September 25, 2011

Eagle Glacier Retreat, Juneau Icefield Alaska

Posted by Mauri Pelto

eagle glacier changeAbove is a paired Landsat image from 1984 left and 2013 right indicating the 1100 m retreat during this period of Eagle Glacier.My first visit to the Eagle Glacier was in 1982 with the, ongoing and important, Juneau Icefield Research Program, that summer I just skied on the glacier. In 1984 we put a test pit at 5000 feet near the crest of the Eagle Glacier to assess the snowpack depth. This was in late July and the snowpack depth both years was 4.3 meters, checking this depth in nearby crevasses yielded a range from 4-4.5 meters. In 1984 the snowline at the end of the summer melt season in early September was at 1050 meters. In the image below the glacier is outlined in green, the snowpit location is indicated by a star and the snowline that is needed for the glacier to be in equilibrium at 1025 meters is indicated.
Eagle Glacier has experienced a significant and sustained retreat since 1948. The first map image below is of the glacier in 1948, at this time the glacier ended at the south end of a yet to be formed glacier lake. By 1982 when I first saw the glacier and when it was mapped again by the USGS (second image) the glacier had retreated to the north end of this 1 kilometer long lake. In the sequence of images the red line is the 1948 terminus, the magenta line the 1982 terminus, the green line 2005 terminus and the orange line the 2011 terminus. From 1982 to the 2005 image used in Google Earth the glacier retreated 500 meters, 21 meters/year (next image). The bottom image is from a 2011 Landsat image in May and indicates the terminus position once again with an additional retreat in six years of 400 meters, 65 meters/year. Going back to the 1948 map the terminus in 2011 is located where the ice was 500-600 feet thick in 1948The more rapid retreat follows the pattern of more negative balances experienced by the glaciers of the Juneau Icefield (Pelto et al, 2008). The Equilibrium line altitude which marks the boundary between the accumulation and the ablation zone each year is a good marker of this. On Eagle Glacier to have an equilibrium the glacier needs to have an ELA of 1050 meters. At this elevation more than 60% of the glacier is in the accumulation zone. Satellite imagery allows identification of the ELA each year, seen below is the elevation in 1984, 1998, 1999 and each year since 2003. The number of years where the ELA is well above 1050 meters dominate leading to mass loss, thinning and glacier retreat. This follows the pattern of Lemon Creek Glacier that is monitored directly for mass balance, which has lost 26 meters of thickness on average since 1953.