The impact of snow line altitude on subglacial sediment export

Large Alpine glaciers typically export significant quantities of subglacially eroded sediment to their downstream environments via meltwater streams. The supply of this glaciogenically-produced sediment has an extensive impact on landform genesis and evolution; downstream ecology and ecosystem succession; river morphology and flood risk management; hydropower installations and industrial sediment extraction. However, little is known about the processes and mechanisms driving the entrainment and evacuation of subglacial sediment by meltwater flow. Controls on the rate of sediment export remain poorly constrained, limiting the reliability of future sediment yield predictions. Although the fine (suspended) fraction of meltwater-borne subglacial sediment export is relatively well studied, records of suspended sediment load are commonly obtained from distal proglacial locations and may consequently be affected by filtering effects in proglacial rivers and forefields. Additionally, almost nothing is known about the coarse (bedload) component and its relationship to suspended sediment transport due to the difficulties in monitoring bedload flux. In this study, we present a first-of-its-kind dataset from Glacier d’Otemma, Switzerland, capturing three years (2020, 2021 and 2022) of continuous suspended sediment load, bedload and water discharge data from a monitoring site immediately in front of the glacier meltwater portal. Measurements of coarse sediment flux were performed using recently developed environmental seismological monitoring techniques. Data from all three years show that bedload delivery to the glacier margin switches from being transport-limited to being supply-limited during the melt season and was driven by snow-line retreat. Bedload evacuation reduces to very low levels as soon as the snow-line reaches the upper parts of the glacier, which it does increasingly often due to climate warming. We hypothesise that the position of the snow line controls the degree of channelisation – or the efficiency – of the subglacial drainage network above and below it, with consequences for both sediment availability and channel competence. Meltwater in the non-channelised parts of the glacier struggles to transport bedload. As the channelisation develops, accumulated bedload is evacuated until the point at which the subglacial channel network reaches its maximum extent or, due to progressive channel division in a dendritic network, is developing channels that are too small to readily transport it. Thus, recent reports of increases in bedload yield from Alpine glaciers may not reflect increases in glacial erosion but simply an increase in the upstream elevation of snow-lines, the associated subglacial channelization and hence subglacial bedload export capacity.