## 6 Discussion
### Variations in Ice Dynamics during 2011-2016
Feeding the known variables in the Equation (7) indicates that the ice might be floating if \(H_{f}\) is less than \(\sim\)50 m. Hence, we analyzed those areas where less \(H_{f}\) is less than 50 m during 2011-2016 (Figure 5) to check whether the retreat was triggered in those areas. The terminus of the main branch was almost stable during 2011-2012 and positive elevation changes were observed in this period (Figure 2e). Only few areas showed \(H_{f}\) less than 50 m (panel 06/15/2012 in Figure 5). However, more areas had thinned during 2014-2016 (panels 06/22/2014, 11/04/2015, 05/20/2016 in Figure 5). These thinned areas (zones C and D in Figure 4) have undergone significant retreat during 2014-2016. Zones A and D even retreated during 2011-2014 but the retreat was insignificant (Figure 4). In contrary to this, zone B had been almost stable during 2011-2016 which can be explained by its high surface elevation (\(H_{f}>50\)). We expect that extreme near-front thinning caused the main branch to retreat at times. At the west branch, we also observed a close link between frontal retreat, thinning and ice velocity. The branch retreated almost twice as much in 2011 as compared to the retreat in 2012 (Figure 3c). This is most likely because the ice was thinner at more areas in 2011 compared to 2012 (panels 06/18/2011 and 06/15/2012 in Figure 6). In 2013 (panel 06/13/2013 in Figure 6), thinning increased, which might have triggered a very strong frontal retreat lasting almost 2 years (2013-2015). This major retreat sequence even broke the seasonality pattern of the terminus position (advance during winter-spring and retreat during summer-fall). The terminus of the west branch did not retreat much during summer-fall of 2015 and was almost balanced after the major retreat event. The velocities also reduced after the summers of 2015 and did not reach as high values as in 2014. This is also in line with the surface elevations in 2015-2016 (panels 04/11/2015, 05/20/2016 in Figure 6) when fewer areas had elevations below 50 m.  
We give a hypothesis in order to explain such events at both the branches when very high surface velocities were associated with major retreating sequences and strong near-front thinning. We expect that the retreat was probably triggered by strong ice thinning which also caused reduction of resistive stress greater than the reduction of driving stress. Consequently friction at the bed decreased, inducing speedup. Similar conclusions were obtained by [PERSON] et al. (2008) [6] in Greenland. At Columbia Glacier, [PERSON] et al. (2005) [33] also found little correspondence between driving stress and ice velocity but the basal drag (a part of resistive stress) played a significant role for the same. The controlling processes for calving are an active area of research [38]. We stress that the role of driving and resistive stresses, bedrock topography, ice-ocean interaction, tidal influence and changes in water column temperature on the ice dynamics of Columbia Glacier need to be further investigated. Figure 5: The colors indicate the ice surface elevation (H\({}_{f}\) in Equation (7)), above the sea surface, of the main branch, ranging from 0 to 50 m, on the respective dates. The dates are mentioned on the top of each panel. The white line represents the front position of the main branch on 06/18/2011. Figure 6: The colors indicate the surface elevation (H\({}_{f}\) in Equation (7)), above the sea surface, of the west branch, ranging from 0 to 50 m, on the respective dates. The dates are mentioned on the top of each panel. The white line from (**i**-**vi**) and (**vii**–**x**v**) represents the front position of the west branch on 06/18/2011 and 06/13/2013 respectively.