95

Variation in Suspended Sediment Concentration ofSupraglacial Lakes on Debris-covered Area of
the Lirung Glacier in the Nepal Himalayas
Nozomu TAKEUCHI1*, Akiko SAKAI2, Shiro KOHSHIMA3,
Koji FUJITA2 and Masayoshi NAKAWO4
1

Department of Earth Sciences, Graduate School of Science, Chiba University,
1-33, Yayoicho, Inage-ku, Chiba-city, Chiba 263-8522, Japan
2
Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan3
Wildlife Research Center, Kyoto University, Kyoto, Japan
4
National Institutes for the Humanities, Tokyo, Japan*e-mail: ntakeuch@faculty.chiba-u.jp

Abstract
Spatial and temporal variations in the suspended sediment concentration (SSC) of supraglacial lakes indebris-covered areas were investigated on the Lirung Glacier in the Langtang region of central Nepal from
May to Octorber 1996. Twenty-eight lakes of various sizes were observed on the glacier. SSC varied widely
among the lakes from 0 to 364 mg l-1. Monthly observations showed that SSC of lakes also varied temporally,
but there was no common trend in seasonality among the lakes. Measurement of the settling rate of suspended
sediment in lake water indicated that SSC rapidly decreased from 37 mg l-1 to 4 mg l-1 within a day. Our results
suggest that in lakes where SSC is high it is maintained mainly by a supply of high SSC meltwater to the lakes
and that the low SSC of other lakes is due to little meltwater inflow to the lakes. Variations in SSC amonglakes probably result from heterogeneous distribution of debris thickness and ice cliffs that affect the meltingof glacial ice around the lakes. Seasonal changes in SSC are probably due to changing meteorologicalconditions and/or debris cover and ice cliffs around the lakes. The availability of lake turbidity would enable
quicker and safer determination of the meltwater supply of lakes via remote-sensing techniques, and may be
useful for elucidating the status of supraglacial lakes and glacier melting on debris-covered glaciers.
Key words: debris-covered glacier, Himalayas, supraglacial lake, suspended sediment concentration,
turbidity

1. Introduction
Debris-covered glaciers with rock debris covering theablation area are common in the Himalayan region (Fujii& Higuchi, 1977). Debris areas usually have complex
surface morphologies including supraglacial lakes,
debris-covered cones, large hollows, ice cliffs andstreams (Iwata et al., 1980). Since the thick debris coversuppresses surface melting of glaciers, supraglacial lakes
and ice cliffs play an important role in the complicatedmelting processes of debris-covered areas on this type ofglacier (e.g., Inoue & Yoshida, 1980; Sakai et al., 2000).
Recently, some supraglacial lakes on debris-covered
glaciers have expanded rapidly and outbursted in the
Himalayan region. Big floods caused by these outbursts(glacier lake outburst floods, GLOFs) have killed inhabitants and destroyed the natural environment downGlobal Environmental Research
16/2012: 95-104
printed in Japan

stream (Yamada & Sharma, 1993). In order to understandthe process of lake expansion in small supraglacial lakes,
it is important to quantify their physical characteristics.
Water sources and meltwater supply processes areespecially important to consider because lake expansion
results from much glacial meltwater input, and/or frommelting of the glacier ice at the sides and bottom of thelakes (Chikita et al., 1997). Furthermore, the characteristics of supraglacial lakes are important to know as they
provide an understanding of the chemical composition ofdischarge meltwater and the biological community living
there. Recent studies have revealed that supraglaciallakes on debris-covered glaciers play a role in thehydrolysis of fresh reactive minerals (Bhatt et al., 2007).
The lakes are also inhabited by diverse organisms, suchas insects, copepods and algae (Takeuchi & Kohshima,2000). Their community structure varies widely among©2012 AIRIES

96

N. TAKEUCHI et al.

the lakes, probably due to physical and/or chemical conditions of the lakes’ water. However, few studies ofsupraglacial lakes have been done, since unstable debris
on such glaciers makes them a dangerous environment
for field investigations.
Variation in water turbidity of supraglacial lakes isvisibly significant on debris covered glaciers. Iwata et al.
(1980) reported variation in water turbidity of supraglacial lakes on the Khumbu Glacier in Eastern Nepal. They
found that glacial lakes on upper parts with relatively thindebris-cover tended to have turbid water, whereas terminal lakes on glaciers with thick debris cover tended tohave clear water. In areas with thin debris cover, surface
melting and the meltwater supply are higher than in areas
with thick debris cover (Nakawo & Young, 1981).
Therefore lake water turbidity may be positively related
to meltwater supply. However, causes of such variation inlake water turbidity are still unknown. Since turbidity
levels can be quickly estimated visually as brightness orcoloration of water (e.g., Ritchie & Cooper, 1988;
Roberts et al., 1995), the discovery and characterization/quantification of a relationship between turbidity
and meltwater supply would enable quicker and safer
determination of lake status via remote-sensing techniques. In fact, variations in turbidity of supraglaciallakes on a debris-covered area of a Himalayan glacier
were successfully detected using ASTER satellite images(Wessels et al., 2002).
This study aims to describe spatial and temporalvariations in the suspended sediment concentration(SSC) of surface water of supraglacial lakes and to
determine factors in the variations on the Lirung Glacierof the Langtang region of Nepal, which is a typical
Himalayan debris-covered glacier. SSC, electrical conductivity (EC) and water temperature in all the lakes on
the debris-covered area of the glacier were measured
during the melting season (May - October) of 1996. The
settling rate of suspended sediment of the lake water anddiurnal variations of lake water levels were also measured at selected lakes to understand the relationshipbetween lake water SSC and meltwater supply to thelakes.

