Changes in Surface Morphology and Glacial Lake Development of Chamlang South Glacier in the Eastern Nepal Himalaya Since 1964

Changes in Surface Morphology and Glacial Lake Development of Chamlang South Glacier in the Eastern Nepal Himalaya since 1964

1* 2 3 1 Takanobu SAWAGAKI , Damodar LAMSAL , Alton C BYERS and Teiji WATANABE

1Faculty of Environmental Earth Science, Hokkaido University N10, W5, Sapporo, 060-0810, Japan 2Graduate School of Environmental Science, Hokkaido University N10, W5, Sapporo, 060-0810, Japan 3The Mountain Institute, 100 Campus Drive, 108 LA Elkins, WV 26241, USA *e-mail: [email protected]

Abstract We carried out multi-date morphological mappings to document the development of the glacial lake ‘Chamlang South Tsho’ in the eastern Nepal Himalaya over four decades. High-resolution Corona KH-4A and Advanced Land Observing Satellite (ALOS) PRISM stereo-data taken in 1964 and 2006 were processed in the Leica Photogrammetric Suite (LPS) to generate digital terrain models (DTMs). The DTMs produced topographic maps representing elevations and morphology of the glacier surface with a maximum error of +/- 10 m. A bathymetric map was also produced based on sonar sounding data obtained in November 2009. Extensive surface lowering was found to have occurred since 1964, as high as 156.9 m in the upper glacier area. The average lowering of the glacier for the entire 42-year period from 1964 to 2006 is 37.5 m, with the average surface-lowering rate calculated at 0.9 m/year. The average surface lowering for the 45 years from the glacier surface in 1964 to the lake bottom in 2009 was 99.5 m at a rate of 2.2 m/year. The minimum and maximum surface lowering during that period were 12 m and 153.8 m, respectively. The area surrounding the largest supraglacial pond in 1964 exhibited a low surface gradient, and there had already been a large degree of ice melting that favoured further lake expansion. The larger lowering rate at the lake area supports the previously presented idea that ice calving into the pond triggered the larger and faster up-glacial lake expansion.

Key words: ALOS PRISM, Chamlang South Glacier, Corona, digital terrain model, glacial lakes, Nepal Himalaya, photogrammetry, satellite images, surface morphology, topographic map

1. Introduction favorable (Reynolds, 2000; Quincey et al., 2007), i.e., exhibiting a glacier surface gradient of less than 2°, low Glaciers in the Hindu Kush-Himalaya have been ice velocity, and/or complete stagnation. Sakai and Fujita receding and melting since the end of the Little Ice Age (2010) discussed the importance of the glacial surface in 1850 (Fujita et al., 2001; Bolch et al., 2008a, 2011; lowering since the Little Ice Age. Glacier recession and Shrestha & Aryal, 2010; Fujita & Nuimura, 2011). One associated phenomena have multiple impacts and sig- of the consequences has been the formation and expan- nificances, all of which need to be considered in an inte- sion of glacial lakes, particularly during the last half grated manner (Haeberli et al., 2007; Hegglin & Huggel, century, as a result of glacial retreat that is most likely in 2008). Most importantly, an understanding of the reces- response to contemporary global warming and regional sion of debris-covered glaciers is absolutely necessary in warming trends (Yamada & Sharma, 1993; Mool et al., terms of hazards assessment, primarily because the 2001; Bajracharya et al., 2007; Watanabe et al., 2009). glacial lakes that form on them sometimes produce As a result, the potential for glacial lake outburst floods devastating GLOFs (Clague & Evans, 2000; Richardson (GLOFs) is increasing throughout the Hindu Kush- & Reynolds, 2000a, 2000b; Ives et al., 2010), which are Himalaya (e.g., Watanabe et al., 1994). often several times larger than normal climatic floods Supraglacial ponds and lakes are likely to form on the (Cenderelli & Wohl, 2001). GLOFs can cause wide- surface of debris-covered glaciers when conditions are spread damage and loss of life, property, infrastructure,

