Field Glacier, Alaska Retreat, Lake expansion, tributary separation

Above is a paired Landsat image from 1984 left and 2013 right, indicating a 2300 m retreat in this period, below is further detailed examination.
The Field Glacier flows from the northwest side of the Juneau Icefield, and is named for Alaskan glaciologist and American Geographical Society leader William O. Field. Bill also helped initiate the Juneau Icefield Research Program, which Maynard Miller then ably managed for more than 50 years. The JIRP program is still thriving today. In 1981, as a part of JIRP, I had my first experience on this glacier. It was early August and there was new snowfall everyday that week. Jabe Blumenthal, Dan Byrne and myself undertook a ski journey to examine the geology on several of the exposed ridges and peaks, note the burgundy line and X’s on image above. This was truly a remote area. The glacier begins from the high ice region above 1800 meters, there are several icefalls near the snowline at 1350 meters, and then it descends the valley ending at 100 meters. The runoff descends the Lace River into Berners Bay. This post focuses on the significant changes occurring at the front of the Field Glacier. The development of a proglacial lake at the terminus is accelerating and spreading into the main southern tributary of the glacier. This lake is going to quickly expand and develop a second arm in that valley. The USGS map from 1948 imagery and the 1984 imagery indicate little change in the terminus position, with only a small lake at the terminus. . After 1984 the mass balance of the Juneau Icefield became more negative, this was apparent from the rise in the snowline elevation on all the glaciers and by the increasing losses and resultant thinning typified by the Lemon Creek Glacier (Miller and Pelto, 1999). The Field Glacier by 2006 had developed a proglacial lake at the terminus that averaged 1.6 km in length, with the east side being longer. There are several small incipient lakes forming at the margin of the glacier above the main lake, each lake indicated by black and orange arrow. In 2009 the lake had expanded to 2.0 km long and was beginning to incorporate the incipient lake on the west side of the main glacier tongue. By 2011 the main lake has nearly reached the southern tributary lakes. The lake has expanded to 2.6 km long, with the west side having caught up with the east side, and an area of 4.0 square kilometers. In addition the main lake has joined with the fringing lake on the south side of the south tributary. There is also a lake on the north side of this tributary. This lake should soon fill the valley of the south tributary and fully merge with the main, as yet unnamed lake at the terminus, maybe this should be Field Lake. Below in order is the 2006, 2009 and 2011 Landsat images. This glacier is experiencing retreat and lake expansion like several other glaciers in the icefield, Gilkey Glacier, Eagle Glacier, and Antler Glacier.

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Eagle Glacier Retreat, Juneau Icefield Alaska

Above 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.