alaska glacier snowline – From a Glaciers Perspective

Oscar Lake growth on the east margin of Brady Glacier in Landsat images from 2000-2020. Point A indicates glacier tongue that becomes iceberg. Blue arrows indicate flow direction.

Brady Glacier, is a large Alaskan tidewater glacier, in the Glacier Bay region that is beginning a period of substantial retreat Pelto et al (2013). The glacier has a number of expanding lakes that are expanding as the secondary termini feeding them retreat. The lakes Trick, North Deception, Dixon, Bearhole, Spur, Oscar, and Abyss continue to evolve. Pelto et al (2013) noted that the end of season observed transient snowline averaged 725 m from 2003-2011, well above the 600 m that represents the equilibrium snowline elevation.In 2018 and 2019 the melt season has been intense for the Brady Glacier in Alaska reaching 1100-1200 m both years. Here we examine the expansion of Oscar Lake from 2000-2020.

In 2000 the lake was just developing and had an area of ~0.5 km². In 2004 the lake had expanded to ~0.8 km². In 2006 the glacier was 1.0 km² in area, Capps et al (2010) reported the maximum lake depth measured with a remote control boat at 140 m near the ice front. The glacier still reaches the east margin of the basin separating the lake into a northern and southern section. They further noted that nearby Abyss Lake had begun to drain subglacially into Oscar Lake. In 2010 Oscar Lake had doubled since 2006 to an area of 2 km². In 2004 the glacier tongue that extends to the east margin of the lake is still in place, but is too narrow to be stable. In 2016, 2018 and 2019 very high snowlines led to extensive melt and glacier thinning, reported in 2016 (Pelto, 2016), and on nearby Taku Glacier setting a record (Pelto, 2019). In 2018 and 2019 a debris covered tongue, Point A, remained attached to the main glacier. In July 2020 this tongue has broken free. The lake now has an area of 4 km². The high snowline in 2019 exposed many firn layers from previous years. These layers were the retained snowcovered from previous winters, that had survived summer and been buried by the next years snowfall. The collective melt of the recent years is exposing the layers.

Oscar Lake growth on the east margin of Brady Glacier in Landsat images from 2000-2010 Blue arrows indicate flow direction. There is a southern and northern part of the lake separated by the glacier tongue during this period.

Oscar Lake in 2014 in a Digital Globe image. Note unstable tongue exteding to east end of basin.

Brady Glacier in 98/2019 Landsat image indicating snowline at 1100-1200 m with purple dots. S=Spur Lake, O=Oscar Lake, A=Abyss Lake, F=Firn lines, D=Dixon Lake, B=Bearhole Lake, N=North Deception Lake, T=Trick Lake

Firn layers on upper Brady Glacier in 9/8/2019 Landsat image.

Davidson Glacier is a large glacier that flows east from the Chilkat Range to the foreland along Chilkat Inlet and Lynn Canal in southeast Alaska. As a result it has a long history of observation of change. Molnia (2008) noted that from 1889 to 1946 the glacier retreated 400 m and a lake had developed at the terminus. By 1978 the glacier had retreated another 700 m with the proglacial lake at the terminus further expanding. Molnia (2008) futher observed a 700 m retreat by 2004. Here we examine Landsat imagery from 1993 to 2014 to identify recent changes of the glacier.

USGS map

Google Earth Image

In 1984 the glacier terminated at the end of a peninsula extending from the south side of the proglacial lake, red arrow. The purple arrow is the snowline at 1100 m. The orange arrows indicate two tributaries feeding the main glacier. By 2001 the terminus has retreated 500 m into a narrower western section of the lake. In 2004 the snowline is at 1250 m, leaving little accumulation area. In 2009 the snowline is at 1200 m. The terminus has retreated from the proglacial lake. In 2013 the snowline is at 1100 m, there is a river connecting the terminus at the yellow arrow to the proglacial lake. The glacier no longer reaches the foreland having retreated into a mountain valley. In 2014 the snowline was at 1300 m at the start of August with a month left in the melt season. The two tributaries at the orange arrows only have a thin connection to the main glacier. The terminus has retreated 800 meters from 1984 to 2014. The retreat will continue due to the high snowlines in recent years and tributary separation. The retreat is less than most nearby glaciers such as Sinclair Glacier or Ferebee Glacier, just east across Lynn Canal.

Johnson et al (2013) compare changes in Davidson and Casement Glaciers that share a flow divide at 1200 m, Casement Glacier flows west and Davidson Glacier flows east. Both glaciers thinned at a rate of 1 m per year at the flow divide from 1995-2011. This is an indication of the high snowlines and negative glacier mass balance. Casement Glacier had a much greater thinning below 600m than Davidson Glacier, which leads to greater retreat. The difference is that Davidson Glacier has a steeper gradient from the terminus than most glaciers.
1984 Landsat image

2001 Landsat image

2004 Landsat image

2009 Landsat image

2013 Landsat image

2014 Landsat image