ASSESSMENT OF DRAINAGE PATHWAYS FROM GLACIAL LAKE AGASSIZ

Geomorphic reconstructions are an often used tool to recreate the former extent of large proglacial lakes. Such reconstructions hinge on the regional uplift patterns over time as well as the position of the dam formed by the ice margin. New chronology about the lake levels in the glacial Lake Agassiz basin and new chronology about ice margin positions on former divides provide new boundary conditions for such reconstructions. For the glacial Lake Agassiz basin there is also a low-water phase known as the Moorehead. Current hypothesis attribute this low level to shifting of drainage routes. However, even with the above mentioned improvements unknowns remain and thus a unique reconstruction for specific time intervals is not yet possible. So here we examine multiple proposed uplift histories and pair them with a range of possible ice margin positions to find the combinations that preclude or permit the shifting drainage hypothesis. Specifically, we undertake this work to assess possible diversions of drainage from the Mississippi River basin to either the Atlantic or Arctic watersheds.

For this exercise we pick two former lake levels the Upham/ Tintah (~13.6 ka), which represents the lowered level prior to the Moorehead and the Campbell (~10.5 ka) which represents the first post low-water level. For these two times we used three uplift models based both on former shorelines and one from an ice sheet modeling exercise. We use the Dyke (2004) ice margins as well as a range of syntactic ice margins with similar properties.

For both time intervals, most of the rebound models generated topography that would allow drainage to all three basins of interest. Rather the drainage actually occurred then depends on the location of the ice margin. This study shows that despite unknowns about the rebound model, the ice margin location is a more critical control to assess any former drainage pathways form glacial Lake Agassiz.