June 27, 2011

Brady Glacier Retreat Leads to Rapid Lake Expansion 2004-2010

Posted by Mauri Pelto

This post details changes in Brady Glacier between the 2004 Google Earth imagery and a 2010 Landsat image and that are examined in more detail by (Pelto et al, 2013). A detailed look at retreat up to 2004 is detailed in a previous post on Brady Glacier.. Brady Glacier is a large glacier at the south end of the Glacier Bay region, Alaska. When first seen by George Vancouver it was a calving tidewater glacier in 1794 filling Taylor Bay with ice. Brady Glacier ceased calving and advanced approximately 8 km during the 19th century (Klotz, 1899). As Bengston (1962) notes, the advance is likely another example of an advance following a change from tidal to non-tidal status rather than that of a more positive mass balance. Bengston (1962) further notes that the massive outwash plain at the terminus is primarily responsible for Brady glacier maintaining itself well other glaciers in the Glacier Bay region retreat. The ELA on this glacier is 800 m, the line above which snow persists even at the end of the average summer, this is one of the lowest in Alaska. The main terminus was still advancing in the 1960’s and 1970’s and has managed a 250-300 meter advance since the USGS map of the 1950’s. The main terminus is not advancing any longer and has begun to retreat, the retreat to date is less than 300 meters. The glacier has been thinning and this has caused many of the subsidiary termini to retreat significantly.
There have been significant changes cause by continued retreat since 2004. For North Tripp Lake, labelled A in images. The glacier lake has separated into two parts since 2004, at the red arrow in the images below, as the glacier margin has pulled back an additional 250 meters. The lake level has dropped resulting in this separation. The drop in elevation is due to drainage south from the lake adjacent to the glacier underneath and beside the glacier, instead of solely to the west. This new drainage path was evident in the 2004 image and noted in the previous post. The color of the two lakes is markedly different, the deeper blue of the lake more distant from the glacier indicates less glacier eroded material in suspension. . Deception Lake, labelled B, has expanded by 500 meters as the glacier has retreated this same amount. At this rate this lake may soon follow the pattern of North Tripp Lake. Across the glacier, an unnamed arm, labeled C has begun to disintegrate. The white arrows in the 2010 image indicate the new open lake area. The area of this lake is now 1.5 square kilometers. North of Lake C is Abyss Lake, labelled D, this lake has expanded due to the 240 meters of retreat of the glacier in the 2004-2010 period. . To the west of Brady Glacier an unnamed glacier arm that used to join a branch of the Brady Glacier has now retreated fully from lake created by its retreat. Note point A in the time sequence below. The retreat from 1950 to 2004 was 3600 meters northeast up the lake basin, top image map from 1950, middle image Google Earth 2004. From 2004 to 2010 the glacier has retreated an additional 600 meters the retreat turning the elbow and now progressing northwest, bottom image 2010 Landsat image. Clearly the Brady Glacier thinning as indicated by tributaries on both its east and west side, continue do demonstrate that a rapid retreat of the main terminus will begin.