WATER SOURCE FOR TUNNEL CHANNEL FORMATION IN THE GREEN BAY LOBE, WI

Conditions near the western margin of the Green Bay Lobe of the Laurentide Ice Sheet were appropriate for the sudden release of large volumes of water that flowed along and deeply incised bed material, forming tunnel channels. Hydropotential modeling indicates there were numerous locations where subglacial water could have ponded near the margin of the lobe, but the volume capable of being stored in those subglacial lakes was insufficient to produce the discharge necessary to carve the channels. Rather, water is needed from both subglacial and supraglacial sources, much like observations in present day Greenland. However, tunnel channels are absent along other locations along the western and southern boundary of the Green Bay Lobe despite the fact that similar surface conditions would have led to supraglacial melt. The general occurrence of tunnel channels correlates spatially with regions with adverse bed slopes indicating that basal conditions played a role in the ability for supraglacial waters to pond. We propose that the presence of subglacial lakes sufficiently altered basal traction to the point that glacier surface profiles were locally depressed serving as basins for supraglacial waters. This is because a reduction in basal traction on the upice boundary of the subglacial lake would lead to a longitudinal extension of ice flow, thereby locally thinning the ice. Additionally, the return to high basal traction on the downice side of the subglacial lake would result in an increase in basal traction thereby thickening the ice. Thinning upice and thickening downice of the lake would result in a local reverse of the surface slope that in three dimensions potentially produce a surface depression. These surface depressions would serve as basins for storing supraglacial waters that periodically drained to the bed. Upon drainage of the supraglacial waters to the bed a sudden increase in basal water pressure would ensue opening a basal channel, thereby leading to rapid simultaneous drainage of both the supraglacial and subglacial waters that create the tunnel channels.