Global Environmental Research ©2012 AIRIES 16/2012: 83-94 printed in Japan 84 T. SAWAGAKI et al. and the geomorphic environment (Ives, 1986; Vuichard complex geometry, lack metadata (ephemeris) pertaining & Zimmermann, 1987; Cenderelli & Wohl, 2003; to image acquisition, and require highly sophisticated Narama et al., 2010). A thorough understanding of image- processing software to extract 3D information. glacial recession is also critical to the estimation of the Despite this, Corona data provide an invaluable source glacier mass balance for water resource management for investigation in various fields such as glaciology and within a given region (Bolch et al., 2008b; Immerzeel archaeology. Detailed information on Corona and ALOS et al., 2010). PRISM data and their processing is described well by Mapping of glacier area change can be carried out Lamsal et al. (2011). with non-stereo orthorectified imagery; however, multi- During the past several decades in the Mt. Everest and date, stereo-capable data are required in order to calculate Makalu-Barun national parks of Nepal, 24 new glacial volumetric changes of glaciers, especially for the debris- lakes have formed and 34 major lakes are reported to covered type. Multi-date stereo-images with long-period have grown substantially as a result of climate change time differences as well as high spatial resolution are and regional warming trends (Bajracharya et al., 2007). needed to produce accurate and detailed 3D topographic Recent analyses of comparative satellite imagery have or terrain maps, that in turn can expand our under- suggested that at least twelve of the new or growing lakes standing of the morphological changes of a glacier’s within the Dudh Kosi watershed, nine of which are surface. Photographic products from the Corona program located in the remote Hongu valley of Makalu-Barun (~1960 to 1972) are the oldest remote sensing stereo National Park, are ‘potentially dangerous’ based on their images publicly available with a wide geographic rapid growth over the past several decades (Bajracharya coverage and high spatial resolution (~1.8 to 7.6 m). et al., 2007; Bajracharya & Mool, 2009). However, Likewise, the Advanced Land Observing Satellite despite the large amount of national and international (ALOS) PRISM, is a relatively new remote sensing media attention recently generated by these new and/or satellite program (launched in 2006) that has stereo growing lakes in the Hongu valley, relatively little was capability capable of generating digital terrain models known about them in 2009 because of their extreme (DTMs) and 3D maps, and which also offers high spatial remoteness and difficult access. resolution stereo-data (2.5 m). Several studies have In response, we carried out the first scientific field investigated volumetric changes in glaciers in the investigation of these glacial lakes in October-November Himalaya using either Corona or ALOS data (e.g., Bolch of 2009 (Fig. 1). At the time, a glacial lake situated at the et al., 2008a; Lamsal et al., 2011). Corona data have a southwestern foot of Mt. Chamlang was considered to be

Fig. 1 Location of the Chamlang South Tsho in the Hongu valley, eastern Nepal Himalaya. CST: Chamlang South Tsho, CNT: Chamlang North Tsho.

