April 26, 2022

East Twin Glacier Retreats from Twin Lake with Developing Icefield Disconnection

Posted by Mauri Pelto

East Twin Glacier in Sentinel 2 images from 2017, 2019 and 2021. Point A marks the threshold, B the terminus contact with lake in 2017, Point 1 is the first ogive above the terminus.

East Twin Glacier is a narrow valley outlet glacier from the Juneau Icefield.  The glacier descends from the icefield through an icefall at 975 m- 600 m that generate ogives at the icefall base. The extensive crevassing begins at 975 m with a threshold at 900 m.  Davies et al (2022) examination of the Juneau Icefield found 63 glaciers had disappeared since a 2005 inventory, with a 10% reduction of glacier area. This study noted the importance of glacier disconnections occurring which separates the accumulation and ablation zones,  leading to stagnation of the glacier segment below.  We found 176 such disconnections in the outlet and valley glaciers of the Juneau Icefield Davies et al (2022). The focus of this post is on the development of a disconnection on the main stem of the East Twin Glacier.

In 1984 I had a chance to complete mass balance observations on the glacier.  The terminus in the lake was 600 m wide,  and the threshold at 900 m was also 600 m wide in 1984. The glacier retreated 900 m from 1984 to 2015 (Pelto, 2017). The terminus has calved into Twin Lake for over a century, but by 2015 the width of the terminus calving into the lake has declined by 75% since 1984, to 150 m. In 2017 there is still a very narrow steepened calving front. By 2019 the terminus no longer has a calving front, but was in contact with the lake.  By 2021 the glacier terminates 200 m from the lake on the west side and 100 m from the lake on the east side. The total retreat from 1984-2021 is 650 m.  In 2018 the snowline reached ~1250 m, 300 m higher than the long term average.  In 2019 the snowline again reached this level. The result is an accelerated reduction in accumulation flowing towards the top of the icefall, along with glacier thinning at the threshold, which enables the disconnection to rapidly develop. The high snowline elevations and exceptional melt in 2018 and 2019 helped to further narrow the glacier at the threshold to 240 m in 2021. The bedrock threshold is quickly cutting across the glacier, this is limiting flow through the icefall and may have shut off the production of new ogives.

The declining mass balance of the Juneau Icefield indicated by the high snowlines is driving thinning, disconnections and  retreat (Pelto 2019).

East Twin Glacier in 2018 and 2019 the highest snowlines since observations began on the Juneau Icefield in 2018. Snowline is the purple dots at 1250-1275 m, well above the threshold at Point A which had just below the mean 950 m snow line position from 1946-2000. 

East Twin Glacier terminus change from 1984-2021 in Landsat images. Red arrow is the 1984 terminus, yellow arrow the 2021 terminus.