While perusing recent images of the Southern Patagonian Icefield, I spotted a relatively recent debris flow on the eastern flank of Glaciar Viedma, down from Punta Heron (see position in OpenStreetMap). This was part of the same exercise in which I saw the debris lump on the surface of Glaciar Río Túnel Inferior described in the previous post.

This debris flow is clearly visible on a Sentinel 2B image of 2017-11-04 and later images, while it does not appear on the cloud-free image that is closest back in time (and that I could find), a Landsat 8 scene from 2017-02-04.

debris flow viedma glacier 2017
Landsat 8 acquired on 2017-02-04 (left) and Sentinel 2 acquired on 2017-11-04 (right), bracketing the time in which the debris flow happened. Sentinel image is courtesy of Copernicus Sentinel Data 2021.

It is difficult to pinpoint when exactly did this debris flow happen. All available images between the two dates above are affected by nearly complete cloud cover. A couple of Sentinel images taken in October 2017 are apparently cloud-free over the spot, but the data granules are apparently corrupted and I was not able to properly access the data. Based on the available information, we can conclude that the debris flow occurred at some point between late summer and late spring 2017. To obtain a more exact date we will have to wait until more data is available.

The flow has left a signature of an elongated debris fan approximately 1700 meters long and 400 meters wide. with many typical surface features like channels, levees, and a three distinct frontal arcs. With an area of 0.6 km2, and conservatively assuming a (geometric) mean debris thickness of 0.3 m, this gives a rock volume of about 180000 m3.

debris flow viedma heron detail 2017
Subset of Sentinel 2 image aquired on 2021-03-03 showing the deposit left by the debris flow in 2017. See text for details.

It is not entirely clear from the image where exactly did the flow start. An speculative scarp is marked in the image below with a dotted line. It is also not clear what might have triggered the debris flow, although as is usual in these cases there might have existed some geological precondition, like relatively loose material lying on an unstable slope, together with some meteorological event, like increased snow melt in spring, for example.

An interesting note is that the flow stopped about 500 m from a placed marked in OpenStreetMap as a camp site in one of the traverse routes over the icefield (marked with a triangle). I am not sure how accurate this is, or how often the camp site is occupied, and by how many people. I guess the number is small but not totally insignificant, as the area is located along one of the most traversed hiking routes over the icefield, just West of the famed Cerro Torre. It is thus easy to imagine that it is not entirely impossible for an event like this to pose some danger to climbers and hikers. It is also possible that the event may have been noticed by people in the field.

debris flow viedma 2017 enlarged detail
Subset of Sentinel 2 image aquired on 2021-03-03 showing the deposit left by the debris flow in 2017. See text for details.

Have you been in the area? Have you seen this debris flow. Feel free to comment below.

Thanks for reading.