Periodic drainage of ice-dammed lakes as a result of variations in glacier velocity

Previous discussions of the catastrophic drainage of ice‐dammed lakes have centred on mechanisms where characteristics of the lake are crucial to drainage initiation, for example dam flotation or tunnel formation at a critical lake depth. This paper describes a mechanism for lake drainage where drainage initiation depends on the characteristics of the glacier and is independent of the characteristics of the lake. Prediction of this mechanism must be based on glacier dynamics, whereas the mechanisms most commonly discussed previously are best predicted primarily on the basis of lake evolution. An ice‐dammed lake at the margin of the glacier Solheimajokull, in southern Iceland, was observed to drain rapidly into the sub‐ or englacial drainage system, supplying water and debris to the bed or interior of the glacier. Geomorphological evidence suggests that the lake drains and refills periodically, discharging up to 13300 m3 of water into the glacier‐hydrological system. The depth of the maximum lake is insufficient to cause either flotation of the ice margin or tunnel opening by plastic deformation of the ice, and we suggest that sudden drainage is related to ice‐bed separations associated with specific glacier flow states rather than to a critical lake depth threshold. This mechanism of lake drainage has implications for conditions at the glacier bed, for the development of basal ice and for the entrainment of debris into the glacier, as well as for the prediction of potentially hazardous catastrophic drainage events and jokulhlaups from ice‐dammed lakes.