Dawes Glacier, Alaska Retreat Driven Separation

On June 17, 2021April 23, 2024 By mspeltoIn alaska glacier retreat1 Comment

Dawes Glacier retreat in 1985 and 2020 Landsat images. Red arrow 1985 terminus, yellow arrow 2020 terminus. Point 1-3 are tributaries joining the main glacier. The glacier is about to separate into two calving termini.

Dawes Glacier terminates at the head of Endicott Arm in the Tracy Arm-Fords Terror Wilderness of southeast Alaska. Endicott Arm is a fjord that has been extending with glacier retreat, and is now 58 km long. Dawes is a major outlet glacier of the rapidly thinning Stikine Icefield. Melkonian et al (2016) observed a rapid thinning of the Stikine Icefield of -0.57 m/year from 2000-2014. Here we compare Landsat imagery to identify changes from 1985-2020. Endicott Arm is host to a population of harbor seals that prefer hauling out on icebergs during the day supplied by Dawes Glacier (Blundell and Pendleton, 2015)

In 1985 the glacier terminated at the red arrow in each image, the tributaries at points 1,2 and 4 connected with the main glacier. Point 3 is the junction point of two tribuataries. The northern arm is 1.3 km wide and the eastern arm is 2.5 km wide. The snowline was at 1100 m. In 1987 the snowline on the glacier was at 1150 m. By 1999 the glacier had retreated 900 m since 1985 and the snowline was at 1300 m. In 2019 the tributaries at Point 1, 2 and 4 have detached from the the main glacier. At Point 3 the northern arm has declined to 0.7 km wide and the eastern arm is 1.8 km wide. In 2019 the snowline is at a record 1450-1500 m.This fragmentation of Dawes Glacier will continue, which leads to a reduced ice flux to the terminus reach. By 2020 Dawes Glacier has retreated 3.8 km since 1985, a rate of 105 m/year. The snowline is again exceptionally high at 1400-1450 m. Of equal importance the glacier terminus is separating into two individual calving termini, that could become fully separate this summer of 2021.

McNabb et al (2014) reported a thinning of 62 m/year from 1985-2013. The reduced inflow and up glacier thinning is ongoing and has driven the increased retreat rate despite a reduction in water depth at the cavling front. A key mechanism for retreat over the last century has been calving. The 2007 Hydrographic map of the area indicates water depth at the calving front still over 100 m, with a depth of 150 m 1 km down fjord of the terminus (see bottom image). The depth more recently has declined to 60 m at the calving front in 2013 (Melkonian et al 2016), yet retreat has increased driven by enhanced melting. The glacier thinning is continuing, but the retreat rate will decline as the fjord head is approached.

At the glacier front the velocity was 13 m/day in 1985, increasing to 18 m/day by 1999 and declining to 5 m/day by 2014 (Melkonian et al 2016).

This reduction will reduce calving and iceberg production. As icebergs are reduced harbor seals will be disappointed as they prefer icebergs to haul out on. The Alaska Department of Fish and Game has been monitoring harbor seals in the fjord and noting that females travel to pup on the icebergs in the spring and also utilize them for mating. ADFG attached satellite tags to harbor seals to monitor their movements beyond the breeding and puppin season finding that that adult and sub-adult seals captured in Endicott Arm spent the late summer and fall months in Stephens Passage, Frederick Sound, and Chatham Strait. How will a reduction in icebergs affect this population overall?

The retreat leading to separation is also happening at other outlets of Stikine Icefield such as Baird Glacier and Sawyer Glacier.

Dawes Glacier retreat in 1985 and 2020 Landsat images. Red arrow 1985 terminus, yellow arrow 2020 terminus and purple dots the snowline. Point 1-4 are tributaries joining the main glacier.

Dawes Glacier in June 2021 Sentinel image. Indicating the impending separation of the terminus.

Hydrograh of Endioctt Arm from 2007.

Nellie Juan Glacier Loses Contact with Contact Glacier, Alaska

On March 2, 2021April 23, 2024 By mspeltoIn alaska glacier retreat1 Comment

Nellie Juan Glacier (NJ) and Contact Glacier (C) in 1986 and 2020 Landsat images. Red arrow is the 1986 terminus location of both glaciers. Yellow arrow marks the terminus location in 2020 after glacier separation and purple dots mark the upper limit of Contact glacier at that time.

Nellie Juan Glacier is a tidewater outlet glacier of the Sargent Icefield, Alaska. Just after 1935 the glacier retreated from moraine shoal into deeper water of the fjord leading to a rapid calving retreat of 2250 m from 1950-2000, a rate of ~45 m/year (Barclay et al 2003). The rate of retreat increased to ~124 m/year from 2006-2018 (Maraldo, 2020). Harbor seals enjoy Port Nellie Juan and the icebergs from the glacier, with a population of ~44,000 identified in 2019 for the greater Prince William Sound region. From 1950-2018 Port Nellie Juan was fed in part by the Contact Glacier, a tributary that also had a separate terminus. Here we examine the changes of Nellie Juan and Contact Glacier using Landsat imagery from 1986-2020.

