May 24, 2014

Dead Branch of Norris Glacier Decay, Alaska

Posted by Mauri Pelto

The Dead Branch of the Norris Glacier is a side valley of the Norris Glacier fed by the main glacier. Instead of tributary it is a distributary tongue. As the Norris Glacier has thinned and retreated 100 m from 1984-2013 recently the inflow had declined. The Dead Branch has retreated and a subglacial lake forms each spring that lifts the glacier causing concentric crevasses to form. The lake then drains later in the summer and the glacier subsides. The crevassing has gotten much worse since I first saw this glacier in 1984. Here we look at Landsat and Google Earth imagery from 1984-2013.

Below is a sequence of three images from 2010 indicating the key feature in the center of the glacier. This dark feature, purple arrow amidst the snow covered glacier in July is caused by large crevasses due to glacier uplift by the sub-glacial lake. The water table rises to near the glacier surface bringing with it sediment that then stains the surrounding snow it infiltrates. There is one large north trending crevasse that connects to a lake that is partly on and partly adjacent to the Dead Branch. The next two images are closeups. The concentric crevasses and radial crevasses, that look like spokes, are indicative of lake uplift in this otherwise flat stagnant reach of the glacier. The expansion of this network suggests that the thinning glacier is easier to lift over a larger area.

dead branch 2010
Google Earth 2010 image

dead branchclose
2010 Closeup annotated- rred arrow concentric crevasses, blue arrow main connection crevasses, purple arrows radial crevasses.

dead branch crevasses 2010 Digital Globe

The set of images below are from Landsat images from 19884-2013 the yellow arrow indicates the western terminus of the glacier tongue. This has retreated 400-500 m during this period. The red arrows indicate two prominent features on the side of the glacier which indicate that there is some thinning and lateral retreat, though not nearly as much as on the feeder Norris Glacier. The large crevasses do not show up well in the late summer 2013 Landsat as the lake has drained and the glacier subsided closing many, and due to the reduced resolution of the image. This branch is similar to that of the Witches Cauldron of Baird Glacier. Both appear poised to expand into large lakes in isolated valleys. In early summer 2014 we will closely watch the lake formation and glacier response .

dead branch 1984
1984 Landsat image

dead branch 1998
1998 Landsat image

dead branch 2132013
2013-July Landsat image

dead branch september 2013
2013-September Landsat image