Esker formation due to supraglacial lake drainage to the bed of the Cordilleran Ice Sheet

A positive relation between increased surface melting and ice acceleration at land-terminating sections of the Greenland Ice Sheet (GrIS), particularly during supraglacial lake drainage, has been suggested as a mechanism for enhanced ice sheet decay. However, the paucity of direct measurements at the ice sheet bed restricts our understanding of subglacial drainage system evolution in response to transient meltwater inputs. On the Fraser Plateau, south-central British Columbia, a single thread ~40 km long and ~0.25-1 km wide erosional corridor, with upslope sections along its long profile, was generated subglacially in response to supraglacial lake drainage during late Cordilleran Ice Sheet (CIS) decay. Corridor fill includes gravel sheets and dunes (>9 m thick), as well as an esker system. In 2010, we collected >4.5 km of ground-penetrating radar (GPR) line and >0.7 km of electrical resistivity (ER) line on the corridor fill. Our data reveal that the esker, which is composed of ridge-wide macroforms that indicate high-magnitude deposition, is draped onto the broader corridor fill. Given the lack of late stage reworking of the corridor fill surrounding the esker, we suggest that waning stage floodwaters were constrained within the esker-forming conduit, following rapid collapse from the corridor-wide flow that eroded the tunnel channel and deposited the broad fill. However, our data reveal that whilst the esker formed subglacially and synchronously, up flow sections locally unroofed during the latest stages of esker formation, resulting in local deposition within subaerial ice-walled channels. This, combined with a lack of corridor fill deformation expected from ice-bed recoupling, indicate that the CIS was thin at the time of supraglacial lake drainage. Our data reveal a relatively simple event sequence that includes evolution from a large erosional channel that discharged rising stage floodwaters following initial supraglacial lake drainage, to a narrower R-channel that efficiently evacuated waning stage flow. Such rapid channelization suggests that thin ice conditions are not conducive to broad ice-bed decoupling and ice acceleration.