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Landscape and sediment processes in a proglacial valley, the Mittivakkat Glacier area, Southeast Greenland

Abstract Geografisk Tidsskrift, Danish Journal of Geography 108(1):97–110, 2008 During the Little Ice Age (LIA), the present proglacial Mittivakkat Valley, stretching 1.5 km ENE-WSW from the terminus of the Mittivakkat Glacier tongue to a delta terminating in the Sermilik Fjord, Southeast Greenland, was transgressed by the glacier as indicated by a terminal moraine near the valley mouth. The first recordings of the glacier terminus in 1933 show a frontal retreat of about 300 m since the Little Ice Age (LIA). Since then, the glacier has retreated another 1200 m leaving a valley train characterized by steep slopes flanking a 150–300 m wide flat-bottomed, sediment-floored trough. The larger part of the valley is floored by fluvial sediments and shows a typical valley sandur stream, the Mittivakkat stream, with one or two main water-channels, which branches out in a braided stream system with numerous intervening channel bars. In the outer part of the valley train, the Mittivakkat stream is erosive and divides around remnants of ground moraine, end-moraines, and outwash with abandoned meltwater channels. In this area some bank erosion is seen, but rejuvenation of older stream channels takes place during spring time when snowbridges cover the recent outlet and force the meltwater to follow the older channels at a higher level. Most recently, the seaward side of the outermost end-moraine ridge has been eroded by the sea, probably because the delta area is degrading. This could be a result of a decreased delivery of glaciofluvial sediments, due to the sink effect of the proglacial valley, but also because of less sea ice during summer periods and a more recent shift in wind directions during open water periods towards south and west. Geoelectric measurements (Schlumberger) along a number of traverses show sediment thicknesses in the valley trough of 6–23 m, which suggest that 2.2 x 106 m3 of sediment has been deposited in the valley. The estimated volume of glaciofluvial sediments deposited in the valley and delta is 9.9 x 106 m3. Recent sediment transport from the glacier basin of 18.4 km2 through the Mittivakkat Valley to the Sermilik Fjord at the mouth of the valley has been monitored. The annual average is around 7,000 m−3. The investigations confirm that a slight net deposition takes place in the upper part of the valley, showing that the proglacial valley will trap more of the sediment released by glacial erosion if the Mittivakkat Glacier continues to retreat. Thus, the investigation demonstrates the close interaction between the different morphological zones in a proglacial sediment transport system.