The hydrology of glacier‐bed overdeepenings: Sediment transport mechanics, drainage system morphology, and geomorphological implications

Abstract

Evacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesized to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelized flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock-floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice-bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates.

Warming-driven erosion and sediment transport in cold regions

“…The importance of subglacial drainage systems in glacier basal sliding, subglacial erosion, and sediment transport has been increasingly emphasized 74,75,101 . Subglacial channelized drainage systems are effective in evacuating subglacially eroded sediment, especially coarse sediment [101][102][103] , although they are spatially limited and will shrink as glaciers thin.…”

Sharp Increase of Extreme Turbidity Events Due To Deglaciation in the Subtropical Andes

“…Seasonal melt affects sediment export by means of changes in the temporality and quantity of the winter snowfall, and in the summer temperatures that determine the potential melting rate. Sediment availability can be altered by several factors, including (a) a greater access of meltwater to the upper reaches of the subglacial system (Delaney & Adhikari, 2020), (b) variations in the relative position of glacier fronts with respect to overdeepenings (Swift et al., 2021), (c) an increase of landslides deposits as a consequence of permafrost degradation and glaciers retreat associated with temperature rise (Gruber & Haeberli, 2007; Krautblatter et al., 2013), and (d) changes in sediment protection against rain and melting due to changes in vegetation, snow and glacial covers (Costa et al., 2018). As these processes are climate‐controlled, they tend to vary simultaneously, which makes it difficult to differentiate the various signals.…”

Topographic Controls on Channelized Meltwater in the Subglacial Environment

“…We expect the hydraulic gradient to shallow in response to the reverse bed slope, perhaps allowing drainage pathways to migrate more freely. Bed erosion and sediment deposition in response to these factors (Swift et al., 2021) may result in gains and losses specifically in the downward‐incised component (i.e., the morphological record) of the channel system (e.g., Greenwood et al., 2017; Livingstone et al., 2016). Additionally, we expect there were positive feedbacks between channel growth, porewater siphoning, basal melt, and fluvial erosion as channel surface area increased at the ice‐bed interface.…”