Reply on RC2

Abstract

Glacial lake outburst floods (GLOFs) or ‘jökulhlaups’ from ice-dammed lakes are frequent in Greenland and can influence local ice dynamics, bedrock displacement, geomorphological changes and flooding hazards. Multidecadal time series of lake drainage dates, drainage volumes and flood outlets are rare but essential for understanding the impact on and interaction with the surroundings, identifying drainage mechanisms, and for mitigating downstream flood effects. In this study, we use ultra-high-resolution structure-from-motion (SfM) digital elevation models (DEM) and orthophotos from unmanned aerial vehicle field surveys in combination with optical satellite imagery to reconstruct robust lake volume changes associated with 14 GLOFs between 2007 and 2021 at Russell Glacier, West Greenland. This makes it, one of the most comprehensive and longest records of ice-dammed lake drainages in Greenland. We find a mean difference of 10 % between the lake drainage volumes compared with estimates derived from a gauged hydrograph 27 km downstream. Due to ice dam thinning, the potential maximum drainage volume in 2021 is c. 60 % smaller than that estimated to have drained in 2007. Our time series reveals variations in the drainage dates ranging from late May to mid-September and moreover that drained volumes range between 0.9–37.7 M m3. We attribute these fluctuations between short periods of relatively high and low drainage volumes to a weakening of the ice dam and an incomplete sealing of the englacial tunnel following the large GLOFs. The syphoning drainage mechanism is triggered by a reduction in englacial meltwater, likely driven by late seasonal drainages and sudden temperature reductions, as well as annual variations in the glacial drainage system. Furthermore, we provide geomorphological evidence of an additional drainage route first observed following the 2021 GLOF with a sub- or englacial and supraglacial water flow across the ice margin. It seems probable that the new drainage route will become dominant in the future which will drive changes in the downstream geomorphology and raise the risk of flooding-related hazards as the existing buffering outlet lakes will be bypassed.