (Collins, 1989; 1998), and 3) history of hydrological events (Collins,
1989) that all coincide with each other.

Rapid changes in suspended sediment ﬂux can provide an indirect view
into the development of the subglacial drainage system (Collins, 1998). For
example, an increase in melt-water discharge can result in subglacial stream
migration allowing water to invade unworked pockets of sediment stored at the
ice-bedrock interface and provide a short lived supply of suspended sediment
(Collins, 1998; 1991). At the Matanuska Glacier, the 1998 and 1999 ablation
seasons have the highest recorded pulses of suspended sediment whereas the
2000, 2001, and 2002 ablation seasons have the smallest recorded pulses of

suspended sediment. The scale and occurrence of the sediment pulses

32

during the 1998 and 1999 ablation seasons could have evacuated most of the
stored sediment leaving small amounts for the 2000, 2001, and 2002 ablation
seasons to erode. According to Collins (1998), Hallet et al., (1996), and Collins
(1991) the evacuation of suspended sediment into the suglacial drainage
system is variably with time and depends on the erodibility of the bedrock and
the amount of sediment stored at the ice-bedrock interface. The variation at
the Matanuska suggests that the rate of supply and rate of sediment removal
from storage are not equal over a period of six years and might explain the
instability relationship between annual suspended-sediment ﬂux vs. discharge.
Along with seasonal changes, scale, and occurrence of the discharge
and sediment pulses, the hydrologic history may also explain the instability
between annual suspended-sediment ﬂux vs. discharge. Prior to 1997, a 100-
year ﬂood event occurred in September 1995 (Denner, et al., 1999) and could
have ﬂushed considerable sediment from the system, explaining the small
amount of suspended sediment during 1997, which was the year for highest

recorded discharge.

Comparison of Glacier Erosion Rates

The average annual minimum effective erosion rate of 1.13 mm yr'1
calculated for the Matanuska Glacier is intermediate to rates recorded for other
glaciers around the world (Table 1). Compared to the Nigardsbreen Glacier,
Norway, it is much greater, probably because the Matanuska Glacier 1) ﬂows

over meta-sedimentary bedrock that is weaker than the metamorphic bedrock

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34

underlying the Nigardsbreen Glacier (Ostrem, 1975), 2) is located in a
tectonically more active area (Hallet et al., 1996), and 3) possibly has a higher
glacier mass balance. Compared to the Batura Glacier, Pakistan, however it is
subsequently less, probably because the Matanuska Glacier 1) lies in a
tectonically less active region than the Batura Glacier and 2) has a lower
glacier mass balance than the Batura Glacier (Collins, 1998). In addition,
compared to glaciers in southeastern Alaska, the average annual minimum
effective erosion rate for the Matanuska Glacier is small, probably because it
has a lower glacier mass balance, overrides harder bedrock, and lies in a

region that is tectonically less active (Hunter et al., 1996).

Role of an Overdeepening in Sediment Transport

Within 1/2 km of the terminus of the Matanuska Glacier lies a head of
outwash that contains coarse sediment ranging from large pebbles to boulders.
It indicates that sometime in the recent past the glacier terminus extended
further north and melt-water emerging from the terminus was transporting
considerable bed load. This is in contradiction to Pierce et al., ‘s (2003)
observation that bed load currently in streams draining the Matanuska Glacier
is <1 % of the total clastic load and is strongly supply limited. A mechanism
proposed by Alley et al., (in review) that might account for the apparent
decrease in stream bed load is presented in ﬁgure 14 and involves two

geometric conditions occurring just inside the ice margin.

35

Condition A): The ice-bedrock adverse slope is s 20%

of ice-surface slope. The glacier is at or below the
“supercooling threshold” and has a channelized drainage
system that is under capacity which leads to erosion

at the glacier bed.

Condition B): The ice-bedrock adverse slope is 20-70%
steeper than the ice-surface slope. The glacier is

above the “supercooling threshold”, which leads to

ice growth in channels causing them to decrease in

size and close off. The channels evolve into a distributed
subglacial drainage system, transport capacity drops off,
and leads to deposition of course material at the glacier

bed.

Today, the geometric condition of the Matanuska Glacier margin is

similar to condition B. Recent geophysical data show that located just inside

the margin is a signiﬁcant overdeepening (Alley et al., 1999; Lawson et al.,

1998; Baker, unpublished data). Observational evidence also shows that

supercooling is occurring (Evenson et a., 1999; Lawson et al., 1999;

Ensminger et al., 2002) and that the subglacial drainage system consists of a

complex network of conduits that emerge at the ice margin as vents that turn

on and off throughout the ablation season (Lawson et al., 1993,1998).