Changes in Surface Morphology and Glacial Lake Development of Chamlang South Glacier 85 among the most dangerous of the nine ‘potentially Chamlang South Tsho has developed rapidly between dangerous’ lakes within the valley (Mool et al., 2001; 1964 and 2000 on the debris-covered Chamlang South Bajrachayra et al., 2007; ICIMOD, 2011), prompting the Glacier (Fig. 2). The altitude of the lake surface is research team to focus its activities on the Chamlang 4,940 m. South Glacier while conducting preliminary analyses of the other eight lakes. Respecting its positional relation- 3. Data and Methods ship with the peak, we call this lake ‘Chamlang South Tsho’ although it has been referred to by various other 3.1 Image processing names. The aim of our study was to examine changes in Two sets of stereo-data (ALOS PRISM and Corona) surface morphology of the Chamlang glacier and the lake with a time span of over four decades were used. ALOS between 1964 and 2006. PRISM data (acquired on 4 December 2006; Path: 140, Row: 41, spatial resolution: 2.5 m) with processing level 2. Study Area 1B1 (radiometrically calibrated data) plus Rational Polynomial Coefficient (RPC) data files were obtained Chamlang South Tsho (longitude: 86˚57’33”E, lati- from the Remote Sensing Technology Center of Japan tude: 27˚45’24”N), has been called different names by (RESTEC). ALOS PRISM acquires data in triple mode F, different researchers, e.g., Chamlang Tsho in the 1:50000 N, and B (forward, nadir, and backward). With these data, Schneider map, Chamlang Pokhari by Byers et al. (2012), <10 m geometric accuracy is achievable (Tadono et al., and most commonly West Chamlang in the 1:50000 topo- 2009). graphic map by the Survey Department of Nepal and in Images acquired from Corona camera systems KH-1, Mool et al. (2001) and Bajracharya et al. (2007), which KH-2, KH-3, KH-4, KH-4A, and KH-4B were declassi- also provided an identification number of kdu_gl 466. fied in 1995 and became available in a digital format in Mt. Chamlang has two glacial lakes on the westward side 2003 (McDonald, 1995; Galiatsatos et al., 2008). The of the peak: one in the northwest, with no local name but early systems of KH-1, KH-2, and KH-3 were equipped an identification number of kdu_gl 464 (Bajracharya with a single panoramic camera, while the latter systems et al., 2007), and the other in the southwest region of the of KH-4, KH-4A, and KH-4B were equipped with both peak (Fig. 1). To avoid confusion, we propose the use of forward- and backward-looking cameras. The images the name of Chamlang South Tsho for the lake located in taken by the KH-4, KH-4A, and KH-4B camera systems, the southwest region. which have a stereo capability, a higher spatial resolution

Fig. 2 Landform features and surrounding geomorphic environments of the Chamlang South Glacier. Corona KH-4A image depicting the supraglacial ponds and glacier surface in 1964 (a) and IKONOS image acquired in 2000 (b) showing the morphological change of a large lake (Chamlang South Tsho) from 1964 to 2000. C1–C5: Abandoned channels, P: Current largest pond.

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(3.00-7.60, 2.70-7.60 and 1.8-7.60 m, respectively) and ALOS PRISM images (F, N and B) with RPC file and wide area coverage offer the oldest available stereo data. three GCPs (collected from a map of the Everest area Corona panoramic filmstrips contain inherent geometric compiled by the National Geographic Society in 1988) distortions, which need to be rectified to extract reliable were employed to triangulate (RMSE ~0.40 pixel) stereo information from its images (Slama, 1980; Altmaier & images to create a stereo model and were used for DTM Kany, 2002). Typical geometric distortions of a Corona generation. A stereo model was generated for the entire image gradually increase from the centre to the extreme area covered by the scenes; however, only a DTM of the edges of an image along the track (Slama, 1980; Altmaier study area was created. The vector DTM was edited upon & Kany, 2002; Lamsal et al., 2011). The processing of viewing the stereo model as a base. Extensive editing was Corona stereo images for DTM generation has been well carried out upon viewing the stereo model until satisfac- described by Altmaier and Kany (2002), Bolch et al. tory terrain representation with mass points was achieved. (2008a) and Lamsal et al. (2011). Corona KH-4A ste- Thick or thin distribution of mass points that did not truly reo-pair images (acquired on 26 November 1964, entity represent ridges, ponds, and depressions were also edited ID: DS1014-2118) of the study site were obtained from to ensure that enough had been accurately represented. the USGS in a digital format scanned at 3,600 dpi Densification and thinning of mass points were mainly (7 microns). carried out during terrain editing, so that the produced Stereo images were processed with the aid of the DTM would duly represent the actual terrain surface. Leica Photogrammetric Suite (LPS). DTMs and topo- After rigorous terrain editing a promising DTM was graphic maps were produced after rigorous editing of the produced (Fig. 3); image processing tasks, including triangulated irregular network (TIN) model. Stereo DTM editing, are well described by Lamsal et al. (2011). MirrorTM/3D Monitor and Leica 3D TopoMouse greatly To correct the geometric distortions of the Corona facilitated and enhanced the tasks of image processing images, a non-metric camera model was selected in the such as ground control point (GCP) collection and terrain LPS platform. This requires only the focal length and editing. A fully editable Leica Terrain Format (LTF) and pixel size of the scanned image, and the flying height of a TIN (vector DTM) model was created for each dataset, the camera platform as input for interior orientation so that the editing of inherent errors took place with parameters (IOPs) (Altmaier & Kany, 2002; Casana & automatically generated DTM to produce accurate 3D Cothren, 2008). Acquiring reliable GCPs from identical terrain maps. locations is one of the most crucial tasks in

Fig. 3 Location of the ground control points (GCPs) denoted by white triangles (a). The red box shows the area shown in (b). Figure 3b shows a representation of landform by mass points, triangles and contour lines in vector DTM (LTF).