Fig. 1 Location of the Lirung Glacier in the Nepal Himalayas.

2. Study Site and General Morphology of theLakes
All investigations were carried out on the Lirung
Glacier in the Langtang region of central Nepal (Fig. 1)
from May 5, 1996 to October 18, 1996. The study included the monsoon season, which occurs between Julyand September. The 4 km long glacier flows south (from7,200 m down to 3,900 m a.s.l., Fig. 2a) and is located on
the southeastern side of Mt. Langtang Lirung. The
equilibrium line was located at approximately 5,500 m inthe study period. The glacier is separated into two parts at
4,200 m a.s.l.: the upper part and lower part (Fig. 3).
Between the two parts, basal rock is exposed for approximately 100 m. The debris free upper part flows from the
top of Mt. Langtang Lirung down to 4,800 m on a steep

Fig. 2 (a) The Lirung Glacier in May, 1996. (b) Supraglaciallakes on the debris covered surface. A large (L24) andsmall (L25) lake on the upperpart of the glacier.

Variation in Suspended Sediment Concentration of Supraglacial Lakes on Debris-covered Area

Fig. 3 Distribution of supraglacial lakes on the Lirung Glacierin May 1996.

97

slope. The lower part flows along the valley bottom andis mostly covered with thick debris. Snow and ice aresupplied from the upper part to the lower part mostly byavalanches. Meltwater flowing into the lower part fromthe upper part was observed on the exposed basal rock.
The debris surface of the lower part of the glacier ismarked by morphologies such as cones, hollows and ice
cliffs; these are similar to those observed on the Khumbu
Glacier (Iwata et al., 1980). The surface level is lower
than the lateral and terminal moraines. The debris surface
from the terminus up to about 1 km upstream is relatively
flat with some vegetation.
Twenty-eight supraglacial lakes or ponds were foundon the lower part of the Lirung Glacier in the
pre-monsoon season (Table 1). For simplicity, all will bereferred to as lakes. Figure 3 shows the supraglacial lake
distribution. All the lakes were located at the bottom of
surface depressions on the debris-covered area. Lake
sizes ranged from approximately 2 to 100 m in diameter.
The largest and probably deepest lake (L24) was morethan 100 m in diameter and 8 m in depth. The smallest
lake (L10) was 2 m in diameter and 1 m in depth. Though
the depth was not measured in any other of the lakes, they
appeared to be less than eight meters deep (the depth ofthe largest lake, L24).
More than half of the lakes (17 out of 28) had ice cliffs

Table 1 SSC and characteristics of supraglacial lakes on the Lirung Glacier. The times shown are Nepali standard time
(GMT+5:45). The size of each lake is represented by L: ≥10,000 m2, M: 10,000 - 100 m2, S: <100 m2. The size change
is represented by +: enlarged, 0: no change, –: reduced.

13:14
12:59
12:27
12:08
10:10
11:38
8:44
9:26

11:51
12:05
11:29
9:34
10:17
12:14
10:00

Algae

10/12
10/12
10/12
10/12
10/12
10/12
10/12
10/12
10/12
10/12
10/12
10/13
10/13
10/13
10/13
10/13
10/13
10/13
10/13
10/13
10/13
10/14
10/16
10/14
10/14
10/14
10/14
10/16

Midge

1.3
11.5
6.4
3.9
8.2
3.0
6.9
6.6
2.7
3.9
0.8
2.0
2.7
4.8
4.6
4.7
5.7
3.3
2.5
2.1
1.8
2.4
0.4
4.0
5.0
4.5
4.4
1.4
4.0

+
+
+

+
+

Size change

21.0
42.4
13.9
11.4
12.8
11.0
15.5
19.3
9.5
8.3
15.6
8.1
17.9
15.9
11.3
10.6
6.3
8.6
3.9
5.7
10.6
10.1
13.8
8.8
14.2
9.3
7.5
12.0
12.7

Water temp. (˚C)

