Paleogeographic reconstruction of glacial Lake Great Falls in west-central Montana, USA

A series of proglacial lakes were present during the maximum extent of continental glaciation. The paleogeographic reconstruction of the ice-dammed glacial Lake Great Falls, one of the westernmost headwaters of the full glacial Laurentide Ice Sheet-meltwater system, is used to propose a conceptual model for the nature of drainage of the proglacial lake. Based on the stratigraphy of lacustrine sediments as well as erosional and depositional features within the lake basin and outlet channels, it is proposed that flow from glacial Lake Great Falls occurred through three channels as the Laurentide Ice Sheet receded and lake levels lowered. Morphologic and sedimentologic evidence suggests the outlet channels were eroded by multiple large outburst floods from glacial Lake Great Falls when the ice-dam was compromised. Flow estimates range between 0.49 to 3.43 x 105 m3s -1 with flow depths between 10--23 m. The formation of glacial Lake Great Falls required an ice-dam from a continental ice sheet; consequently, the lake history can be used to infer the extent of continental glaciations. The chronology of the proglacial lake is constrained to the late Wisconsin based on luminescence ages as well as a U-series and radiocarbon date. The chronology of glacial Lake Great Falls indicates that the late Wisconsin glaciation was the most extensive in west-central Montana. This study will have considerable relevance in the interpretation of other downstream glacial lakes and their flow releases in eastern Montana and North Dakota which could potentially release large volumes of fresh meltwater into the Mississippi River System.