Knickpoint evolution in a supraglacial stream

Despite numerous studies of knickpoints in bedrock and alluvial channels, no detailed description of knickpoint change on ice has been reported in the literature to date. This paper presents the first investigation of knickpoint evolution within a supraglacial stream on Vadrec del Forno, Switzerland. Repeat longitudinal profile surveys of the knickpoint reveal an increase in step height of 115 mm and upstream migration of 0.26 m over a period of three days during the 2017 ablation season. Rates and magnitudes of erosion vary spatially across the knickpoint in relation to differing discharge regimes. At high discharges (~ 0.013 m3 s-1), erosion is focused at the step base; at low discharges (~ 0.003 m3 s-1), erosion is focused on the reach upstream of the knickpoint, at the step lip and the step riser face. This gives rise to replacement of knickpoint morphology, driven by frictional thermal erosion and hydraulic action. The formation of a pool further influences step morphology, inducing secondary circulation and increased melt at the base of the step riser, causing steepening. Results highlight the complexities of water flow over knickpoints, demonstrating that the stream power law does not accurately characterise changing knickpoint morphology or predict rates of retreat. Although morphological similarities have been reported between supraglacial channels and bedrock/alluvial streams, knickpoints in non ice walled channels will not necessarily respond to discharge similarly to those in ice because of the different erosion processes involved.