Study Sites and Observations

Tsho Rolpa (water level, 4,580m a.s.l.; 27°51'N, 86°29'E) and Imja (5,009m a.s.l.; 27°54'N, 86°55 E) in the Nepal Himalayas and Lugge (4,539 m a.s.l.; 28°5'N, 90°18'E) in the Bhutan Himalayas (Fig. 1) commonly contact both side moraines and the cliff-shaped glacial terminus at uplake (Fig. 2A). Imja and Lugge abut on the dead-ice zone at downlake, which is 20 to 30m higher than the lake level, but Tsho Rolpa touches the end moraine near the lake level (Fig. 2B).

The histories of the lake expansion for Tsho Rolpa and Imja are shown as an increase of the lake surface area in Fig. 3. It should be noted that an increasing rate of the surface area is clearly linear for each lake. Tsho Rolpa and Imja laterally contacted both side moraines in the 1960s and 1980s, respectively. The width of the water surface is longitudinally roughly constant for each lake (Fig. 1). The linearity in Fig. 3 thus means that the rate of the glacial retreat is roughly constant after the 1960s or 1980s. However, it is unknown why the glacial retreat occurs at a constant rate for each lake. The increasing rate for Tsho Rolpa is consistently higher than that for Imja. Sakai et al. [11] indicated that Imja increased in surface

area and water volume from 0.60 km and 2.8 x 10 m in 1992 to 0.86 km and 3.6 x 10 m in 2002, respectively. The mean water depth of Imja is thus 46.7m in 1992 and 41.9m in 2002. Hence, the expansion of the lake basin likely prevails horizontally more than vertically. Lugge has also been expanding by the glacial retreat of approximately 200 m in 1993 to 2004 [12]. However, its increasing rate is less than 0.01 km2/yr [13], which is smaller than that of Imja. This small rate is probably due to the relatively large downstream movement of Lugge Glacier. By observing the movements of Lugge Glacier and the neighboring Thorthormi Glacier with no moraine-dammed lake, Naito et al. [14] suggests that the calving at the Lugge glacier terminus facilitates the downward glacial movement in the downstream.

Sachigo Lake Ground Temperature Map
Figure 1. Location of three moraine-dammed lakes: (A) Tsho Rolpa; (B) Imja; and (C) Lugge, and observation sites on the bathymetric maps. The bathymetries of Imja, Tsho Rolpa, and Lugge were obtained in 1992 [8], 1993 [9] and 2002 [10], respectively.
Figure 2. Tsho Rolpa Glacial Lake: (A) cliff-shaped glacial terminus in contact with the lake water at uplake (27 May 1995) and (B) the end moraine only 2 to 3m higher than the water level (26 May 1995). The glacial terminus above the lake level is ca. 20m high.

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Ji?ti I'X.I I 1470 l'»75 I vxi I IW5 1991 [145 2(11)1) 2(1 H5 2d ill

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Ji?ti I'X.I I 1470 l'»75 I vxi I IW5 1991 [145 2(11)1) 2(1 H5 2d ill

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Figure 3. Transition of the surface area for Tsho Rolpa and Imja (modified after [4, 17]). The black and white circles for Imja were obtained by topographic surveys and satellite images (SPOT with resolution 10m or Landsat7 with resolution 15m), respectively.

Physical conditions of Tsho Rolpa, Imja, and Lugge were examined in the pre-monsoon seasons of 1996 (Tsho Rolpa) and 1997 (Imja) and in the post-monsoon season of 2002 (Lugge) by using a TTD (temperature-turbidity-depth) profiler (model ATU200-PK, Alec Electronics, Inc., Japan; 0.2m pitch) and sampling lake water [15, 16, 17]. The vertical measurements of water temperature and turbidity were longitudinally carried out three or four times at some points along the centerline of the elongated lakes (Fig. 1). The water turbidity (ppm) was converted into suspended sediment concentration (SSC) (g/L) by using the significant correlation (r = 0.836 to 0.996) between water turbidity and SSC. The "surplus density", a, of lake water was numerically obtained by G = (pec -1000) x 10, where pec is the bulk density (kg/m3) of lake water at SSC, C (g/L), water temperature, 9 (°C), and dissolved solids, D (g/L). The bulk density pec is defined by pec = (1 - C / ps )pe + C, where ps is the particle density of suspended sediment (= 2,730 kg/m3 for Tsho Rolpa, 2,750

kg/m3 for Imja, and 2,760 kg/m3 for Lugge [10, 16, 17]) andp9 is the water density at 9 and D. Dissolved solids in lake water were nearly uniform at ca. 0.030 g/L for Tsho Rolpa, at ca. 0.017 g/L for Imja and at ca. 0.025 g/L for Lugge [18]. Thus, the effect of dissolved solids on the water volume is negligibly small.

The monitoring of water temperature, turbidity, and flow velocity was simultaneously conducted in Tsho Rolpa by mooring turbidimeters and current meters at some depths [16]. The particle size of suspended sediment was analyzed for less than 44pm grains by the photo-extinction method and for more than 44pm grains by sieving [19].

The meteorology (wind, air temperature, relative humidity, rainfall, air pressure, and solar radiation) was measured at site AWS on the end moraine of Imja, near the end moraine of Tsho Rolpa or on the dead-ice zone of Lugge (Fig. 1) [16, 17, 20]. Using the meteorological and hydrological data, Sakai et al. [2] estimated the heat balance of Tsho Rolpa and thereby the ice-melting rate.

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