81
0
4
66
9
37
39
2
36
0
29
89
28
25
58
40
0
44
15
27
364
11
182
37
14
9
5
31
46

EC (µS cm)

7:43
16:00
8:29
8:49
13:29
9:45
10:55
10:11
12:38
9:29
10:28
12:47
11:00
12:01
12:22
13:13
11:28
11:11
9:44
10:16
10:40
12:43
13:43
13:12
12:05
11:26
11:47
10:50

137 30.1
1 26.0
21
8.7
35
7.7
0
3.2
30
4.4
disappeared
disappeared
36
7.9
5
4.9
disappeared
disappeared
disappeared
10
8.8
52
6.2
disappeared31
4.8
disappeared0
4.2
disappeared40
5.9
disappeared
60 18.625
9.0
disappeared
disappeared
disappeared
disappeared
32 10.0

5.9
9.0
4.5
4.2
2.1
2.4

0
0
+
+
-

0.2
1.2

+

+

4.5
3.2

0
0

+
+
+

1.5

+

1.8

-

0.6

-

0.6
5.3

-

-1

+

5/8
5/9
5/8
5/8
5/10
5/8
5/9
5/8
5/9
5/13
5/13
5/13
5/13
5/13
5/13
5/13
6/15
6/15
6/15
6/15
6/15
5/11
5/11
5/11
5/11
5/11
5/11
5/11

SSC (mg l )

+
+

Time

+

Date

+
+
+
+
+

Water Temp .(˚C)

+
+
+
+
+
+

EC (µS cm)

+

-1

+
+

SSC (mg l )

+

Time

+

Post-monsoon

Date

Ice Cliff

L
M
M
M
M
M
S
S
M
S
M
M
M
M
M
S
M
M
M
M
M
M
M
L
M
M
M
M

Water Flow

Size of lake

Lake No.
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
L12
L13
L14
L15
L16
L17
L18
L19
L20
L21
L22
L23
L24
L25
L26
L27
L28
Mean

Pre-monsoon

+

+

3.1

+

98

N. TAKEUCHI et al.

larger than 2 m × 2 m around them. In the daytime, a large
amount of meltwater was observed flowing into the lakes
from these ice cliffs. Nine supraglacial streams were
present in the debris-covered area (F1-F9, Fig. 3). Since
all of these streams flowed out from openings in the ice
and into other openings in the ice or debris, the streams
were likely to be connected with englacial drainage
channels. Three long-shaped lakes located near theterminus or margin of the glacier (L1, L23, L28) wereassociated with supraglacial streams. In these three lakes,
the surface water moved downstream slowly. In the other
25 lakes, no inlet or outlet channels and no obvious surface water flow were observed.

3. Measurements
3.1 Water characteristics
For each lake studied, suspended sediment concentration (SSC, mg l-1), electrical conductivity (EC, µS
cm-1) and water temperature (˚C) were measured near thelake surface (5 - 10 cm in depth) at the lakeside. The
measuring/sampling at all of the lakes was carried out
during the daytime (9:00-13:00) in the pre-monsoon
season (May and June) and post-monsoon season(October). EC and water temperature was measured with
an EC meter (Model SC82, YOKOGAWA Co.). EC values were compensated to 25˚C. The water samples weretransported to Japan and analyzed at the Tokyo Instituteof Technology. In order to measure SSC, 100 ml of sample water was first filtered through a pre-weighted
membrane filter (Millipore HAWP047XX, pore size:
0.45 µm). Then, the filters were dried (65˚C, 24h), andthe weight of the particles on the filter was measured with
an electric balance (METTLER AE240, 0.01 mg resolution). Although these measured values are all point
measurements and do not indicate the average water
characteristics of the lakes, daytime variation of each
Table 2 Water characteristics of streams and ice on the Lirung
Glacier.
No.

Date Time

EC

SSC

Water

-1

(µS cm )Temp.(˚C)(mg l )

