Lake Great Falls was a prehistoric proglacial lake in central Montana near modern Great Falls, existing from 15,000 BCE to 11,000 BCE. It extended from the Cut Bank area south to Holter Lake, and at present-day Great Falls reached depths of about 600 feet. The lake formed during major drainage changes in the last glacial period, after ice sheets forced earlier north-flowing rivers southward and the Laurentide Ice Sheet blocked the Missouri River. Around 13,000 BCE, glacial retreat triggered a catastrophic outburst flood that sent meltwater through the Highwood Mountains, carving the 100-mile-long, 500-foot-deep Shonkin Sag, one of the world’s notable prehistoric meltwater channels.