Changes in Surface Morphology and Glacial Lake Development of Chamlang South Glacier 87 photogrammetric operations for producing quality DTMs The use of Corona data to generate a DTM has been and topographical maps. The GCPs for the Corona extremely limited in the scientific literature. The few images (Fig. 3) were taken from corrected ALOS stereo studies that do exist include Altmaier and Kany (2002) models, with meticulous care given to their accuracy. with Corona KH-4B; Bolch et al. (2008a, 2011) with Afterwards, approximately 100 tie-points were auto- Corona (KH-4) and ASTER; and Lamsal et al. (2011) matically generated with an image matching technique with Corona KH-4A and ALOS PRISM. We employed a for stereo data based on the provided IOPs and exterior combination of Corona KH-4A (spatial resolution, orientation parameters (EOPs). All tie points were 2.7-7.6 m) and ALOS PRISM (spatial resolution, 2.5 m) automatically placed at the correct locations on both data as used by Lamsal et al. (2011) to generate DTMs images (forward and backward). Aerial triangulation was and topographic maps. Three-dimensional topographic then performed (RMSE ~1.25 pixel) to produce a vector mappings with Corona KH-4A and ALOS PRISM stereo DTM. Later, substantial DTM editing was carried out to imageries can represent glacial surfaces, including obtain a representative terrain surface as explained micro-landforms, such as supraglacial ponds, depres- above. sions, moraine ridges, and ice cliffs satisfactorily well with a high accuracy. However, the DTMs produced with 3.2 Evaluating the accuracy of the produced DTMs automatic procedures were largely unsatisfactory, which and topographic maps led to extensive DTM editing to remove unnecessarily The pond/lake boundary of Chamlang South Tsho thick mass points or to add mass points where their was delineated from the surrounding moraines and distribution was thin. Despite careful editing of DTMs, glacier while viewing the DTM superimposed over the there may still be small errors (a maximum of +/– 10 m). stereo model of the ALOS PRISM data. The surround- However, representation of the terrain of the glacier sur- ings are higher than the lake surface, and the texture of face was entirely satisfactory for the present purpose. the lake is also different. The flat lake surface is easily identifiable on the stereo model. Uncertainty in delineat- ing the precise lake boundary over the stereo model is believed to be very small, i.e., less than one meter. a) The accuracy of the produced DTMs and topographic maps of the Chamlang South Glacier was also assessed by differentiating the 2006 ALOS DTM from the 1964 Corona DTM in the unaltered area outside the lateral and terminal moraine boundary. As shown in Fig. 2, the Chamlang South Glacier is surrounded by lateral and terminal moraines. The glacier surface within the lateral and terminal moraines is subject to surface lowering because of glacier melt. It is assumed that the surface beyond the lateral moraines has remained unchanged. This unchanged surface (buffer of ~0.55 km2) beyond the lateral/terminal moraines is used for assessing the consistency among the DTMs and the topographic maps. Figure 4a is a portion of the elevation change map beyond the lateral moraines (unchanged ground) of the b) Chamlang South Glacier. The glacier surface area within the lateral moraines was masked for the calculation. The difference between the Corona and ALOS DTM ranges from 11 to –13 m (Fig. 4a). To quantify this DTM error more precisely, a histogram of DTM differences was prepared (Fig. 4b). The calculations were carried out on a 3 × 3 m grid, and the total pixel count was 59,001. Sub- sequently, the pixel count for each class was converted to an error percentage. The classes with the most dominant DTM difference were –1-1 m (26%) and 1-3 m (26%), while the classes with negligible influential difference were –13 - (–11) m (0.1%), –11 - (–9) m (0.2%), –9 - (–7) m (0.4%), 7 - 9 m (1%), and 9 - 11 m (0.2%). The range of DEM differences from –5 to 5 m accounts for 90.9% and the range from –1 to 3 m comprises 52.0%. As a result, it is concluded that a maximum relative DEM error of 10 m Fig. 4 Portion of the elevation change map beyond the lateral and terminal moraines of the Chamlang South may exist in the generated DTMs and the topographic Glacier, showing unchanged areas (a), and a maps of the Chamlang South Glacier. histogram of DEM differences (b).

