June 20, 2012

Great Glacier Retreat, Stikine Icefield, BC Canada

Posted by Mauri Pelto

Great Glacier is the largest outlet glacier of the Stikine Icefield terminating in Canada. The name came from the large expanse of the glacier in the lowlands of the Stikine River during the late 19th and early 20th century, that has now become a large lake. The glacier filled what is now a large lake at the terminus of the glacier pushing the Stikine River to the east side of the valley. In 1914 the glacier was easy to ascend from the banks of the Stikine River, the picture below is from the National Railroad Archive. By 1965 the new lake had formed, but the glacier still reached the far side of the lake in several places as indicated by the 1965 Canadian Topographic Map, green arrows. A comparison of 1986 Landsat, 2005 Google Earth and 2011 Landsat imagery illustrates the retreat. The yellow arrow indicates a glacier dammed lake, the violet arrows the snowline and the red arrow the northeast tributary. By 1986 the new lake had largely developed, and the glacier was beginning to retreat into the mountain valley above the lake. Retreat from the moraines of the late 19th century was 3200 m. By 2011 the glacier had retreated further into valley, 900 m retreat from 1986-2011. Great Glacier is following the pattern of behavior of other Stikine Icefield glaciers such as Sawyer Glacier There is a glacier dammed lake that has to date changed little at the yellow arrow, this lake fills and drains under the glacier periodically, top image below. A view of the glacier from across the lake today indicates the distance to the now valley confined glacier, and the trimlines of the former ice surface, yellow arrows in middle image The Great Glacier has one major tributary on the northeast tributary that is very low in elevation with a top elevation of 800 m. Given the regional snowline of 1100-1200 meters (Pelto, 1987)this is too low to retain snowcover through the summer and will lead to progressive thinning. This branch of the glacier has and will thin faster than the rest of the glacier and is doomed given its limited top elevation. The proglacial lakes on its periphery will continue to grow as this downwastes, green arrow bottom image.