36

 

 
   

Air
Melt
water Glacier

  
    
  

—>

Bedrock

 

Sediment

 

 

 

Condition A.

 

 

 

 

 

 

Condition B

Figure 13: Overdeepening model (modiﬁed from Alley et al., in review),
Mechanism for trapping coarse meterial at the ice-bedrock interface.
Condition A. Glacier lacking supercooling. Condition B. Glacier far into the
supercooling phase.

If it is assumed that debris within basal ice and morainal sediment
represents the main debris reservoir within the glacier and that it supplies most
of the melt-water stream load, it may be possible to estimate the amount of
coarse material being retained by the glacier in an overdeepening relative to
what is leaving the glacier in the form of suspended load. For example, grain-
size analysis of debris in basal ice and morainal sediment show that 57 to 72%
of the debris has a grain-size diameter of > 0 phi and is capable of being
placed in suspension by subglacial melt-water streams. The remaining 18 to
43% of the debris has a diameter of s O phi and, on the other hand, is not
capable of being placed in suspension and is subject to being trapped in an
overdeepening. The O phi cutoff between debris in suspension and debris not
in suspension was used because grain-size analysis of suspended sediment
data show that the largest particle in suspension throughout the ablation
season is 0.5 phi. Given the average yearly amount of suspended load leaving
the glacier is 1.14E+09kg for the period of 1997 to 2002, it is logical to
conclude that the average amount of coarse material being trapped is on the
order of 2.85E+08 kg to 8.61E+08 kg for the same period using the Flux of bed
load equation.

FBL = Fss *(M _>. (DI M s (D) eq.(1)
Where FBL is the ﬂux of bed load or the amount of material being trapped, the
F33 is the ﬂux of suspended sediment or average yearly amount of suspended

load leaving the glacier, and (M 2 (DI M s (D) is the ratio between debris in

38

suspension and debris not in suspension. This amount of coarse material
trapped is applied to a Rate of Accretion equation

RA = FBL/ (1-n)A eq. (2)
where RA is the rate of accretion or thickness of coarse material trapped over a
given area, FBI is ﬂux of bed load or amount of coarse material trapped, 1-n is
the porosity, and A is the area. Using the assumed porosity for gravel of 30%(
Fetter, 2001), the amount of coarse material trapped could deposit a layer of
2.9 to 9.3 cm a year over an area of 5km2 on the ice-bedrock adverse slope.

If the grain size cutoff between debris in suspension and debris not in
suspension is shifted to include debris s 3.5 phi because the grain-size
analysis of suspended sediment from the discharge vent show that < 1% of
sands is coming out of the vent, then the coarse material trapped may also
include ﬁne sands with the gravel. When the cutoff is shifted, the grain-size
analysis of debris within basal ice and morainal sediment show that 30 to 51%
of the debris has a grain-size analysis > 3.5 phi and is capable of being placed
in suspension by subglacial melt-water streams. The remaining 49 to 70% of
the debris has a diameter of s 3.5 phi and is not capable of being placed in
suspension is subject to being trapped in an overdeepening. Given the same
variables for the same equations and period of time, it is logical to conclude
that the average amount of coarse material being trapped is on the order of
1.10E+09 kg to 2.66E+O9 kg. This amount of coarse material trapped, which
assumes a porosity of 20% (Fetter, 2001), could deposit a layer 10.3 to 25.1

cm a year over an area of 5km2 on the ice-bedrock adverse slope.

39

The thickness of the coarse trapped material includes major
assumptions. The ﬁrst is that coarse material is being accreted as a thin sheet
or layer along the entire adverse slope of the glacier. How the coarse material
might be distributed however would depend on the geometry of the adverse
slope(s) and the type and pattern of the subglacial system. The second is that
the area in which coarse material is being accreted is limited to 5km2.

The third assumption is that the debris within basal ice and morainal
sediment represent the main debris reservoir within the glacier. Compared to
the supraglacial debris, the debris within basal ice and morainal sediment have
the higher amount of sediment > O phi. The supraglacial debris does not have
enough ﬁnes to supply the amount of suspended sediment coming out of the
glacier. Also, the debris within basal ice and morainal sediment are of
subglacial origin whereas the melt-out till and lacustrine sediment are located
in the proglacial environment. Lodgement till is of subglacial origin but was not
used because it fell within the grain-size range of debris within basal ice and
morainal sediment. The exact composition of the material being trapped is
unknown and further studies are needed to quantify the exact amount being

retained by the overdeepening.

