OberaargletscherOn Oberaargletscher, boulder migration was first measured in 2017 using 4 stakes each which had been drilled into the ice around each boulder. In 2021 a new method was tested, using no stakes at all. For this, fist-sized, flat rocks were spray-painted black. Four of these were placed in a pattern around each boulder. A vertical aerial photo of each arrangement was then recorded using a drone (DJI Mavic Pro 2). To better identify the marker stones, coloured plastic sheets where temporarily placed under these. Three boulders were equipped in this way on 29.7.2021, after a late start to the ablation season had made an earlier visit impracticable. In addition, an automatic camera was installed recording hourly photos from the ground. The boulders were next visited on October 2021, when measurements and drone photos were repeated and the automatic camera was retrieved. The method produced excellent results. 15 additional boulders, also visible in the aerial photos, could also be tracked. Tape measurements between the marker stones confirmed that these, in contrast to the boulders, had not moved horizontally relative to the glacier ice. | |||
 Hydropower dam and Oberaargletscher on July 29th, 2021 as seen from the ENE. The study area is on the glacier tongue, roughly inside the bend of the prominent middle moraine. |  Group of boulders on the lowest part of the glacier tongue, looking upglacier towards Oberaarjoch. Each boulder has formed a glacier table and is about to slip off its pedestal towards the left (south; photo taken on July 29th, 2021). |  This boulder is weathering rapidly. Debris which has fallen off it is mostly on the left (north) side, because the boulder has recently advanced, relative to the ice, towards the south. Smaller debris does not follow the boulder's migration, as it cannot form glacier tables (July 29th, 2021). |  Boulder sorting near the study area. Only large boulders have migrated away from the medial moraine visible on the right. Oberaarjoch, the col visible in the background, is indicating a WSW direction (photo taken on July 29th, 2021). |
 Boulders 1, 2 and 3 which were equipped with smaller marker stones in order to measure southward migration in relation to the glacier ice. Each boulder has developed an ice tail showing previous positions (1.10.2021). |  Vertical drone photo of boulder 1 on 29.7.2021 (left) and 1.10.2021 (right). Red, blue, green and yellow markers indicate the positions of marker stones. In the left picture, each marker stone is 4 meters from the edge of the boulder. Glacier flow is towards the top of the frame, the yellow arrow points south. Boulder 1 moved south, boulder 1a southeast, in relation to the glacier ice, until it nearly collided with boulder 1. |  Vertical drone photo of boulder 2 on 29.7.2021 (left) and 1.10.2021 (right). Red, blue, green and yellow markers indicate the positions of marker stones. In the left picture, each marker stone is 4 metres from the edge of the boulder. Glacier flow is towards the top of the frame, the yellow arrow points south. Boulder 2 moved southeast in relation to the glacier ice. |  Vertical drone photo of boulder 3 on 29.7.2021 (left) and 1.10.2021 (right). Red, blue, green and yellow markers indicate the positions of marker stones. In the left picture, each marker stone is 4 metres from the edge of the boulder. Glacier flow is towards the top of the frame, the yellow arrow points south. Boulder 3 moved east in relation to the glacier ice. |
 Drone panorama from above the glacier portal, looking upglacier towards the study area and Oberaarjoch, the col in the far background. Note several mudflows which have descended from the lateral moraine on the left (29.7.2021). |  Drone panorama from above boulders 1, 2 and 3, looking upglacier towards Oberaarjoch, i.e. towards WSW (29.7.2021). |  Drone panorama from above boulders 1, 2 and 3, looking downglacier towards Oberaarsee, i.e. towards ENE (29.7.2021). |  Drone panorama from a position roughly 500m upglacier from the study area (marked in red), looking downglacier, i.e. towards ENE. Boulders start migrating away from the southern, true right) margin of the medial moraine only some distance downglacier from this position (29.7.2021). |
 Vertical view of all boulders recorded by drone observation in 2021. Note that in this representation north is up. The pale blue arrow shows the flow direction of the glacier. The base image shows the glacier surface on 29.7.2021. Displacement of each boulder is indicated by a black arrow. Four lines each, extending from boulders 1, 2 and 3 (at the starting position) go to positions of small painted marker stones which are considered stationary relative to the glacier flow. Each of these lines is 4m long. | |||