Hidden Creek Lake (HCL), an ice-marginal lake impounded by Kennicott Glacier, Wrangell Mountains, Alaska, fills annually to ~20 to 30 ¿ 106 m3 and then drains subglacially within 2 to 3 days. During the 1999 and 2000 j¿kulhlaups, we carried out a series of planned observations around the lake and in the Kennicott River, which drains the glacier. Approximately 20% of the lake volume was contained within a subglacial water wedge beneath the ice dam. The entire volume of the lake drains through the wedge; hydraulic head loss through this constriction may be responsible for the fairly symmetrical shape of the HCL outflow hydrographs, deduced from lake level records, basin hypsometry, and collapse of the ice dam. The flood hydrographs in the Kennicott River are similar in shape to the outflow hydrographs, and within error, lake volume matched the river flood volume in both years. Up to 12 ¿ 106 m3 of water was temporarily stored within the glacier during the 2000 j¿kulhlaup. During the 2000 j¿kulhlaup the background flow in the Kennicott River shifted to a dilute chemical composition. As the HCL j¿kulhlaup progressed, Donoho Falls Lake filled with water whose chemistry was closer to that of the background flow in Kennicott River than to HCL water. Comparison of these chemical signals with typical summer variations in Kennicott River chemistry suggests that the j¿kulhlaup created high subglacial water pressure that impeded normal drainage of solute-rich water from a distributed drainage system into a conduit system at the glacier bed and even caused flow direction locally to reverse.