In 1986 the glacier terminated in a 0.5-km-wide calving front 1 km down fjord from the junction with Contact Glacier, which was 2 km wide. The snowline is at 500 m, the upper margin of Contact Glacier is indicated by purple dots and ranges from 400-500 m. The retreat by 1994 is ~300 m, the snowline is at 500 m and Contact Glacier has almost no snowcover. In 2000 the connection with Contact Glacier is 1.9 km wide. By 2018 terminus retreat in the center of Nellie Juan Glacier since 1986 is 2500 m. There is fringing connection of ice 300 m wide with Contact Glacier. Contact Glacier upper margin is now at 250 m, resulting in a more rapid retreat of the head of the glacier on the northern arm 1800 m since 1986, than that of the main terminus of Contact Glacier of 500 m. The snowline in 2018 is at 900 m, leaving the primary accumulation zone of Nellie Juan Glacier without snowcover. In 2019 there is still a narrow connection between Nellie Juan and Contact Glacier. In 2020 the two glaciers have separated. Contact Glacier, which had an area of 6.5 km² in 1986, in 2020 has an area of 3.5 km². Contact Glacier has lacked an accumulation zone during most years in this period and cannot be sustained. Nellie Juan Glacier has retreated 2800 m along the former centerline and what is now its northern margin since 1986. In 2020 the main accumulation area of Nellie Juan Glacier is again without snowcover, with a snowline above 900 m.

The glacier is terminating near a region of prominent crevassing indicating a bedrock step that may mark the head of the fjord. This will lead to an end to calving retreat and iceberg production, which will impact harbor seal haul out in the fjord (Jansen et al 2015). The retreat of Nellie Juan is less extensive than at Excelsior Glacier or Ellsworth Glacier draining the south side of the icefield. The percent loss in area of Contact Glacier is greater than other regional examples.

Nellie Juan Glacier (NJ) and Contact Glacier (C) in 1994 and 2019 Landsat images. Red arrow is the 1986 terminus location of both glaciers. Yellow arrow marks the terminus location in 2020 after glacier separation and purple dots mark the upper limit of Contact glacier at that time. In 2019 the glaciers are still connected.

Nellie Juan Glacier (NJ) and Contact Glacier (C) in 2000 and 2018 Landsat images. Red arrow is the 1986 terminus location of both glaciers. Yellow arrow marks the terminus location in 2020 after glacier separation and purple dots mark the upper limit of Contact glacier at that time. In 2018 the glaciers are still connected

Dawes Glacier, Alaska Retreat and Harbor Seals

On December 17, 2015May 2, 2024 By mspeltoIn climate change glacier retreat, glacier climate change, Glacier Observations

Comparison of 1987 and 2015 Landsat images of Dawes Glacier. Red arrow 1987 terminus, yellow arrow 2015 terminus, pink arrow location where tributaries separated.

Dawes Glacier terminates at the head of Endicott Arm, a 55 km long fjord in southeast Alaska. Dawes is a major outlet glacier of the Stikine Icefield. Larsen et al (2007) observed a rapid thinning of the Stikine Icefield and that Dawes was thinning faster than all but Muir Glacier in Southeast Alaska during the 1948-2000 period. During the period from 1891 when first mapped and 1967 the glacier retreated 6.8 km (Molnia,2008). The retreat has been driven by rising snowlines in the region that has driven the retreat of North Dawes, Baird and Sawyer Glacier.

A comparison of 1987 and 2015 Landsat images illustrate recent retreat and thinning of the glacier. The main terminus retreated 1100 m during this interval, a reduced rate from the previous period from 1978 to 1987 the glacier retreated 2.8 km. Key tributaries at the purple and green arrow each have a 30% decline in width. At the pink arrows are three tributaries that fed the Dawes Glacier in 1987 and are now detached. This fragmentation will continue. The reduced inflow and up glacier thinning is ongoing as will the retreat. A key mechanism for retreat over the last century has been calving. The calving rate has declined of late, possibly due to reduced water depth. The 2007 Hydrographic map of the area indicates water depth at the calving front still over 100 m., with a depth of 150 m 1 km down fjord of the terminus (see bottom image). Examination of surface elevation portrayed in Google Earth indicate a relatively sharp rise near the first junction, the surface elevation being at 1400 feet. The trimline is noted with blue arrows, note how much higher above the ice the tramline is at the terminus than at 1400 feet. At this point the northern arm would appear to have a bed above sea level and the main arm at least a much shallower bed. Pelto and Warren (1991) observed the calving rate reduction with water depth in the area. Note the ogives, curved bands, on the northern arm that form once per year at the base of icefall due to seasonal velocity change. The glacier thinning is continuing, but the retreat rate will decline as the fjord head is approached. As calving is reduced harbor seals will be disappointed as they like us are drawn to glaciers.

Google Earth image of Dawes Glacier in 2013. Blue arrows indicate trillion and number are elevation in feet.

The Alaska Department of Fish and Game has been monitoring harbor seals in the fjord and noting their use of icebergs and proximal glacier regions. The noted that females travel to pup on the icebergs in the spring and also utilize the are for mating. Because there was little information on where seals that use glacial habitat during pupping and mating season spend the remainder of the year, ADFG attached satellite tags to harbor seals to monitor their movements. In 2008 this data indicated that that adult and sub-adult seals captured in Endicott Arm early summer spent the late summer and fall months in Stephens Passage, Frederick Sound, Chatham Strait, This study is in part prompted by a decline of harbor seals in the Glacier Bay region where they also utilize icebergs, as NPS biologist Jamie Womble explained at the AGU 2015 meetin

1978 Landsat image, blue arrow 1978 terminus, red arrow 1987 terminus and 2015 terminus yellow arrow. Note the improvement in the Landsat imagery.