1Centre for Glaciology, Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, UK
2Glaciology Group, College of Science, Swansea University, Swansea, SA2 8PP, UK
3Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden
4Department of Geography, The Ohio State University, 1036 Derby Hall, 154 North Oval Mall, Columbus, Ohio 43210-1361, USA
*now at: International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, USA
Abstract. The hydraulic fracture of ice during the rapid tapping of supraglacial lakes is proposed as one mechanism to establish efficient surface-to-bed hydraulic pathways through kilometre-thick ice. This study presents detailed records of lake discharge, ice motion, and passive seismicity capturing the behaviour and processes preceding, during and following the rapid (~2 h) tapping of a large (~4 km2) supraglacial lake through 1.1 km of the western margin of the Greenland Ice Sheet. Peak discharge (3300 m3s−1) was coincident with maximal rates of horizontal displacement and vertical uplift, indicating that surface water accessed the ice-bed interface causing widespread hydraulic separation and enhanced basal motion. The differential motion of four GPS located around the lake, record the opening and closure of fractures suggesting that on short time-scales the brittle fracture of ice dominates ice flow. We hypothesise that during lake tapping, drainage occurred through a ~3 km long longitudinal fracture with a mean width of ~0.4 m. The perennial location of the supraglacial lake and the observed pattern of fracturing and surface uplift evince control by the local subglacial topography and the gradient of hydraulic potential. Our observations support the assertion that water-filled crevasses can propagate without longitudinal extension. The tapping of the lake coincided with the rapid drainage of a cluster of supraglacial lakes located within the same elevation band coincident with a notable and isolated peak in the catchment-wide, proglacial Watson River hydrograph.