Geochronology and Geomorphology of the Brule Spillway of Glacial Lake Duluth

During the Late Pleistocene and Early Holocene, a series of dyssynchronous advances and retreats resulted in the evolution of the Upper St. Croix River Valley. Proglacial lakes forming at the margins of the retreating ice within the Lake Superior Basin and subsequent multiple drainage events through the Brule Spillway have also drastically shaped the landscape. As a result, the Brule Spillway and broader St. Croix River valley presents an important case study to spillover processes occurring in proglacial landscapes and the reorganization of regional drainage networks and creation of a new hydrogeomorphic regime. While prior literature on the Spillway exists, the paucity of data related to the timing, duration, and magnitude of these drainage events leaves many questions. As a result, this study focuses on a reach of the Upper St. Croix River Valley and Brule Spillway in order to examine the geomorphic impact downstream in the Lower St. Croix River Valley, relation to known lake levels in the Superior Basin, and lastly to test the morphology seen as a result of spillover and outburst flood events in proglacial drainage settings. Methodology to achieve this includes the use of geomorphic mapping, coring, ground penetrating radar (GPR), optically stimulated luminescence dating (OSL), 10Be cosmogenic depth profiling, radiocarbon and X-ray fluorescence (XRF). Based on the results of this research, we are able to add further geochronologic and geomorphic constraints to how the Upper St. Croix River Valley was shaped over time as a result of both ice marginal and proglacial drainage processes. As a result, we hypothesize a series of at least two distinct incisional events within the Brule Spillway. This includes spillover resulting in the initial incision and aggradation prior to the Marquette readvance of the Superior Lobe and a second incisional event resulting from the gradual retreat of the Marquette margins lending to a transgressive ice-marginal outburst flood and the final drainage sequences through the Brule Spillway.