## 6 Discussion
### Variations in Ice Dynamics during 2011-2016  
We observed distinct seasonal velocity patterns at the main and the west branch of Columbia Glacier during 2011-2016. The main branch flowed with high velocities during late winter to midsummer but the velocity declined towards autumn with a subsequent increase during winter again. The west branch has also seasonal velocity fluctuations with comparably lower magnitudes. We hypothesize that the development of the basal drainage system primarily controlled these seasonal velocity patterns. We expect that the subglacial drainage system was not yet sufficiently developed to fully route the meltwater in winter and the early melt season [10]. This increased the basal water pressure leading to an increase in basal motion and surface velocity of the glacier. During late summer the drainage system was fully formed to channelize the flow of the water. This caused a drop in basal water pressure at the bed outside the channels and correspondingly reduced velocity. As the season changed from autumn to winter, the drainage channels tended to close due to viscous deformation [34]. Consequently, the summer meltwater, retained in the firn and ice body could not be drained effectively through the channels and lubricated the ice-bed interface, leading to an increase in velocity. The loop closed when the drainage system started to develop again in the consecutive spring. Previous studies at Columbia Glacier [33; 35; 36] support our hypothesis by showing a link between velocity variations and subglacial hydrology. In order to show this link, [PERSON] et al. (2005) and [PERSON] et al. (1994) [33; 36] observed ice velocity, basal water pressure and rainfall data at Columbia Glacier on a diurnal scale between 07/05/1987 and 08/31/1987. Similarly, [PERSON] et al. (2011) [9] observed the coupling of ice dynamics to basal water pressure on Perito Moreno Glacier in Patagonia. The observed flow regime of the main branch is similar to the type 3 behavior of tidewater glaciers in Greenland [7]. These authors also attributed type 3 behavior to seasonal alteration between inefficient to efficient subglacial drainage system. Although the seasonal behavior was consistent in every year of our observations, the magnitude of interannual surface velocity varied (e.g., the terminus velocity of \(\sim\)6.5 m day\({}^{-1}\) and \(\sim\)1 m day\({}^{-1}\) in autumn of 2011 and 2012, respectively). We could not observe a direct link or correlation to the meteorological variables. Also, no melt amount was available to test the hypothesis of [PERSON] et al. (2013) [8] of summer melt regulating winter glacier flow. One explanation might be that the variation in interannual velocity depended on the hydraulic capacity of the drainage system and its connectivity to the unchannelized drainage system [11]. After 2014, the rise in surface flow of the main branch coincided with its strong frontal retreat (Figure 2c,f). To explain the frontal retreat, we assume that the ice might be floating, and hence prone to retreat, if the near front surface elevation (\(H_{f}\)), above the sea surface, satisfies Equation (7), defined as  
\[H_{f}<(\frac{\rho_{water}}{\rho_{ice}}-1)*H_{s} \tag{7}\]where (\(H_{s}\)) is the depth of submerged ice, (\(\rho_{ice}\)) and (\(\rho_{water}\)) are the densities of ice (917 kg m\({}^{-3}\)) and water (1030 kg m\({}^{-3}\)) respectively. [PERSON] (2001) [4] reported up to 400 m water depths of Columbia Glacier forebay based on boat-borne fathometer observations during 1995-1997 (his Figure 18). Recently, [PERSON] et al. (2016) [37] also showed an updated bathymetry of the fjord (2011) of maximum 420 m below sea level (their Figure 1). The terminus has retreated further behind their [4; 37] water depth estimates and no other bathymetric measurements, especially nearby the terminus position during 2011-2016, are available in Columbia Bay. Therefore, we assumed a maximum water depth of 400 m which is equal to the depth of submerged ice (\(H_{s}\)) when the ice is grounded.