The primary portion of the Douglas Glacier was a debris covered valley tongue that is separated from the slopes feeding the terminus reach. The feeder glacier tongues, pink arrows, end on the bedrock slopes above a steep cliff and do not reach the valley glacier below, blue arrows. One section of the glacier, the furthest west portion noted by a pink arrow, the Douglas Neve flows down a steep mountains side. The bedrock slope at the base of the glacier is particularly smooth, which combined with the steep slope,, 40% grade or 22 degree slope, enhances basal sliding. On small alpine glacier the resulting high velocity leads to extensive crevassing. This crevassing can literally penetrate to the base of the glacier near the thin terminus. This leads to portions of the glacier simply separating from the rest of the glacier and avalanching down the slope or melting in place. Here we utilize Landsat images from 2000 and 2012 and Google Earth imagery from 2004 and 2009 to examine the retreat of this glacier. The sequence of images below are in order 2000, 2004, 2009 and 2012. In 2000 the terminus of the glacier terminates at a prominent bedrock fracture at 1640 meters above sea level. In 2004 the terminus still reaches this fracture. The green line in the Google Earth imagery is the 2004 terminus and the burgundy line the 2009 terminus. By 2009 the terminus has retreated 400 meters, and consists of two unsustainable narrow tongues, both less than 100 meterw side. By 2012 the two narrow tongues have been lost, resulting in a 700 m retreat from 2000 to 2012 with the terminus now at 1800 meters. As the retreat of an alpine glacier progresses crevassing typically is reduced as glacier speed declines. Here we see an increase in crevassing from 2004 above to 2009 below in the terminus area, suggesting that the retreat will continue via pieces of the glacier separating from the glacier and avalanching. This process is a much different setting, but similar in practice to ice shelf loss through rifting that reaches the critical point where the rifts lead to icebergs breaking off. At this point the terminus remains unsustainable. This retreat is similar to that of New Zealand glaciers in general as noted by the NIWA and Trevor Chinn, and examined in detail on Murchison Glacier, Mueller Glacier and Gunn Glacier
Murchison Glacier drains southeast from the Mount Cook region, one valley east of Tasman Glacier. The end of the glacier terminates in a lake that is rapidly developing as the glacier retreats. This retreat will become rapid as 2010 imagery indicates other proglacial lakes have now developed 3.5 km above the actual terminus. These lakes are at a higher elevation and may not endure but do help increase ablation, and in the image below show a glacier that is too narrow to provide flow to the lower 3.5 km. The increased retreat has been forecast by the NIWAand Dykes et al (2009) This lower section is debris covered, stagnant, relatively flat and will not survive long. The demise of the lower section of this glacier will parallel that of Tasman Glacier. The glacier has retreated 2200 meters from the moraines at the south end of the lake. There was not a lake in the 1972 map of the region. A comparison of 2006 and 2010 imagery indicates the decrease in glaciated area in the lake basin. The bottom image is from NASA after the Feb. 2011 earthquake near Christchurch that led to a major calving event of a portion of the rotten stagnant terminus reach of the Tasman Glacier. There is no evident calving event from Murchison Glacier. The lake on the western margin of the valley, separated from the main lake has since April 2010 expanded notably. The glacier still has a significant accumulation area above 1650 m to survive at a smaller size. The lower debris covered tongue is 6 km long and extends from the terminus at 1050 meters to 1200 meters, a very low gradient to supply healthy flow from the accumulation area. The ongoing retreat is triggered by warming and a rise in the snowline in the New Zealand Alps observed by the NIWA.
Donne Glacier descends the spectacular east face of Mount Tutoka in southwest New Zealand. This glacier has been undergoing rapid retreat this decade creating a new alpine lake. The National Institute of Water & Atmospheric Research (NIWA) conducts an annual survey of the snowline of New Zealand glaciers. In order to thrive a glacier must have at 50-70% of its area snowcovered at the end of the summer melt season. For NZ glacier NIWA has noted 67% as the key to equilibrium conditions. If then snowline is above normal the glacier will lose mass, if the snowline is lower than normal the glacier will gain mass. Since 2000 the snowline has been above normal in nine of the ten years, only in 2005 was the snowline slightly lower than normal (NIWA, 2010). In 2009 the snowline was the highest of any of the years The result of a decade of high snowline’s is glacier mass loss and retreat. Below is a sequence of images from 2000, 2003, 2006 and 2009 of Donne Glacier the first and last images are from NIWA and the middle two are Google Earth images.
In 2000 the glacier reaches almost all the way across the newly forming unnamed lake. By 2003 the large debris covered section has detached and the lake has doubled in size. In 2006 the faint orange line indicates the 2003 terminus position. The retreat of 100 meters has led to further lake expansion. In the 2009 images the glacier is still ending in the expanding lake, and is still actively flowing. The number of crevasses and the snowcover existing even in poor snow years such as 2003, 2006 and 2009 indicate the glacier still has a persistent accumulation zone. The glacier begins near 2200 meters and descends to about 1300 meters in 2 kilometers. A persistent accumulation zone is key to survival. The retreat and formation of new alpine lakes is also occurring at two nearby glaciers that NIWA observes. Gunn Glacier (below) and Park Pass Glacier (above), in the Google Earth images. Both glaciers end in lakes still occupied by icebergs that used to be part the terminus of the glacier. The icebergs did not calve off so much as representing disintegration of the terminus. The tongue visible on Park Pass Glacier in the middle of the lake is now gone.