Chilkoot inlet glacier retreat – From a Glaciers Perspective

Meade Glacier in Landsat images from 1986 and 2018. The red arrow indicates the 1986 terminus, pink arrow the 2014 terminus, yellow arrow the 2018 terminus, orange arrows tow tributaries to Meade Glacier and the purple dots the snowline.

Meade Glacier drains the northwest portion of the Juneau Icefield, with meltwater entering the Katzehin River and then Chilkoot Inlet. The glacier begins in British Columbia and ends in Alaska. Here we use Landsat imagery to examine changes in the glacier from 1986 to 2018. The glacier experienced a slow continuous retreat from 1948 to 1986 of 400 m, with the glacier ending on an outwash plain.

In 1986 the terminus is indicated by a red arrow, the snowline is at 1250 m in 1986, there is no evident lake at the terminus of glacier just an expanding outwash plain. Both tributaries from the south, orange arrows, are 750 m+ wide where they join Meade Glacier. By 2004 a 400 m long proglacial lake has formed at the terminus. The two tributaries from the south, at the orange arrows, no longer are connected to the glacier. The snowline is at 1450-1500 m. In 2014 the proglacial lake is 3.5 km long, the entire lower 2.5 km of the glacier has collapsed since 2004. There is still considerable relict ice floating in the lake. There is a substantial lake along the southern margin of the glacier where a tributary streams enters the main valley. This indicates the glacier will quickly retreat to this point by further collapse into the lake. The snowline in 2014 is at 1450 m on Aug. 2, the date of the imagery, the high snowline ensures continued mass loss and glacier retreat. By 2018 Mead Glacier has retreated 4.1 km since 1986. The snowline is at 1450 m on October 2, when fall snow should have already begun. A third tributary entering the glacier from the east at 1200 m no longer reaches the main stem. Based on surface slope changes the glacier appears to be within 1 km of the inland limit of the proglacial lake. The inland limit should be near the prominent bedrock knob on the south side of the glacier a short distance inland of the current terminus. When this is reached the glacier retreat will be reduced. The retreat parallels that of most Juneau Icefield glaciers including the next glaciers to the south Field Glacier and Gilkey Glacier. The glacier shares a divide with Warm Creek Glacier that terminates in British Columbia that is also retreating rapidly in an expanding lake.

Meade Glacier in Landsat image from 2004. The red arrow indicates the 1986 terminus, pink arrow the 2014 terminus, yellow arrow the 2018 terminus, orange arrows tow tributaries to Meade Glacier and the purple dots the snowline.

Meade Glacier in Landsat image from 2014. The red arrow indicates the 1986 terminus, pink arrow the 2014 terminus, yellow arrow the 2018 terminus, orange arrows tow tributaries to Meade Glacier and the purple dots the snowline.

Meade Glacier in Landsat image from 2018. The red arrow indicates the 1986 terminus, pink arrow the 2014 terminus, yellow arrow the 2018 terminus, orange arrows tow tributaries to Meade Glacier and the purple dots the snowline.

Ferebee Glacier is in the Coast Range of Alaska 15 km northeast of Skagway. The Ferebee River that drains the glacier flows into the Chilkoot Inlet. The glacier begins at 1800 m in the Klukwah Mountains and flowed south and terminated on an outwash plain at 200 m in the USGS map of the region from 1955. Little change in the terminus occurred prior to 1981. I observed the glacier from the air in 1981 and there was no lake at the terminus and only a minor several hundred meter wide devegatated zone from recent retreat. Here we examine Landsat imagery from 1986-2014 to identify recent change. A landslide occurred onto Ferebee Glacier in 2014, that will add debris cover to the surface that is limited today.

Google Earth image

USGS map of area

In each image the red arrow marks the 1986 terminus, the yellow arrow the 2014 terminus and the purple dots the snowline. In 1986 the glacier still terminated on the outwash plain at the south end of what is a newly forming lake small pockets of open water are evident on the lateral margins. The snowline is at the base of the lower icefall at 900 m. By 1999 a lake has formed at the terminus, that is 1.2 km long, a retreat of nearly 100 meters per year. The snowline is at 1000 m in 1999. A 2004 Google Earth image of the terminus area indicates two regions of concentric crevassing upglacier of the terminus, one adjacent to the yellow arrow, this indicates ice that has been lifted and then dropped by water, which only happens if the ice is thin enough for flotation. By 2013 the lake has more than doubled in length and the snowline is near the top of a pair of icefalls at 1300 m. In 2014 the lake is 2.7 km long on the eastern shore and 2.5 km long on the western shore. The glacier has retreated 2.6 km in 28 years, still nearly 100 m/year. The lake is not becoming narrower and there is no elevation step on the glacier, to suggest the end of the developing lake is near. The snowline in 2014 even in early August is at 1400 m. This glacier will continue to retreat as long as the snowline is above the top of the icefalls at 1250 m. The retreat of this glacier is like the Meade Glacier across Chilkoot Inlet, LeBlondeau Glacier just to the west and Gilkey Glacier to the south.

1986 Landsat image