GLACIAL LAKE GREAT FALLS AND THE EROSION OF ITS OUTLET CHANNEL (SHONKIN SPILLWAY)

Glacial Lake Great Falls was the westernmost headwaters of the late Wisconsin full glacial Laurentide Ice Sheet – meltwater system. On the basis of a buried tephra, interpreted as Mount St. Helens Set S, in the lakebed sediments and optically stimulated luminescence (OSL), the age of the lake and floods are constrained to 15 500 cal. yr. BP. The lake outlet (Shonkin spillway), a large (750 m wide and 80 m deep), anomalous, dry valley, was eroded into the lower slopes of the Highwood Mountains. Morphologic and sedimentologic evidence suggests this spillway was eroded by non-catastrophic flows from glacial Lake Great Falls and subglacial discharge. Paleo-tributary valleys entering the floodway suggest additional subglacial meltwater inputs from the Laurentide Ice as distance increases from glacial Lake Great Falls. In the spillway, boulder-gravel flood beds in terraces suggest large flows where each terrace represents an incision phase. Electrical resistivity ground images suggest at least 10 m of post-flood-fill on the valley floor. This study will have considerable relevance in the interpretation of other downstream glacial lakes and there flood releases in eastern Montana and North Dakota, possibly releasing large volumes of fresh meltwater into the Mississippi River system.