-1

F1

5/9

8:18

65

13.9

0.3

F2

5/9

9:20

261

13.7

0.2

F3

5/8

9:36

185

14.1

0.3

F4

5/9

11:30

182

14.8

0.3

F5

5/9

11:55

221

15.0

0.2

F6

5/8

12:22

219

14.3

0.2

F7

5/13

9:35

114

14.8

0.4

I1

6/17

110

11.5

-

I2

5/29

557

2.6

-

I3

6/17

91

2.4

-

I4

6/15

549

3.5

-

I5

6/15

127

1.3

-

measurement appears to be smaller than the ranges ofmeasured values. For example, the daytime variation of
measured characters in L1 on May 30, 1996 (7:00-16:00)
ranged from 111 mg l-1 to 158 mg l-1 (± 17%) for SSC,
and 34-37 µS cm-1 for EC (± 4%). Thus, we assumed thatthese data were approximate to the characteristics of eachlake.
Organisms living at lake bottoms were sought using a
net of fine mesh at each lake. Lake water including
benthos was collected by a pipette at each lake from thelakeside. The water was kept in 50 ml polyethylene bottles and preserved as a 3% formalin solution. The samples were transported to Japan where organisms (microbes) in the water and benthos were analyzed with amicroscope.
Water characteristics of stream water (seven sites) andglacial ice (five sites) were also measured in the sameway (Table 2, Fig. 3). EC and water temperature of
stream water were measured directly from the streamside.
One hundred ml of the water was collected in plastic
bottles for SSC measurement. Ice samples were collected
in plastic bags with an ice axe from ice cliffs after surface
debris on the ice was washed off. Air was excluded as theice melted in the plastic bags. The EC of the melted water
was measured. One hundred ml of each meltwater sample
was collected in plastic bottles for SSC measurements.
To measure seasonal changes in lake water SSC,
100 ml of surface lake water was collected from thelakeside of six lakes (L1, L2, L3, L4, L6, and L24),
which were selected based on safe accessibility. The
sampling was done monthly during the study period(May 8, June 13, July 14, August 15, September 14 andOctober 13). The lake water was measured at a consistenttime (noon) for all the monthly samples.
3.2 Settling rate of suspended sediment in lake water
To measure the settling rate of suspended sediment of
turbid lake water, SSC in a lake water sample was measured over time in a container without disturbance.
Surface lake-water (about 65 l) was collected from thelakeside of L6, which is a representative turbid lake andsafely accessible (May 30, 1996). The water was kept
undisturbed in a plastic container (54 cm × 34 cm ×38 cm, length, width, and height, respectively). Then,
100 ml was sampled from surface of the water at noon
every day until June 17 (19 days) to measure SSC.
3.3 Water levels of lakes
At nine lakes that were safely accessible (L1, L2, L3,
L4, L6, L9, L15, L24, L28 in Fig. 3), the water level was
automatically recorded every 30 minutes (single point
measurements). The water level was measured with awater pressure sensor (VPRNP Hirose-Rika Co., output:
5 mV = 1 cm deep). The data were recorded in dataloggers (DATAMARK, Hakusankogyo Co., recording
resolution = 1 mV). The sensors were placed about 10 cm
beneath the minimum water level, and were shaded fromsunlight. The measurements were carried out during a
3-7 day period for each lake (Table 3) in the pre-monsoon

Variation in Suspended Sediment Concentration of Supraglacial Lakes on Debris-covered Area

99

Table 3 Water level changes and water characteristics of nine lakes on the Lirung Glacier.
Lake
No.

MeasuredperiodTime of max.
water level

Time of min.
water level

Amplitude of the change (cm)Min-Max (Mean)

Surfacewater flow

SSC
-1
(mg l )

L1

5/7-6/30

18:00-23:00 (6:00-8:00)

9:00-13:00

5.0-39 (18.7)

+

81

21.0

L2

6/19-6/22

No Diurnal change

-

0

42.4

L3

5/10-5/16

No Diurnal change

-

4

13.9

L4

5/10-5/17

13:00-14:00

6:00-7:00

1.7-4.0 (2.3)

-

66

11.4

L6

5/28-6/4

13:00-14:30

6:00-8:00

2.8-7.5 (4.7)

-

37

11.0

L9

5/28-6/3

13:00-16:30

6:30-9:30

3.0-7.3 (5.1)

-

36

9.5

L15

7/1-7/5

10:30-13:30

6:00-8:00

1.4-1.5 (1.4)

-

58

11.3

L24

6/16-6/18

13:00-16:00

11:00

1.0-1.8 (1.4)

-

37

8.8

L28

6/9-6/11

16:30-17:00

8:00-9:30

2.5-8.2 (5.4)

+

31

12.0

season (from May 5 to July 10, 1996). For L1 (Fig. 3), thewater level and water temperature were recorded every
hour (single point measurements) with a water level
meter (MD-8C, CTI Science Co. at 1 cm increments,
0.1˚C increments, respectively) from May 5 to October
30.

EC
-1
(µS cm )(a) Pre-monsoon (May)

