## 4 Results  
### Annual Deposition of Impurities  
At many locations of 79 NG we find the glacier surface to be clean after the drainage of supraglacial lakes. Some examples are shown in Figure 2. For quantifying the change in reflectance over time, we found unfortunately only one appropriate lake, due to constraints such as S2 launch, cloud free coverage and snow free coverage with frozen surface. This lake is shown in Figure 3. We named this lake 'Lake iCUPE'. Lake iCUPE has drained annually in the past years on: 21 August 2015, 17 July 2016, 16 July 2017, 1 August 2018, 17 July 2019 and 22 June 2020. After the drainage the surface appears bright, while over time this bright spots darkens, as can be seen in the right panel of Figure 3. As in 2019 major cracks further south appeared, which reached into our lake ground, the undisturbed time period ends 2018. In 2015 there has been no S2 coverage, thus we end up with the time period 2016-2018 to quantify the change in reflectance. From a Sentinel-2 scene acquired on 28 July 2017 three brighter spots were digitized manually and the timing of their center positions was quantified from a flowline analysis (Figure 3). By projecting the locations of the bright spots back in time we found the centers of the two former lake grounds to be at the actual lake position on 11 August 2015 and 24 July 2016. As the glacier moves with a speed of about 1000 m/a, the lake ground (\(\sim\)800-900 m length) of year 2 is separated from the year 1's lake ground surface by about 100-150 m. A comparison of the reflectance of the lake ground over four subsequent patches of former lake ground indicates the in-situ darkening by deposition: the reflectance drops within the first year after drainage from 0.8 to 0.68 and further to 0.62 two yearsafter drainage (Figure 3). After that, the former lake ground cannot be distinguished from surrounding areas. Here we assume that snow albedo is connected to reflectance as analogous but we are only making qualitative use of the terms. We infer that the change in reflectance is due to local deposition of dust, as we can rule out lag-deposit to govern this effect: a comparison with a further upstream situated lake, Lake CE (Figure 1), shows that in that area, changes of reflectance with distance (time) is not significant (see Figure S1). Ablation rates and flow speed together amounts to roughly the same amount of melt at the surface, leaving to similar amounts of lag-deposit. This is in conflict to the strong change in reflectance at the floating tongue (Lake iCUPE), that can only be achieved by deposition in addition to lag-deposits. Figure 3: **(a)** Spectral behaviour of three different lake floors formed in three different years. Coloured markers have been averaged considering the respective region of interests on Sentinel-2 BOA reflectance image (panel **b**, image acquired on 28 July 2017). Black lines are the spectral Vis-SWIR acquisitions performed at the surface by [PERSON] et al. (2017): A—surface cryoconite; B—uniform dark; C—intermediate dark; D—light dark; E—Pleistocene ice; F—Holocene ice; G—snow. (**c**) sketch explaining the process from lake drainage to subsequent dirt deposition. Figure 2: Former lake grounds (bright areas) in the study area as seen by Sentinel-2 on 28 July 2017 (a), 31 July 2016 (b), 5 August 2018 (c) and by aerial photography on 8 August 2013 (d). The white box marks the location of former Lake iCUPE from 2015 to 2017, for more details see Figure 3.  
We hypothesize that this deposition is caused by surrounding soil and mountains (despite that we have not analysed any samples), as [PERSON] et al. (2018) have shown dust to come indeed from nearby mountains. Although Lake iCUPE is somewhat away from the former field site of the study of ([PERSON], 2018), it is very likely that at about the same elevation the source of the dust is similar. Fresh wet and dry deposition from non-local sources is anticipated to have orders of magnitude lower impact and is thus unlikely to cause such severe change in reflectance. Ref.