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3.3 Bathymetric survey which have already been abandoned (Fig. 2a). These Many researchers have examined the areal expansion channels suggest that the former melt-water drainage rates of glacial lakes in the Himalaya by using photo- over- spilled when the glacier surface was much higher graphs, maps and satellite images (Watanabe et al., 1994; than today. The magnitude of the overspill was deter- Yamada, 1998; Ageta et al., 2000; Haeberli et al., 2001; mined not to be large, based on the small size of the Yabuki, 2003; Byers, 2007; Sakai et al., 2007; depositional landforms on the terminal moraines. The Racoviteanu et al., 2008, 2009; Byers et al., 2012). largest channel located to the northwest of the terminal However, few studies have investigated bathymetric moraine currently has surface water (a small river), the changes in the supraglacial or proglacial lakes and ponds uppermost of which is subsurface flow. The water of this of concern (cf. Benn et al., 2001; Fujita et al., 2009), small river is most likely derived from the pond shown as partly because it is extremely difficult to gain access to P in Fig. 2b. most of the high altitude glacier areas in the Himalayan Morphologies shown in Fig. 5 were converted to mountains. Recurrent measurements have also been DEMs (Fig. 7) for the purpose of raster-based calcula- restricted or are completely absent. Still, it is important to tions to quantify glacial morphology and its change by measure the water depth of glacial lakes in order to have producing elevation change maps and surface profiles. a better understanding of the expansion process Here, we use the term ‘lowering’ in a descriptive manner, mechanisms involved (e.g., Röhl, 2008; Sakai et al., without suggesting specific processes, because this is 2009). more favorable to the description of morphological We carried out geomorphic observations and a bathy- changes that occurred between 1964 and 2006. metric survey at and around Chamlang South Tsho between 26 and 29 October 2009, at the end of the ice-melting period. The lake water depth was confirmed continuously using sonar sounding from a boat. Fishing sonar (Fish Elite 500C) was utilized for sounding the depth. Depth measurement points were simultaneously recorded by GPS.

4. Results and Discussion

Figure 5 shows the surface morphologies around the Chamlang South Glacier with topographic maps in 1964 and 2006 at contour intervals of 2 m (faint lines), 10 m (lighter bold lines), and 20 m (bold lines), mapped by Corona KH-9 and ALOS PRISM stereo images, respectively. As can be seen when comparing the two images, noticeable glacial surface morphology change occurred during the 42-year period from 1964 to 2006. Corona KH-4A image shows a few supraglacial ponds in 1964 (Fig. 2a). These ponds were most probably very small in 1962 when a climbing expedition team Fig. 5 Topographic maps of the Chamlang South Glacier in from Hokkaido University scaled the summit of Mt. 1964 (a) and 2006 (b). Contour intervals: 2 m (faint Chamlang. The largest pond was located at an altitude of lines), 10 m (lighter bold lines), and 20 m (bold lines). 4,952 m, and the glacial surface upward from the pond The red lines show the lateral and terminal moraines. had a relatively steep gradient in 1964 (Fig. 5a). The steep section of the glacier had completely melted out by 2006 resulting in the formation of a large lake, ‘Chamlang South Tsho’, that had a lake water surface altitude of 4,940 m in 2006 (Fig. 5b). This suggests that some 12 m of lake surface lowering had occurred during the period between 1964 and 2006. As shown in the bathymetric map (Fig. 6), the current lake is approximately 550 m wide (north-south) and about 1,650 m long (east-west), with the bathymetric survey suggesting that the maximum water depth is 87 m. The current lake area, 8.7 × 107 m2, is roughly six times larger than that shown on the Schneider map (surveyed between 1955 and 1974). 6 3 The total water volume is estimated to be 35.6 × 10 m . The terminal moraine of the Chamlang South Glacier Fig. 6 Bathymetric DEM of the lake in 2009. has six dissected channels (Figs. 2a and 2b), five of