4. Results
4.1 SSC, EC and temperature of the surfacelake-water, stream and ice
SSC of the lake water in the pre-monsoon season
ranged widely among the lakes from 0 to 364 mg l-1(mean = 45.8 mg l-1, Table 1). A histogram of SSC
distribution shows that there were two frequent rangesof lake-water SSC, which were 0 - 10 mg l-1 and 30 40 mg l-1 (Fig. 4a). The variation in SSC appeared to beconsistent with the visible water turbidity of each lake.
The water of the higher SSC lakes was visibly turbidfrom a distance. The water color of these lakes wascreamy yellow or creamy green (Fig. 5a). In contrast, thelower SSC lakes (e.g., L2, L10, L17) were clear blue
(Fig. 5b). Although the turbidity of some lakes appeared
to vary spatially within the lake, the spatial variation
seemed to be obviously smaller than the turbidity variation among the lakes. The spatial distribution of lakewater SSC on the debris-covered area showed no clear
trend along the glacier (Fig. 6a). Both higher and lower
SSC lakes were distributed from the lower to upper partsof the debris- covered area.
Lake water EC was generally found to be low (3.9 21.0 µS cm-1, mean = 12.7 µS cm-1, SE = 1.3, n = 28)
except for L2 (42.4 µS cm-1) in lakes near the terminus.
EC values tended to be higher (> 15 µS cm-1) in lakes
located in the lower half of the debris-covered area, andlower (< 10 µS cm-1) in lakes located in the upper half ofit (Fig. 3). Temperatures of the surface lake-water also
varied among the lakes (0.4˚C - 11.5˚C, mean = 4.0˚C,
SE = 0.46, n = 28, Table 1).
In the post-monsoon season (October), 15 of the 28
lakes were observed at the same position wheareas theother 13 lakes had disappeared (Fig. 6, Table 1). There
were no newly appearing lakes. Some of the lakes
changed in size (area). Four of the 15 lakes had expanded
whereas seven lakes had shrunk and the remaining four(b) Post-monsoon (October)Fig. 4 Histograms of lake water SSC on the Lirung Glacier inthe pre- and post-monsoon seasons of 1996.

lakes showed no significant change (Table 1). Lake water
SSC varied from 0 to 137 mg l-1 in this season (mean =32.0 mg l-1, Table 1). A histogram of lake water SSC
shows a similar distribution to that of the pre-monsoon
season (Fig. 4b). There were the same two frequent
ranges of lake water SSC, which were 0 - 10 mg l-1 and30 - 40 mg l-1. EC and water temperature varied from 3.2
to 30.1 µS cm-1 (mean: 10.0 µS cm-1) and 0.2˚C to 9.0˚C(mean: 3.1˚C), respectively. There were no significant
differences in the mean of SSC, EC, or water temperature
between pre- and post-monsoon seasons.
Living organisms were found in the lake water and at
the bottom of lakes in both pre- and post-monsoon seasons. A midge and algae were observed in two lakes (L2and L3) and the midge only was observed in eight lakes(L4, L6, L9, L14, L15, L16, L22, L24, Table 1). No
organism was found in the other 18 lakes. The algae
observed included several taxa of green algae, diatoms,
and cyanobacteria.

100

N. TAKEUCHI et al.

Fig. 7 Seasonal changes of SSC at six lakes on the Lirung
Glacier. The values of each month are SSC on May 8,
June 13, July 14, August 15, September 14 and October
13, respectively.

Fig. 5 Photographs of two supraglacial lakes of different
turbidity. The SSC level can be recognized by water
turbidity: (a) high-SSC water appears creamy yellow(L20, SSC = 364 mg l-1, width = 2 m). (b) almost-zeroSSC water appears clear blue (L10, SSC = 0 mg l-1, width= 2 m).

Fig. 8 Changes in the SSC of turbid water from Lake L6
in a container.

Fig. 6 Spatial distribution of lake water SSC on the
Lirung Glacier in the pre- and post-monsoon
seasons of 1996.

The water characteristics of streams and ice meltwater on this glacier are shown in Table 2. Compared to lakewater characteristics, the water of seven streams (F1-F7)
had higher mean SSCs (178 mg l-1 versus 45.8 mg l-1,
statistical t-test: t value (t) = –4.363, probability (P) =0.000 < 0.05), but the EC means did not differ significantly (14.4 µS cm-1 versus 12.7 µS cm-1, t = – 1.25, P =0.222 > 0.05). The mean water temperature of thestreams was low (0.27˚C, range = 0.2˚C - 0.4˚C, SE =0.029, n = 7). Ice samples collected from five ice cliffs on
the glacier had higher mean SSCs (287 mg l-1 versus
45.8 mg l-1, t = –4.48, P = 0.000 < 0.05), but lower mean
EC (4.3 µS cm-1 versus 12.7 µS cm-1, t = 2.54, P = 0.016< 0.05) relative to the lake water means.

101

Relative water level (cm)Variation in Suspended Sediment Concentration of Supraglacial Lakes on Debris-covered Area

Fig. 9 Diurnal changes in relative water level of three supraglacial lakes (L3, L4, and L1) on the Lirung Glacierfrom May 10 to 16, 1996.

