## 6 Discussion  
### Terminus Change Rates, Velocity, Mass Flux and Elevation Change  
Columbia Glacier has lost area at a rate of 0.64% yr\({}^{-1}\) during 1980-2007 [4; 16]. The west branch showed an even higher loss rate (1.56% yr\({}^{-1}\)) as compared to the main branch (0.17% yr\({}^{-1}\)) during 2011-2016. This corresponds to 2.81 km and 1.08 km mean length change of the terminus of the main branch and the west branch respectively. The glacier's speed up since 1982 is quite evident and strongly linked with the fast glacier retreat [4]. The velocity of [PERSON] (2001) [4]'s measurement point L\({}_{50}\) (red circle in Figure 4) increased from \(<\)2 m day\({}^{-1}\) in 1982 to \(\sim\)8 m day\({}^{-1}\) in 1998. Similarly, high velocities near its terminus was also noticed during 1980-2001 by [PERSON] et al. (2005) [33]. Our results are in agreement with the observations of speedup and strong frontal retreat [4; 16; 33].  
Nevertheless, the velocity observations by [PERSON] (2001) and [PERSON] et al. (2005) [4; 33] are unevenly spaced in time and limited to few velocity estimates on an annual course. They, therefore, do not necessarily reflect any seasonal variability. Moreover, previous measurements during different time spans make direct comparison difficult. From 1985 to 2013, [PERSON] et al. (2015) [3] reported a mean ice flux of 2.6 G yr\({}^{-1}\). They used the mean surface velocity (3.75 m day\({}^{-1}\)) through the flux gate and applied linear rates of surface change to fill temporal gaps of the DEMs during the study period. Their reported mass flux values are higher than those observed by us at the main branch (\(\sim\)1.18 \(\pm\) 0.30 G yr\({}^{-1}\)). There may be various reasons for this: (i) our mass flux estimates correspond only to the main branch; (ii) different observation periods and most likely a different position of the flux gate used in [PERSON] et al. (2015) [3]; (iii) the seasonal variability of surface velocities were estimated using only few annual estimates; (iv) with the retreat on bedrock and split into two branches, surface lowering rates increased and flux was correspondingly reduced as well.  
Figure 4: The surface velocity of Columbia Glacier in the lower catchment area during 09/30/2011–10/11/2011. The branches are indicated by their names “Main Branch” and “West Branch”. A, B, C and D depict the imaginary zones of the main branch based on different surface velocities and frontal retreat. Red circle (L\({}_{50}\)) indicates the measurement point in [4].  
At the main branch, our mean elevation change of \(-\)3.09 m yr\({}^{-1}\) (2011-2014) is slightly more negative compared to the long-term (1957-2007) change of \(-\)2.76 m yr\({}^{-1}\) presented by [PERSON] et al. (2012) [13] for the entire glacier. Elevation change at the west branch is almost double (\(-\)5.65 m yr\({}^{-1}\)) during 2011-2014 than that observed by [PERSON] et al. (2012) [13]. [PERSON] et al. (2005) [33] also reported an elevation change of \(\sim\)12.5 m yr\({}^{-1}\) during \(\sim\)1977-2001, close to L\({}_{50}\) (Figure 4). We observed a large interannual variability of elevation change with values ranging from +7 m yr\({}^{-1}\) to \(-\)40 m yr\({}^{-1}\) at the terminus. These strong interannual variations are most likely caused by the highly varying precipitation in Alaska's coastal region as highlighted by [PERSON] (2012) [16] during 1978-2010. Our differences with the published elevation change measurements in [PERSON] et al. (2012) and [PERSON] (2001) [13; 33] can be attributed primarily to different observation periods and measurement techniques. We showed short-term changes which these studies lack.