## 7 Conclusions  
We presented the analysis of a new dataset (TanDEM-X) on changes in surface velocity, mass flux (only at the main branch during 2011-2014), surface elevation and front position of Columbia Glacier, Alaska during 2011-2016. Our results support the hypothesis that the transition from an inefficient to an efficient subglacial drainage system and associated changes in basal water pressure are primarily responsible for seasonal changes of the surface velocity of Columbia Glacier. This hypothesis is supported by the field investigations carried out at Columbia Glacier during summers of 1987. However, for short durations (2014-2015 and 2015-2016 at the main branch, and 2013-2014 at the west branch), an abrupt rise in surface velocity was coincident with strong frontal retreat and extreme near-front thinning. It appears that these retreat sequences were strong enough to break the seasonality of the front position of the west branch, in particular during 2013-2014 when it's near front thinned incredibly (Figure 6). We expect that these strong retreat sequences initiated by near-front thinning which further reduced resistive stress and effective pressure, inducing the glacier flow. However, we stress that the influence of other physical variables (e.g., bedrock topography, tidal influence) on the ice dynamics of Columbia Glacier need further consideration and investigation. The mass flux from the main branch varied with the surface velocity. We stress that the seasonality and interannual velocity changes should be considered in order to precisely estimate frontal ablation including ice discharge.  
We also conclude that short observation intervals are required in order to avoid decorrelation due to structural changes, especially for fast flowing glaciers like Columbia Glacier. The high temporal and spatial resolution of the German TanDEM-X mission is of additional benefit for tracking surface structures, especially for fast moving glaciers like Columbia Glacier, and delineating the front position. The time series of bistatic acquisitions proved to be highly suitable in deriving high-resolution DEMs (surface elevation) without the limitation of temporal decorrelation and estimating relative elevation changes during melt seasons. The X-band signal of TanDEM-X can be strongly influenced by the prevailing surface conditions (melt and freeze of firn and snow) and thus limits elevation change measurements.  
The supplementary information is available online at www.mdpi.com/2072-4292/9/6/635/s1. The TanDEM-X DEMs and glacier surface velocity generated for this study are available via the PANGAEA database ([[https://doi.pangaae.de/10.1594/PANGAEA.876219](https://doi.pangaae.de/10.1594/PANGAEA.876219)]([https://doi.pangaae.de/10.1594/PANGAEA.876219](https://doi.pangaae.de/10.1594/PANGAEA.876219))).  
The study was funded by the HGF Alliance \"Remote Sensing and Earth System Dynamics\". The publishing of this paper was financially supported by the FAU open access fund provided through the German Research Foundation (DFG). The TanDEM-X data used in the study was kindly provided under DLR AO XTI_GLAC0264 and two pairs of TerraSAR-X data were obtained under DLR TSX AO LAN_0164. The auxiliary data was kindly provided by NASA, USGS, NSIDC, GLMS and [PERSON] (University of Alaska Fairbanks). The authors would like to thank [PERSON] and [PERSON] (both FAU) and 3 anonymous reviewers for their useful suggestions.  
[PERSON] processed and analyzed the data and jointly wrote the manuscript with [PERSON]. [PERSON] initiated and supervised the study.  
The authors declare no conflict of interest.