DYNAMICS OF THE 1986 AND 2002 HUBBARD GLACIER JöKULHLAUPS

Two of the largest glacier outburst floods in Earth’s post-Pleistocene history occurred in 1986 and 2002 at Hubbard Glacier, the largest marine tidewater glacier in Alaska. Hubbard, an advancing glacier sits at the head of Disenchantment Bay. In late May 1986, Hubbard Glacier blocked the mouth of Russell Fiord, a 60-km-long fiord that connects to Disenchantment Bay from the east, forming Russell Lake. Runoff entering from an 1,800 km² drainage area caused the lake level to rise as much as 0.25 m/d. About 5.5 km³ of water was introduced into the lake at its maximum stage. On October 7, 1986, after 132 days of closure and a water level rise of 25.8 m, the ice dam failed. The lake drained catastrophically, with peak discharge being reached in 6 hours. The flood was over within 24 hours. Average discharge was about 1.1 x 10⁵ km³/s with peak discharge of 1.04 x 10⁶ km³/s.

A similar closure occurred in 2002. By June 13, 2002, Hubbard was pushing sediment against the south wall of Russell Fiord. By early July 2002, the height of the growing moraine prevented tidal exchange between the lake and the bay. By early August, a delicate balance existed between the height of the growing moraine and the volume and height of water in Russell Lake. Drain­age occurred through a narrow channel cut into the top of the moraine. Lakw filling was rapid, averaging 0.22 m/d. The volume of water introduced into the lake by the time the lake reached its maximum stage was about 3.1 km³. Maximum inflow was about 2.6 x 10³ m³/s on 12 August 2002.

On August 14, 2002, Russell Lake reached a peak stage of 15.1 m. As much as 15 cm of precipitation fell during the two previous days, resulting in an inflow nearly half an order of magnitude greater than the average influx for the previous 60 days. While the 1986 flood resulted from an abrupt ice-dam failure, the 2002 flood resulted from prolonged erosion of a deepening and widening channel cut in the moraine dam. About 9 hours after flood onset, lake stage had fallen only 0.68 m, and discharge was about 8.6 x 10³ m³/s. About 15 hours after onset, the lake stage had fallen 2.56 m and the discharge was about 3.3 x 10⁴ m³/s. Peak discharge, about 5.2 x 10⁴ m³/s, was reached after about 21 hours. The flood lasted about 33 hours.