Heard it was the melting ice shelf that created a dam effect anda huge rush of water right thru the middle of the country like a tsunami
The Missouri River originally flowed north. The Ice sheet in the last major advance blocked that flow, until the water found a way out to the Southeast. The north shore of Lake Sakakawea is roughly the southernmost extent of that ice sheet. There are rattlesnakes on the South side of the Lake/River in Western North Dakota, but not on the north side (where the ice was).