Digital landscapes of deglaciation: Identifying Late Quaternary glacial lake outburst

High resolution DEMs obtained from LiDAR topographic data have led to 9 improved landform inventories (e.g. landslides and fault scarps) and understanding of 10 geomorphic event frequency. Here we use airborne LiDAR mapping to investigate 11 meltwater pathways associated with the Tweed Valley palaeo ice-stream (UK). In particular 12 we focus on a gorge downstream of Palaeolake Milfield, previously mapped as a sub13 glacial meltwater channel, where the identification of abandoned headcut channels, run-up 14 bars, rock-cut terrace surfaces and eddy flow features attest to formation by a sub-aerial 15 glacial lake outburst flood (GLOF) caused by breaching of a sediment dam, likely an esker 16 ridge. Mapping of these landforms combined with analysis of the gorge rim elevations and 17 cross-section variability revealed a two phase event with another breach site downstream 18 following flow blockage by high relief drumlin topography. We estimate the magnitude of 19 peak flow to be 1-3 x 10 m/s, duration of the event to range from 16-155 days, and a 20 specific sediment yield of 10-10 m/km/yr. We identified other outburst pathways in the 21 lower Tweed basin that help delineate an ice margin position of the retreating Tweed Valley 22 ice stream. The results suggest that low magnitude outburst floods are under-represented 23 in Quaternary geomorphological maps. We therefore recommend regional LiDAR mapping 24 of meltwater pathways to identify other GLOFs in order to better quantify the pattern of 25 freshwater and sediment fluxes from melting ice sheets to oceans. Despite the relatively 26 low magnitude of the Till outburst event, it had a significant impact on the landscape 27 development of the lower Tweed Valley through the creation of a new tributary pathway and 28 triggering of rapid knickpoint retreat encouraging new regional models of post-glacial fluvial 29 landscape response. 30