## 1 Introduction  
Although Alaskan glaciers comprise \(\sim\)12% of the world's glaciated areas outside the large ice sheets [1], they contributed only 8% (0.07 mm \(\,\mathrm{SLE\,yr^{-1}}\)) to the globally observed sea-level rise during 1901-2009 [2]. Columbia Glacier is a large marine-terminating glacier in Alaska, which has rapidly retreated, accelerated and contributed significantly to sea level rise since the early 1980s [3, 4]. Out of 27 marine-terminating Alaskan glaciers, Columbia Glacier accounted for about one-fourth (0.010 \(\pm\) 0.002 \(\,\mathrm{mm\,SLE\,yr^{-1}}\)) of the mass loss due to frontal ablation during 1985-2013 [3]. Dynamic discharge from marine-terminating glaciers is also a significant source of sea-level rise [5]. The magnitude of frontal ablation including ice discharge depends on ice velocity, which itself is influenced by the subglacial drainage system, surface melt and runoff, and dynamic adjustment to thinning and retreat of the ice front [6; 7]. It has been reported that marine-terminating glaciers can exhibit strong seasonal surface velocity variations due to above-mentioned physical processes (e.g., [7]). For Alaska, [PERSON] et al. (2013) [8] reported that wintertime velocity variations are regulated by summer melt. [PERSON] et al. (2011) [9] observed, at Perito Moreno Glacier in Patagonia, a close relation between basal water pressure and surface velocity. They attributed this process to the input of meltwater exceeding the capacity of the subglacial drainage system as the melt season advances. During late summer, the drainage system is completely developed and thus efficiently routes the meltwater, causing water pressure and velocity to drop [10]. The water pressure and velocity might even drop further when a high capacity drainage system evacuates water from the unchannelized regions [11]. [PERSON] et al. (2014) [7] presented seasonal velocity variations of marine-terminating glaciers in Greenland using time series of TerraSAR-X satellite data. Their observed glaciers exhibited strong seasonal variations driven by meltwater availability. A group of glaciers, abbreviated as type 2, showed velocities which are relatively stable from late summer through spring, with a strong early summer speedup and midsummer slowdown [7]. In most cases, winter speed remained lower than the peak summer speed. The velocity of other set of glaciers, abbreviated as type 3, started to decrease during high runoff periods and achieved a pronounced minimum during late summer, when runoff is low [7]. In addition to basal hydrology, [PERSON] et al. (2008) [6] inferred that the terminus retreat can also contribute to the surface flow by weakening the resistive stress. The authors also concluded that the terminus retreat might be triggered in case of significant near-front thinning. [PERSON] (1996) [12] empirically linked the terminus retreat with the terminal thickness in case of Columbia Glacier and concluded that the terminus retreat is likely to trigger when the terminal thickness becomes less than 50 m above the (local) flotation thickness. Columbia Glacier accelerated by a factor of six between 1982 and 2000 [4]. Measurements based on optical imagery during 1985-2013 [3] and TerraSAR-X in 2011 [13] confirmed the near-front speedup even in recent years. However, the very high surface velocities of Columbia Glacier limited the utility of many satellite systems due to their insufficient temporal resolution. For instance, [PERSON] (2013) [14] could not retrieve the surface velocities for Columbia Glacier using ALOS PALSAR imagery with a repeat cycle of 46 days. Therefore, most of the available surface velocity measurements on Columbia Glacier have been based on single or very few image pairs falling within a narrow period of a given year. The observed values were generally considered representative for an annual scale and do not necessarily reflect any seasonal variability. In 2010, Columbia Glacier split into two branches, abbreviated as the main and the west branch (Figure 1).