Conclusion
Knowledge of glacier erosion rates have become increasingly important
in understanding the tectonic evolution of glaciated mountain belts (Alley et al.,

in review; Collins, 1998; Hallet et al., 1996). The Matanuska Glacier average,

40

annual, minimum effective erosion rate of 1.13 mmyr’1 for the period of 1997-
2002 is intermediate to other rates recorded for glaciers around the world
because of the amount of tectonic activity in the area, the amount of mass
balance, and the erodibility of the bedrock. For the same period, the average,
annual total sediment ﬂux is 1 .14E+09 kg. The amount of coarse material
trapped due to the overdeepending is on the order of 2.85E+08 kg to 8.61E+08
kg for the same period. Finally, the fate of this coarse material could deposit a
layer of 2.9 to 9.3 cm a year over an area of 5km2 on the ice-bedrock adverse

slope.

41

REFERENCES CITED

Alley, R.B., Lawson, D.E., Evenson, E.B., Strasser J.C., and Larson, G.J. (1998)
Glaciohydraulic supercooling: A freeze-on mechanishm to create stratiﬁed,
debris-righ basal ice. 2. Theory, Journal of Glaciology, 44 563-569

Alley, R.B., Lawson, D.E., Evenson, E.B., Larson, G.J. (1999) Some glaciological and
geological implications of basal-ice accretion in overdeepening. ln (eds
Michelson, OM. and Attig, J.W.) Glacial Processes Past and Present,
Geological Society of America Special Paper 337, 1-9.

Alley R.B., Lawson, D.E., Evenson, E.B., Larson, G.J., Baker, G.(in review).
Graded Glaciers.

Andrews, J.T., Milliman, J.D., Jennings, A.E., Rynes, N.. and Dwyer, J. (1994)
Sediment thicknesses and Holocene glacial marine sedimentation rates in
three east Greenland fjords. Journal of Geology. 102: 669-683.

Bezinge, A. (1987) Glacial melt-water stream, hydrology, and sediment transport the
Case of the Grande Dixen hydroelectricity scheme. In: A.M. Gurnell and M.J.
Clark eds. Glade-ﬂuvial Sediment Transfer. \Nlley, Chichester, 473-494.

Bezinge, A., Clark, M.J., Gurnell, A.M., Warburton, J.(1989) The management of
sediment transported by glacial melt-water streams and its signiﬁcance for the
estimation of sediment yield. Annals of Glaciology, 13: 1-5.

Bogen, J. (1989) Glacial sediment production and development of hydro-electric
power in glacierised areas. Annals of Glaciology. 13: 6-11.

Bogen, J. (1996) Erosion rates and sediment yields of glaciers. Annals of Glaciology,
22: 48-52.

Boulton, 6.8., (1971) Tlll genesis and fabric in Svalbaeb, Spitsbergen. In Goldthwait
R.P. (ed) “ﬁll: a symposium. Columbus, OH, Ohio State University Press. 742

Collins, D. (1989) Seasonal Development of Subglacial Drainage and Suspended
Sediment Delivery to Melt Waters Beneath an Alpine Glacier. Annals of
Glaciology, 13: 45-50.

Collins, 0. (1990) Seasonal and annual variations of suspended sediment transport
in meltwaters draining from an Alpine glacier. lAHS Publicelions, 193: 439-446.

Collins, D. (1996) A conceptual based model of the interaction between ﬂowing
meltwater and subglacial sediment Annals of Glaciology, 22: 224-232.

Collins, D. (1998) Suspended Sediment Flux in Meltwaters Draining from Batura

Glacier as an Indicator of the Rate of Glacial Erosion in the Karakoram
Mountains. Quatemery Proceedings, 6: 1-10.

42

Denner, J.C, Lawson, D.E., Larson, G.J., Evenson, E.B., Alley, R.B., Strasser, J.C.,
Kopcznski, S., (1999) Seasonal variability in hydrologic system response to
intense rain events, Matanuska Glacier, Alaska. Annals of Glaciology 28.

Evenson, E.B., Lawson, D.E., Strasser, J.C., Larson, G.J., Alley, R.B., Ensminger,
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