Changes in Surface Morphology and Glacial Lake Development of Chamlang South Glacier 89

In Fig. 7, the surface morphologies for both years are represented in eleven classes (ranging from 4,710 - 5,150 m at 40-m class intervals) with the same 11 ranges and color indexes used to depict comprehensive surface morphology comparisons. The actual calculation was done using numeric values of the DEM. The two most salient features on the morphological maps of the glacier, in terms of DTMs from 1964 and 2006, include the following: (i) there was an elongated lower surface or depression along the middle of the glacier in 1964 (Figs. 5a and 7a) as depicted by the light green colour in the 4,950-4,990 m class range, and (ii) the glacier surface above the elongated depression surface was relatively steep in 1964, but subsequently melted out causing substantial surface lowering in the area by 2006. Topographic maps (Fig. 5) and DEMs (Fig. 7) clearly show that the elevations of the debris-covered glacier surface have substantially changed. A map of elevation changes for the entire area (Fig. 8) was produced by subtracting the DEM values for 2006 (Fig. 7b) from those Fig. 7 Digital Elevation Model (DEM) of the Chamlang South for 1964 (Fig. 7a). The values for elevation change are Glacier in 1964 (a), and DEM for 2006 (b). Terrain presented in nine classes (Fig. 8). Extensive surface elevation values are grouped in eleven classes with the lowering, e.g., as high as 156.9 m, is visible in the same colors at 40 m intervals for each year. Red line up-glacier area, just up from the 2006 east lakeshore, shows the lakeshore in 2006. from melting of the steep glacier surface that existed in 1964. This characteristic is similar to that of the Imja Glacier Lake (Lamsal et al., 2011). The average lowering of the glacier surface (calculated for 3 × 3 m grids using the elevation change map, Fig. 8) for the 42-year period from 1964 to 2006 over the entire area is 37.5 m, with the average rate of surface lowering being 0.9 m/year. Figure 8 clearly demonstrates that surface lowering at the dead-ice and nearby area is smaller, whereas the lowering gradually increases in the up-glacier area and reaches its maximum in the uppermost area. This suggests that less glacier melting has occurred in the down-glacier area when compared with the up-glacier area. Fig. 8 Elevation change map showing the surface lowering of The elevation change map in the lake area of 2009 the Chamlang South Glacier for 42 years from 1964 to (Fig. 9) was produced by subtracting the 2009 lake bot- 2006. tom values (bathymetric DEM, Fig. 6) from the 1964 glacier surface values (topographic DEM, Fig. 10b). The surface lowering values are grouped into eight classes to depict lowering extent. The average surface lowering for the 45 years from 1964 to 2009 was 99.5 m at a rate of 2.2 m/year; the minimum and maximum surface lowering during this period were 12 m and 153.8 m, respectively. The explicit influence of a steeper surface gradient in the upper part of the up-glacier area in the 1964 map, as well as the deepest area in the 2009 bathymetric map, are manifested in the elevation change map of the lake: i.e., greater surface lowering has occurred in the area speci- fied above (Fig. 9). The longitudinal profile L–L’ (Fig. 11) shows the extent of the surface lowering along the glacier of 1964, Fig. 9 Surface lowering from the glacier surface in 1964 and the lake area and dead-ice area in 2006 together with (topographic DEM) to the lake bottom in 2009 the steepness of the glacial surface gradient for both years. (bathymetric DEM). Along profile L–L’ (Fig. 11), the maximum surface lowering in the dead-ice area for the 42-year period from

