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Anderson, Suzanne P. ¹ ; Bartholomaus, Timothy C. ² ; Anderson, Robert S. ³

¹ CU
² CU
³ CU

Observations at Kennicott Glacier, Alaska, suggest that glacier sliding may trigger drainage of Hidden Creek Lake, an ice-dammed lake that annually produces a jökulhlaup. Glaciers slide when basal water pressures are elevated over a broad region of the bed. At Kennicott Glacier we have used solute concentrations, particularly of chloride (Cl-), in the outlet stream as a proxy for mean subglacial water pressures. Chloride concentrations rise and fall over periods of 10-14 days in a pattern slightly lagged with respect to variations in river discharge. Water appears to obtain high Cl- concentration during long, close interaction with the glacier bed, while low Cl- concentrations reflect rain and melt waters that transit the glacier rapidly with little bed interaction. Low Cl- concentration in the outlet stream occur during periods of rising discharge and during the annual outburst of Hidden Creek Lake, times we infer to be characterized by elevated subglacial water pressures that reduce outflow of water from the glacier bed.

Optical surveying of two stakes on the glacier near Hidden Creek Lake in 2000 showed a brief speed-up and sustained ice surface uplift starting 10 days before lake drainage. The timing of surface uplift corresponded to low Cl- in the outlet stream. Although the stakes showed that the ice surface remained elevated, Cl- concentrations climbed by an order of magnitude in the days before lake drainage occurred. We infer that subglacial water pressures remained elevated, but with a head gradient that allowed drainage of subglacial water into conduits and out of the glacier. Drainage of Hidden Creek Lake was accompanied by further uplift and speed-up of the ice surface and by a precipitous drop in river Cl- concentrations. In 2006, differential GPS surveying of five stations along the glacier show that a speed-up began 5 days before the lake drained, again corresponding to low Cl- in the outlet stream. This event differed from 2000 in that Cl- concentrations remained low and lake drainage followed the speed-up event closely in time (5 d versus 10 d). In both years, Cl- concentrations rose following the jökulhlaup and ice velocity and surface uplift returned to pre-flood conditions within a day after lake drainage. The flood precursor common to both years is an ice speed-up and surface uplift event accompanied by a chemical signal of elevated subglacial water pressures. We infer that cavities opened by sliding provided a leak that led to lake drainage.