Differential impact of isolated topographic bumps on glacial ice flow and subglacial processes

Abstract. Topographic highs (“bumps”) across glaciated landscapes have the potential to temporarily slow glacial ice flow or, conversely, increase ice flow through strain heating and subglacial meltwater production. Isolated bumps of variable size across the deglaciated landscape of the Cordilleran Ice Sheet (CIS) of Washington state present an opportunity to assess the influence of topographic highs on ice-bed interactions and ice flow organization. This work utilizes semi-automatic mapping techniques of subglacial bedforms to characterize the morphology of streamlined subglacial bedforms including elongation, surface relief, and orientation – all of which provide insight into subglacial processes during post-Last Glacial Maximum deglaciation of the landscape. We identify a bump-size threshold of ~ 4.5 km³ in which bumps larger than this size will consistently and significantly disrupt both ice-flow organization and subglacial sedimentary processes – fundamental to the genesis of streamlined subglacial bedforms. Additionally, sedimentary processes are most mature downstream of bumps as reflected by enhanced bedform elongation and reduced surface relief, likely due to increased availability and production of subglacial sediment and meltwater. While isolated topography is found to play a role in disrupting ice flow, not all bumps have the same degree of impact. The variable influence of isolated topographic bumps on ice flow in this system has significance for outlet glaciers of the Greenland Ice Sheet (GrIS) due to general topographic similarities.