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ABSTRACT

The effects of glacier ice block grounding on the morphology and sedimentology of proglacial fluvial outwash were examined during a glacier outburst flood or jökulhlaup, near Søndre Strømfjord, west Greenland. Observations made during and after the 1987 jökulhlaup both on the surface of an ice contact delta and within a confined valley sandur plain provided information about the formation of ice block obstacle marks and the significance of these bedforms for sandur morphology and sedimentology. Flow directions determined from obstacle mark morphology have been used successfully to chart flow direction changes on the falling limb of the jökulhlaup. Maximum flow depths for scour around stranded ice blocks may be given by 0·5–0·9 times the diameter of the ice block, as estimated from the depth of scour, the height of the obstacle shadow or the extent of ice block meltout sediments. Minimum flow depths can be represented by the height of the obstacle shadow above the mean bed level. The internal composition of the shadow indicates the ability of the flow to transport various sizes of material into the lee of obstacles. Ice block obstacle marks within the distal portion of the sandur initiated waning stage channel change. Proximal and lateral erosion around stranded ice blocks extended downstream from the ice block, forming chute channels which then captured waning stage flows, resulting in significant bar incision with associated deposition of lobate or deltaic deposits. It is suggested that ice block obstacle marks are important in terms of channel morphology, channel morphological change and their usefulness as palaeohydrological indicators.