3.1 Klutlan Glacier
3.1.1 Surface displacement
During the study, 41 and 28 surface displacement maps of Klutlan Glacier were produced from 1990 to 2019 using COSI-Corr and CIAS software (Figure 3-6; Figures S3 and S4) respectively. Between 1990 and 1998, the maximum surface displacement was 0.4 ± 0.03 m d-1in the upper reservoir zone (43 km up-glacier from terminus) and less than 0.1 ± 0.05 m d-1in the from terminus to ~25 km up-glacier the maximum surface displacement remained. However, during 1996-1997 surface displacement increased slightly (0.3 ± 0.04 m d-1), ~30 km up-glacier from terminus (Figure 2a). Between1 July 1998 and 5 August 2002, no change was observed in surface displacement (0.4 ± 0.03 m d-1) in the upper reservoir zone (Figure 2b).
Between 5 August 2002 and23 July 2006, the surface displacement increased from 0.4 to 0.5 ± 0.04 m d-1between ~35-40 km up-glacier from the terminus (Figure 2c). The surface displacement profile steepened and became slightlyconvexbetween 25 km and 50 km up-glacier from terminus during 2002-2007 in comparison with 1999-2002 (Figure 2b & c). The steepening of the surface displacement profile can be attributedto mass accumulation in the reservoir zone, which leads to an increase in the basal shear stress (Dolgoushin and Osipova, 1975; Jiskoot, 2011). Between2007 to 2011 the surface flow speedwas 0.3-0.4 ± 0.04 m d-1 at ~35 km up-glacier from the terminus (Figure 2dand 5a) and between 2011 and 2013, the flow speed increased to 0.6 ± 0.02 m d-1between25-45 km up-glacier from the terminus. However, the surface flow speed near the glacier terminusremained consistently low during these six years (~ 0.05 ± 0.02 m d-1) (Figure 2d).
The increase in surface speed(maximum 0.95 ± 0.02 m d-1) continued in 2013-2014, between 25 and 35 km up-glacier from the terminus. Between21 August 2014 to08 August 2015, the maximum surface speeddoubled to 2.1 ± 0.02 m d-1 (Figures3a and 5c) and between 2015 and 2016, the maximum surface displacement increased further to 3.6 ± 0.02 m d-1, ~15 km up-glacier from terminus (Figure 3b and 5d). The first maximum surface displacement(surge peak6.2 ± 0.14 m d-1) observed between 15 May and 09 July 2016, at ~15 km from terminus (Figure 4). However, the maximum surface displacement decreased (2 ± 0.23 m d-1)gradually in July to October 2016 (Figure 3c).
The high flow speed continued untilJuly 2018. Between 18 June and 6 August 2017, the maximum surface displacement again increased (4.5 ± 0.1 m d-1), ~25 km up-glacier from terminus (Figure 3d). and between 22 June and 24 July 2018 we observed a second peak in flow speed (~5 ± 0.23 m d-1) near theterminus (Figure 4). The surge front reached the terminus inthis period and the surge continued until October 2018 with slow speed (~1 ± 0.3 m d-1) (Figure 3d). During 03 August to09 September 2019, the maximum surface displacement decreased gradually (0.7 ± 0.23 m d-1)~40 km up-glacier from terminus (Figure 3e).
3.1.2 Elevation change
Seven elevation change maps of Klutlan Glacier using ASTER DEMs were generated (Figure 6, 7) to understand glacier surface elevation changes before, during and after the surge. All the ASTER DEMs were subtracted from the 2002 ASTER DEM to understand the mass transfer from reservoir zone to receiving zone. The location of DBL changed frequently during the surge phase and moved significantly down-glacier from 2015 to 2019 (Figure 6 b-f). Between 2009 and 2002, the receiving zone (from 15 to 20 km up-glacier from terminus) maximum lowered by -40 ± 30m and the reservoir zone (between 40 and 50 km up-glacier from terminus)maximum thickened by +20 ± 30m. During 2015 the DBL moved ~6.1 km(1800 masl.) down-glacier in comparison to 2004 (1990 masl.). Also, lower receiving zone (up to 10 km up-glacier from terminus)maximum thinned by-80 ±28m and the surge front achieved the height of ~38 ± 28 m during this period, at ~22 km up-glacier from the terminus in this period.
In 2016, the lower receiving zone (0-5 km up-glacier from terminus) exhibited maximum lowering of -70 ±22 m whereas the surge front thickened by +45 ±22 m, ~17 km up-glacier from the terminus. During 2018, the thickened region moved down-glaciertowards terminus and the receiving zone (~12 km up-glacier from terminus) gainedmaximum thickening of +52±23 m. In 2019, the lower receiving zone (0-5 km up-glacier from terminus) showeda maximum gain of +31 ±40 m whereas the reservoir zone (~38 km up-glacier from terminus) maximum thinned by -65 ± 33 m. On average, in2019 the DBL shifted ~16 km (788 masl.) down-glacier in comparison to 2004 (1990 masl.).