4.2 Seasonal changes in lake water SSC
The monthly observations revealed that lake water
SSC changed temporally in the selected lakes from thepre-monsoon to post-monsoon season, but there was nocommon trend among the lakes (Fig. 7). The SSCs of L1and L3 increased during that period whereas that of L4
decreased. The SSCs of L6 and L24 decreased once insummer and increased again by October. The SSC of L2,
of which was a lower SSC, kept its low value and showedno significant change during the study period.
4.3 Settling rate of suspended sediment in lake water
Figure 8 shows the change in SSC of water in a container collected from the turbid lake water of L6. In
undisturbed water, SSC rapidly decreased from 37 mg l-1
to 4 mg l-1 within a day. From there, SSC gradually
decreased and finally become zero after eleven days.
Sediments were observed at the bottom of the container
after the second day and were likely to be deposits of thesuspended particles in the turbid lake water.
4.4 Water level changes
Measurements of the water levels of nine lakes
showed various patterns of temporal change. Seven of thenine lakes studied (L1, L4, L6, L9, L15, L24, L28)
showed clear diurnal patterns, and two lakes (L2, L3)
showed no diurnal patterns (Table 3). Figure 9 contrasts
water-level changes measured simultaneously at threelakes (L1, L3 and L4). The L1 and L4 water levels
showed clear diurnal changes, whereas the L3 waterlevels did not. The trend of the water level of L4 appeared
to be to increase from early morning, becoming maximal
in the afternoon, and decreasing to a minimum in themorning. This pattern was observed at five other lakes(L6, L9, L15, L24, L28). The diurnal pattern of L1 differed from that of the other five lakes. It showed anirregular pattern, which had a maximal level late at night
or in the morning, and a minimum in the afternoon. L1 is
only the lake that showed an irregular diurnal pattern.
The pattern of water level changes was found to be

associated with lake water SSC. Table 3 shows SSC andEC for the lakes at which the water level was measured.
The mean SSC was significantly higher for the sevenlakes with diurnal changes than that of the two lakes
without diurnal changes in water level (49.4 mg l-1 versus
2.0 mg l-1, t = 3.35, P = 0.012 < 0.05). No significant
difference was found in EC between the two lake types(12.1 µS cm-1 versus 28.2 µS cm-1, t = 2.36, P = 0.051 >0.05).

5. Discussion
5.1 Lake water SSC and meltwater supply
The two peaks in the distribution of lake water SSC(Fig. 4) indicate that there are two distincitive lake types,
which are the low-SSC type (0 - 10 mg l-1) and high-SSC
Type (> 10 mg l-1). Lake water SSC was not correlatedwith EC or water temperature of the lakes. EC and watertemperature of all the lakes were generally low, suggesting that the main source of lake water of both types was
glacial meltwater. The SSC type may be due to local
conditions of each lake regarding meltwater supply.
The measurement of the settling rate of suspended
particles (Fig. 8) suggests that a high SSC of lake watercannot be maintained without water disturbance, and/or
quick turnover of the lake water, being replaced by anappreciable supply of high SSC water. The supposed
appreciable and constant supply of high SSC water ismeltwater from ice cliffs and drainage water systems.
The stream water and glacial ice of the ice cliffs on thisglacier were found to have higher SSCs (Table 2). Since
the streams were thought to be part of the glacial drainage
systems, and the ice cliffs were thought to represent meltwater, these potential lake water sources may have higher
SSC compared to the lake water.
The higher SSC of the lakes with diurnal water levelchange (i.e., a suspected large supply of meltwater) relative to the lake without diurnal change (i.e., a suspectedscant supply of meltwater, Table 3) supports the idea thatthe high water SSC of lakes could be maintained by the

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N. TAKEUCHI et al.

meltwater supply. Since net radiation accounts for morethan 90% of the incoming heat in Himalayan glaciers(e.g., Takeuchi et al., 2000) and it reaches a maximum ataround noon time, the diurnal cycle of water level with amaximum in the afternoon suggests that the lakes aremainly fed by surface meltwater from their local watersheds; including the ice cliffs around the lake. The irregular diurnal pattern of the L1 water level suggests that thelake was mainly fed through drainage water systems withmeltwater that originated from an area far upside theglacier. The traveling time of water through drainage
water systems has been reported to vary depending on thewater amount (Collins, 1995), and the water flow indrainage water systems is likely to increase suddenly due
to the eventual collapse of lakes located above.
Disturbance of the lake water could be caused bywind, debris falls or inflow of water. It is unlikely thatwind is responsible for maintaining SSC, since high andlow-SSC lakes were located side by side (for example L3and L4). Disturbance caused by falling debris or water
inflowing from ice cliffs seemed to be more important inmaintaining high SSC of the lake water. The higher
turbidity of lake water near ice cliffs (e.g., L18), suggeststhat the high SSC had been kept by disturbance frommeltwater inflow and debris falling from ice cliffs into
the lakes. The in situ measurement of ice cliff melt rates
on this glacier was 7.2 cm day-1 (Sakai et al., 1998).
Debris falls from ice cliffs were often observed as ice
cliff melting proceeded. The lakes that had ice cliffs orsurface water flow, had significantly higher SSC than thelakes which had neither ice cliffs nor surface-water flow
(62.7 mg l-1 versus 3.63 mg l-1, t = 3.27, P = 0.004 < 0.05).
This fact supports the idea that the disturbances caused
by water flow and debris falls from ice cliffs could
maintain high SSC levels.
These observations of water supplies and disturbances of lake water suggest that high SSC in supraglacial lakes is likely to be maintained by the following
factors: (1) a large supply of high SSC water from theglacial ice surface and/or glacial drainage water system,and (2) disturbance of lake water by these water flows
and/or by debris falling into the lakes from the ice cliffs.
In contrast, the low-SSC type is likely to be receiving
little meltwater.
5.2 Factors of spatial and temporal variations inlake water SSC
The major factor affecting variations in lake water
SSC is likely to be the amount of meltwater supplied to
the lakes. As suggested by Takeuchi and Kohshima(2000), the amount of meltwater supplied probably depends on the melting activity of the glacier surface
around the lake. Since the thick debris inhibits glaciermelting (e.g., Nakawo & Young, 1981), the melting
activity is likely to be related to debris thickness on theice surface. Lakes with a large meltwater supply would
be located in areas with thin debris, whereas lakes withlittle meltwater would be in an area with thick debris.
Furthermore, lakes that have ice cliffs at their side would

