March 9, 2012

McDonald Glacier, Montana Retreat

Posted by Mauri Pelto

McDonald Glacier is in the Mission Range of the Montana southeast of Flathead Lake. It is the largest and one of only two significant glaciers in this range. The glacier is tucked under the north side of McDonald Peak. The glacier was over 1 kilometer long in the 1966 USGS map of the region. By 2005 the glacier has lost 45% of its area, retreating 200 meters on average and losing one of its accumulation areas. A comparison of the map image, 2003 and 2005 image illustrate this retreat, orange line is the map terminus, black lines the terminus in 2003 and 2005. Two closeup views indicate a key exposure of rock in the midst of the glacier, black arrow. Three former areas of accumulation A,B and C are also noted. At this point area C is no longer part of the glacier. A and B both still indicate some minor crevassing indicating the glacier is not stagnant, and that these areas have been an accumulation area in the years prior to 2005. Accumulation area B in the first closeup looks to have a minimal connection to the main glacier, and is such a small area, that it is on the path of accumulation area C to disappearance. In Montana there are many glaciers that are rapidly disappearing (Hopper Glacier, and a few that are only shrinking slowly (Harrison Glacier). McDonald Glacier is in between these two paths retreating steadily, but not on the verge of disappearing, Sperry Glacier is another example of this response type.