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Imja glacial lake to the north (Fujita et al., 2009). If ice a) calving had not been the main mechanism, downward lake expansion in Chamlang South Tsho would have been larger. Profile A–A’ (Fig. 11) in the dead-ice area demonstrates that the glacial surface, even in 1964, was significantly lower than its lateral moraines, although the glacial surface in the 1960s had been close to, or even higher than, the lateral moraines in the lower area of the Imja Glacier (Lamsal et al., 2011). Thus, the surface lowering in the lower area of the Chamlang South Glacier from 1964 to 2006 was small, suggesting that substantial b) ice melting in the area had already occurred by 1964. Profile B–B’ (Fig. 11) reveals that glacial ice melting was less in the pond area, as well as the section towards the left lateral moraine than the section towards the right lateral moraine. In this particular profile section (Profile B–B’), the glacier surface in 1964 was already consider- ably lower than the lateral moraines. Furthermore, the undulating surface structure under the lake in 2006 seems unlikely to indicate bedrock, thus suggesting that it most probably consists of glacial ice and/or debris deposits on the ice. Two other profiles C–C’ and D–D’ (Fig. 11) Fig. 10 Topographic contour map (a) and DEM (b) of the 1964 exhibit progressively larger glacial surface lowering in glacier surface in the 2009 lake area, extracted from the up-glacial area. Figs. 5a and 8 respectively. Few studies reporting morphological changes of glaciers (e.g., Bolch et al., 2008a) and glacial lakes (e.g., 1964 to 2006 was 29 m (from 4,954 m to 4,925 m), while Lamsal et al., 2011) in the Nepal Himalaya exist, par- the maximum surface lowering in the lake area for ticularly those concerned with pre- and post-glacial lake the 45-year period from 1964 to 2009 was 134 m (from development and expansion as presented here. Prior to 5,026 m to 4,892 m). Figure 5a clearly shows that the the current study, no field research at the Chamlang area around the largest supraglacial pond had a low South Glacier had been conducted, so a comparison of surface gradient by 1964, and that there had already been glacier surface lowering of the glacier and glacial lake to large melting, which favoured further lake expansion: the its own previous period is not possible. However, the relative height between the lake surface and the lateral glacier surface lowering at the Chamlang South Glacier moraine (DGM of Sakai & Fujita, 2010) exceeded can be compared with two of the most thoroughly 100 m, so the lower area of the Chamlang South Glacier researched glaciers in the same region, i.e., the Khumbu in the 1960s can be categorized as a glacier of the type and Imja glaciers (Bolch et al., 2008a; Lamsal et al., ‘with glacial lake’ of Sakai and Fujita (2010). The area 2011; Nuimura et al., 2011). The results found at the down-glacier from the pond melted less whereas the Chamlang South Glacier agree well with the conclusions steeper up-glacial area melted progressively more, of these studies. For example, Nuimura et al. (2011) accelerating between 1964 and 2006. Considering the calculated a surface-lowering rate of 0.6-0.8 m/year in steeper up-glacier surface and the water-surface level of 17 years from 1978 to 1995 in the ablation area of the the largest supraglacial pond in 1964, it is suggested that Khumbu Glacier, and Bolch et al. (2011) also reported ice calving into the pond triggered the larger and faster 0.5-1.2 m/year of surface lowering in the 40-year period up-glacial lake expansion (Kirkbride & Waren, 1997; from 1962 to 2002 for the same glacier. Lamsal et al. Röhl, 2008; Fujita et al., 2009; Sakai et al., 2009). One (2011) found surface lowering of the dead-ice and reason for low surface lowering in the dead- ice area as up-glacier area of the Imja Glacier for the 42-year period well as non-downward lake expansion could be due to a from 1964 to 2006 at the rate of 0.4 m/year and thicker debris cover there, as debris thickness increases 1.1 m/year, respectively. Further, Fujita et al. (2009) progressively down-glacier, hampering ice melting showed that the lowering rate of the dead-ice area of the (Mattson et al., 1993; Kayastha, 2000; Benn et al., 2001). same glacier for the period from 2001 to 2007 was 0.06 to Thus, ice calving is undoubtedly a main mechanism of 1.03 m/year. Of particular interest is the fact that the rate lake expansion, especially for the further expansion of a of lowering in the lake area of Imja Tsho for 38 years large glacial lake once it has formed. In contrast, direct was the same as that of the Chamlang South Tsho at ice melting, including processes such as water-line 2.2 m/year. Furthermore, Fujita et al. (2009) estimate that melting, aqueous and subaqueous melting, remain largely direct ice melting at the lake bottom of Imja Tsho for the significantly less influential than ice calving processes. past several years is not significant. Therefore, the Subaqueous melting is also very small in the case of the evidence strongly suggests that the larger lowering rate

