The role of englacial hydrology and hydromechanical processes during an outburst flood cycle from an ice-dammed lake.

In a time of retreating glaciers, outburst floods from ice-marginal lakes are poised to becomemore prevalent as glaciers thin and tributaries detach. A detailed field-based investigation ofthe filling and drainage of one such lake, dammed by the 70-km-long Kaskawulsh Glacier inYukon, Canada, was conducted to characterize the role of the little-studied englacialhydrological system during these events. We deployed a variety of geophysical andhydrometeorological instruments in and around the ∼1 km2 lake to monitor thehydrology and dynamics of the lake—glacier system. By integrating the resultsfrom all instruments and surveys, we develop a conceptual model that describes theevolution of various storage reservoirs leading up to, during and following lakedrainage. From the beginning of the instrument record in June 2017, the subaerial lake filled at anaverage rate of 0.5 m d−1 before reaching a maximum volume of 9.9x106m3 on 17 August.GPS and timelapse imagery reveal vertical glacier displacements exceeding 25 m near the icefront and 3 m at a kilometer distance, as water is injected beneath a partially floating ice shelf.These data are used to estimate the evolution of the subglacial hydraulic potential, fromwhich a hydraulic seal and likely flood flowpaths are delineated. Abrupt changes in ice-shelfuplift rates, associated with the formation of fractures and faulting, are linked to aredistribution of englacial water. Near the surface, water pressures in multiple boreholesexhibit sudden changes of up to 15 m of head, while at depth, ice-penetrating radarreflection-power measurements indicate fast and slow adjustments in englacialwater storage. The onset of drainage begins within six days of a reversal in thesubglacial hydraulic gradient near the lake, whereby flow across 55% of the lakecatchment area is redirected away from the lake and toward the Kaskawulsh Glacier.Lake outflow discharge appears to increase exponentially over the course of ∼19days, before reaching an estimated maximum of 75-110 m3s−1 on 4 September.Radar data collected after the drainage event suggest that the englacial reservoir didnot empty entirely, hinting at a possible buffering role for the englacial drainagesystem. According to water-balance calculations, the subglacial and englacial reservoirs storeapproximately 55% and 22%, respectively, of the water in the catchment at peak lake level,compared to 23% in the subaerially exposed lake. In our conceptual model, the subaerial,subglacial and deep/shallow englacial reservoirs connect abruptly in a series ofhydromechanical events detected across multiple sensor types. The dynamic coupling ofthese reservoirs and the abrupt nature of connections between them represent anadvance in our conceptual understanding of outburst floods from ice-marginal lakes. [ABSTRACT FROM AUTHOR]