February 5, 2018
Double Glacier, Alaska in 1986 and 2017 Landsat images. The 1986 terminus location of the northern Big River Lobe and Southern Drift River Lobe are shown with red arrows. Yellow arrows indicate the 2017 terminus locations. Purple dots indicate the snowline. Point A indicates a prominent and expanding nunatak that is below the snowline.
Double Glacier is the largest glacier contained within the Lake Clark National Park (LCNP) with an area of 137 km2 (Arendt et al, 2012). The glacier has a double eastern terminus with the northern or Big River Lobe terminus feeding the South Branch Big River and the southern terminus or Drift River Lobe feeding the Drift River. From 2001-2008 Arendt et al (2012) found that Double Glacier had large elevation losses of ~2 m/year below 1400 m. From 1956-2007 Double Glacier lost 7% of its area, which was below average in the region (NPS, 2012). Here we examine Landsat imagery from1986-2017 to identify changes of the glacier including the surface impact of Mt. Redoubt volcano 2009 eruption.
In the 1956 Kenai Topographic map the Drift River Lobe extends to an elevation of 150 m almost to the Drift River Valley bottom. The Big River Lobe terminates near the eastern end of the proglacial lake that is just a small fringing water body. In 1986 the Big River Lobe reached the western margin of a 2.5 km long proglacial lake (Big River Lobe Lake), red arrow. The Drift River Lobe terminated at a ridge at 300 m, red arrow. A prominent nunatak, Point A extends 4 km up the middle of the glacier from 850 m to 1050 m in elevation. The snowline is at 1050 m. In 2009 the most striking visual is volcanic ash covers the entire glacier. This is from Redoubt volcano which the Alaskan Volcano Observatory reports on the 2009 eruption of this stratovolcano on the west side of Cook Inlet beginning in March 2009. Nineteen major ash-producing explosions generated ash clouds that reached heights between 5200 m and 18900 m. During ash fall in Anchorage, the Ted Stevens International Airport was shut down on for part of March 28 and March 29. The explosive phase ended on April 4 with a dome collapse and an ash cloud that reached 15,200 m and travelled southeast, depositing up to 2 mm of ash fall in Homer, Anchor Point, and Seldovia. The final lava dome ceased growth by July 1, 2009. In 2009 the Big River Lobe has retreated 500 m from the 1986 position. The Drift River Lobe has had a 1600 m retreat since 1986. This retreat was not driven by the ash that fell just months before the Landsat image was acquired. In 2016 the snowline is at 1050 m. The ash remains evident in the ablation zone. In 2017 the snowline is again at 1050 m in late July, with six-eight weeks left in the melt season. The Drift River Lobe terminus retreat from 1986-2017 has been 1700 m. The Big River Lobe terminus retreat from 1986-2017 has been 1400 m. The decreased albedo from the ash between 1986 and 2017 is evident and will lead to enhanced ablation zone melting and retreat. The Nunatak, Point A is located below the snowline each year. Glacier thinning has led to the expansion in width and vertical relief from the glacier of this nunatak. The lowest 1 km of the glacier today is narrow and fed by a thin ice tongue. The retreat of this glacier is similar to that of Blockade Glacier, Hallo and Spotted Glacier in the same region, each have had substantial retreats with lake expansion.
Double Glacier, Alaska in 2009 and 2016 Landsat images. The 2009 image indicates ash fall from Redoubt Volcano covering the glacier. The 1986 terminus location of the northern Big River Lobe and Southern Drift River Lobe are shown with red arrows. Yellow arrows indicate the 2017 terminus locations. Purple dots indicate the snowline. Point A indicates a prominent and expanding nunatak that is below the snowline.
Kenai Topographic map indicating glacier margins in 1956.