March 19, 2015
Downwasting Tributary-Glacier Dammed Lake Formation at HPN4 Glacier, Patagonia, Chile
Posted by Mauri Pelto
We often are more concerned with what is happening at the terminus of a glacier; however, often key changes are happening up glacier some distance. This is the case with the following example. The Northern Patagonia Icefield (Hielo Patagónico Norte, HPN) is one of the two main icefields in Patagonia. The remoteness of the region is evidenced by the number of significant lakes and glaciers that remain unnamed. This remoteness has led to several valuable detailed recent studies utilizing satellite imagery on glacier extent (Davies and Glasser, 2012), glacier thickness change (Willis et al, 2012) and glacier velocity (Mouginot and Rignot, 2015) Here we focus on a downwasting tributary to an unnamed glacier listed as HPN4 Glacier in the aforementioned studies. Davies and Glasser, (2012) identify this region of the icefield as retreating faster from 2001-2011 than during any measured period since 1870. Willis et al, (2012) in their Figure 2, seen below, identify this an area of pronounced thinning, approximately 5 m/year from 2000 to 2011. Why such rapid thinning in an area without calving? Mouginot and Rignot, (2015) indicate that this area is not an area of rapid flow, and given the thinning it should be an area of diminishing flow. Here we examine changes from 1987 to 2014 using Landsat imagery.
Digital Globe image of southern section of Northern Patagonia Icefield, with black arrow indicating downwasting tributary from the east flowing into HPN4 Glacier.
Figure 2 from Willis et al (2012) indicating thinning of NPI glaciers from 2000-2011, the blue arrow indicates the downwasting tributary of interest flowing into HPN4 Glacier from the east.
Ice Flow direction for HPN4 Glacier and the downwasting tributary (DT). The flow diagram above indicates the converging flow of the downwasting tributary and HPN4, that meet at the medial moraine.
In 1987 there are five contributing glacier tongues to the downwasting tributary, each indicated with a red arrow. It is like a bathtub being filled with five taps at once. The yellow arrow indicates a medial moraine at the mouth of the valley, signalling the lack of current contribution of the downwasting tributary to HPN4 Glacier. By 2004 only three of the contributing glacier tongues still merged with the downwasting tributary, and two of these are much more tentative. The medial moraine has shifted east indicating that the main HPN4 Glacier is now flowing into the valley instead of the downwasting tributary being a contributing tributary to HPN4. By 2014 there is only one contributing glacier tongue to the downwasting tributary, only one tap for this draining bathtub, the other four contributing tongues have retreated from contact with the downwasting tributary. The medial moraine has spread eastward and some fringing proglacial/subglacial lakes are evident. In 2015 the only change is that the rifting near the medial moraine is more pronounced. A closeup 2013 Digital Globe image indicates both fringing ponds-blue arrows, rifts caused by varying flotation-green arrows and expanding supraglacial ponds, red arrows. The rifts are a sign of instability and typically lead to break up of this portion of the terminus. The downwasting tributary continues to demise faster than HPN4 Glacier, which crosses the valley mouth, hence it is likely that a glacier dammed lake will form and that HPN4 Glacier will continue to flow further east up this valley, which could offset some of the downwasting and lake development. In either case this redirected flow of HPN4 into a high ablation valley, will help encourage a faster retreat of the main terminus. How large the lakes gets and how much of the time it is filled are difficult to speculate upon. Analogs for this type of lake are seen at. Allemania Glacier (Lago Martinic) and Baird Glacier (Witches Cauldron) . Schaefer et al (2013) discuss the HPN4 Glacier because the main terminus has changed little given its modelled mass balance, and the modelled mass balance to the east appears too negative, which they suggest indicates wind redistribution from the HPN4 to the Pared Sud Glacier just east. That is a challenge to sort out without some ground truth.
1987 Landsat image
2004 Landsat image
2014 Landsat image-not the fringing blue indicating trapped sub-glacial/proglacial lake.
2015 Landsat image
Google Earth image 2013