have an abundant meltwater supply. Thus, differing
meltwater supplies among the lakes is probably due tospatial variation in debris thickness and ice cliffs on theglacier surface.
The lack of spatial trends in lake water SSC on the
Lirung Glacier is probably due to distribution of debris
thickness. A previous study on the Khumbu Glacier ineastern Nepal showed that low-SSC type lakes weregenerally distributed in the downstream area of theglacier, since the debris thickness in the area was morethan 2 m on average (Takeuchi & Kohshima, 2000). On
the Lirung Glacier, the distribution of low-SSC typelakes showed no such trend. They were located from theupper to the lower parts of the debris-covered area
(Fig. 6). This is probably due to little area with thickdebris cover of more than 2 m in the downstream areaand heterogeneous distribution of debris thickness on thisglacier.
The seasonal change in lake water SSC is probably
explained by changes in meteorological conditions
and/or debris cover and ice cliffs around the lakes. Since
observations have revealed that the melting activity ofthis glacier is highest in the period from May to June
(Rana, 1997), the meltwater supply is likely to reach amaximum in that season. Thus, surface water SSC oflakes would increase in the pre-monsoon season and thendecrease after the season. The changes in lake-water SSC
of L4 roughly agreed with the seasonal changes in melting activity of the glacier (Fig. 7). However, the SSC of
the other lakes showed no such seasonality. In these lakes,
changes in debris cover and ice cliffs are likely to be
more significant compared with the meteorologicalconditions during the period. The average melt rate of ice
cliffs was 7.2 cm day-1 during the monsoon season on thisglacier (Sakai et al., 1998). Melting of ice cliffs may
significantly alter the area of ice cliffs and the morphology and distribution of debris cover, affecting the amountof meltwater supplied to the lake. Glacial flow would
also change the morphology of the glacial surface. In fact,changes in debris cover and ice cliffs were ubiquitously
observed during the study period. L2 was only the lakethat kept a low SSC value without significant seasonalchange. This lake is probably located on thick, stabledebris cover. Thus, an increase of lake water SSC may becaused by the expansion of ice cliffs and/or thinning of
the debris cover, whereas a decrease may be caused by areduction of ice cliffs and/or thickening of the debris
cover around lakes.
The number of lakes on the debris-covered area decreased during the study period, indicating that lakes
drastically form and collapse in the melting season. Since
low-SSC type lakes should be located on relatively thickdebris cover, they could be expected to have a longer life
span. However, the SSCs of the disappearing lakes varied
widely from 2 to 89 mg l-1, thus the life span of lakes
appears not to be related with lake water SSC on thisglacier. The distribution of the disappearing lakes and thespatial distribution of lake water SSC on this glacierindicate that most of the debris-covered area on theVariation in Suspended Sediment Concentration of Supraglacial Lakes on Debris-covered Area

glacier is actively deforming during the melting season.
The living organisms found in the lakes mostly match
those in supraglacial lakes on the Khumbu Glacier ofeastern Nepal (Takeuchi & Kohshima, 2000). Based on
the biological categories in Takeuchi and Kohshima(2000), two lakes, L2 and L3, correspond to Type A lakes(lakes with animals (the midge) and algae), eight lakes toType B lakes (with animals (the midge) only), and theothers to Type C lakes (without living organisms). The
lake water SSCs of each lake type are consistent with theprevious finding on the Khumbu Glacier: a lower SSC
supports Type A and a higher SSC, Types B and C (Table
1). Only two Type A lakes existed on the Lirung Glacier(14 lakes on the Khumbu Glacier in contrast), suggestingthat a thick, stable debris cover, which allows inhabitation by diverse organisms, is limited on the Lirung
Glacier.

