January 30, 2014
North Alsek Glacier comparison in 1984 and 2015 Landsat images. Illustrating the formation of proglacial lake due to glacier retreat. Red arrow is the 1984 terminus and yellow arrow 2013 terminus. Point A is and island.
The Alsek Glacier is a large glacier draining into Alsek Lake and the Alsek River. The first glacier upriver of Alsek Glacier flowing from the east and ending on the Alsek River valley floor is an unnamed glacier, here named North Alsek Glacier. The USGS topographic map compiled from a 1958 aerial image indicates a piedmont lobe spread out on the Alsek River lowland, without a lake, and a series of moraine ridges between the glacier terminus and the Alsek River. This glacier drains a series of peaks of 2000 m in elevation and drains directly west toward the Alsek River, blue arrows indicate glacier flow.
Here we examine Landsat imagery to identify the change in terminus position of the glacier from 1984-2015. In 1984 a small lake has developed along the north shore of the lake that is 1000 m by 500 m. The glacier has retreated to a newly exposed knob, possibly an island, at the red arrow. The red arrow in each image below indicates the location of this knob, the yellow arrow indicates the 2015 terminus location on the south side of the glacier near the end of a peninsula. There is no lake downglacier of the yellow arrow in 1984. By 2011 the glacier retreat has led to development of a substantial lake that is 2.5 km north to south and 1.1 to 1.5 km side from east to west. In 2013 the central tongue of the glacier has continued to thin and breakup. In 2015 the last tongue of ice extending into the lake south of the island has been lost. The glacier has lost an area of 6.5 square kilometers. Retreat has been 2300 m on the northern margin and 2400 m on the central and southern margin from 1958-2015. The majority of the retreat at the northern margin occurred between 1958-1984, while nearly all the retreat occurred after 1984 for the central and southern portion of the glacier. The glacier will continue to retreat out of the lake basin. The retreat has been nearly identical to nearby Walker Glacier that also had a piedmont lobe, but less than the nearby East Novatak Glacier and Yakutat Glacier.