Lake Agassiz was a large proglacial lake that existed in central North America during the late Pleistocene, fed by meltwater from the retreating Laurentide Ice Sheet at the end of the last glacial per
Traverse Gap in the riverbed of Glacial River Warren. The former southern outlet of Lake Agassiz and source of River Warren is at Lake Traverse at the bottom of the photo; the flooded valley in the center (now Browns Valley) and Big Stone Lake in the distance are relics of the river.
Lockhart Phase of Lake Agassiz, c. 13,000 YBP. Teller and Leverington, 2004 (U.S. Geological Survey)