6. Conclusions
Measurements of the settling rate of suspended particles and water level changes of supraglacial lakes in thedebris-covered area of the Lirung Glacier revealed that ahigh SSC level in supraglacial lakes is likely to be maintained by an abundant supply of high-SSC meltwaterfrom the glacial ice surface and/or glacial drainage water
system, and/or disturbance of lake-water by these waterflows and/or by debris stones falling into the lakes fromthe ice cliffs. In contrast, the low-SSC type is likely toreceive little meltwater. No clear trend in spatial variationin lake water SSC was found, suggesting a heterogeneous
distribution of debris thickness and ice cliffs on thisglacier. Lake water SSC also varied seasonally and it isprobably explained by changes in meteorological conditions and/or debris cover and ice cliffs around the lakes.
A positive relationship between SSC and meltwatersupply implies that supraglacial lakes can be distinguished by lake water turbidity between lakes with andwithout meltwater supply. Turbid lakes and non-turbid
lakes can be regarded as those supplied with meltwaterand those supplied with little meltwater, respectively. The
availability of lake turbidity would enable quicker andsafer determinations of meltwater supply of lakes via
remote-sensing techniques, and may be useful for
elucidating the status of supraglacial lakes and glaciermelting. For example, the turbidity of lakes can be used
as an indicator of probability of lake expansion. The
expansion of supraglacial lakes may occur more in highly
turbid cases (meltwater-supplied lakes) relative to nonturbid lakes (no meltwater supply), because lake expansion results from a large water supply to the lakes, and/orfrom melting of glacier ice at the side and bottom of thelakes (Chikita et al., 1997). Furthermore, lake-water
turbidity may be used as a rough indicator of melting
activity on debris-covered glaciers. Distribution of turbid
lakes may indicate a melting area on a debris-covered
surface, because the meltwater supply to the lakes may be
the result of melting activity around the lake, except forlakes fed with englacial water systems. Thus, turbidity of103

supraglacial lakes may be a useful indicator of roughmelting conditions of debris-covered areas.

Acknowledgments
We would like to express our thanks to the Department of Hydrology and Meteorology, Ministry of Water
Resources, Government of Nepal. We thank all of the
Sherpa people and the members of the Cryosphere
Research in the Himalayas (CREH) project for their help
supporting our field work in Nepal. We also thank two
anonymous reviewers for suggestions on the manuscript.
The expenses of this research were supported by aGrant-in-Aid for Scientific Research (No. 06041051,
09490018 and Aid for JSPS Research Fellow).
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Nozomu TAKEUCHI
Dr. Nozomu Takeuchi is an Associate Professor at
the Department of Earth Sciences, Graduate
School of Science, Chiba University, Japan. He
received his PhD from the Tokyo Institute ofTechnology in 1999. His doctoral thesis wasentitled “Studies on albedo-modification of snow
and ice by cryo-microbial activities.” He is interested in the ecology ofliving organisms on glaciers and their effect on glacial systems. He has
been working on glaciers worldwide including those of the Himalayas,
Tienshan, Altai, Pamir, the Arctic, Patagonia and Alaska. His research isbased on field work and biogeochemical analysis in the laboratory tounderstand glacier ecosystems.

Akiko SAKAI
Akiko SAKAI is an Assistant Professor at theGraduate School of Environmental Studies,
Nagoya University. She has been studying glaciallakes in the Himalayas since 1993, and has also
carried out observations of the ablation process ofdebris covered glaciers since 1995. She has
carried out field observations at Tsho Rolpa and Imja Glacial Lakes and at
Lirung and Khumbu Glaciers in the Nepalese Himalayas, and hydrologicalobservations at Qilian Shan in China, and is studying the discharges fromglaciers in arid regions. At present, she is working on developing a GlacierInventory of Asian Mountains.

Shiro KOHSHIMADr. Shiro KOHSHIMA is a Professor at the
Wildlife Research Center, Kyoto University,
Japan. His research interests are glacial ecology
and animal ethology.

Koji FUJITA
Koji FUJITA is an Associate Professor at theGraduate School of Environmental Studies,
Nagoya University. He received his PhD (inScience) from Nagoya University. He has undertaken various field work in Nepal, Tibet, Bhutan,
Kyrgyzstan and Tajikistan, as well as overwintering at Dome Fuji Station, which is inland in east Antarctica. His research
has focused on fluctuations of Himalayan glaciers, ice-core based
paleoclimate reconstruction in the Asian highlands, and stable water
isotope climatology in Antarctica.

Masayoshi NAKAWO
Dr. Masayoshi NAKAWO is Executive Directorof the National Institutes for the Humanities,
Japan. His research interests span the environmental issues regarding water resources inEurasia.

(Received 28 November 2011, Accepted 13 February 2012)