Changes in Surface Morphology and Glacial Lake Development of Chamlang South Glacier 91

Fig. 11 Cross sections (A–A’, B–B’, C–C’, D–D’) and a longitudinal section (L–L’) showing glacier topographies in 1964 and 2006, and lake bathymetry in 2009. of the Chamlang South Tsho is attributable to ice-calving Lamsal et al., 2011) in the representation of the glacial processes. morphology and surrounding terrain morphology, as well The applicability of repeat remote sensing data, espe- as the documentation of changes over time. Clearly, cially DTMs, in the study of glacial lake dynamics has coarse resolution data such as ASTER and SRTM been described elsewhere (e.g., Kääb, 2005; Surazakov (15/30 m and 90 m spatial resolutions, respectively) & Aizen, 2006; Fujita et al., 2008). Nevertheless, results inhibit the detection of micro-landform features (Fujita from this study are distinguished from the previous et al., 2008). However, our research further demonstrated research through the use of remote sensing data (e.g., and corroborated the previous conclusions of Lamsal

92 T. SAWAGAKI et al. et al. (2011) concerning the use of high-resolution Centre for Integrated Field Environmental Science’ at Corona and ALOS data in the LPS platform, and its Hokkaido University; the National Geographic Society- production of sufficiently accurate DTMs and detailed Waitt Grant Programme, Washington, DC; The Mountain topographic maps that represent changes in glacier Institute, Washington, DC; and Himalayan Research surfaces, including micro-landform features such as Expeditions, Kathmandu, Nepal. supraglacial ponds, ice cliffs, and moraine ridges.

5. Conclusions References

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Takanobu SAWAGAKI

Takanobu SAWAGAKI is an Assistant Professor of the Faculty of environmental Earth Science, Hokkaido University. His major fields of study are Glacial Geology, Quaternary Science and Geographical Information Systems. He has broad experience in circumpolar and alpine regions as a member of Japanese Antarctic research expeditions, the Academic Alpine Club of Hokkaido, and his own research expeditions in cold regions around the world.

Damodar LAMSAL

Damodar LAMSAL obtained his Ph.D. at the Graduate School of Environmental Science, Hokkaido University, in September 2011. His major areas of interest include modeling of glacier surface morphology, glacial lake development, and evaluation of the potentiality of glacial lake outburst floods (GLOFs) and their hazards. He has expertise in geographic information systems (GIS) and remote sensing (RS), especially digital photogrammetry.

Alton C BYERS

Alton C. BYERS, Ph.D., is a mountain geographer, climber and photographer specializing in applied

research, integrated conservation and development programs. He received his doctorate from the University of Colorado in 1987, focus- ing on landscape change, soil erosion, and vegetation dynamics in Sagarmatha (Mt. Everest) National Park, Khumbu, Nepal. He joined The Mountain Institute (TMI) in 1990 as Environmental Advisor, and has since worked as Co-manager of the Makalu-Barun National Park (Nepal Programs), Director of Appalachian Programs, Director of TMI’s 400-acre Spruce Knob Mountain Center in West

Virginia, and Director of Science and Research at TMI since 2004.

Teiji WATANABE

Teiji WATANABE is a Professor of the Faculty of Environmental Earth Science and the Graduate School of Environmental Science, Hokkaido University. Having a background in alpine geomorphology, he specializes in mountain geo- ecology, with interests in interaction of human- geo-ecosystems in high mountain areas and natural resource management in protected areas. He has conducted extensive fieldwork in the Himalayas, the Karakorums, the Alps, the Pamirs, and Japanese mountains.

(Received 20 November 2011, Accepted 13 March 2012)