Geomorphic control on regional glacier lake outburst flood and debris flow activity over northern Tien Shan

Glacier lake outburst floods (GLOFs) and related debris flows (DF) are significant natural threats in the Tien Shan Mountains. Their occurrence is favoured by the formation of new glacier lakes and the destabilization of moraines and hillslopes due to climate warming. Understanding the frequency-magnitude of these processes is essential for the implementation of Disaster Risk Reduction strategies. Yet, long-term records of past GLOFs or DF in the region are almost completely missing, which renders rational hazard and risk assessments difficult. Here, we present a unique, multi-century dataset of regional GLOF-DF reconstruction for the Tien Shan based on tree-ring analyses from six different torrential fans, and provide insights on regional processes activity. Based on this dataset, we also test whether GLOF-DF activity is related to glacier degradation and changes in geomorphic characteristics at the catchment scale. Results from >430 disturbed trees growing on six different DF fans suggest frequent GLOF-DF activity since the 19th century, which is consistent with available historical records. We also observe an increase in process activity during the mid-20th century coinciding with phases of glacier stagnation or even slight glacier advances. This means GLOF-DF activity in northern Tien Shan is inversely related to moraine-glacier ratios as well as to glacier area shrinkage rates and fan characteristics (such as slope and depositional area). These findings imply that glaciological and geomorphic features could be used for regional susceptibility assessments in the future. Results presented here are the longest, annually resolved GLOF-DF series in the region, if not worldwide, and constitute a unique dataset to understand process variability. As such, they should be used for further climate change adaptation to mitigate natural hazard and risk in the region