Jökulhlaup triggering: Observations at Kennicott Glacier, Alaska

Jökulhlaups, sudden releases of water impounded by a glacier, produce large floods unrelated to weather events. We draw on observations from 90 years of annual jökulhlaups from Hidden Creek Lake, Kennicott Glacier, Alaska and from detailed monitoring in 1999 and 2000, to examine conditions that trigger outburst floods. For the class of jökulhlaups caused by subglacial drainage, the trigger must be related to formation of subglacial conduits, a pivotal problem in glaciology. Hidden Creek Lake water level at drainage has declined over the last century, during which time the glacier has thinned. The water level trend is mirrored by a tendency toward earlier release dates in the summer. These observations suggest that a minimum threshold lake level must be exceeded for drainage to occur, and that this threshold is related to ice thickness. The release date varies by over a month, however, and lake level varies by as much as 10 m over spans of a few years, which indicates that more is involved than simple exceedance of a threshold. Kennicott Glacier impounds several other small lakes. In two summers with fairly complete observations of their behavior, these lakes drained in sequence from nearest to furthest from the terminus. More frequent observations have been made of drainage of one of these lakes: Erie Lake, located roughly the same distance from the terminus as Hidden Creek Lake, along a major tributary to the Kennicott Glacier, usually drains within days of Hidden Creek Lake. These patterns are consistent with a trigger that is related to glacier-wide evolution of the hydrologic system, rather than each drainage reflecting purely local conditions. Perhaps there is a linkage between timing of outbursts and seasonal upglacier extension of the subglacial conduit system. While we have no direct information on annual evolution of the conduit system at Kennicott Glacier, some have suggested that conduits and the snowline move upglacier in tandem. In 1999 and 2000, the snowline had retreated far upglacier, beyond Hidden Creek Lake, before it drained. Estimated melt rates in the days preceding Hidden Creek Lake outbursts, calculated with a degree day model, show no pattern: lake drainage has occurred during times of both low and high melt production. Jökulhlaup triggering appears to be controlled by a number of conditions, among them the height of the ice barrier, organization of the subglacial hydrologic system, and specific, probably transient, conditions within that system at the time of drainage.