3.2 Fisher Glacier
3.2.1 Surface displacement
We generated42 surface displacement maps of FisherGlacier using COSI-Corr from 1984 to 2019 which covers quiescence, build-up and active phase (Figure 8-10).Also, 15 surface displacement maps of Fisher Glacier were generated using CIASbetween2000and2018to cross-check results (Supplementary Fig. S5 and S6).During 28 July 1984 to 09 September 1988, surface displacement wasslow (<0.10 ± 0.04 m d-1) throughout the glacier. However, we observed a slight increase (0.20 ± 0.04 m d-1) in surface displacement in upper reservoir zone (25 km up-glacier from terminus) during 09 September 1988 to 14 August 1990(Figure 8a). Between 30 June 1991 and 31 July 1999, the surface displacement gradually increased from 0.1 to 0.2 ±0.04 m d-1 between 25 and 30 km up-glacier from terminus (Figure 8b).However, we did not observe any change in surface displacement (0.20 m d-1) in the reservoir zone (~20 km up-glacier from terminus) between 31 July 1999 and 19 June 2007 (Figure 8c) butthisslightly increased (0.2 ± 0.04 m d-1), at ~12.5km up-glacier from terminus during 02 July 2006 to 19 June 2007.Surface displacement profile gradually steepens along the central linebetween 1984 and 2007, especially in the upper reaches of the glacier (Figure 8a, 8c). Such steepening of surface displacement profile can be attributed toice mass accumulation in the reservoir zone which leads to increase inspeed of surface displacement(Dolgoushin and Osipova, 1975).
We observed the pre-surge accelerationbetween 2007 and 2013 when the maximum surface displacementincreased consistently from 0.20 to 0.40 m d-1 in upper reservoir zone (~25 km up-glacier from terminus)(Figure 8d, 10). Several studies have been considered such a gradual increase in surface displacement across the glacier before the main surge phase as a pre-surge acceleration. For instance, Round et al., (2017) reported about 2.5 years long pre-surge acceleration of Kyagar Glacier in the KarakoramfromSeptember 2011 to February 2014.
The maximum surface displacement doubled (0.80 m d-1) during 2013-2014,at ~15 km up-glacier from terminus (Figure 9a). However, we did not observe any advance in terminus till 2014. We observedan active phase in early June 2014 when maximum surface velocity reached 2.5 ± 0.2 m d-1(Figure 10). In the first week of August 2015, surge peak (~7 ±0.45 m d-1)was observed near the glacier tongue. The surge of Fisher Glacier producedeight times acceleration in surface displacement from July 2014 and a seventeen timesacceleration since August 2013(Figure 10). However, after then the surge continued with a diminishing rate of surface displacement (1.5 ± 0.13 m d-1), until 27August 2016 (Figure 9c).
Between 27 August 2016 and 08 August 2017, the surface displacementwas low (~0.2± 0.01 m d-1)and decreased further to~0.05± 0.01 m d-1from 30 September 2017 to 03 August 2019(Figure9d, 10). Many studies also reported such slow speed (0.05 m d-1) of glacial surface occurred in quiescent phase (Bevington and Copland, 2014; Kochtitzky et al., 2019).
3.2.2 Elevation change
The six ASTER DEMs were subtracted from 2003 ASTER DEM to understand the mass transfer from the reservoir zone to the receiving zone(Figure 11). Due to limited coverage of ASTER DEMs, the DEM differencing results cover parts of Fisher Glacier but includessurface elevation changes associatedwith before, during and after the surge (Figure 12).During 2016, the reservoir zone (between 35 and 40 km up-glacier from terminus) maximum thinned by -67± 27m and the lower receiving zone (~10 km up-glacier from terminus) gained maximum thickness of +41± 27 m. In2018, the receiving zone exhibited average thickening of +88 ± 25 mwhereas the surge front achieved a gain of+110 ± 25m elevationnear the terminus. During 2019, the DBL moved ~3 km (959 masl.) down-glacier in comparison to 2016 (1006 masl.) meanwhile the reservoir zone (~35 km up-glacier from terminus) experienced -60± 22mmass loss.
3.2.3 Terminus and frontal lakechanges
We used the 1973 Landsat MSS image as a reference to estimate the change in glacier length of Fisher Glacier (Figure 13). The terminus of Fisher Glacier retreated -422±121m and -2012±57m in1984 and 2008 respectively(Fig. S9).Although the surface speedof Fisher Glacier gradually increased since2008 and the surge initiated in 2013,the terminus of Fisher Glacierreceded -536± 57 m between2008 and 2014. The surge advancebetween 2014 and 2016 amounted to +1356±29m (Supplementary Table S4).The frontal lake area increased from 0.54 km2 to 2.8 km2 between 1984 and 2010 and increased further to 3.2 km2 until 2014. Due to the advancing terminus the like size reduced by 50%, to 1.6 km2, between 2014 and 2016 (Supplementary Table S8).